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Network Working Group M. Nystroem Request for Comments: 4758 RSA Security Category: Informational November 2006

     Cryptographic Token Key Initialization Protocol (CT-KIP)
                       Version 1.0 Revision 1

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The IETF Trust (2006).

Abstract

 This document constitutes Revision 1 of Cryptographic Token Key
 Initialization Protocol (CT-KIP) Version 1.0 from RSA Laboratories'
 One-Time Password Specifications (OTPS) series.  The body of this
 document, except for the intellectual property considerations
 section, is taken from the CT-KIP Version 1.0 document, but comments
 received during the IETF review are reflected; hence, the status of a
 revised version.  As no "bits-on-the-wire" have changed, the protocol
 specified herein is compatible with CT-KIP Version 1.0.
 CT-KIP is a client-server protocol for initialization (and
 configuration) of cryptographic tokens.  The protocol requires
 neither private-key capabilities in the cryptographic tokens, nor an
 established public-key infrastructure.  Provisioned (or generated)
 secrets will only be available to the server and the cryptographic
 token itself.

Nystroem Informational [Page 1] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

Table of Contents

 1. Introduction ....................................................4
    1.1. Scope ......................................................4
    1.2. Background .................................................4
    1.3. Document Organization ......................................5
 2. Acronyms and Notation ...........................................5
    2.1. Acronyms ...................................................5
    2.2. Notation ...................................................5
 3. CT-KIP ..........................................................6
    3.1. Overview ...................................................6
    3.2. Entities ...................................................7
    3.3. Principles of Operation ....................................7
    3.4. The CT-KIP One-Way Pseudorandom Function, CT-KIP-PRF ......10
         3.4.1. Introduction .......................................10
         3.4.2. Declaration ........................................11
    3.5. Generation of Cryptographic Keys for Tokens ...............11
    3.6. Encryption of Pseudorandom Nonces Sent from the
         CT-KIP Client .............................................12
    3.7. CT-KIP Schema Basics ......................................13
         3.7.1. Introduction .......................................13
         3.7.2. General XML Schema Requirements ....................13
         3.7.3. The AbstractRequestType Type .......................13
         3.7.4. The AbstractResponseType type ......................14
         3.7.5. The StatusCode Type ................................14
         3.7.6. The IdentifierType Type ............................16
         3.7.7. The NonceType Type .................................16
         3.7.8. The ExtensionsType and the
                AbstractExtensionType Types ........................17
    3.8. CT-KIP Messages ...........................................17
         3.8.1. Introduction .......................................17
         3.8.2. CT-KIP Initialization ..............................17
         3.8.3. The CT-KIP Client's Initial PDU ....................18
         3.8.4. The CT-KIP server's initial PDU ....................20
         3.8.5. The CT-KIP Client's Second PDU .....................23
         3.8.6. The CT-KIP Server's Final PDU ......................24
    3.9. Protocol Extensions .......................................27
         3.9.1. The ClientInfoType Type ............................27
         3.9.2. The ServerInfoType Type ............................28
         3.9.3. The OTPKeyConfigurationDataType Type ...............28
 4. Protocol Bindings ..............................................29
    4.1. General Requirement .......................................29
    4.2. HTTP/1.1 binding for CT-KIP ...............................29
         4.2.1. Introduction .......................................29
         4.2.2. Identification of CT-KIP Messages ..................29
         4.2.3. HTTP Headers .......................................29
         4.2.4. HTTP Operations ....................................30
         4.2.5. HTTP Status Codes ..................................30

Nystroem Informational [Page 2] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

         4.2.6. HTTP Authentication ................................31
         4.2.7. Initialization of CT-KIP ...........................31
         4.2.8. Example Messages ...................................31
 5. Security considerations ........................................32
    5.1. General ...................................................32
    5.2. Active Attacks ............................................32
         5.2.1. Introduction .......................................32
         5.2.2. Message Modifications ..............................32
         5.2.3. Message Deletion ...................................34
         5.2.4. Message Insertion ..................................34
         5.2.5. Message Replay .....................................34
         5.2.6. Message Reordering .................................35
         5.2.7. Man in the Middle ..................................35
    5.3. Passive Attacks ...........................................35
    5.4. Cryptographic Attacks .....................................35
    5.5. Attacks on the Interaction between CT-KIP and User
         Authentication ............................................36
 6. Intellectual Property Considerations ...........................36
 7. References .....................................................37
    7.1. Normative References ......................................37
    7.2. Informative References ....................................37
 Appendix A. CT-KIP Schema .........................................39
 Appendix B. Examples of CT-KIP Messages ...........................46
    B.1. Introduction ..............................................46
    B.2. Example of a CT-KIP Initialization (Trigger) Message ......46
    B.3. Example of a <ClientHello> Message ........................46
    B.4. Example of a <ServerHello> Message ........................47
    B.5. Example of a <ClientNonce> Message ........................47
    B.6. Example of a <ServerFinished> Message .....................48
 Appendix C. Integration with PKCS #11 .............................48
 Appendix D. Example CT-KIP-PRF Realizations .......................48
    D.1. Introduction ..............................................48
    D.2. CT-KIP-PRF-AES ............................................48
         D.2.1. Identification .....................................48
         D.2.2. Definition .........................................49
         D.2.3. Example ............................................50
    D.3. CT-KIP-PRF-SHA256 .........................................50
         D.3.1. Identification .....................................50
         D.3.2. Definition .........................................51
         D.3.3. Example ............................................52
 Appendix E. About OTPS ............................................53

Nystroem Informational [Page 3] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

1. Introduction

 Note: This document is Revision 1 of CT-KIP Version 1.0 [12] from RSA
 Laboratories' OTPS series.

1.1. Scope

 This document describes a client-server protocol for initialization
 (and configuration) of cryptographic tokens.  The protocol requires
 neither private-key capabilities in the cryptographic tokens, nor an
 established public-key infrastructure.
 The objectives of this protocol are:
 o  To provide a secure method of initializing cryptographic tokens
    with secret keys without exposing generated, secret material to
    any other entities than the server and the cryptographic token
    itself,
 o  To avoid, as much as possible, any impact on existing
    cryptographic token manufacturing processes,
 o  To provide a solution that is easy to administer and scales well.
 The mechanism is intended for general use within computer and
 communications systems employing connected cryptographic tokens (or
 software emulations thereof).

1.2. Background

 A cryptographic token may be a handheld hardware device, a hardware
 device connected to a personal computer through an electronic
 interface such as USB, or a software module resident on a personal
 computer, which offers cryptographic functionality that may be used,
 e.g., to authenticate a user towards some service.  Increasingly,
 these tokens work in a connected fashion, enabling their programmatic
 initialization as well as programmatic retrieval of their output
 values.  This document intends to meet the need for an open and
 interoperable mechanism to programmatically initialize and configure
 connected cryptographic tokens.  A companion document entitled "A
 PKCS #11 Mechanism for the Cryptographic Token Key Initialization
 Protocol" [2] describes an application-programming interface suitable
 for use with this mechanism.

Nystroem Informational [Page 4] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

1.3. Document Organization

 The organization of this document is as follows:
 o  Section 1 is an introduction.
 o  Section 2 defines some notation used in this document.
 o  Section 3 defines the protocol mechanism in detail.
 o  Section 4 defines a binding of the protocol to transports.
 o  Section 5 provides security considerations.
 o  Appendix A defines the XML schema for the protocol mechanism,
    Appendix B gives example messages, and Appendix C discusses
    integration with PKCS #11 [3].
 o  Appendix D provides example realizations of an abstract
    pseudorandom function defined in Section 3.
 o  Appendix E provides general information about the One-Time
    Password Specifications.

2. Acronyms and Notation

2.1. Acronyms

 MAC      Message Authentication Code
 PDU      Protocol Data Unit
 PRF      Pseudo-Random Function
 CT-KIP   Cryptographic Token Key Initialization Protocol (the
          protocol mechanism described herein)

2.2. Notation

 ||       String concatenation
 [x]      Optional element x
 A ^ B    Exclusive-or operation on strings A and B (A and B of equal
          length)
 K_AUTH   Secret key used for authentication purposes

Nystroem Informational [Page 5] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 K_TOKEN  Secret key used for token computations, generated in CT-KIP
 K_SERVER Public key of CT-KIP server
 K_SHARED Secret key shared between the cryptographic token and the
          CT-KIP server
 K        Key used to encrypt R_C (either K_SERVER or K_SHARED)
 R        Pseudorandom value chosen by the cryptographic token and
          used for MAC computations
 R_C      Pseudorandom value chosen by the cryptographic token
 R_S      Pseudorandom value chosen by the CT-KIP server
 The following typographical convention is used in the body of the
 text: <XMLElement>.

3. CT-KIP

3.1. Overview

 The CT-KIP is a client-server protocol for the secure initialization
 of cryptographic tokens.  The protocol is meant to provide high
 assurance for both the server and the client (cryptographic token)
 that generated keys have been correctly and randomly generated and
 not exposed to other entities.  The protocol does not require the
 existence of a public-key infrastructure.
 +---------------+                            +---------------+
 |               |                            |               |
 | CT-KIP client |                            | CT-KIP server |
 |               |                            |               |
 +---------------+                            +---------------+
         |                                            |
         |        [ <---- CT-KIP trigger ---- ]       |
         |                                            |
         |        ------- Client Hello ------->       |
         |                                            |
         |        <------ Server Hello --------       |
         |                                            |
         |        ------- Client Nonce ------->       |
         |                                            |
         |        <----- Server Finished ------       |
 Figure 1: The 4-pass CT-KIP protocol (with optional preceding
 trigger)

Nystroem Informational [Page 6] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

3.2. Entities

 In principle, the protocol involves a CT-KIP client and a CT-KIP
 server.
 It is assumed that a desktop/laptop or a wireless device (e.g., a
 mobile phone or a PDA) will host an application communicating with
 the CT-KIP server as well as the cryptographic token, and
 collectively, the cryptographic token and the communicating
 application form the CT-KIP client.  When there is a need to point
 out if an action is to be performed by the communicating application
 or by the token the text will make this explicit.
 The manner in which the communicating application will transfer CT-
 KIP protocol elements to and from the cryptographic token is
 transparent to the CT-KIP server.  One method for this transfer is
 described in [2].

3.3. Principles of Operation

 To initiate a CT-KIP session, a user may use a browser to connect to
 a web server running on some host.  The user may then identify (and
 authenticate) herself (through some means that essentially are out of
 scope for this document) and possibly indicate how the CT-KIP client
 shall contact the CT-KIP server.  There are also other alternatives
 for CT-KIP session initiation, such as the CT-KIP client being pre-
 configured to contact a certain CT-KIP server, or the user being
 informed out-of-band about the location of the CT-KIP server.  In any
 event, once the location of the CT-KIP server is known, the CT-KIP
 client and the CT-KIP server engage in a 4-pass protocol in which:
 a.  The CT-KIP client provides information to the CT-KIP server about
     the cryptographic token's identity, supported CT-KIP versions,
     cryptographic algorithms supported by the token and for which
     keys may be generated using this protocol, and encryption and MAC
     algorithms supported by the cryptographic token for the purposes
     of this protocol.
 b.  Based on this information, the CT-KIP server provides a random
     nonce, R_S, to the CT-KIP client, along with information about
     the type of key to generate, the encryption algorithm chosen to
     protect sensitive data sent in the protocol.  In addition, it
     provides either information about a shared secret key to use for
     encrypting the cryptographic token's random nonce (see below), or
     its own public key.  The length of the nonce R_S may depend on
     the selected key type.

Nystroem Informational [Page 7] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 c.  The cryptographic token generates a random nonce R_C and encrypts
     it using the selected encryption algorithm and with a key K that
     is either the CT-KIP server's public key K_SERVER, or a shared
     secret key K_SHARED as indicated by the CT-KIP server.  The
     length of the nonce R_C may depend on the selected key type.  The
     CT-KIP client then sends the encrypted random nonce to the CT-KIP
     server.  The token also calculates a cryptographic key K_TOKEN of
     the selected type from the combination of the two random nonces
     R_S and R_C, the encryption key K, and possibly some other data,
     using the CT-KIP-PRF function defined herein.
 d.  The CT-KIP server decrypts R_C, calculates K_TOKEN from the
     combination of the two random nonces R_S and R_C, the encryption
     key K, and possibly some other data, using the CT-KIP-PRF
     function defined herein.  The server then associates K_TOKEN with
     the cryptographic token in a server-side data store.  The intent
     is that the data store later on will be used by some service that
     needs to verify or decrypt data produced by the cryptographic
     token and the key.
 e.  Once the association has been made, the CT-KIP server sends a
     confirmation message to the CT-KIP client.  The confirmation
     message includes an identifier for the generated key and may also
     contain additional configuration information, e.g., the identity
     of the CT-KIP server.
 f.  Upon receipt of the CT-KIP server's confirmation message, the
     cryptographic token associates the provided key identifier with
     the generated key K_TOKEN, and stores the provided configuration
     data, if any.
 Note: Conceptually, although R_C is one pseudorandom string, it may
 be viewed as consisting of two components, R_C1 and R_C2, where R_C1
 is generated during the protocol run, and R_C2 can be generated at
 the cryptographic token manufacturing time and stored in the
 cryptographic token.  In that case, the latter string, R_C2, should
 be unique for each cryptographic token for a given manufacturer.

Nystroem Informational [Page 8] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 +----------------------+    +-------+     +----------------------+
 |    +------------+    |    |       |     |                      |
 |    | Server key |    |    |       |     |                      |
 | +<-|  Public    |------>------------->-------------+---------+ |
 | |  |  Private   |    |    |       |     |          |         | |
 | |  +------------+    |    |       |     |          |         | |
 | |        |           |    |       |     |          |         | |
 | V        V           |    |       |     |          V         V |
 | |   +---------+      |    |       |     |        +---------+ | |
 | |   | Decrypt |<-------<-------------<-----------| Encrypt | | |
 | |   +---------+      |    |       |     |        +---------+ | |
 | |      |  +--------+ |    |       |     |            ^       | |
 | |      |  | Server | |    |       |     |            |       | |
 | |      |  | Random |--->------------->------+  +----------+  | |
 | |      |  +--------+ |    |       |     |   |  | Client   |  | |
 | |      |      |      |    |       |     |   |  | Random   |  | |
 | |      |      |      |    |       |     |   |  +----------+  | |
 | |      |      |      |    |       |     |   |        |       | |
 | |      V      V      |    |       |     |   V        V       | |
 | |   +------------+   |    |       |     | +------------+     | |
 | +-->| CT-KIP PRF |   |    |       |     | | CT-KIP PRF |<----+ |
 |     +------------+   |    |       |     | +------------+       |
 |           |          |    |       |     |       |              |
 |           V          |    |       |     |       V              |
 |       +-------+      |    |       |     |   +-------+          |
 |       |  Key  |      |    |       |     |   |  Key  |          |
 |       +-------+      |    |       |     |   +-------+          |
 |       +-------+      |    |       |     |   +-------+          |
 |       |Key Id |-------->------------->------|Key Id |          |
 |       +-------+      |    |       |     |   +-------+          |
 +----------------------+    +-------+     +----------------------+
      CT-KIP Server        CT-KIP Client     CT-KIP Client (Token)
                             (PC Host)
 Figure 2: Principal data flow for CT-KIP key generation - using
 public server key
 The inclusion of the two random nonces R_S and R_C in the key
 generation provides assurance to both sides (the token and the CT-KIP
 server) that they have contributed to the key's randomness and that
 the key is unique.  The inclusion of the encryption key K ensures
 that no man-in-the-middle may be present, or else the cryptographic
 token will end up with a key different from the one stored by the
 legitimate CT-KIP server.
 Note: A man-in-the middle (in the form of corrupt client software or
 a mistakenly contacted server) may present his own public key to the
 token.  This will enable the attacker to learn the client's version

Nystroem Informational [Page 9] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 of K_TOKEN.  However, the attacker is not able to persuade the
 legitimate server to derive the same value for K_TOKEN, since K_TOKEN
 is a function of the public key involved, and the attacker's public
 key must be different than the correct server's (or else the attacker
 would not be able to decrypt the information received from the
 client).  Therefore, once the attacker is no longer "in the middle",
 the client and server will detect that they are "out of synch" when
 they try to use their keys.  Therefore, in the case of encrypting R_C
 with K_SERVER, it is important to verify that K_SERVER really is the
 legitimate server's key.  One way to do this is to independently
 validate a newly generated K_TOKEN against some validation service at
 the server (e.g., by using a connection independent from the one used
 for the key generation).
 The CT-KIP server may couple an initial user authentication to the
 CT-KIP execution in several ways to ensure that a generated K_TOKEN
 ends up associated with the correct token and user.  One way is to
 provide a one-time value to the user or CT-KIP client after
 successful user authentication and require this value to be used when
 contacting the CT-KIP service (in effect coupling the user
 authentication with the subsequent CT-KIP protocol run).  This value
 could, for example, be placed in a <TriggerNonce> element of the CT-
 KIP initialization trigger (if triggers are used; see Section 4.2.7).
 Another way is for the user to provide a token identifier which will
 later be used in the CT-KIP protocol to the server during the
 authentication phase.  The server may then include this token
 identifier in the CT-KIP initialization trigger.  It is also
 legitimate for a CT-KIP client to initiate a CT-KIP protocol run
 without having received an initialization message from a server, but
 in this case any provided token identifier shall not be accepted by
 the server unless the server has access to a unique token key for the
 identified token and that key will be used in the protocol.  Whatever
 the method, the CT-KIP server must ensure that a generated key is
 associated with the correct token and, if applicable, the correct
 user.  For a further discussion of this and threats related to man-
 in-the-middle attacks in this context, see Section 5.5.

3.4. The CT-KIP One-Way Pseudorandom Function, CT-KIP-PRF

3.4.1. Introduction

 The general requirements on CT-KIP-PRF are the same as on keyed hash
 functions: It shall take an arbitrary length input, and be one-way
 and collision-free (for a definition of these terms, see, e.g., [4]).
 Further, the CT-KIP-PRF function shall be capable of generating a
 variable-length output, and its output shall be unpredictable even if
 other outputs for the same key are known.

Nystroem Informational [Page 10] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 It is assumed that any realization of CT-KIP-PRF takes three input
 parameters: A secret key k, some combination of variable data, and
 the desired length of the output.  Examples of the variable data
 include, but are not limited to, a current token counter value, the
 current token time, and a challenge.  The combination of variable
 data can, without loss of generalization, be considered as a salt
 value (see PKCS #5 Version 2.0 [5], Section 4), and this
 characterization of CT-KIP-PRF should fit all actual PRF algorithms
 implemented by tokens.  From the point of view of this specification,
 CT-KIP-PRF is a "black-box" function that, given the inputs,
 generates a pseudorandom value.
 Separate specifications may define the implementation of CT-KIP-PRF
 for various types of cryptographic tokens.  Appendix D contains two
 example realizations of CT-KIP-PRF.

3.4.2. Declaration

 CT-KIP-PRF (k, s, dsLen)
 Input:
 k     secret key in octet string format
 s     octet string of varying length consisting of variable data
       distinguishing the particular string being derived
 dsLen desired length of the output
 Output:
 DS    pseudorandom string, dsLen-octets long
 For the purposes of this document, the secret key k shall be 16
 octets long.

3.5. Generation of Cryptographic Keys for Tokens

 In CT-KIP, keys are generated using the CT-KIP-PRF function, a secret
 random value R_C chosen by the CT-KIP client, a random value R_S
 chosen by the CT-KIP server, and the key k used to encrypt R_C.  The
 input parameter s of CT-KIP-PRF is set to the concatenation of the
 (ASCII) string "Key generation", k, and R_S, and the input parameter
 dsLen is set to the desired length of the key, K_TOKEN (the length of
 K_TOKEN is given by the key's type):

Nystroem Informational [Page 11] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 dsLen = (desired length of K_TOKEN)
 K_TOKEN = CT-KIP-PRF (R_C, "Key generation" || k || R_S, dsLen)
 When computing K_TOKEN above, the output of CT-KIP-PRF may be subject
 to an algorithm-dependent transform before being adopted as a key of
 the selected type.  One example of this is the need for parity in DES
 keys.

3.6. Encryption of Pseudorandom Nonces Sent from the CT-KIP Client

 CT-KIP client random nonce(s) are either encrypted with the public
 key provided by the CT-KIP server or by a shared secret key.  For
 example, in the case of a public RSA key, an RSA encryption scheme
 from PKCS #1 [6] may be used.
 In the case of a shared secret key, to avoid dependence on other
 algorithms, the CT-KIP client may use the CT-KIP-PRF function
 described herein with the shared secret key K_SHARED as input
 parameter k (in this case, K_SHARED should be used solely for this
 purpose), the concatenation of the (ASCII) string "Encryption" and
 the server's nonce R_S as input parameter s, and dsLen set to the
 length of R_C:
 dsLen = len(R_C)
 DS = CT-KIP-PRF(K_SHARED, "Encryption" || R_S, dsLen)
 This will produce a pseudorandom string DS of length equal to R_C.
 Encryption of R_C may then be achieved by XOR-ing DS with R_C:
 Enc-R_C = DS ^ R_C
 The CT-KIP server will then perform the reverse operation to extract
 R_C from Enc-R_C.
 Note: It may appear that an attacker, who learns a previous value of
 R_C, may be able to replay the corresponding R_S and, hence, learn a
 new R_C as well.  However, this attack is mitigated by the
 requirement for a server to show knowledge of K_AUTH (see below) in
 order to successfully complete a key re-generation.

Nystroem Informational [Page 12] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

3.7. CT-KIP Schema Basics

3.7.1. Introduction

 Core parts of the XML schema for CT-KIP, found in Appendix A, are
 explained in this section.  Specific protocol message elements are
 defined in Section 3.8.  Examples can be found in Appendix B.
 The XML format for CT-KIP messages have been designed to be
 extensible.  However, it is possible that the use of extensions will
 harm interoperability; therefore, any use of extensions should be
 carefully considered.  For example, if a particular implementation
 relies on the presence of a proprietary extension, then it may not be
 able to interoperate with independent implementations that have no
 knowledge of this extension.
 XML types defined in this sub-section are not CT-KIP messages; rather
 they provide building blocks that are used by CT-KIP messages.

3.7.2. General XML Schema Requirements

 Some CT-KIP elements rely on the parties being able to compare
 received values with stored values.  Unless otherwise noted, all
 elements in this document that have the XML Schema "xs:string" type,
 or a type derived from it, must be compared using an exact binary
 comparison.  In particular, CT-KIP implementations must not depend on
 case-insensitive string comparisons, normalization or trimming of
 white space, or conversion of locale-specific formats such as
 numbers.
 Implementations that compare values that are represented using
 different character encodings must use a comparison method that
 returns the same result as converting both values to the Unicode
 character encoding, Normalization Form C [1], and then performing an
 exact binary comparison.
 No collation or sorting order for attributes or element values is
 defined.  Therefore, CT-KIP implementations must not depend on
 specific sorting orders for values.

3.7.3. The AbstractRequestType Type

 All CT-KIP requests are defined as extensions to the abstract
 AbstractRequestType type.  The elements of the AbstractRequestType,
 therefore, apply to all CT-KIP requests.  All CT-KIP requests must
 contain a Version attribute.  For this version of this specification,
 Version shall be set to "1.0".

Nystroem Informational [Page 13] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 <xs:complexType name="AbstractRequestType" abstract="true">
   <xs:attribute name="Version" type="VersionType"
    use="required"/>
 </xs:complexType>

3.7.4. The AbstractResponseType type

 All CT-KIP responses are defined as extensions to the abstract
 AbstractResponseType type.  The elements of the AbstractResponseType,
 therefore, apply to all CT-KIP responses.  All CT-KIP responses
 contain a Version attribute indicating the version that was used.  A
 Status attribute, which indicates whether the preceding request was
 successful or not must also be present.  Finally, all responses may
 contain a SessionID attribute identifying the particular CT-KIP
 session.  The SessionID attribute needs only be present if more than
 one roundtrip is required for a successful protocol run (this is the
 case with the protocol version described herein).
 <xs:complexType name="AbstractResponseType" abstract="true">
   <xs:attribute name="Version" type="VersionType" use="required"/>
   <xs:attribute name="SessionID" type="IdentifierType"/>
   <xs:attribute name="Status" type="StatusCode" use="required"/>
 </xs:complexType>

3.7.5. The StatusCode Type

 The StatusCode type enumerates all possible return codes:
 <xs:simpleType name="StatusCode">
   <xs:restriction base="xs:string">
     <xs:enumeration value="Continue"/>
     <xs:enumeration value="Success"/>
     <xs:enumeration value="Abort"/>
     <xs:enumeration value="AccessDenied"/>
     <xs:enumeration value="MalformedRequest"/>
     <xs:enumeration value="UnknownRequest"/>
     <xs:enumeration value="UnknownCriticalExtension"/>
     <xs:enumeration value="UnsupportedVersion"/>
     <xs:enumeration value="NoSupportedKeyTypes"/>
     <xs:enumeration value="NoSupportedEncryptionAlgorithms"/>
     <xs:enumeration value="NoSupportedMACAlgorithms"/>
     <xs:enumeration value="InitializationFailed"/>
   </xs:restriction>
 </xs:simpleType>
 Upon transmission or receipt of a message for which the Status
 attribute's value is not "Success" or "Continue", the default
 behavior, unless explicitly stated otherwise below, is that both the

Nystroem Informational [Page 14] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 CT-KIP server and the CT-KIP client shall immediately terminate the
 CT-KIP session.  CT-KIP servers and CT-KIP clients must delete any
 secret values generated as a result of failed runs of the CT-KIP
 protocol.  Session identifiers may be retained from successful or
 failed protocol runs for replay detection purposes, but such retained
 identifiers shall not be reused for subsequent runs of the protocol.
 When possible, the CT-KIP client should present an appropriate error
 message to the user.
 These status codes are valid in all CT-KIP-Response messages unless
 explicitly stated otherwise.
 o  "Continue" indicates that the CT-KIP server is ready for a
    subsequent request from the CT-KIP client.  It cannot be sent in
    the server's final message.
 o  "Success" indicates successful completion of the CT-KIP session.
    It can only be sent in the server's final message.
 o  "Abort" indicates that the CT-KIP server rejected the CT-KIP
    client's request for unspecified reasons.
 o  "AccessDenied" indicates that the CT-KIP client is not authorized
    to contact this CT-KIP server.
 o  "MalformedRequest" indicates that the CT-KIP server failed to
    parse the CT-KIP client's request.
 o  "UnknownRequest" indicates that the CT-KIP client made a request
    that is unknown to the CT-KIP server.
 o  "UnknownCriticalExtension" indicates that a critical CT-KIP
    extension (see below) used by the CT-KIP client was not supported
    or recognized by the CT-KIP server.
 o  "UnsupportedVersion" indicates that the CT-KIP client used a CT-
    KIP protocol version not supported by the CT-KIP server.  This
    error is only valid in the CT-KIP server's first response message.
 o  "NoSupportedKeyTypes" indicates that the CT-KIP client only
    suggested key types that are not supported by the CT-KIP server.
    This error is only valid in the CT-KIP server's first response
    message.  Note that the error will only occur if the CT-KIP server
    does not support any of the CT-KIP client's suggested key types.

Nystroem Informational [Page 15] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 o  "NoSupportedEncryptionAlgorithms" indicates that the CT-KIP client
    only suggested encryption algorithms that are not supported by the
    CT-KIP server.  This error is only valid in the CT-KIP server's
    first response message.  Note that the error will only occur if
    the CT-KIP server does not support any of the CT-KIP client's
    suggested encryption algorithms.
 o  "NoSupportedMACAlgorithms" indicates that the CT-KIP client only
    suggested MAC algorithms that are not supported by the CT-KIP
    server.  This error is only valid in the CT-KIP server's first
    response message.  Note that the error will only occur if the CT-
    KIP server does not support any of the CT-KIP client's suggested
    MAC algorithms.
 o  "InitializationFailed" indicates that the CT-KIP server could not
    generate a valid key given the provided data.  When this status
    code is received, the CT-KIP client should try to restart CT-KIP,
    as it is possible that a new run will succeed.

3.7.6. The IdentifierType Type

 The IdentifierType type is used to identify various CT-KIP elements,
 such as sessions, users, and services.  Identifiers must not be
 longer than 128 octets.
 <xs:simpleType name="IdentifierType">
   <xs:restriction base="xs:string">
     <xs:maxLength value="128"/>
   </xs:restriction>
 </xs:simpleType>

3.7.7. The NonceType Type

 The NonceType type is used to carry pseudorandom values in CT-KIP
 messages.  A nonce, as the name implies, must be used only once.  For
 each CT-KIP message that requires a nonce element to be sent, a fresh
 nonce shall be generated each time.  Nonce values must be at least 16
 octets long.
 <xs:simpleType name="NonceType">
   <xs:restriction base="xs:base64Binary">
     <xs:minLength value="16"/>
   </xs:restriction>
 </xs:simpleType>

Nystroem Informational [Page 16] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

3.7.8. The ExtensionsType and the AbstractExtensionType Types

 The ExtensionsType type is a list of type-value pairs that define
 optional CT-KIP features supported by a CT-KIP client or server.
 Extensions may be sent with any CT-KIP message.  Please see the
 description of individual CT-KIP messages in Section 3.8 of this
 document for applicable extensions.  Unless an extension is marked as
 Critical, a receiving party need not be able to interpret it.  A
 receiving party is always free to disregard any (non-critical)
 extensions.
 <xs:complexType name="AbstractExtensionsType">
   <xs:sequence maxOccurs="unbounded">
     <xs:element name="Extension" type="AbstractExtensionType"/>
   </xs:sequence>
 </xs:complexType>
 <xs:complexType name="AbstractExtensionType" abstract="true">
   <xs:attribute name="Critical" type="xs:boolean"/>
 </xs:complexType>

3.8. CT-KIP Messages

3.8.1. Introduction

 In this section, CT-KIP messages, including their parameters,
 encodings and semantics are defined.

3.8.2. CT-KIP Initialization

 The CT-KIP server may initialize the CT-KIP protocol by sending a
 <CT-KIPTrigger> message.  This message may, e.g., be sent in response
 to a user requesting token initialization in a browsing session.
 <xs:complexType name="InitializationTriggerType">
   <xs:sequence>
     <xs:element name="TokenID" type="xs:base64Binary" minOccurs="0"/>
     <xs:element name="KeyID" type="xs:base64Binary" minOccurs="0"/>
     <xs:element name="TokenPlatformInfo"
       type="TokenPlatformInfoType" minOccurs="0"/>
     <xs:element name="TriggerNonce" type="NonceType"/>
     <xs:element name="CT-KIPURL" type="xs:anyURI" minOccurs="0"/>
     <xs:any namespace="##other" processContents="strict"
       minOccurs="0"/>
   </xs:sequence>
   <xs:attribute name="id" type="xs:ID"/>
 </xs:complexType>

Nystroem Informational [Page 17] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 <xs:element name="CT-KIPTrigger" type="CT-KIPTriggerType"/>
 <xs:complexType name="CT-KIPTriggerType">
   <xs:annotation>
     <xs:documentation xml:lang="en">
        Message used to trigger the device to initiate a
        CT-KIP run.
     </xs:documentation>
   </xs:annotation>
   <xs:sequence>
     <xs:choice>
       <xs:element name="InitializationTrigger"
         type="InitializationTriggerType"/>
       <xs:any nameSpace="##other" processContents="strict"/>
     </xs:choice>
   </xs:sequence>
   <xs:attribute name="Version" type="ct-kip:VersionType"/>
 </xs:complexType>
 The <CT-KIPTrigger> element is intended for the CT-KIP client and may
 inform the CT-KIP client about the identifier for the token that is
 to be initialized, and, optionally, of the identifier for the key on
 that token.  The latter would apply when re-seeding.  The trigger
 always contains a nonce to allow the server to couple the trigger
 with a later CT-KIP <ClientHello> request.  Finally, the trigger may
 contain a URL to use when contacting the CT-KIP server.  The <xs:any>
 elements are for future extensibility.  Any provided <TokenID> or
 <KeyID> values shall be used by the CT-KIP client in the subsequent
 <ClientHello> request.  The optional <TokenPlatformInfo> element
 informs the CT-KIP client about the characteristics of the intended
 token platform, and applies in the public-key variant of CT-KIP in
 situations when the client potentially needs to decide which one of
 several tokens to initialize.
 The Version attribute shall be set to "1.0" for this version of CT-
 KIP.

3.8.3. The CT-KIP Client's Initial PDU

 This message is the initial message sent from the CT-KIP client to
 the CT-KIP server.
 <xs:element name="ClientHello" type="ClientHelloPDU"/>
 <xs:complexType name="ClientHelloPDU">
   <xs:annotation>
     <xs:documentation xml:lang="en">
        Message sent from CT-KIP client to CT-KIP server to

Nystroem Informational [Page 18] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

        initiate a CT-KIP session.
     </xs:documentation>
   </xs:annotation>
   <xs:complexContent>
     <xs:extension base="AbstractRequestType">
       <xs:sequence>
         <xs:element name="TokenID"
           type="xs:base64Binary" minOccurs="0"/>
         <xs:element name="KeyID"
           type="xs:base64Binary" minOccurs="0"/>
         <xs:element name="ClientNonce"
           type="NonceType" minOccurs="0"/>
         <xs:element name= "TriggerNonce"
           type="NonceType" minOccurs="0"/>
         <xs:element name="SupportedKeyTypes"
           type="AlgorithmsType"/>
         <xs:element name="SupportedEncryptionAlgorithms"
           type="AlgorithmsType"/>
         <xs:element name="SupportedMACAlgorithms"
           type="AlgorithmsType"/>
         <xs:element name="Extensions"
           type="ExtensionsType" minOccurs="0"/>
       </xs:sequence>
     </xs:extension>
   </xs:complexContent>
 </xs:complexType>
 The components of this message have the following meaning:
 o  Version: (attribute inherited from the AbstractRequestType type)
    The highest version of this protocol the client supports.  Only
    version one ("1.0") is currently specified.
 o  <TokenID>: An identifier for the cryptographic token (allows the
    server to find, e.g., a correct shared secret for MACing
    purposes).  The identifier shall only be present if such shared
    secrets exist or if the identifier was provided by the server in a
    <CT-KIPTrigger> element (see Section 4.2.7 below).  In the latter
    case, it must have the same value as the identifier provided in
    that element.
 o  <KeyID>: An identifier for the key that will be overwritten if the
    protocol run is successful.  The identifier shall only be present
    if the key exists or was provided by the server in a
    <CT-KIPTrigger> element (see Section 4.2.7 below).  In the latter
    case, it must have the same value as the identifier provided in
    that element.

Nystroem Informational [Page 19] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 o  <ClientNonce>: This is the nonce R, which, when present, shall be
    used by the server when calculating MAC values (see below).  It is
    recommended that clients include this element whenever the <KeyID>
    element is present.
 o  <TriggerNonce>: This optional element shall be present if and only
    if the CT-KIP run was initialized with a <CT-KIPTrigger> message
    (see Section 4.2.7 below), and shall, in that case, have the same
    value as the <TriggerNonce> child of that message.  A server using
    nonces in this way must verify that the nonce is valid and that
    any token or key identifier values provided in the <CT-KIPTrigger>
    message match the corresponding identifier values in the
    <ClientHello> message.
 o  <SupportedKeyTypes>: A sequence of URIs indicating the key types
    for which the token is willing to generate keys through CT-KIP.
 o  <SupportedEncryptionAlgorithms>: A sequence of URIs indicating the
    encryption algorithms supported by the cryptographic token for the
    purposes of CT-KIP.  The CT-KIP client may indicate the same
    algorithm both as a supported key type and as an encryption
    algorithm.
 o  <SupportedMACAlgorithms>: A sequence of URIs indicating the MAC
    algorithms supported by the cryptographic token for the purposes
    of CT-KIP.  The CT-KIP client may indicate the same algorithm both
    as an encryption algorithm and as a MAC algorithm (e.g., http://
    www.rsasecurity.com/rsalabs/otps/schemas/2005/12/
    ct-kip#ct-kip-prf-aes defined in Appendix D)
 o  <Extensions>: A sequence of extensions.  One extension is defined
    for this message in this version of CT-KIP: the ClientInfoType
    (see Section 3.9.1).

3.8.4. The CT-KIP server's initial PDU

 This message is the first message sent from the CT-KIP server to the
 CT-KIP client (assuming a trigger message has not been sent to
 initiate the protocol, in which case, this message is the second
 message sent from the CT-KIP server to the CT-KIP client).  It is
 sent upon reception of a <ClientHello> message.
 <xs:element name="ServerHello" type="ServerHelloPDU"/>
 <xs:complexType name="ServerHelloPDU">
   <xs:annotation>
     <xs:documentation xml:lang="en">
       Message sent from CT-KIP server to CT-KIP

Nystroem Informational [Page 20] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

       client in response to a received ClientHello
       PDU.
     </xs:documentation>
   </xs:annotation>
   <xs:complexContent>
     <xs:extension base="AbstractResponseType">
       <xs:sequence minOccurs="0">
         <xs:element name="KeyType"
           type="AlgorithmType"/>
         <xs:element name="EncryptionAlgorithm"
           type="AlgorithmType"/>
         <xs:element name="MacAlgorithm"
           type="AlgorithmType"/>
         <xs:element name="EncryptionKey"
           type="ds:KeyInfoType"/>
         <xs:element name="Payload"
           type="PayloadType"/>
         <xs:element name="Extensions"
           type="ExtensionsType" minOccurs="0"/>
         <xs:element name="Mac" type="MacType"
           minOccurs="0"/>
       </xs:sequence>
     </xs:extension>
   </xs:complexContent>
 </xs:complexType>
 <xs:complexType name="PayloadType">
   <xs:annotation>
     <xs:documentation xml:lang="en">
       Currently, only the nonce is defined.  In future versions,
       other payloads may be defined, e.g., for one-roundtrip
       initialization protocols.
     </xs:documentation>
   </xs:annotation>
   <xs:choice>
     <xs:element name="Nonce" type="NonceType"/>
     <any namespace="##other" processContents="strict"/>
   </xs:choice>
 </xs:complexType>
 <xs:complexType name="MacType">
   <xs:simpleContent>
     <xs:extension base="xs:base64Binary">
       <xs:attribute name="MacAlgorithm" type="xs:anyURI"/>
     </xs:extension>
   </xs:simpleContent>
 </xs:complexType>

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 The components of this message have the following meaning:
 o  Version: (attribute inherited from the AbstractResponseType type)
    The version selected by the CT-KIP server.  May be lower than the
    version indicated by the CT-KIP client, in which case, local
    policy at the client will determine whether or not to continue the
    session.
 o  SessionID: (attribute inherited from the AbstractResponseType
    type) An identifier for this session.
 o  Status: (attribute inherited from the abstract
    AbstractResponseType type) Return code for the <ClientHello>.  If
    Status is not "Continue", only the Status and Version attributes
    will be present; otherwise, all the other elements must be present
    as well.
 o  <KeyType>: The type of the key to be generated.
 o  <EncryptionAlgorithm>: The encryption algorithm to use when
    protecting R_C.
 o  <MacAlgorithm>: The MAC algorithm to be used by the CT-KIP server.
 o  <EncryptionKey>: Information about the key to use when encrypting
    R_C.  It will either be the server's public key (the <ds:KeyValue>
    alternative of ds:KeyInfoType) or an identifier for a shared
    secret key (the <ds:KeyName> alternative of ds:KeyInfoType).
 o  <Payload>: The actual payload.  For this version of the protocol,
    only one payload is defined: the pseudorandom string R_S.
 o  <Extensions>: A list of server extensions.  Two extensions are
    defined for this message in this version of CT-KIP: the
    ClientInfoType and the ServerInfoType (see Section 3.9).
 o  <Mac>: The MAC must be present if the CT-KIP run will result in
    the replacement of an existing token key with a new one (i.e., if
    the <KeyID> element was present in the <ClientHello> message).  In
    this case, the CT-KIP server must prove to the cryptographic token
    that it is authorized to replace it.  The MAC value shall be
    computed on the (ASCII) string "MAC 1 computation", the client's
    nonce R (if sent), and the server's nonce R_S using an
    authentication key K_AUTH that should be a special authentication
    key used only for this purpose but may be the current K_TOKEN.

Nystroem Informational [Page 22] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

    The MAC value may be computed by using the CT-KIP-PRF function of
    Section 3.4, in which case the input parameter s shall be set to
    the concatenation of the (ASCII) string "MAC 1 computation", R (if
    sent by the client), and R_S, and k shall be set to K_AUTH.  The
    input parameter dsLen shall be set to the length of R_S:
    dsLen = len(R_S)
    MAC = CT-KIP-PRF (K_AUTH, "MAC 1 computation" || [R ||] R_S,
    dsLen)
    The CT-KIP client must verify the MAC if the successful execution
    of the protocol will result in the replacement of an existing
    token key with a newly generated one.  The CT-KIP client must
    terminate the CT-KIP session if the MAC does not verify, and must
    delete any nonces, keys, and/or secrets associated with the failed
    run of the CT-KIP protocol.
    The MacType's MacAlgorithm attribute shall, when present, identify
    the negotiated MAC algorithm.

3.8.5. The CT-KIP Client's Second PDU

 This message contains the nonce chosen by the cryptographic token,
 R_C, encrypted by the specified encryption key and encryption
 algorithm.
 <xs:element name="ClientNonce" type="ClientNoncePDU"/>
 <xs:complexType name="ClientNoncePDU">
   <xs:annotation>
     <xs:documentation xml:lang="en">
       Second message sent from CT-KIP client to
       CT-KIP server in a CT-KIP session.
     </xs:documentation>
   </xs:annotation>
   <xs:complexContent>
     <xs:extension base="AbstractRequestType">
       <xs:sequence>
         <xs:element name="EncryptedNonce"
           type="xs:base64Binary"/>
         <xs:element name="Extensions"
           type="ExtensionsType" minOccurs="0"/>
       </xs:sequence>
       <xs:attribute name="SessionID" type="IdentifierType"
         use="required"/>
     </xs:extension>
   </xs:complexContent>

Nystroem Informational [Page 23] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 </xs:complexType>
 The components of this message have the following meaning:
 o  Version: (inherited from the AbstractRequestType type) Shall be
    the same version as in the <ServerHello> message.
 o  SessionID: Shall have the same value as the SessionID attribute in
    the received <ServerHello> message.
 o  <EncryptedNonce>: The nonce generated and encrypted by the token.
    The encryption shall be made using the selected encryption
    algorithm and identified key, and as specified in Section 3.4.
 o  <Extensions>: A list of extensions.  Two extensions are defined
    for this message in this version of CT-KIP: the ClientInfoType and
    the ServerInfoType (see Section 3.9).

3.8.6. The CT-KIP Server's Final PDU

 This message is the last message of a two roundtrip CT-KIP exchange.
 The CT-KIP server sends this message to the CT-KIP client in response
 to the <ClientNonce> message.
 <xs:element name="ServerFinished" type="ServerFinishedPDU"/>
 <xs:complexType name="ServerFinishedPDU">
   <xs:annotation>
     <xs:documentation xml:lang="en">
       Final message sent from CT-KIP server to
       CT-KIP client in a CT-KIP session.
     </xs:documentation>
   </xs:annotation>
   <xs:complexContent>
     <xs:extension base="AbstractResponseType">
       <xs:sequence minOccurs="0">
         <xs:element name="TokenID"
           type="xs:base64Binary"/>
         <xs:element name="KeyID"
           type="xs:base64Binary"/>
         <xs:element name="KeyExpiryDate"
           type="xs:dateTime" minOccurs="0"/>
         <xs:element name="ServiceID"
           type="IdentifierType" minOccurs="0"/>
         <xs:element name="ServiceLogo"
           type="LogoType" minOccurs="0"/>
         <xs:element name="UserID"
           type="IdentifierType" minOccurs="0"/>

Nystroem Informational [Page 24] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

         <xs:element name="Extensions"
           type="ExtensionsType" minOccurs="0"/>
         <xs:element name="Mac"
           type="MacType"/>
       </xs:sequence>
     </xs:extension>
   </xs:complexContent>
 </xs:complexType>
 The components of this message have the following meaning:
 o  Version: (inherited from the AbstractResponseType type) The CT-KIP
    version used in this session.
 o  SessionID: (inherited from the AbstractResponseType type) The
    previously established identifier for this session.
 o  Status: (inherited from the AbstractResponseType type) Return code
    for the <ServerFinished> message.  If Status is not "Success",
    only the Status, SessionID, and Version attributes will be present
    (the presence of the SessionID attribute is dependent on the type
    of reported error); otherwise, all the other elements must be
    present as well.  In this latter case, the <ServerFinished>
    message can be seen as a "Commit" message, instructing the
    cryptographic token to store the generated key and associate the
    given key identifier with this key.
 o  <TokenID>: An identifier for the token carrying the generated key.
    Must have the same value as the <TokenID> element of the
    <ClientHello> message, if one was provided.  When assigned by the
    CT-KIP server, the <TokenID> element shall be unique within the
    domain of the CT-KIP server.
 o  <KeyID>: An identifier for the newly generated key.  The
    identifier shall be globally unique.  Must have the same value as
    any key identifier provided by the CT-KIP client in the
    <ClientHello> message.
    The reason for requiring globally unique key identifiers is that
    it avoids potential conflicts when associating key holders with
    key identifiers.  One way of achieving global uniqueness with
    reasonable certainty is to hash the combination of the issuer's
    fully qualified domain name with an (issuer-specific) serial
    number, assuming that each issuer makes sure their serial numbers
    never repeat.

Nystroem Informational [Page 25] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

    CT-KIP clients must support key identifiers at least 64 octets
    long.  CT-KIP servers should not generate key identifiers longer
    than 64 octets.
 o  <KeyExpiryDate>: This optional element provides the date and time
    after which the generated key should be treated as expired and
    invalid.
 o  <ServiceID>: An optional identifier for the service that has
    stored the generated key.  The cryptographic token may store this
    identifier associated with the key in order to simplify later
    lookups.  The identifier shall be a printable string.
 o  <ServiceLogo>: This optional element provides a graphical logo
    image for the service that can be displayed in user interfaces,
    e.g., to help a user select a certain key.  The logo should
    contain an image within the size range of 60 pixels wide by 45
    pixels high, and 200 pixels wide by 150 pixels high.  The required
    MimeType attribute of this type provides information about the
    MIME type of the image.  This specification supports both the JPEG
    and GIF image formats (with MIME types of "image/jpeg" and "image/
    gif").
 o  <UserID>: An optional identifier for the user associated with the
    generated key in the authentication service.  The cryptographic
    token may store this identifier associated with the generated key
    in order to enhance later user experiences.  The identifier shall
    be a printable string.
 o  <Extensions>: A list of extensions chosen by the CT-KIP server.
    For this message, this version of CT-KIP defines two extensions,
    the OTPKeyConfigurationDataType and the ClientInfoType (see
    Section 3.9).
 o  <Mac>: To avoid a false "Commit" message causing the token to end
    up in an initialized state for which the server does not know the
    stored key, <ServerFinished> messages must always be authenticated
    with a MAC.  The MAC shall be made using the already established
    MAC algorithm.  The MAC value shall be computed on the (ASCII)
    string "MAC 2 computation" and R_C using an authentication key
    K_AUTH.  Again, this should be a special authentication key used
    only for this purpose, but may also be an existing K_TOKEN.  (In
    this case, implementations must protect against attacks where
    K_TOKEN is used to pre-compute MAC values.)  If no authentication
    key is present in the token, and no K_TOKEN existed before the CT-
    KIP run, K_AUTH shall be the newly generated K_TOKEN.

Nystroem Informational [Page 26] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

    If CT-KIP-PRF is used as the MAC algorithm, then the input
    parameter s shall consist of the concatenation of the (ASCII)
    string "MAC 2 computation" and R_C, and the parameter dsLen shall
    be set to the length of R_C:
    dsLen = len(R_C)
    MAC = CT-KIP-PRF (K_AUTH, "MAC 2 computation" || R_C, dsLen)
    When receiving a <ServerFinished> message with Status = "Success"
    for which the MAC verifies, the CT-KIP client shall associate the
    generated key K_TOKEN with the provided key identifier and store
    this data permanently.  After this operation, it shall not be
    possible to overwrite the key unless knowledge of an authorizing
    key is proven through a MAC on a later <ServerHello> (and
    <ServerFinished>) message.
    The CT-KIP client must verify the MAC.  The CT-KIP client must
    terminate the CT-KIP session if the MAC does not verify, and must,
    in this case, also delete any nonces, keys, and/or secrets
    associated with the failed run of the CT-KIP protocol.
    The MacType's MacAlgorithm attribute shall, when present, identify
    the negotiated MAC algorithm.

3.9. Protocol Extensions

3.9.1. The ClientInfoType Type

 When present in a <ClientHello> or a <ClientNonce> message, the
 optional ClientInfoType extension contains CT-KIP client-specific
 information.  CT-KIP servers must support this extension.  CT-KIP
 servers must not attempt to interpret the data it carries and, if
 received, must include it unmodified in the current protocol run's
 next server response.  Servers need not retain the ClientInfoType's
 data after that response has been generated.
 <xs:complexType name="ClientInfoType">
   <xs:complexContent>
     <xs:extension base="AbstractExtensionType">
       <xs:sequence>
         <xs:element name="Data"
           type="xs:base64Binary"/>
       </xs:sequence>
     </xs:extension>
   </xs:complexContent>
 </xs:complexType>

Nystroem Informational [Page 27] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

3.9.2. The ServerInfoType Type

 When present, the optional ServerInfoType extension contains CT-KIP
 server-specific information.  This extension is only valid in
 <ServerHello> messages for which Status = "Continue".  CT-KIP clients
 must support this extension.  CT-KIP clients must not attempt to
 interpret the data it carries and, if received, must include it
 unmodified in the current protocol run's next client request (i.e.,
 the <ClientNonce> message).  CT-KIP clients need not retain the
 ServerInfoType's data after that request has been generated.  This
 extension may be used, e.g., for state management in the CT-KIP
 server.
 <xs:complexType name="ServerInfoType">
   <xs:complexContent>
     <xs:extension base="AbstractExtensionType">
       <xs:sequence>
         <xs:element name="Data"
           type="xs:base64Binary"/>
       </xs:sequence>
     </xs:extension>
   </xs:complexContent>
 </xs:complexType>

3.9.3. The OTPKeyConfigurationDataType Type

 The optional OTPKeyConfigurationDataType extension contains
 additional key configuration data for OTP keys:
 <xs:complexType name="OTPKeyConfigurationDataType">
   <xs:annotation>
     <xs:documentation xml:lang="en">
       This extension is only valid in ServerFinished
       PDUs.  It carries additional configuration data
       that an OTP token should use (subject to local
       policy) when generating OTP values with a newly
       generated OTP key.
     </xs:documentation>
   </xs:annotation>
   <xs:complexContent>
     <xs:extension base="ExtensionType">
       <xs:sequence>
         <xs:element name="OTPFormat"
           type="OTPFormatType"/>
         <xs:element name="OTPLength"
           type="xs:positiveInteger"/>
         <xs:element name="OTPMode"
           type="OTPModeType" minOccurs="0"/>

Nystroem Informational [Page 28] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

       </xs:sequence>
     </xs:extension>
   </xs:complexContent>
 </xs:complexType>
 This extension is only valid in <ServerFinished> messages.  It
 carries additional configuration data that the cryptographic token
 should use (subject to local policy) when generating OTP values from
 the newly generated OTP key.  The components of this extension have
 the following meaning:
 o  OTPFormat: The default format of OTPs produced with this key.
 o  OTPLength: The default length of OTPs produced with this key.
 o  OTPMode: The default mode of operation when producing OTPs with
    this key.

4. Protocol Bindings

4.1. General Requirement

 CT-KIP assumes a reliable transport.

4.2. HTTP/1.1 binding for CT-KIP

4.2.1. Introduction

 This section presents a binding of the previous messages to HTTP/1.1
 [7].  Note that the HTTP client normally will be different from the
 CT-KIP client, i.e., the HTTP client will only exist to "proxy" CT-
 KIP messages from the CT-KIP client to the CT-KIP server.  Likewise,
 on the HTTP server side, the CT-KIP server may receive CT-KIP PDUs
 from a "front-end" HTTP server.

4.2.2. Identification of CT-KIP Messages

 The MIME-type for all CT-KIP messages shall be
 application/vnd.otps.ct-kip+xml

4.2.3. HTTP Headers

 HTTP proxies must not cache responses carrying CT-KIP messages.  For
 this reason, the following holds:

Nystroem Informational [Page 29] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 o  When using HTTP/1.1, requesters should:
  • Include a Cache-Control header field set to "no-cache,

no-store".

  • Include a Pragma header field set to "no-cache".
 o  When using HTTP/1.1, responders should:
  • Include a Cache-Control header field set to "no-cache,

no-must-revalidate, private".

  • Include a Pragma header field set to "no-cache".
  • NOT include a Validator, such as a Last-Modified or ETag

header.

 There are no other restrictions on HTTP headers, besides the
 requirement to set the Content-Type header value to application/
 vnd.otps.ct-kip+xml.

4.2.4. HTTP Operations

 Persistent connections as defined in HTTP/1.1 are assumed but not
 required.  CT-KIP requests are mapped to HTTP POST operations.  CT-
 KIP responses are mapped to HTTP responses.

4.2.5. HTTP Status Codes

 A CT-KIP HTTP responder that refuses to perform a message exchange
 with a CT-KIP HTTP requester should return a 403 (Forbidden)
 response.  In this case, the content of the HTTP body is not
 significant.  In the case of an HTTP error while processing a CT-KIP
 request, the HTTP server must return a 500 (Internal Server Error)
 response.  This type of error should be returned for HTTP-related
 errors detected before control is passed to the CT-KIP processor, or
 when the CT-KIP processor reports an internal error (for example, the
 CT-KIP XML namespace is incorrect, or the CT-KIP schema cannot be
 located).  If the type of a CT-KIP request cannot be determined, the
 CT-KIP responder must return a 400 (Bad request) response.
 In these cases (i.e., when the HTTP response code is 4xx or 5xx), the
 content of the HTTP body is not significant.
 Redirection status codes (3xx) apply as usual.

Nystroem Informational [Page 30] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 Whenever the HTTP POST is successfully invoked, the CT-KIP HTTP
 responder must use the 200 status code and provide a suitable CT-KIP
 message (possibly with CT-KIP error information included) in the HTTP
 body.

4.2.6. HTTP Authentication

 No support for HTTP/1.1 authentication is assumed.

4.2.7. Initialization of CT-KIP

 The CT-KIP server may initialize the CT-KIP protocol by sending an
 HTTP response with Content-Type set to application/
 vnd.otps.ct-kip+xml and response code set to 200 (OK).  This message
 may, e.g., be sent in response to a user requesting token
 initialization in a browsing session.  The initialization message may
 carry data in its body.  If this is the case, the data shall be a
 valid instance of a <CT-KIPTrigger> element.

4.2.8. Example Messages

 a.  Initialization from CT-KIP server:
 HTTP/1.1 200 OK
 Cache-Control: no-store
 Content-Type: application/vnd.otps.ct-kip+xml
 Content-Length: <some value>
 CT-KIP initialization data in XML form...
 b.  Initial request from CT-KIP client:
 POST http://example.com/cgi-bin/CT-KIP-server HTTP/1.1
 Cache-Control: no-store
 Pragma: no-cache
 Host: example.com
 Content-Type: application/vnd.otps.ct-kip+xml
 Content-Length: <some value>
 CT-KIP data in XML form (supported version, supported algorithms...)

Nystroem Informational [Page 31] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 c.  Initial response from CT-KIP server:
 HTTP/1.1 200 OK
 Cache-Control: no-store
 Content-Type: application/vnd.otps.ct-kip+xml
 Content-Length: <some other value>
 CT-KIP data in XML form (server random nonce, server public key, ...)

5. Security considerations

5.1. General

 CT-KIP is designed to protect generated key material from exposure.
 No other entities than the CT-KIP server and the cryptographic token
 will have access to a generated K_TOKEN if the cryptographic
 algorithms used are of sufficient strength and, on the CT-KIP client
 side, generation and encryption of R_C and generation of K_TOKEN take
 place as specified and in the token.  This applies even if malicious
 software is present in the CT-KIP client.  However, as discussed in
 the following, CT-KIP does not protect against certain other threats
 resulting from man-in-the-middle attacks and other forms of attacks.
 CT-KIP should, therefore, be run over a transport providing privacy
 and integrity, such as HTTP over Transport Layer Security (TLS) with
 a suitable ciphersuite, when such threats are a concern.  Note that
 TLS ciphersuites with anonymous key exchanges are not suitable in
 those situations.

5.2. Active Attacks

5.2.1. Introduction

 An active attacker may attempt to modify, delete, insert, replay or
 reorder messages for a variety of purposes including service denial
 and compromise of generated key material.  Sections 5.2.2 through
 5.2.7 analyze these attack scenarios.

5.2.2. Message Modifications

 Modifications to a <CT-KIPTrigger> message will either cause denial-
 of-service (modifications of any of the identifiers or the nonce) or
 the CT-KIP client to contact the wrong CT-KIP server.  The latter is
 in effect a man-in-the-middle attack and is discussed further in
 Section 5.2.7.
 An attacker may modify a <ClientHello> message.  This means that the
 attacker could indicate a different key or token than the one
 intended by the CT-KIP client, and could also suggest other

Nystroem Informational [Page 32] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 cryptographic algorithms than the ones preferred by the CT-KIP
 client, e.g., cryptographically weaker ones.  The attacker could also
 suggest earlier versions of the CT-KIP protocol, in case these
 versions have been shown to have vulnerabilities.  These
 modifications could lead to an attacker succeeding in initializing or
 modifying another token than the one intended (i.e., the server
 assigning the generated key to the wrong token), or gaining access to
 a generated key through the use of weak cryptographic algorithms or
 protocol versions.  CT-KIP implementations may protect against the
 latter by having strict policies about what versions and algorithms
 they support and accept.  The former threat (assignment of a
 generated key to the wrong token) is not possible when the shared-key
 variant of CT-KIP is employed (assuming existing shared keys are
 unique per token) but is possible in the public-key variant.
 Therefore, CT-KIP servers must not accept unilaterally provided token
 identifiers in the public-key variant.  This is also indicated in the
 protocol description.  In the shared-key variant, however, an
 attacker may be able to provide the wrong identifier (possibly also
 leading to the incorrect user being associated with the generated
 key) if the attacker has real-time access to the token with the
 identified key.  In other words, the generated key is associated with
 the correct token but the token is associated with the incorrect
 user.  See further Section 5.5 for a discussion of this threat and
 possible countermeasures.
 An attacker may also modify a <ServerHello> message.  This means that
 the attacker could indicate different key types, algorithms, or
 protocol versions than the legitimate server would, e.g.,
 cryptographically weaker ones.  The attacker could also provide a
 different nonce than the one sent by the legitimate server.  Clients
 will protect against the former through strict adherence to policies
 regarding permissible algorithms and protocol versions.  The latter
 (wrong nonce) will not constitute a security problem, as a generated
 key will not match the key generated on the legitimate server.  Also,
 whenever the CT-KIP run would result in the replacement of an
 existing key, the <Mac> element protects against modifications of
 R_S.
 Modifications of <ClientNonce> messages are also possible.  If an
 attacker modifies the SessionID attribute, then, in effect, a switch
 to another session will occur at the server, assuming the new
 SessionID is valid at that time on the server.  It still will not
 allow the attacker to learn a generated K_TOKEN since R_C has been
 wrapped for the legitimate server.  Modifications of the
 <EncryptedNonce> element, e.g., replacing it with a value for which
 the attacker knows an underlying R'C, will not result in the client
 changing its pre-CT-KIP state, since the server will be unable to
 provide a valid MAC in its final message to the client.  The server

Nystroem Informational [Page 33] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 may, however, end up storing K'TOKEN rather than K_TOKEN.  If the
 token has been associated with a particular user, then this could
 constitute a security problem.  For a further discussion about this
 threat, and a possible countermeasure, see Section 5.5 below.  Note
 that use of Secure Socket Layer (SSL) or TLS does not protect against
 this attack if the attacker has access to the CT-KIP client (e.g.,
 through malicious software, "trojans").
 Finally, attackers may also modify the <ServerFinished> message.
 Replacing the <Mac> element will only result in denial-of-service.
 Replacement of any other element may cause the CT-KIP client to
 associate, e.g., the wrong service with the generated key.  CT-KIP
 should be run over a transport providing privacy and integrity when
 this is a concern.

5.2.3. Message Deletion

 Message deletion will not cause any other harm than denial-of-
 service, since a token shall not change its state (i.e., "commit" to
 a generated key) until it receives the final message from the CT-KIP
 server and successfully has processed that message, including
 validation of its MAC.  A deleted <ServerFinished> message will not
 cause the server to end up in an inconsistent state vis-a-vis the
 token if the server implements the suggestions in Section 5.5.

5.2.4. Message Insertion

 An active attacker may initiate a CT-KIP run at any time, and suggest
 any token identifier.  CT-KIP server implementations may receive some
 protection against inadvertently initializing a token or
 inadvertently replacing an existing key or assigning a key to a token
 by initializing the CT-KIP run by use of the <CT-KIPTrigger>.  The
 <TriggerNonce> element allows the server to associate a CT-KIP
 protocol run with, e.g., an earlier user-authenticated session.  The
 security of this method, therefore, depends on the ability to protect
 the <TriggerNonce> element in the CT-KIP initialization message.  If
 an eavesdropper is able to capture this message, he may race the
 legitimate user for a key initialization.  CT-KIP over a transport
 providing privacy and integrity, coupled with the recommendations in
 Section 5.5, is recommended when this is a concern.
 Insertion of other messages into an existing protocol run is seen as
 equivalent to modification of legitimately sent messages.

5.2.5. Message Replay

 Attempts to replay a previously recorded CT-KIP message will be
 detected, as the use of nonces ensures that both parties are live.

Nystroem Informational [Page 34] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

5.2.6. Message Reordering

 An attacker may attempt to re-order messages but this will be
 detected, as each message is of a unique type.

5.2.7. Man in the Middle

 In addition to other active attacks, an attacker posing as a man in
 the middle may be able to provide his own public key to the CT-KIP
 client.  This threat and countermeasures to it are discussed in
 Section 3.3.  An attacker posing as a man-in-the-middle may also be
 acting as a proxy and, hence, may not interfere with CT-KIP runs but
 still learn valuable information; see Section 5.3.

5.3. Passive Attacks

 Passive attackers may eavesdrop on CT-KIP runs to learn information
 that later on may be used to impersonate users, mount active attacks,
 etc.
 If CT-KIP is not run over a transport providing privacy, a passive
 attacker may learn:
 o  What tokens a particular user is in possession of;
 o  The identifiers of keys on those tokens and other attributes
    pertaining to those keys, e.g., the lifetime of the keys; and
 o  CT-KIP versions and cryptographic algorithms supported by a
    particular CT-KIP client or server.
 Whenever the above is a concern, CT-KIP should be run over a
 transport providing privacy.  If man-in-the-middle attacks for the
 purposes described above are a concern, the transport should also
 offer server-side authentication.

5.4. Cryptographic Attacks

 An attacker with unlimited access to an initialized token may use the
 token as an "oracle" to pre-compute values that later on may be used
 to impersonate the CT-KIP server.  Sections 3.6 and 3.8 contain
 discussions of this threat and steps recommended to protect against
 it.

Nystroem Informational [Page 35] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

5.5. Attacks on the Interaction between CT-KIP and User Authentication

 If keys generated in CT-KIP will be associated with a particular user
 at the CT-KIP server (or a server trusted by, and communicating with
 the CT-KIP server), then in order to protect against threats where an
 attacker replaces a client-provided encrypted R_C with his own R'C
 (regardless of whether the public-key variant or the shared-secret
 variant of CT-KIP is employed to encrypt the client nonce), the
 server should not commit to associate a generated K_TOKEN with the
 given token (user) until the user simultaneously has proven both
 possession of a token containing K_TOKEN and some out-of-band
 provided authenticating information (e.g., a temporary password).
 For example, if the token is a one-time password token, the user
 could be required to authenticate with both a one-time password
 generated by the token and an out-of-band provided temporary PIN in
 order to have the server "commit" to the generated token value for
 the given user.  Preferably, the user should perform this operation
 from another host than the one used to initialize the token, in order
 to minimize the risk of malicious software on the client interfering
 with the process.
 Another threat arises when an attacker is able to trick a user to
 authenticate to the attacker rather than to the legitimate service
 before the CT-KIP protocol run.  If successful, the attacker will
 then be able to impersonate the user towards the legitimate service,
 and subsequently receive a valid CT-KIP trigger.  If the public-key
 variant of CT-KIP is used, this may result in the attacker being able
 to (after a successful CT-KIP protocol run) impersonate the user.
 Ordinary precautions must, therefore, be in place to ensure that
 users authenticate only to legitimate services.

6. Intellectual Property Considerations

 RSA and SecurID are registered trademarks or trademarks of RSA
 Security Inc. in the United States and/or other countries.  The names
 of other products and services mentioned may be the trademarks of
 their respective owners.

Nystroem Informational [Page 36] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

7. References

7.1. Normative References

 [1]   Davis, M. and M. Duerst, "Unicode Normalization Forms",
       March 2001,
       <http://www.unicode.org/unicode/reports/tr15/tr15-21.html>.

7.2. Informative References

 [2]   RSA Laboratories, "PKCS #11 Mechanisms for the Cryptographic
       Token Key Initialization Protocol", PKCS #11 Version 2.20
       Amendment 2, December 2005, <ftp://ftp.rsasecurity.com/pub/
       pkcs/pkcs-11/v2-20/pkcs-11v2-20a2.pdf>.
 [3]   RSA Laboratories, "Cryptographic Token Interface Standard",
       PKCS #11 Version 2.20, June 2004, <ftp://ftp.rsasecurity.com/
       pub/pkcs/pkcs-11/v2-20/pkcs-11v2-20.pdf>.
 [4]   RSA Laboratories, "Frequently Asked Questions About Today's
       Cryptography. Version 4.1", 2000, <http://www.rsasecurity.com/
       rsalabs/faq/files/rsalabs_faq41.pdf>.
 [5]   RSA Laboratories, "Password-Based Cryptography Standard",
       PKCS #5 Version 2.0, March 1999,
       <ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-5v2/pkcs5v2-0.pdf>.
 [6]   RSA Laboratories, "RSA Cryptography Standard", PKCS #1 Version
       2.1, June 2002,
       <ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-1/pkcs-1v2-1.pdf>.
 [7]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
       Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
       HTTP/1.1", RFC 2616, June 1999.
 [8]   National Institute of Standards and Technology, "Specification
       for the Advanced Encryption Standard (AES)", FIPS 197,
       November 2001,
       <http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf>.
 [9]   Krawzcyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
       for Message Authentication", RFC 2104, February 1997.
 [10]  Iwata, T. and K. Kurosawa, "OMAC: One-Key CBC MAC.  In Fast
       Software Encryption, FSE 2003, pages 129 - 153.
       Springer-Verlag", 2003,
       <http://crypt.cis.ibaraki.ac.jp/omac/docs/omac.pdf>.

Nystroem Informational [Page 37] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 [11]  National Institute of Standards and Technology, "Secure Hash
       Standard", FIPS 197, February 2004, <http://csrc.nist.gov/
       publications/fips/fips180-2/fips180-2withchangenotice.pdf>.
 [12]  RSA Laboratories, "Cryptographic Token Key Initialization
       Protocol", OTPS Version 1.0, December 2005,
       <ftp://ftp.rsasecurity.com/pub/otps/ct-kip/ct-kip-v1-0.pdf>.

Nystroem Informational [Page 38] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

Appendix A. CT-KIP Schema

 <xs:schema
   targetNamespace=
   "http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/ct-kip#"
   xmlns:xs="http://www.w3.org/2001/XMLSchema"
   xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
   xmlns=
   "http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/ct-kip#">
 <xs:import namespace="http://www.w3.org/2000/09/xmldsig#"
   schemaLocation=
   "http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/
 xmldsig-core-schema.xsd"/>
 <!-- Basic types -->
 <xs:complexType name="AbstractRequestType" abstract="true">
   <xs:attribute name="Version" type="VersionType" use="required"/>
 </xs:complexType>
 <xs:complexType name="AbstractResponseType" abstract="true">
   <xs:attribute name="Version" type="VersionType" use="required"/>
   <xs:attribute name="SessionID" type="IdentifierType"/>
   <xs:attribute name="Status" type="StatusCode" use="required"/>
 </xs:complexType>
 <xs:simpleType name="StatusCode">
   <xs:restriction base="xs:string">
     <xs:enumeration value="Continue"/>
     <xs:enumeration value="Success"/>
     <xs:enumeration value="Abort"/>
     <xs:enumeration value="AccessDenied"/>
     <xs:enumeration value="MalformedRequest"/>
     <xs:enumeration value="UnknownRequest"/>
     <xs:enumeration value="UnknownCriticalExtension"/>
     <xs:enumeration value="UnsupportedVersion"/>
     <xs:enumeration value="NoSupportedKeyTypes"/>
     <xs:enumeration value="NoSupportedEncryptionAlgorithms"/>
     <xs:enumeration value="NoSupportedMACAlgorithms"/>
     <xs:enumeration value="InitializationFailed"/>
   </xs:restriction>
 </xs:simpleType>
 <xs:simpleType name="VersionType">
   <xs:restriction base="xs:string">
     <xs:pattern value="\d{1,2}\.\d{1,3}"/>
   </xs:restriction>

Nystroem Informational [Page 39] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 </xs:simpleType>
 <xs:simpleType name="IdentifierType">
   <xs:restriction base="xs:string">
     <xs:maxLength value="128"/>
   </xs:restriction>
 </xs:simpleType>
 <xs:simpleType name="NonceType">
   <xs:restriction base="xs:base64Binary">
     <xs:length value="16"/>
   </xs:restriction>
 </xs:simpleType>
 <xs:complexType name="LogoType">
   <xs:simpleContent>
     <xs:extension base="xs:base64Binary">
       <xs:attribute name="MimeType" type="MimeTypeType"
       use="required"/>
     </xs:extension>
   </xs:simpleContent>
 </xs:complexType>
 <xs:simpleType name="MimeTypeType">
   <xs:restriction base="xs:string">
     <xs:enumeration value="image/jpeg"/>
     <xs:enumeration value="image/gif"/>
   </xs:restriction>
 </xs:simpleType>
 <!-- Algorithms are identified through URIs -->
 <xs:complexType name="AlgorithmsType">
   <xs:sequence maxOccurs="unbounded">
     <xs:element name="Algorithm" type="AlgorithmType"/>
   </xs:sequence>
 </xs:complexType>
 <xs:simpleType name="AlgorithmType">
   <xs:restriction base="xs:anyURI"/>
 </xs:simpleType>
 <xs:complexType name="MacType">
   <xs:simpleContent>
     <xs:extension base="xs:base64Binary">
       <xs:attribute name="MacAlgorithm"
       type="xs:anyURI"/>
     </xs:extension>
   </xs:simpleContent>

Nystroem Informational [Page 40] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 </xs:complexType>
 <!-- CT-KIP extensions (for future use) -->
 <xs:complexType name="ExtensionsType">
   <xs:sequence maxOccurs="unbounded">
     <xs:element name="Extension" type="AbstractExtensionType"/>
   </xs:sequence>
 </xs:complexType>
 <xs:complexType name="AbstractExtensionType" abstract="true">
   <xs:attribute name="Critical" type="xs:boolean"/>
 </xs:complexType>
 <xs:complexType name="ClientInfoType">
   <xs:complexContent>
     <xs:extension base="AbstractExtensionType">
       <xs:sequence>
         <xs:element name="Data" type="xs:base64Binary"/>
       </xs:sequence>
     </xs:extension>
   </xs:complexContent>
 </xs:complexType>
 <xs:complexType name="ServerInfoType">
   <xs:complexContent>
     <xs:extension base="AbstractExtensionType">
       <xs:sequence>
         <xs:element name="Data" type="xs:base64Binary"/>
       </xs:sequence>
     </xs:extension>
   </xs:complexContent>
 </xs:complexType>
 <xs:complexType name="OTPKeyConfigurationDataType">
   <xs:annotation>
     <xs:documentation xml:lang="en">
       This extension is only valid in ServerFinished PDUs.  It
       carries additional configuration data that an OTP token should
       use (subject to local policy) when generating OTP values from a
       newly generated OTP key.
     </xs:documentation>
   </xs:annotation>
   <xs:complexContent>
     <xs:extension base="AbstractExtensionType">
       <xs:sequence>
         <xs:element name="OTPFormat" type="OTPFormatType"/>
         <xs:element name="OTPLength" type="xs:positiveInteger"/>
         <xs:element name="OTPMode" type="OTPModeType" minOccurs="0"/>

Nystroem Informational [Page 41] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

       </xs:sequence>
     </xs:extension>
   </xs:complexContent>
 </xs:complexType>
 <xs:simpleType name="OTPFormatType">
   <xs:restriction base="xs:string">
     <xs:enumeration value="Decimal"/>
     <xs:enumeration value="Hexadecimal"/>
     <xs:enumeration value="Alphanumeric"/>
     <xs:enumeration value="Binary"/>
   </xs:restriction>
 </xs:simpleType>
 <xs:complexType name="OTPModeType">
   <xs:choice maxOccurs="unbounded">
     <xs:element name="Time" type="TimeType"/>
     <xs:element name="Counter"/>
     <xs:element name="Challenge"/>
     <xs:any namespace="##other" processContents="strict"/>
   </xs:choice>
 </xs:complexType>
 <xs:complexType name="TimeType">
   <xs:complexContent>
     <xs:restriction base="xs:anyType">
       <xs:attribute name="TimeInterval" type="xs:positiveInteger"/>
     </xs:restriction>
   </xs:complexContent>
 </xs:complexType>
 <xs:complexType name="PayloadType">
   <xs:annotation>
     <xs:documentation xml:lang="en">
     </xs:documentation>
   </xs:annotation>
   <xs:choice>
     <xs:element name="Nonce" type="NonceType"/>
     <xs:any namespace="##other" processContents="strict"/>
   </xs:choice>
 </xs:complexType>
 <xs:simpleType name="PlatformType">
   <xs:restriction base="xs:string">
     <xs:enumeration value="Hardware"/>
     <xs:enumeration value="Software"/>
     <xs:enumeration value="Unspecified"/>
   </xs:restriction>

Nystroem Informational [Page 42] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 </xs:simpleType>
 <xs:complexType name="TokenPlatformInfoType">
   <xs:annotation>
     <xs:documentation xml:lang="en">
       Carries token platform information helping the client to select
       a suitable token.
     </xs:documentation>
   </xs:annotation>
   <xs:attribute name="KeyLocation" type="PlatformType"/>
   <xs:attribute name="AlgorithmLocation" type="PlatformType"/>
 </xs:complexType>
 <xs:complexType name="InitializationTriggerType">
   <xs:sequence>
     <xs:element name="TokenID" type="xs:base64Binary" minOccurs="0"/>
     <xs:element name="KeyID" type="xs:base64Binary" minOccurs="0"/>
     <xs:element name="TokenPlatformInfo" type="TokenPlatformInfoType"
       minOccurs="0"/>
     <xs:element name="TriggerNonce" type="NonceType"/>
     <xs:element name="CT-KIPURL" type="xs:anyURI" minOccurs="0"/>
     <xs:any namespace="##other" processContents="strict"
       minOccurs="0"/>
   </xs:sequence>
 </xs:complexType>
 <!-- CT-KIP PDUs -->
 <!-- CT-KIP trigger -->
 <xs:element name="CT-KIPTrigger" type="CT-KIPTriggerType"/>
 <xs:complexType name="CT-KIPTriggerType">
   <xs:annotation>
     <xs:documentation xml:lang="en">
       Message used to trigger the device to initiate a CT-KIP run.
     </xs:documentation>
   </xs:annotation>
   <xs:sequence>
     <xs:choice>
       <xs:element name="InitializationTrigger"
       type="InitializationTriggerType"/>
       <xs:any namespace="##other" processContents="strict"/>
     </xs:choice>
   </xs:sequence>
   <xs:attribute name="Version" type="VersionType"/>
 </xs:complexType>
 <!-- ClientHello PDU -->

Nystroem Informational [Page 43] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 <xs:element name="ClientHello" type="ClientHelloPDU"/>
 <xs:complexType name="ClientHelloPDU">
   <xs:annotation>
     <xs:documentation xml:lang="en">
       Message sent from CT-KIP client to CT-KIP server to initiate an
       CT-KIP session.
     </xs:documentation>
   </xs:annotation>
   <xs:complexContent>
     <xs:extension base="AbstractRequestType">
       <xs:sequence>
         <xs:element name="TokenID" type="xs:base64Binary"
           minOccurs="0"/>
         <xs:element name="KeyID" type="xs:base64Binary"
           minOccurs="0"/>
         <xs:element name="ClientNonce" type="NonceType"
           minOccurs="0"/>
         <xs:element name="TriggerNonce" type="NonceType"
           minOccurs="0"/>
         <xs:element name="SupportedKeyTypes" type="AlgorithmsType"/>
         <xs:element name="SupportedEncryptionAlgorithms"
           type="AlgorithmsType"/>
         <xs:element name="SupportedMACAlgorithms"
           type="AlgorithmsType"/>
         <xs:element name="Extensions" type="ExtensionsType"
           minOccurs="0"/>
       </xs:sequence>
     </xs:extension>
   </xs:complexContent>
 </xs:complexType>
 <!-- ServerHello PDU -->
 <xs:element name="ServerHello" type="ServerHelloPDU"/>
 <xs:complexType name="ServerHelloPDU">
   <xs:annotation>
     <xs:documentation xml:lang="en">
       Message sent from CT-KIP server to CT-KIP client in response to
       a received ClientHello PDU.
     </xs:documentation>
   </xs:annotation>
   <xs:complexContent>
     <xs:extension base="AbstractResponseType">
       <xs:sequence minOccurs="0">
         <xs:element name="KeyType" type="AlgorithmType"/>
         <xs:element name="EncryptionAlgorithm" type="AlgorithmType"/>
         <xs:element name="MacAlgorithm" type="AlgorithmType"/>

Nystroem Informational [Page 44] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

         <xs:element name="EncryptionKey" type="ds:KeyInfoType"/>
         <xs:element name="Payload" type="PayloadType"/>
         <xs:element name="Extensions" type="ExtensionsType"
           minOccurs="0"/>
         <xs:element name="Mac" type="MacType" minOccurs="0"/>
       </xs:sequence>
     </xs:extension>
   </xs:complexContent>
 </xs:complexType>
 <!-- ClientNonce PDU -->
 <xs:element name="ClientNonce" type="ClientNoncePDU"/>
 <xs:complexType name="ClientNoncePDU">
   <xs:annotation>
     <xs:documentation xml:lang="en">
       Second message sent from CT-KIP client to CT-KIP server to
       convey the client's chosen secret.
     </xs:documentation>
   </xs:annotation>
   <xs:complexContent>
     <xs:extension base="AbstractRequestType">
       <xs:sequence>
         <xs:element name="EncryptedNonce" type="xs:base64Binary"/>
         <xs:element name="Extensions" type="ExtensionsType"
           minOccurs="0"/>
       </xs:sequence>
       <xs:attribute name="SessionID" type="IdentifierType"
         use="required"/>
     </xs:extension>
   </xs:complexContent>
 </xs:complexType>
 <!-- ServerFinished PDU -->
 <xs:element name="ServerFinished" type="ServerFinishedPDU"/>
 <xs:complexType name="ServerFinishedPDU">
   <xs:annotation>
     <xs:documentation xml:lang="en">
       Final message sent from CT-KIP server to CT-KIP client in an
       CT-KIP session.
     </xs:documentation>
   </xs:annotation>
   <xs:complexContent>
     <xs:extension base="AbstractResponseType">
       <xs:sequence minOccurs="0">
         <xs:element name="TokenID" type="xs:base64Binary"/>
         <xs:element name="KeyID" type="xs:base64Binary"/>
         <xs:element name="KeyExpiryDate" type="xs:dateTime"

Nystroem Informational [Page 45] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

           minOccurs="0"/>
         <xs:element name="ServiceID" type="IdentifierType"
           minOccurs="0"/>
         <xs:element name="ServiceLogo" type="LogoType"
           minOccurs="0"/>
         <xs:element name="UserID" type="IdentifierType"
           minOccurs="0"/>
         <xs:element name="Extensions" type="ExtensionsType"
           minOccurs="0"/>
         <xs:element name="Mac" type="MacType"/>
       </xs:sequence>
     </xs:extension>
   </xs:complexContent>
 </xs:complexType>
 </xs:schema>

Appendix B. Examples of CT-KIP Messages

B.1. Introduction

 All examples are syntactically correct.  MAC and cipher values are
 fictitious, however.  The examples illustrate a complete CT-KIP
 exchange, starting with an initialization (trigger) message from the
 server.

B.2. Example of a CT-KIP Initialization (Trigger) Message

 <CT-KIPTrigger
   xmlns=
   "http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/ct-kip#"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   Version="1.0">
   <InitializationTrigger>
     <TokenID>12345678</TokenID>
     <TriggerNonce>112dsdfwf312asder394jw==</TriggerNonce>
   </InitializationTrigger>
 </CT-KIPTrigger>

B.3. Example of a <ClientHello> Message

 <?xml version="1.0" encoding="UTF-8"?>
 <ClientHello
   xmlns=
   "http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/ct-kip#"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   Version="1.0">
   <TokenID>12345678</TokenID>

Nystroem Informational [Page 46] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

  <TriggerNonce>112dsdfwf312asder394jw==</TriggerNonce>
   <SupportedKeyTypes>
     <Algorithm>http://www.rsasecurity.com/rsalabs/otps/schemas
 /2005/09/otps-wst#SecurID-AES</Algorithm>
   </SupportedKeyTypes>
   <SupportedEncryptionAlgorithms>
     <Algorithm>http://www.w3.org/2001/04/xmlenc#rsa-1_5</Algorithm>
     <Algorithm>http://www.rsasecurity.com/rsalabs/otps/schemas/
 2005/12/ct-kip#ct-kip-prf-aes</Algorithm>
   </SupportedEncryptionAlgorithms>
   <SupportedMACAlgorithms>
     <Algorithm>http://www.rsasecurity.com/rsalabs/otps/schemas/
 2005/12/ct-kip#ct-kip-prf-aes</Algorithm>
   </SupportedMACAlgorithms>
 </ClientHello>

B.4. Example of a <ServerHello> Message

 <?xml version="1.0" encoding="UTF-8"?>
 <ServerHello
   xmlns=
 "http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/ct-kip#"
   xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   Version="1.0" SessionID="4114" Status="Success">
   <KeyType>http://www.rsasecurity.com/rsalabs/otps/schemas/2005/09/
 otps-wst#SecurID-AES</KeyType>
   <EncryptionAlgorithm>http://www.rsasecurity.com/rsalabs/otps/
 schemas/2005/12/ct-kip#ct-kip-prf-aes</EncryptionAlgorithm>
   <MacAlgorithm>http://www.rsasecurity.com/rsalabs/otps/schemas/
 2005/12/ct-kip#ct-kip-prf-aes</MacAlgorithm>
   <EncryptionKey>
     <ds:KeyName>KEY-1</ds:KeyName>
   </EncryptionKey>
   <Payload>
     <Nonce>qw2ewasde312asder394jw==</Nonce>
   </Payload>
 </ServerHello>

B.5. Example of a <ClientNonce> Message

 <?xml version="1.0" encoding="UTF-8"?>
 <ClientNonce
   xmlns="http://www.rsasecurity.com/rsalabs/otps/schemas/
 2005/12/ct-kip#"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   Version="1.0" SessionID="4114">
   <EncryptedNonce>vXENc+Um/9/NvmYKiHDLaErK0gk=</EncryptedNonce>

Nystroem Informational [Page 47] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 </ClientNonce>

B.6. Example of a <ServerFinished> Message

 <?xml version="1.0" encoding="UTF-8"?>
 <ServerFinished
   xmlns="http://www.rsasecurity.com/rsalabs/otps/schemas/
 2005/12/ct-kip#"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   Version="1.0" SessionID="4114" Status="Success">
   <TokenID>12345678</TokenID>
   <KeyExpiryDate>2009-09-16T03:02:00Z</KeyExpiryDate>
   <KeyID>43212093</KeyID>
   <ServiceID>Example Enterprise Name</ServiceID>
   <UserID>exampleLoginName</UserID>
   <Extensions>
     <Extension xsi:type="OTPKeyConfigurationDataType">
       <OTPFormat>Decimal</OTPFormat>
       <OTPLength>6</OTPLength>
       <OTPMode><Time/></OTPMode>
     </Extension>
   </Extensions>
   <Mac>miidfasde312asder394jw==</Mac>
 </ServerFinished>

Appendix C. Integration with PKCS #11

 A CT-KIP client that needs to communicate with a connected
 cryptographic token to perform a CT-KIP exchange may use PKCS #11 [3]
 as a programming interface.  When performing CT-KIP with a
 cryptographic token using the PKCS #11 programming interface, the
 procedure described in [2], Appendix B, is recommended.

Appendix D. Example CT-KIP-PRF Realizations

D.1. Introduction

 This example appendix defines CT-KIP-PRF in terms of AES [8] and HMAC
 [9].

D.2. CT-KIP-PRF-AES

D.2.1. Identification

 For tokens supporting this realization of CT-KIP-PRF, the following
 URI may be used to identify this algorithm in CT-KIP:
 http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/

Nystroem Informational [Page 48] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 ct-kip#ct-kip-prf-aes
 When this URI is used to identify the encryption algorithm to use,
 the method for encryption of R_C values described in Section 3.6
 shall be used.

D.2.2. Definition

 CT-KIP-PRF-AES (k, s, dsLen)
 Input:
 k     encryption key to use
 s     octet string consisting of randomizing material.  The length of
       the string s is sLen.
 dsLen desired length of the output
 Output:
 DS    a pseudorandom string, dsLen-octets long
 Steps:
 1.  Let bLen be the output block size of AES in octets:
     bLen = (AES output block length in octets)
     (normally, bLen = 16)
 2.  If dsLen > (2**32 - 1) * bLen, output "derived data too long" and
     stop
 3.  Let n be the number of bLen-octet blocks in the output data,
     rounding up, and let j be the number of octets in the last block:
     n = ROUND( dsLen / bLen )
     j = dsLen - (n - 1) * bLen
 4.  For each block of the pseudorandom string DS, apply the function
     F defined below to the key k, the string s and the block index to
     compute the block:
     B1 = F (k, s, 1) ,
     B2 = F (k, s, 2) ,

Nystroem Informational [Page 49] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

     ...
     Bn = F (k, s, n)
 The function F is defined in terms of the OMAC1 construction from
 [10], using AES as the block cipher:
 F (k, s, i) = OMAC1-AES (k, INT (i) || s)
 where INT (i) is a four-octet encoding of the integer i, most
 significant octet first, and the output length of OMAC1 is set to
 bLen.
 Concatenate the blocks and extract the first dsLen octets to produce
 the desired data string DS:
 DS = B1 || B2 || ... || Bn<0..j-1>
 Output the derived data DS.

D.2.3. Example

 If we assume that dsLen = 16, then:
 n = 16 / 16 = 1
 j = 16 - (1 - 1) * 16 = 16
 DS = B1 = F (k, s, 1) = OMAC1-AES (k, INT (1) || S)

D.3. CT-KIP-PRF-SHA256

D.3.1. Identification

 For tokens supporting this realization of CT-KIP-PRF, the following
 URI may be used to identify this algorithm in CT-KIP:
 http://www.rsasecurity.com/rsalabs/otps/schemas/2005/12/
 ct-kip#ct-kip-prf-sha256
 When this URI is used to identify the encryption algorithm to use,
 the method for encryption of R_C values described in Section 3.6
 shall be used.

Nystroem Informational [Page 50] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

D.3.2. Definition

 CT-KIP-PRF-SHA256 (k, s, dsLen)
 Input:
 k     encryption key to use
 s     octet string consisting of randomizing material.  The length of
       the string s is sLen
 dsLen desired length of the output
 Output:
 DS    a pseudorandom string, dsLen-octets long
 Steps:
 1.  Let bLen be the output size in octets of SHA-256 [11] (no
     truncation is done on the HMAC output):
     bLen = 32
 2.  If dsLen > (2**32 - 1) bLen, output "derived data too long" and
     stop
 3.  Let n be the number of bLen-octet blocks in the output data,
     rounding up, and let j be the number of octets in the last block:
     n = ROUND ( dsLen / bLen )
     j = dsLen - (n - 1) * bLen
 4.  For each block of the pseudorandom string DS, apply the function
     F defined below to the key k, the string s and the block index to
     compute the block:
     B1 = F (k, s, 1) ,
     B2 = F (k, s, 2) ,
     ...
     Bn = F (k, s, n)

Nystroem Informational [Page 51] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

 The function F is defined in terms of the HMAC construction from [9],
 using SHA-256 as the digest algorithm:
 F (k, s, i) = HMAC-SHA256 (k, INT (i) || s)
 where INT (i) is a four-octet encoding of the integer i, most
 significant octet first, and the output length of HMAC is set to
 bLen.
 Concatenate the blocks and extract the first dsLen octets to produce
 the desired data string DS:
 DS = B1 || B2 || ... || Bn<0..j-1>
 Output the derived data DS.

D.3.3. Example

 If we assume that sLen = 256 (two 128-octet long values) and dsLen =
 16, then:
 n = ROUND ( 16 / 32 ) = 1
 j = 16 - (1 - 1) * 32 = 16
 B1 = F (k, s, 1) = HMAC-SHA256 (k, INT (1) || s )
 DS = B1<0 ... 15>
 That is, the result will be the first 16 octets of the HMAC output.

Nystroem Informational [Page 52] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

Appendix E. About OTPS

 The One-Time Password Specifications are documents produced by RSA
 Laboratories in cooperation with secure systems developers for the
 purpose of simplifying integration and management of strong
 authentication technology into secure applications, and to enhance
 the user experience of this technology.
 Further development of the OTPS series will occur through mailing
 list discussions and occasional workshops, and suggestions for
 improvement are welcome.  As for our PKCS documents, results may also
 be submitted to standards forums.  For more information, contact:
 OTPS Editor
 RSA Laboratories
 174 Middlesex Turnpike
 Bedford, MA  01730 USA
 otps-editor@rsasecurity.com
 http://www.rsasecurity.com/rsalabs/

Author's Address

 Magnus Nystroem
 RSA Security
 EMail: magnus@rsasecurity.com

Nystroem Informational [Page 53] RFC 4758 CT-KIP Version 1.0 Revision 1 November 2006

Full Copyright Statement

 Copyright (C) The IETF Trust (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST,
 AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
 EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
 THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY
 IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
 PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
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 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
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 rights that may cover technology that may be required to implement
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Nystroem Informational [Page 54]

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