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rfc:rfc6278

Internet Engineering Task Force (IETF) J. Herzog Request for Comments: 6278 R. Khazan Category: Informational MIT Lincoln Laboratory ISSN: 2070-1721 June 2011

Use of Static-Static Elliptic Curve Diffie-Hellman Key Agreement in
                    Cryptographic Message Syntax

Abstract

 This document describes how to use the 'static-static Elliptic Curve
 Diffie-Hellman key-agreement scheme (i.e., Elliptic Curve Diffie-
 Hellman where both participants use static Diffie-Hellman values)
 with the Cryptographic Message Syntax.  In this form of key
 agreement, the Diffie-Hellman values of both the sender and receiver
 are long-term values contained in certificates.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6278.

Herzog & Khazan Informational [Page 1] RFC 6278 Static-Static ECDH in CMS June 2011

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ....................................................2
    1.1. Requirements Terminology ...................................5
 2. EnvelopedData Using Static-Static ECDH ..........................5
    2.1. Fields of the KeyAgreeRecipientInfo ........................5
    2.2. Actions of the Sending Agent ...............................6
    2.3. Actions of the Receiving Agent .............................7
 3. AuthenticatedData Using Static-Static ECDH ......................8
    3.1. Fields of the KeyAgreeRecipientInfo ........................8
    3.2. Actions of the Sending Agent ...............................8
    3.3. Actions of the Receiving Agent .............................9
 4. AuthEnvelopedData Using Static-Static ECDH ......................9
    4.1. Fields of the KeyAgreeRecipientInfo ........................9
    4.2. Actions of the Sending Agent ...............................9
    4.3. Actions of the Receiving Agent .............................9
 5. Comparison to RFC 5753 ..........................................9
 6. Requirements and Recommendations ...............................10
 7. Security Considerations ........................................12
 8. Acknowledgements ...............................................14
 9. References .....................................................14
    9.1. Normative References ......................................14
    9.2. Informative References ....................................15

1. Introduction

 This document describes how to use the static-static Elliptic Curve
 Diffie-Hellman key-agreement scheme (i.e., Elliptic Curve Diffie-
 Hellman [RFC6090] where both participants use static Diffie-Hellman
 values) in the Cryptographic Message Syntax (CMS) [RFC5652].  The CMS
 is a standard notation and representation for cryptographic messages.
 The CMS uses ASN.1 notation [X.680] [X.681] [X.682] [X.683] to define

Herzog & Khazan Informational [Page 2] RFC 6278 Static-Static ECDH in CMS June 2011

 a number of structures that carry both cryptographically protected
 information and key-management information regarding the keys used.
 Of particular interest here are three structures:
 o  EnvelopedData, which holds encrypted (but not necessarily
    authenticated) information [RFC5652],
 o  AuthenticatedData, which holds authenticated (MACed) information
    [RFC5652], and
 o  AuthEnvelopedData, which holds information protected by
    authenticated encryption: a cryptographic scheme that combines
    encryption and authentication [RFC5083].
 All three of these types share the same basic structure.  First, a
 fresh symmetric key is generated.  This symmetric key has a different
 name that reflects its usage in each of the three structures.
 EnvelopedData uses a content-encryption key (CEK); AuthenticatedData
 uses an authentication key; AuthEnvelopedData uses a content-
 authenticated-encryption key.  The originator uses the symmetric key
 to cryptographically protect the content.  The symmetric key is then
 wrapped for each recipient; only the intended recipient has access to
 the private keying material necessary to unwrap the symmetric key.
 Once unwrapped, the recipient uses the symmetric key to decrypt the
 content, check the authenticity of the content, or both.  The CMS
 supports several different approaches to symmetric key wrapping,
 including:
 o  key transport: the symmetric key is encrypted using the public
    encryption key of some recipient,
 o  key-encryption key: the symmetric key is encrypted using a
    previously distributed symmetric key, and
 o  key agreement: the symmetric key is encrypted using a key-
    encryption key (KEK) created using a key-agreement scheme and a
    key-derivation function (KDF).
 One such key-agreement scheme is the Diffie-Hellman algorithm
 [RFC2631], which uses group theory to produce a value known only to
 its two participants.  In this case, the participants are the
 originator and one of the recipients.  Each participant produces a
 private value and a public value, and each participant can produce
 the shared secret value from their own private value and their
 counterpart's public value.  There are some variations on the basic
 algorithm:

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 o  The basic algorithm typically uses the group 'Z mod p', meaning
    the set of integers modulo some prime p.  One can also use an
    elliptic curve group, which allows for shorter messages.
 o  Over elliptic curve groups, the standard algorithm can be extended
    to incorporate the 'cofactor' of the group.  This method, called
    'cofactor Elliptic Curve Diffie-Hellman' [SP800-56A] can prevent
    certain attacks possible in the elliptic curve group.
 o  The participants can generate fresh new public/private values
    (called ephemeral values) for each run of the algorithm, or they
    can re-use long-term values (called static values).  Ephemeral
    values add randomness to the resulting private value, while static
    values can be embedded in certificates.  The two participants do
    not need to use the same kind of value: either participant can use
    either type.  In 'ephemeral-static' Diffie-Hellman, for example,
    the sender uses an ephemeral public/private pair value while the
    receiver uses a static pair.  In 'static-static' Diffie-Hellman,
    on the other hand, both participants use static pairs.  (Receivers
    cannot use ephemeral values in this setting, and so we ignore
    ephemeral-ephemeral and static-ephemeral Diffie-Hellman in this
    document.)
 Several of these variations are already described in existing CMS
 standards; for example, [RFC3370] contains the conventions for using
 ephemeral-static and static-static Diffie-Hellman over the 'basic' (Z
 mod p) group.  [RFC5753] contains the conventions for using
 ephemeral-static Diffie-Hellman over elliptic curves (both standard
 and cofactor methods).  It does not, however, contain conventions for
 using either method of static-static Elliptic Curve Diffie-Hellman,
 preferring to discuss the Elliptic Curve Menezes-Qu-Vanstone (ECMQV)
 algorithm instead.
 In this document, we specify the conventions for using static-static
 Elliptic Curve Diffie-Hellman (ECDH) for both standard and cofactor
 methods.  Our motivation stems from the fact that ECMQV has been
 removed from the National Security Agency's Suite B of cryptographic
 algorithms and will therefore be unavailable to some participants.
 These participants can use ephemeral-static Elliptic Curve Diffie-
 Hellman, of course, but ephemeral-static Diffie-Hellman does not
 provide source authentication.  The CMS does allow the application of
 digital signatures for source authentication, but this alternative is
 available only to those participants with certified signature keys.
 By specifying conventions for static-static Elliptic Curve Diffie-
 Hellman in this document, we present a third alternative for source
 authentication, available to those participants with certified
 Elliptic Curve Diffie-Hellman keys.

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 We note that like ephemeral-static ECDH, static-static ECDH creates a
 secret key shared by the sender and receiver.  Unlike ephemeral-
 static ECDH, however, static-static ECDH uses a static key pair for
 the sender.  Each of the three CMS structures discussed in this
 document (EnvelopedData, AuthenticatedData, and AuthEnvelopedData)
 uses static-static ECDH to achieve different goals:
 o  EnvelopedData uses static-static ECDH to provide data
    confidentiality.  It will not necessarily, however, provide data
    authenticity.
 o  AuthenticatedData uses static-static ECDH to provide data
    authenticity.  It will not provide data confidentiality.
 o  AuthEnvelopedData uses static-static ECDH to provide both
    confidentiality and data authenticity.

1.1. Requirements Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

2. EnvelopedData Using Static-Static ECDH

 If an implementation uses static-static ECDH with the CMS
 EnvelopedData, then the following techniques and formats MUST be
 used.  The fields of EnvelopedData are as in [RFC5652]; as static-
 static ECDH is a key-agreement algorithm, the RecipientInfo 'kari'
 choice is used.  When using static-static ECDH, the EnvelopedData
 originatorInfo field MAY include the certificate(s) for the EC public
 key(s) used in the formation of the pairwise key.

2.1. Fields of the KeyAgreeRecipientInfo

 When using static-static ECDH with EnvelopedData, the fields of
 KeyAgreeRecipientInfo [RFC5652] are as follows:
 o  version MUST be 3.
 o  originator identifies the static EC public key of the sender.  It
    MUST be either issuerAndSerialNumber or subjectKeyIdentifier, and
    it MUST point to one of the sending agent's certificates.
 o  ukm MAY be present or absent.  However, message originators SHOULD
    include the ukm and SHOULD ensure that the value of ukm is unique
    to the message being sent.  As specified in [RFC5652],
    implementations MUST support ukm message recipient processing, so

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    interoperability is not a concern if the ukm is present or absent.
    The use of a fresh value for ukm will ensure that a different key
    is generated for each message between the same sender and
    receiver.  The ukm, if present, is placed in the entityUInfo field
    of the ECC-CMS-SharedInfo structure [RFC5753] and therefore used
    as an input to the key-derivation function.
 o  keyEncryptionAlgorithm MUST contain the object identifier of the
    key-encryption algorithm, which in this case is a key-agreement
    algorithm (see Section 5).  The parameters field contains
    KeyWrapAlgorithm.  The KeyWrapAlgorithm is the algorithm
    identifier that indicates the symmetric encryption algorithm used
    to encrypt the content-encryption key (CEK) with the key-
    encryption key (KEK) and any associated parameters (see
    Section 5).
 o  recipientEncryptedKeys contains an identifier and an encrypted CEK
    for each recipient.  The RecipientEncryptedKey
    KeyAgreeRecipientIdentifier MUST contain either the
    issuerAndSerialNumber identifying the recipient's certificate or
    the RecipientKeyIdentifier containing the subject key identifier
    from the recipient's certificate.  In both cases, the recipient's
    certificate contains the recipient's static ECDH public key.
    RecipientEncryptedKey EncryptedKey MUST contain the content-
    encryption key encrypted with the static-static ECDH-generated
    pairwise key-encryption key using the algorithm specified by the
    KeyWrapAlgorithm.

2.2. Actions of the Sending Agent

 When using static-static ECDH with EnvelopedData, the sending agent
 first obtains the EC public key(s) and domain parameters contained in
 the recipient's certificate.  It MUST confirm the following at least
 once per recipient-certificate:
 o  that both certificates (the recipient's certificate and its own)
    contain public-key values with the same curve parameters, and
 o  that both of these public-key values are marked as appropriate for
    ECDH (that is, marked with algorithm identifiers id-ecPublicKey or
    id-ecDH [RFC5480]).
 The sender then determines whether to use standard or cofactor
 Diffie-Hellman.  After doing so, the sender then determines which
 hash algorithms to use for the key-derivation function.  It then
 chooses the keyEncryptionAlgorithm value that reflects these choices.
 It then determines:

Herzog & Khazan Informational [Page 6] RFC 6278 Static-Static ECDH in CMS June 2011

 o  an integer "keydatalen", which is the KeyWrapAlgorithm symmetric
    key size in bits, and
 o  the value of ukm, if used.
 The sender then determines a bit string "SharedInfo", which is the
 DER encoding of ECC-CMS-SharedInfo (see Section 7.2 of [RFC5753]).
 The sending agent then performs either the Elliptic Curve Diffie-
 Hellman operation of [RFC6090] (for standard Diffie-Hellman) or the
 Elliptic Curve Cryptography Cofactor Diffie-Hellman (ECC CDH)
 Primitive of [SP800-56A] (for cofactor Diffie-Hellman).  The sending
 agent then applies the simple hash-function construct of [X963]
 (using the hash algorithm identified in the key-agreement algorithm)
 to the results of the Diffie-Hellman operation and the SharedInfo
 string.  (This construct is also described in Section 3.6.1 of
 [SEC1].)  As a result, the sending agent obtains a shared secret bit
 string "K", which is used as the pairwise key-encryption key (KEK) to
 wrap the CEK for that recipient, as specified in [RFC5652].

2.3. Actions of the Receiving Agent

 When using static-static ECDH with EnvelopedData, the receiving agent
 retrieves keyEncryptionAlgorithm to determine the key-agreement
 algorithm chosen by the sender, which will identify:
 o  the domain parameters of the curve used,
 o  whether standard or cofactor Diffie-Hellman was used, and
 o  which hash function was used for the KDF.
 The receiver then retrieves the sender's certificate identified in
 the rid field and extracts the EC public key(s) and domain parameters
 contained therein.  It MUST confirm the following at least once per
 sender certificate:
 o  that both certificates (the sender's certificate and its own)
    contain public-key values with the same curve parameters, and
 o  that both of these public-key values are marked as appropriate for
    ECDH (that is, marked with algorithm identifiers id-ecPublicKey or
    id-ecDH [RFC5480]).
 The receiver then determines whether standard or cofactor Diffie-
 Hellman was used.  The receiver then determines a bit string
 "SharedInfo", which is the DER encoding of ECC-CMS-SharedInfo (see
 Section 7.2 of [RFC5753]).  The receiving agent then performs either
 the Elliptic Curve Diffie-Hellman operation of [RFC6090] (for

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 standard Diffie-Hellman) or the Elliptic Curve Cryptography Cofactor
 Diffie-Hellman (ECC CDH) Primitive of [SP800-56A] (for cofactor
 Diffie-Hellman).  The receiving agent then applies the simple hash-
 function construct of [X963] (using the hash algorithm identified in
 the key-agreement algorithm) to the results of the Diffie-Hellman
 operation and the SharedInfo string.  (This construct is also
 described in Section 3.6.1 of [SEC1].)  As a result, the receiving
 agent obtains a shared secret bit string "K", which it uses as the
 pairwise key-encryption key to unwrap the CEK.

3. AuthenticatedData Using Static-Static ECDH

 This section describes how to use the static-static ECDH key-
 agreement algorithm with AuthenticatedData.  When using static-static
 ECDH with AuthenticatedData, the fields of AuthenticatedData are as
 in [RFC5652], but with the following restrictions:
 o  macAlgorithm MUST contain the algorithm identifier of the message
    authentication code (MAC) algorithm.  This algorithm SHOULD be one
    of the following -- id-hmacWITHSHA224, id-hmacWITHSHA256,
    id-hmacWITHSHA384, or id-hmacWITHSHA512 -- and SHOULD NOT be
    hmac-SHA1.  (See Section 5.)
 o  digestAlgorithm MUST contain the algorithm identifier of the hash
    algorithm.  This algorithm SHOULD be one of the following --
    id-sha224, id-sha256, id-sha384, or id-sha512 -- and SHOULD NOT be
    id-sha1.  (See Section 5.)
 As static-static ECDH is a key-agreement algorithm, the RecipientInfo
 kari choice is used in the AuthenticatedData.  When using static-
 static ECDH, the AuthenticatedData originatorInfo field MAY include
 the certificate(s) for the EC public key(s) used in the formation of
 the pairwise key.

3.1. Fields of the KeyAgreeRecipientInfo

 The AuthenticatedData KeyAgreeRecipientInfo fields are used in the
 same manner as the fields for the corresponding EnvelopedData
 KeyAgreeRecipientInfo fields of Section 2.1 of this document.  The
 authentication key is wrapped in the same manner as is described
 there for the content-encryption key.

3.2. Actions of the Sending Agent

 The sending agent uses the same actions as for EnvelopedData with
 static-static ECDH, as specified in Section 2.2 of this document.

Herzog & Khazan Informational [Page 8] RFC 6278 Static-Static ECDH in CMS June 2011

3.3. Actions of the Receiving Agent

 The receiving agent uses the same actions as for EnvelopedData with
 static-static ECDH, as specified in Section 2.3 of this document.

4. AuthEnvelopedData Using Static-Static ECDH

 When using static-static ECDH with AuthEnvelopedData, the fields of
 AuthEnvelopedData are as in [RFC5083].  As static-static ECDH is a
 key-agreement algorithm, the RecipientInfo kari choice is used.  When
 using static-static ECDH, the AuthEnvelopedData originatorInfo field
 MAY include the certificate(s) for the EC public key used in the
 formation of the pairwise key.

4.1. Fields of the KeyAgreeRecipientInfo

 The AuthEnvelopedData KeyAgreeRecipientInfo fields are used in the
 same manner as the fields for the corresponding EnvelopedData
 KeyAgreeRecipientInfo fields of Section 2.1 of this document.  The
 content-authenticated-encryption key is wrapped in the same manner as
 is described there for the content-encryption key.

4.2. Actions of the Sending Agent

 The sending agent uses the same actions as for EnvelopedData with
 static-static ECDH, as specified in Section 2.2 of this document.

4.3. Actions of the Receiving Agent

 The receiving agent uses the same actions as for EnvelopedData with
 static-static ECDH, as specified in Section 2.3 of this document.

5. Comparison to RFC 5753

 This document defines the use of static-static ECDH for
 EnvelopedData, AuthenticatedData, and AuthEnvelopedData.  [RFC5753]
 defines ephemeral-static ECDH for EnvelopedData only.
 With regard to EnvelopedData, this document and [RFC5753] greatly
 parallel each other.  Both specify how to apply Elliptic Curve
 Diffie-Hellman and differ only on how the sender's public value is to
 be communicated to the recipient.  In [RFC5753], the sender provides
 the public value explicitly by including an OriginatorPublicKey value
 in the originator field of KeyAgreeRecipientInfo.  In this document,
 the sender includes a reference to a (certified) public value by
 including either an IssuerAndSerialNumber or SubjectKeyIdentifier
 value in the same field.  Put another way, [RFC5753] provides an
 interpretation of a KeyAgreeRecipientInfo structure where:

Herzog & Khazan Informational [Page 9] RFC 6278 Static-Static ECDH in CMS June 2011

 o  the keyEncryptionAlgorithm value indicates Elliptic Curve Diffie-
    Hellman, and
 o  the originator field contains an OriginatorPublicKey value.
 This document, on the other hand, provides an interpretation of a
 KeyAgreeRecipientInfo structure where:
 o  the keyEncryptionAlgorithm value indicates Elliptic Curve Diffie-
    Hellman, and
 o  the originator field contains either an IssuerAndSerialNumber
    value or a SubjectKeyIdentifier value.
 AuthenticatedData or AuthEnvelopedData messages, on the other hand,
 are not given any form of ECDH by [RFC5753].  This is appropriate:
 that document only defines ephemeral-static Diffie-Hellman, and this
 form of Diffie-Hellman does not (inherently) provide any form of data
 authentication or data-origin authentication.  This document, on the
 other hand, requires that the sender use a certified public value.
 Thus, this form of key agreement provides implicit key authentication
 and, under some limited circumstances, data-origin authentication.
 (See Section 7.)
 This document does not define any new ASN.1 structures or algorithm
 identifiers.  It provides new ways to interpret structures from
 [RFC5652] and [RFC5753], and it allows previously defined algorithms
 to be used under these new interpretations.  Specifically:
 o  The ECDH key-agreement algorithm identifiers from [RFC5753] define
    only how Diffie-Hellman values are processed, and not where these
    values are created.  Therefore, they can be used for static-static
    ECDH with no changes.
 o  The key-wrap, MAC, and digest algorithms referenced in [RFC5753]
    describe how the secret key is to be used but not created.
    Therefore, they can be used with keys from static-static ECDH
    without modification.

6. Requirements and Recommendations

 It is RECOMMENDED that implementations of this specification support
 AuthenticatedData and EnvelopedData.  Support for AuthEnvelopedData
 is OPTIONAL.
 Implementations that support this specification MUST support standard
 Elliptic Curve Diffie-Hellman, and these implementations MAY also
 support cofactor Elliptic Curve Diffie-Hellman.

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 In order to encourage interoperability, implementations SHOULD use
 the elliptic curve domain parameters specified by [RFC5480].
 Implementations that support standard static-static Elliptic Curve
 Diffie-Hellman:
 o  MUST support the dhSinglePass-stdDH-sha256kdf-scheme key-
    agreement algorithm;
 o  MAY support the dhSinglePass-stdDH-sha224kdf-scheme,
    dhSinglePass-stdDH-sha384kdf-scheme, and
    dhSinglePass-stdDH-sha512kdf-scheme key-agreement algorithms; and
 o  SHOULD NOT support the dhSinglePass-stdDH-sha1kdf-scheme
    algorithm.
 Other algorithms MAY also be supported.
 Implementations that support cofactor static-static Elliptic Curve
 Diffie-Hellman:
 o  MUST support the dhSinglePass-cofactorDH-sha256kdf-scheme key-
    agreement algorithm;
 o  MAY support the dhSinglePass-cofactorDH-sha224kdf-scheme,
    dhSinglePass-cofactorDH-sha384kdf-scheme, and
    dhSinglePass-cofactorDH-sha512kdf-scheme key-agreement algorithms;
    and
 o  SHOULD NOT support the dhSinglePass-cofactorDH-sha1kdf-scheme
    algorithm.
 In addition, all implementations:
 o  MUST support the id-aes128-wrap key-wrap algorithm and the
    id-aes128-cbc content-encryption algorithm;
 o  MAY support:
  • the id-aes192-wrap and id-aes256-wrap key-wrap algorithms;
  • the id-aes128-CCM, id-aes192-CCM, id-aes256-CCM, id-aes128-GCM,

id-aes192-GCM, and id-aes256-GCM authenticated-encryption

       algorithms; and
  • the id-aes192-cbc and id-aes256-cbc content-encryption

algorithms.

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 o  SHOULD NOT support the id-alg-CMS3DESwrap key-wrap algorithm or
    the des-ede3-cbc content-encryption algorithms.
 (All algorithms above are defined in [RFC3370], [RFC3565], [RFC5084],
 and [RFC5753].)  Unless otherwise noted above, other algorithms MAY
 also be supported.

7. Security Considerations

 All security considerations in Section 9 of [RFC5753] apply.
 Extreme care must be used when using static-static Diffie-Hellman
 (either standard or cofactor) without the use of some per-message
 value in the ukm.  As described in [RFC5753], the ukm value (if
 present) will be embedded in an ECC-CMS-SharedInfo structure, and the
 DER encoding of this structure will be used as the 'SharedInfo' input
 to the key-derivation function of [X963].  The purpose of this input
 is to add a message-unique value to the key-distribution function so
 that two different sessions of static-static ECDH between a given
 pair of agents result in independent keys.  If the ukm value is not
 used or is re-used, on the other hand, then the ECC-CMS-SharedInfo
 structure (and 'SharedInfo' input) will likely not vary from message
 to message.  In this case, the two agents will re-use the same keying
 material across multiple messages.  This is considered to be bad
 cryptographic practice and may open the sender to attacks on Diffie-
 Hellman (e.g., the 'small subgroup' attack [MenezesUstaoglu] or
 other, yet-undiscovered attacks).
 It is for these reasons that Section 2.1 states that message senders
 SHOULD include the ukm and SHOULD ensure that the value of ukm is
 unique to the message being sent.  One way to ensure the uniqueness
 of the ukm is for the message sender to choose a 'sufficiently long'
 random string for each message (where, as a rule of thumb, a
 'sufficiently long' string is one at least as long as the keys used
 by the key-wrap algorithm identified in the keyEncryptionAlgorithm
 field of the KeyAgreeRecipientInfo structure).  However, other
 methods (such as a counter) are possible.  Also, applications that
 cannot tolerate the inclusion of per-message information in the ukm
 (due to bandwidth requirements, for example) SHOULD NOT use static-
 static ECDH for a recipient without ascertaining that the recipient
 knows the private value associated with their certified Diffie-
 Hellman value.
 Static-static Diffie-Hellman, when used as described in this
 document, does not necessarily provide data-origin authentication.
 Consider, for example, the following sequence of events:

Herzog & Khazan Informational [Page 12] RFC 6278 Static-Static ECDH in CMS June 2011

 o  Alice sends an AuthEnvelopedData message to both Bob and Mallory.
    Furthermore, Alice uses a static-static DH method to transport the
    content-authenticated-encryption key to Bob, and some arbitrary
    method to transport the same key to Mallory.
 o  Mallory intercepts the message and prevents Bob from receiving it.
 o  Mallory recovers the content-authenticated-encryption key from the
    message received from Alice.  Mallory then creates new plaintext
    of her choice, and encrypts it using the same authenticated-
    encryption algorithm and the same content-authenticated-encryption
    key used by Alice.
 o  Mallory then replaces the EncryptedContentInfo and
    MessageAuthenticationCode fields of Alice's message with the
    values Mallory just generated.  She may additionally remove her
    RecipientInfo value from Alice's message.
 o  Mallory sends the modified message to Bob.
 o  Bob receives the message, validates the static-static DH values,
    and decrypts/authenticates the message.
 At this point, Bob has received and validated a message that appears
 to have been sent by Alice, but whose content was chosen by Mallory.
 Mallory may not even be an apparent receiver of the modified message.
 Thus, this use of static-static Diffie-Hellman does not necessarily
 provide data-origin authentication.  (We note that this example does
 not also contradict either confidentiality or data authentication:
 Alice's message was not received by anyone not intended by Alice, and
 Mallory's message was not modified before reaching Bob.)
 More generally, the data origin may not be authenticated unless:
 o  it is a priori guaranteed that the message in question was sent to
    exactly one recipient, or
 o  data-origin authentication is provided by some other mechanism
    (such as digital signatures).
 However, we also note that this lack of authentication is not a
 product of static-static ECDH per se, but is inherent in the way key-
 agreement schemes are used in the AuthenticatedData and
 AuthEnvelopedData structures of the CMS.
 When two parties are communicating using static-static ECDH as
 described in this document, and either party's asymmetric keys have
 been centrally generated, it is possible for that party's central

Herzog & Khazan Informational [Page 13] RFC 6278 Static-Static ECDH in CMS June 2011

 infrastructure to decrypt the communication (for application-layer
 network monitoring or filtering, for example).  By way of contrast:
 were ephemeral-static ECDH to be used instead, such decryption by the
 sender's infrastructure would not be possible (though it would remain
 possible for the infrastructure of any recipient).

8. Acknowledgements and Disclaimer

 This work is sponsored by the United States Air Force under Air Force
 Contract FA8721-05-C-0002.  Opinions, interpretations, conclusions
 and recommendations are those of the authors and are not necessarily
 endorsed by the United States Government.
 The authors would like to thank Jim Schaad, Russ Housley, Sean
 Turner, Brian Weis, Rene Struik, Brian Carpenter, David McGrew, and
 Stephen Farrell for their helpful comments and suggestions.  We would
 also like to thank Jim Schaad for describing to us the attack
 described in Section 7.

9. References

9.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3370]  Housley, R., "Cryptographic Message Syntax (CMS)
            Algorithms", RFC 3370, August 2002.
 [RFC3565]  Schaad, J., "Use of the Advanced Encryption Standard (AES)
            Encryption Algorithm in Cryptographic Message Syntax
            (CMS)", RFC 3565, July 2003.
 [RFC5083]  Housley, R., "Cryptographic Message Syntax (CMS)
            Authenticated-Enveloped-Data Content Type", RFC 5083,
            November 2007.
 [RFC5084]  Housley, R., "Using AES-CCM and AES-GCM Authenticated
            Encryption in the Cryptographic Message Syntax (CMS)",
            RFC 5084, November 2007.
 [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
            "Elliptic Curve Cryptography Subject Public Key
            Information", RFC 5480, March 2009.
 [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
            RFC 5652, September 2009.

Herzog & Khazan Informational [Page 14] RFC 6278 Static-Static ECDH in CMS June 2011

 [RFC5753]  Turner, S. and D. Brown, "Use of Elliptic Curve
            Cryptography (ECC) Algorithms in Cryptographic Message
            Syntax (CMS)", RFC 5753, January 2010.
 [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
            Curve Cryptography Algorithms", RFC 6090, February 2011.
 [SP800-56A]
            Barker, E., Johnson, D., and M. Smid, "Recommendation for
            Pair-Wise Key Establishment Schemes Using Discrete
            Logarithm Cryptography (Revised)", NIST Special
            Publication (SP) 800-56A, March 2007.
 [X963]     "Public Key Cryptography for the Financial Services
            Industry, Key Agreement and Key Transport Using Elliptic
            Curve Cryptography", ANSI X9.63, 2001.

9.2. Informative References

 [MenezesUstaoglu]
            Menezes, A. and B. Ustaoglu, "On Reusing Ephemeral Keys in
            Diffie-Hellman Key Agreement Protocols", International
            Journal of Applied Cryptography, Vol. 2, No. 2, pp. 154-
            158, 2010.
 [RFC2631]  Rescorla, E., "Diffie-Hellman Key Agreement Method",
            RFC 2631, June 1999.
 [SEC1]     Standards for Efficient Cryptography Group (SECG), "SEC 1:
            Elliptic Curve Cryptography", Version 2.0, May 2009.
 [X.680]    ITU-T, "Information Technology - Abstract Syntax Notation
            One: Specification of Basic Notation",
            Recommendation X.680, ISO/IEC 8824-1:2002, 2002.
 [X.681]    ITU-T, "Information Technology - Abstract Syntax Notation
            One: Information Object Specification",
            Recommendation X.681, ISO/IEC 8824-2:2002, 2002.
 [X.682]    ITU-T, "Information Technology - Abstract Syntax Notation
            One: Constraint Specification", Recommendation X.682, ISO/
            IEC 8824-3:2002, 2002.
 [X.683]    ITU-T, "Information Technology - Abstract Syntax Notation
            One: Parameterization of ASN.1 Specifications",
            Recommendation X.683, ISO/IEC 8824-4:2002, 2002.

Herzog & Khazan Informational [Page 15] RFC 6278 Static-Static ECDH in CMS June 2011

Authors' Addresses

 Jonathan C. Herzog
 MIT Lincoln Laboratory
 244 Wood St.
 Lexington, MA  02144
 USA
 EMail: jherzog@ll.mit.edu
 Roger Khazan
 MIT Lincoln Laboratory
 244 Wood St.
 Lexington, MA  02144
 USA
 EMail: rkh@ll.mit.edu

Herzog & Khazan Informational [Page 16]

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