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rfc:rfc4251

Network Working Group T. Ylonen Request for Comments: 4251 SSH Communications Security Corp Category: Standards Track C. Lonvick, Ed.

                                                   Cisco Systems, Inc.
                                                          January 2006
            The Secure Shell (SSH) Protocol Architecture

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 The Secure Shell (SSH) Protocol is a protocol for secure remote login
 and other secure network services over an insecure network.  This
 document describes the architecture of the SSH protocol, as well as
 the notation and terminology used in SSH protocol documents.  It also
 discusses the SSH algorithm naming system that allows local
 extensions.  The SSH protocol consists of three major components: The
 Transport Layer Protocol provides server authentication,
 confidentiality, and integrity with perfect forward secrecy.  The
 User Authentication Protocol authenticates the client to the server.
 The Connection Protocol multiplexes the encrypted tunnel into several
 logical channels.  Details of these protocols are described in
 separate documents.

Ylonen & Lonvick Standards Track [Page 1] RFC 4251 SSH Protocol Architecture January 2006

Table of Contents

 1. Introduction ....................................................3
 2. Contributors ....................................................3
 3. Conventions Used in This Document ...............................4
 4. Architecture ....................................................4
    4.1. Host Keys ..................................................4
    4.2. Extensibility ..............................................6
    4.3. Policy Issues ..............................................6
    4.4. Security Properties ........................................7
    4.5. Localization and Character Set Support .....................7
 5. Data Type Representations Used in the SSH Protocols .............8
 6. Algorithm and Method Naming ....................................10
 7. Message Numbers ................................................11
 8. IANA Considerations ............................................12
 9. Security Considerations ........................................13
    9.1. Pseudo-Random Number Generation ...........................13
    9.2. Control Character Filtering ...............................14
    9.3. Transport .................................................14
         9.3.1. Confidentiality ....................................14
         9.3.2. Data Integrity .....................................16
         9.3.3. Replay .............................................16
         9.3.4. Man-in-the-middle ..................................17
         9.3.5. Denial of Service ..................................19
         9.3.6. Covert Channels ....................................20
         9.3.7. Forward Secrecy ....................................20
         9.3.8. Ordering of Key Exchange Methods ...................20
         9.3.9. Traffic Analysis ...................................21
    9.4. Authentication Protocol ...................................21
         9.4.1. Weak Transport .....................................21
         9.4.2. Debug Messages .....................................22
         9.4.3. Local Security Policy ..............................22
         9.4.4. Public Key Authentication ..........................23
         9.4.5. Password Authentication ............................23
         9.4.6. Host-Based Authentication ..........................23
    9.5. Connection Protocol .......................................24
         9.5.1. End Point Security .................................24
         9.5.2. Proxy Forwarding ...................................24
         9.5.3. X11 Forwarding .....................................24
 10. References ....................................................26
    10.1. Normative References .....................................26
    10.2. Informative References ...................................26
 Authors' Addresses ................................................29
 Trademark Notice ..................................................29

Ylonen & Lonvick Standards Track [Page 2] RFC 4251 SSH Protocol Architecture January 2006

1. Introduction

 Secure Shell (SSH) is a protocol for secure remote login and other
 secure network services over an insecure network.  It consists of
 three major components:
 o  The Transport Layer Protocol [SSH-TRANS] provides server
    authentication, confidentiality, and integrity.  It may optionally
    also provide compression.  The transport layer will typically be
    run over a TCP/IP connection, but might also be used on top of any
    other reliable data stream.
 o  The User Authentication Protocol [SSH-USERAUTH] authenticates the
    client-side user to the server.  It runs over the transport layer
    protocol.
 o  The Connection Protocol [SSH-CONNECT] multiplexes the encrypted
    tunnel into several logical channels.  It runs over the user
    authentication protocol.
 The client sends a service request once a secure transport layer
 connection has been established.  A second service request is sent
 after user authentication is complete.  This allows new protocols to
 be defined and coexist with the protocols listed above.
 The connection protocol provides channels that can be used for a wide
 range of purposes.  Standard methods are provided for setting up
 secure interactive shell sessions and for forwarding ("tunneling")
 arbitrary TCP/IP ports and X11 connections.

2. Contributors

 The major original contributors of this set of documents have been:
 Tatu Ylonen, Tero Kivinen, Timo J. Rinne, Sami Lehtinen (all of SSH
 Communications Security Corp), and Markku-Juhani O. Saarinen
 (University of Jyvaskyla).  Darren Moffat was the original editor of
 this set of documents and also made very substantial contributions.
 Many people contributed to the development of this document over the
 years.  People who should be acknowledged include Mats Andersson, Ben
 Harris, Bill Sommerfeld, Brent McClure, Niels Moller, Damien Miller,
 Derek Fawcus, Frank Cusack, Heikki Nousiainen, Jakob Schlyter, Jeff
 Van Dyke, Jeffrey Altman, Jeffrey Hutzelman, Jon Bright, Joseph
 Galbraith, Ken Hornstein, Markus Friedl, Martin Forssen, Nicolas
 Williams, Niels Provos, Perry Metzger, Peter Gutmann, Simon
 Josefsson, Simon Tatham, Wei Dai, Denis Bider, der Mouse, and
 Tadayoshi Kohno.  Listing their names here does not mean that they
 endorse this document, but that they have contributed to it.

Ylonen & Lonvick Standards Track [Page 3] RFC 4251 SSH Protocol Architecture January 2006

3. Conventions Used in This Document

 All documents related to the SSH protocols shall use the keywords
 "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
 "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" to describe
 requirements.  These keywords are to be interpreted as described in
 [RFC2119].
 The keywords "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME
 FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG
 APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in
 this document when used to describe namespace allocation are to be
 interpreted as described in [RFC2434].
 Protocol fields and possible values to fill them are defined in this
 set of documents.  Protocol fields will be defined in the message
 definitions.  As an example, SSH_MSG_CHANNEL_DATA is defined as
 follows.
    byte      SSH_MSG_CHANNEL_DATA
    uint32    recipient channel
    string    data
 Throughout these documents, when the fields are referenced, they will
 appear within single quotes.  When values to fill those fields are
 referenced, they will appear within double quotes.  Using the above
 example, possible values for 'data' are "foo" and "bar".

4. Architecture

4.1. Host Keys

 Each server host SHOULD have a host key.  Hosts MAY have multiple
 host keys using multiple different algorithms.  Multiple hosts MAY
 share the same host key.  If a host has keys at all, it MUST have at
 least one key that uses each REQUIRED public key algorithm (DSS
 [FIPS-186-2]).
 The server host key is used during key exchange to verify that the
 client is really talking to the correct server.  For this to be
 possible, the client must have a priori knowledge of the server's
 public host key.
 Two different trust models can be used:
 o  The client has a local database that associates each host name (as
    typed by the user) with the corresponding public host key.  This
    method requires no centrally administered infrastructure, and no

Ylonen & Lonvick Standards Track [Page 4] RFC 4251 SSH Protocol Architecture January 2006

    third-party coordination.  The downside is that the database of
    name-to-key associations may become burdensome to maintain.
 o  The host name-to-key association is certified by a trusted
    certification authority (CA).  The client only knows the CA root
    key, and can verify the validity of all host keys certified by
    accepted CAs.
 The second alternative eases the maintenance problem, since ideally
 only a single CA key needs to be securely stored on the client.  On
 the other hand, each host key must be appropriately certified by a
 central authority before authorization is possible.  Also, a lot of
 trust is placed on the central infrastructure.
 The protocol provides the option that the server name - host key
 association is not checked when connecting to the host for the first
 time.  This allows communication without prior communication of host
 keys or certification.  The connection still provides protection
 against passive listening; however, it becomes vulnerable to active
 man-in-the-middle attacks.  Implementations SHOULD NOT normally allow
 such connections by default, as they pose a potential security
 problem.  However, as there is no widely deployed key infrastructure
 available on the Internet at the time of this writing, this option
 makes the protocol much more usable during the transition time until
 such an infrastructure emerges, while still providing a much higher
 level of security than that offered by older solutions (e.g., telnet
 [RFC0854] and rlogin [RFC1282]).
 Implementations SHOULD try to make the best effort to check host
 keys.  An example of a possible strategy is to only accept a host key
 without checking the first time a host is connected, save the key in
 a local database, and compare against that key on all future
 connections to that host.
 Implementations MAY provide additional methods for verifying the
 correctness of host keys, e.g., a hexadecimal fingerprint derived
 from the SHA-1 hash [FIPS-180-2] of the public key.  Such
 fingerprints can easily be verified by using telephone or other
 external communication channels.
 All implementations SHOULD provide an option not to accept host keys
 that cannot be verified.
 The members of this Working Group believe that 'ease of use' is
 critical to end-user acceptance of security solutions, and no
 improvement in security is gained if the new solutions are not used.
 Thus, providing the option not to check the server host key is

Ylonen & Lonvick Standards Track [Page 5] RFC 4251 SSH Protocol Architecture January 2006

 believed to improve the overall security of the Internet, even though
 it reduces the security of the protocol in configurations where it is
 allowed.

4.2. Extensibility

 We believe that the protocol will evolve over time, and some
 organizations will want to use their own encryption, authentication,
 and/or key exchange methods.  Central registration of all extensions
 is cumbersome, especially for experimental or classified features.
 On the other hand, having no central registration leads to conflicts
 in method identifiers, making interoperability difficult.
 We have chosen to identify algorithms, methods, formats, and
 extension protocols with textual names that are of a specific format.
 DNS names are used to create local namespaces where experimental or
 classified extensions can be defined without fear of conflicts with
 other implementations.
 One design goal has been to keep the base protocol as simple as
 possible, and to require as few algorithms as possible.  However, all
 implementations MUST support a minimal set of algorithms to ensure
 interoperability (this does not imply that the local policy on all
 hosts would necessarily allow these algorithms).  The mandatory
 algorithms are specified in the relevant protocol documents.
 Additional algorithms, methods, formats, and extension protocols can
 be defined in separate documents.  See Section 6, Algorithm Naming,
 for more information.

4.3. Policy Issues

 The protocol allows full negotiation of encryption, integrity, key
 exchange, compression, and public key algorithms and formats.
 Encryption, integrity, public key, and compression algorithms can be
 different for each direction.
 The following policy issues SHOULD be addressed in the configuration
 mechanisms of each implementation:
 o  Encryption, integrity, and compression algorithms, separately for
    each direction.  The policy MUST specify which is the preferred
    algorithm (e.g., the first algorithm listed in each category).
 o  Public key algorithms and key exchange method to be used for host
    authentication.  The existence of trusted host keys for different
    public key algorithms also affects this choice.

Ylonen & Lonvick Standards Track [Page 6] RFC 4251 SSH Protocol Architecture January 2006

 o  The authentication methods that are to be required by the server
    for each user.  The server's policy MAY require multiple
    authentication for some or all users.  The required algorithms MAY
    depend on the location from where the user is trying to gain
    access.
 o  The operations that the user is allowed to perform using the
    connection protocol.  Some issues are related to security; for
    example, the policy SHOULD NOT allow the server to start sessions
    or run commands on the client machine, and MUST NOT allow
    connections to the authentication agent unless forwarding such
    connections has been requested.  Other issues, such as which
    TCP/IP ports can be forwarded and by whom, are clearly issues of
    local policy.  Many of these issues may involve traversing or
    bypassing firewalls, and are interrelated with the local security
    policy.

4.4. Security Properties

 The primary goal of the SSH protocol is to improve security on the
 Internet.  It attempts to do this in a way that is easy to deploy,
 even at the cost of absolute security.
 o  All encryption, integrity, and public key algorithms used are
    well-known, well-established algorithms.
 o  All algorithms are used with cryptographically sound key sizes
    that are believed to provide protection against even the strongest
    cryptanalytic attacks for decades.
 o  All algorithms are negotiated, and in case some algorithm is
    broken, it is easy to switch to some other algorithm without
    modifying the base protocol.
 Specific concessions were made to make widespread, fast deployment
 easier.  The particular case where this comes up is verifying that
 the server host key really belongs to the desired host; the protocol
 allows the verification to be left out, but this is NOT RECOMMENDED.
 This is believed to significantly improve usability in the short
 term, until widespread Internet public key infrastructures emerge.

4.5. Localization and Character Set Support

 For the most part, the SSH protocols do not directly pass text that
 would be displayed to the user.  However, there are some places where
 such data might be passed.  When applicable, the character set for

Ylonen & Lonvick Standards Track [Page 7] RFC 4251 SSH Protocol Architecture January 2006

 the data MUST be explicitly specified.  In most places, ISO-10646
 UTF-8 encoding is used [RFC3629].  When applicable, a field is also
 provided for a language tag [RFC3066].
 One big issue is the character set of the interactive session.  There
 is no clear solution, as different applications may display data in
 different formats.  Different types of terminal emulation may also be
 employed in the client, and the character set to be used is
 effectively determined by the terminal emulation.  Thus, no place is
 provided for directly specifying the character set or encoding for
 terminal session data.  However, the terminal emulation type (e.g.,
 "vt100") is transmitted to the remote site, and it implicitly
 specifies the character set and encoding.  Applications typically use
 the terminal type to determine what character set they use, or the
 character set is determined using some external means.  The terminal
 emulation may also allow configuring the default character set.  In
 any case, the character set for the terminal session is considered
 primarily a client local issue.
 Internal names used to identify algorithms or protocols are normally
 never displayed to users, and must be in US-ASCII.
 The client and server user names are inherently constrained by what
 the server is prepared to accept.  They might, however, occasionally
 be displayed in logs, reports, etc.  They MUST be encoded using ISO
 10646 UTF-8, but other encodings may be required in some cases.  It
 is up to the server to decide how to map user names to accepted user
 names.  Straight bit-wise, binary comparison is RECOMMENDED.
 For localization purposes, the protocol attempts to minimize the
 number of textual messages transmitted.  When present, such messages
 typically relate to errors, debugging information, or some externally
 configured data.  For data that is normally displayed, it SHOULD be
 possible to fetch a localized message instead of the transmitted
 message by using a numerical code.  The remaining messages SHOULD be
 configurable.

5. Data Type Representations Used in the SSH Protocols

 byte
    A byte represents an arbitrary 8-bit value (octet).  Fixed length
    data is sometimes represented as an array of bytes, written
    byte[n], where n is the number of bytes in the array.

Ylonen & Lonvick Standards Track [Page 8] RFC 4251 SSH Protocol Architecture January 2006

 boolean
    A boolean value is stored as a single byte.  The value 0
    represents FALSE, and the value 1 represents TRUE.  All non-zero
    values MUST be interpreted as TRUE; however, applications MUST NOT
    store values other than 0 and 1.
 uint32
    Represents a 32-bit unsigned integer.  Stored as four bytes in the
    order of decreasing significance (network byte order).  For
    example: the value 699921578 (0x29b7f4aa) is stored as 29 b7 f4
    aa.
 uint64
    Represents a 64-bit unsigned integer.  Stored as eight bytes in
    the order of decreasing significance (network byte order).
 string
    Arbitrary length binary string.  Strings are allowed to contain
    arbitrary binary data, including null characters and 8-bit
    characters.  They are stored as a uint32 containing its length
    (number of bytes that follow) and zero (= empty string) or more
    bytes that are the value of the string.  Terminating null
    characters are not used.
    Strings are also used to store text.  In that case, US-ASCII is
    used for internal names, and ISO-10646 UTF-8 for text that might
    be displayed to the user.  The terminating null character SHOULD
    NOT normally be stored in the string.  For example: the US-ASCII
    string "testing" is represented as 00 00 00 07 t e s t i n g.  The
    UTF-8 mapping does not alter the encoding of US-ASCII characters.
 mpint
    Represents multiple precision integers in two's complement format,
    stored as a string, 8 bits per byte, MSB first.  Negative numbers
    have the value 1 as the most significant bit of the first byte of
    the data partition.  If the most significant bit would be set for
    a positive number, the number MUST be preceded by a zero byte.
    Unnecessary leading bytes with the value 0 or 255 MUST NOT be
    included.  The value zero MUST be stored as a string with zero
    bytes of data.
    By convention, a number that is used in modular computations in
    Z_n SHOULD be represented in the range 0 <= x < n.

Ylonen & Lonvick Standards Track [Page 9] RFC 4251 SSH Protocol Architecture January 2006

       Examples:
       value (hex)        representation (hex)
       -----------        --------------------
       0                  00 00 00 00
       9a378f9b2e332a7    00 00 00 08 09 a3 78 f9 b2 e3 32 a7
       80                 00 00 00 02 00 80
       -1234              00 00 00 02 ed cc
       -deadbeef          00 00 00 05 ff 21 52 41 11
 name-list
    A string containing a comma-separated list of names.  A name-list
    is represented as a uint32 containing its length (number of bytes
    that follow) followed by a comma-separated list of zero or more
    names.  A name MUST have a non-zero length, and it MUST NOT
    contain a comma (",").  As this is a list of names, all of the
    elements contained are names and MUST be in US-ASCII.  Context may
    impose additional restrictions on the names.  For example, the
    names in a name-list may have to be a list of valid algorithm
    identifiers (see Section 6 below), or a list of [RFC3066] language
    tags.  The order of the names in a name-list may or may not be
    significant.  Again, this depends on the context in which the list
    is used.  Terminating null characters MUST NOT be used, neither
    for the individual names, nor for the list as a whole.
     Examples:
     value                      representation (hex)
     -----                      --------------------
     (), the empty name-list    00 00 00 00
     ("zlib")                   00 00 00 04 7a 6c 69 62
     ("zlib,none")              00 00 00 09 7a 6c 69 62 2c 6e 6f 6e 65

6. Algorithm and Method Naming

 The SSH protocols refer to particular hash, encryption, integrity,
 compression, and key exchange algorithms or methods by name.  There
 are some standard algorithms and methods that all implementations
 MUST support.  There are also algorithms and methods that are defined
 in the protocol specification, but are OPTIONAL.  Furthermore, it is
 expected that some organizations will want to use their own
 algorithms or methods.
 In this protocol, all algorithm and method identifiers MUST be
 printable US-ASCII, non-empty strings no longer than 64 characters.
 Names MUST be case-sensitive.

Ylonen & Lonvick Standards Track [Page 10] RFC 4251 SSH Protocol Architecture January 2006

 There are two formats for algorithm and method names:
 o  Names that do not contain an at-sign ("@") are reserved to be
    assigned by IETF CONSENSUS.  Examples include "3des-cbc", "sha-1",
    "hmac-sha1", and "zlib" (the doublequotes are not part of the
    name).  Names of this format are only valid if they are first
    registered with the IANA.  Registered names MUST NOT contain an
    at-sign ("@"), comma (","), whitespace, control characters (ASCII
    codes 32 or less), or the ASCII code 127 (DEL).  Names are case-
    sensitive, and MUST NOT be longer than 64 characters.
 o  Anyone can define additional algorithms or methods by using names
    in the format name@domainname, e.g., "ourcipher-cbc@example.com".
    The format of the part preceding the at-sign is not specified;
    however, these names MUST be printable US-ASCII strings, and MUST
    NOT contain the comma character (","), whitespace, control
    characters (ASCII codes 32 or less), or the ASCII code 127 (DEL).
    They MUST have only a single at-sign in them.  The part following
    the at-sign MUST be a valid, fully qualified domain name [RFC1034]
    controlled by the person or organization defining the name.  Names
    are case-sensitive, and MUST NOT be longer than 64 characters.  It
    is up to each domain how it manages its local namespace.  It
    should be noted that these names resemble STD 11 [RFC0822] email
    addresses.  This is purely coincidental and has nothing to do with
    STD 11 [RFC0822].

7. Message Numbers

 SSH packets have message numbers in the range 1 to 255.  These
 numbers have been allocated as follows:
 Transport layer protocol:
    1 to 19    Transport layer generic (e.g., disconnect, ignore,
               debug, etc.)
    20 to 29   Algorithm negotiation
    30 to 49   Key exchange method specific (numbers can be reused
               for different authentication methods)
 User authentication protocol:
    50 to 59   User authentication generic
    60 to 79   User authentication method specific (numbers can be
               reused for different authentication methods)

Ylonen & Lonvick Standards Track [Page 11] RFC 4251 SSH Protocol Architecture January 2006

 Connection protocol:
    80 to 89   Connection protocol generic
    90 to 127  Channel related messages
 Reserved for client protocols:
    128 to 191 Reserved
 Local extensions:
    192 to 255 Local extensions

8. IANA Considerations

 This document is part of a set.  The instructions for the IANA for
 the SSH protocol, as defined in this document, [SSH-USERAUTH],
 [SSH-TRANS], and [SSH-CONNECT], are detailed in [SSH-NUMBERS].  The
 following is a brief summary for convenience, but note well that
 [SSH-NUMBERS] contains the actual instructions to the IANA, which may
 be superseded in the future.
 Allocation of the following types of names in the SSH protocols is
 assigned by IETF consensus:
 o  Service Names
    *  Authentication Methods
    *  Connection Protocol Channel Names
    *  Connection Protocol Global Request Names
    *  Connection Protocol Channel Request Names
 o  Key Exchange Method Names
 o  Assigned Algorithm Names
    *  Encryption Algorithm Names
    *  MAC Algorithm Names
    *  Public Key Algorithm Names
    *  Compression Algorithm Names
 These names MUST be printable US-ASCII strings, and MUST NOT contain
 the characters at-sign ("@"), comma (","), whitespace, control
 characters (ASCII codes 32 or less), or the ASCII code 127 (DEL).
 Names are case-sensitive, and MUST NOT be longer than 64 characters.
 Names with the at-sign ("@") are locally defined extensions and are
 not controlled by the IANA.

Ylonen & Lonvick Standards Track [Page 12] RFC 4251 SSH Protocol Architecture January 2006

 Each category of names listed above has a separate namespace.
 However, using the same name in multiple categories SHOULD be avoided
 to minimize confusion.
 Message numbers (see Section 7) in the range of 0 to 191 are
 allocated via IETF CONSENSUS, as described in [RFC2434].  Message
 numbers in the 192 to 255 range (local extensions) are reserved for
 PRIVATE USE, also as described in [RFC2434].

9. Security Considerations

 In order to make the entire body of Security Considerations more
 accessible, Security Considerations for the transport,
 authentication, and connection documents have been gathered here.
 The transport protocol [SSH-TRANS] provides a confidential channel
 over an insecure network.  It performs server host authentication,
 key exchange, encryption, and integrity protection.  It also derives
 a unique session id that may be used by higher-level protocols.
 The authentication protocol [SSH-USERAUTH] provides a suite of
 mechanisms that can be used to authenticate the client user to the
 server.  Individual mechanisms specified in the authentication
 protocol use the session id provided by the transport protocol and/or
 depend on the security and integrity guarantees of the transport
 protocol.
 The connection protocol [SSH-CONNECT] specifies a mechanism to
 multiplex multiple streams (channels) of data over the confidential
 and authenticated transport.  It also specifies channels for
 accessing an interactive shell, for proxy-forwarding various external
 protocols over the secure transport (including arbitrary TCP/IP
 protocols), and for accessing secure subsystems on the server host.

9.1. Pseudo-Random Number Generation

 This protocol binds each session key to the session by including
 random, session specific data in the hash used to produce session
 keys.  Special care should be taken to ensure that all of the random
 numbers are of good quality.  If the random data here (e.g., Diffie-
 Hellman (DH) parameters) are pseudo-random, then the pseudo-random
 number generator should be cryptographically secure (i.e., its next
 output not easily guessed even when knowing all previous outputs)
 and, furthermore, proper entropy needs to be added to the pseudo-
 random number generator.  [RFC4086] offers suggestions for sources of
 random numbers and entropy.  Implementers should note the importance
 of entropy and the well-meant, anecdotal warning about the difficulty
 in properly implementing pseudo-random number generating functions.

Ylonen & Lonvick Standards Track [Page 13] RFC 4251 SSH Protocol Architecture January 2006

 The amount of entropy available to a given client or server may
 sometimes be less than what is required.  In this case, one must
 either resort to pseudo-random number generation regardless of
 insufficient entropy or refuse to run the protocol.  The latter is
 preferable.

9.2. Control Character Filtering

 When displaying text to a user, such as error or debug messages, the
 client software SHOULD replace any control characters (except tab,
 carriage return, and newline) with safe sequences to avoid attacks by
 sending terminal control characters.

9.3. Transport

9.3.1. Confidentiality

 It is beyond the scope of this document and the Secure Shell Working
 Group to analyze or recommend specific ciphers other than the ones
 that have been established and accepted within the industry.  At the
 time of this writing, commonly used ciphers include 3DES, ARCFOUR,
 twofish, serpent, and blowfish.  AES has been published by The US
 Federal Information Processing Standards as [FIPS-197], and the
 cryptographic community has accepted AES as well.  As always,
 implementers and users should check current literature to ensure that
 no recent vulnerabilities have been found in ciphers used within
 products.  Implementers should also check to see which ciphers are
 considered to be relatively stronger than others and should recommend
 their use to users over relatively weaker ciphers.  It would be
 considered good form for an implementation to politely and
 unobtrusively notify a user that a stronger cipher is available and
 should be used when a weaker one is actively chosen.
 The "none" cipher is provided for debugging and SHOULD NOT be used
 except for that purpose.  Its cryptographic properties are
 sufficiently described in [RFC2410], which will show that its use
 does not meet the intent of this protocol.
 The relative merits of these and other ciphers may also be found in
 current literature.  Two references that may provide information on
 the subject are [SCHNEIER] and [KAUFMAN].  Both of these describe the
 CBC mode of operation of certain ciphers and the weakness of this
 scheme.  Essentially, this mode is theoretically vulnerable to chosen
 cipher-text attacks because of the high predictability of the start
 of packet sequence.  However, this attack is deemed difficult and not
 considered fully practicable, especially if relatively long block
 sizes are used.

Ylonen & Lonvick Standards Track [Page 14] RFC 4251 SSH Protocol Architecture January 2006

 Additionally, another CBC mode attack may be mitigated through the
 insertion of packets containing SSH_MSG_IGNORE.  Without this
 technique, a specific attack may be successful.  For this attack
 (commonly known as the Rogaway attack [ROGAWAY], [DAI], [BELLARE]) to
 work, the attacker would need to know the Initialization Vector (IV)
 of the next block that is going to be encrypted.  In CBC mode that is
 the output of the encryption of the previous block.  If the attacker
 does not have any way to see the packet yet (i.e., it is in the
 internal buffers of the SSH implementation or even in the kernel),
 then this attack will not work.  If the last packet has been sent out
 to the network (i.e., the attacker has access to it), then he can use
 the attack.
 In the optimal case, an implementer would need to add an extra packet
 only if the packet has been sent out onto the network and there are
 no other packets waiting for transmission.  Implementers may wish to
 check if there are any unsent packets awaiting transmission;
 unfortunately, it is not normally easy to obtain this information
 from the kernel or buffers.  If there are no unsent packets, then a
 packet containing SSH_MSG_IGNORE SHOULD be sent.  If a new packet is
 added to the stream every time the attacker knows the IV that is
 supposed to be used for the next packet, then the attacker will not
 be able to guess the correct IV, thus the attack will never be
 successful.
 As an example, consider the following case:
    Client                                                  Server
    ------                                                  ------
    TCP(seq=x, len=500)             ---->
     contains Record 1
                        [500 ms passes, no ACK]
    TCP(seq=x, len=1000)            ---->
     contains Records 1,2
                                                              ACK
 1. The Nagle algorithm + TCP retransmits mean that the two records
    get coalesced into a single TCP segment.
 2. Record 2 is not at the beginning of the TCP segment and never will
    be because it gets ACKed.
 3. Yet, the attack is possible because Record 1 has already been
    seen.

Ylonen & Lonvick Standards Track [Page 15] RFC 4251 SSH Protocol Architecture January 2006

 As this example indicates, it is unsafe to use the existence of
 unflushed data in the TCP buffers proper as a guide to whether an
 empty packet is needed, since when the second write() is performed
 the buffers will contain the un-ACKed Record 1.
 On the other hand, it is perfectly safe to have the following
 situation:
    Client                                                  Server
    ------                                                  ------
    TCP(seq=x, len=500)             ---->
       contains SSH_MSG_IGNORE
    TCP(seq=y, len=500)             ---->
       contains Data
    Provided that the IV for the second SSH Record is fixed after the
    data for the Data packet is determined, then the following should
    be performed:
       read from user
       encrypt null packet
       encrypt data packet

9.3.2. Data Integrity

 This protocol does allow the Data Integrity mechanism to be disabled.
 Implementers SHOULD be wary of exposing this feature for any purpose
 other than debugging.  Users and administrators SHOULD be explicitly
 warned anytime the "none" MAC is enabled.
 So long as the "none" MAC is not used, this protocol provides data
 integrity.
 Because MACs use a 32-bit sequence number, they might start to leak
 information after 2**32 packets have been sent.  However, following
 the rekeying recommendations should prevent this attack.  The
 transport protocol [SSH-TRANS] recommends rekeying after one gigabyte
 of data, and the smallest possible packet is 16 bytes.  Therefore,
 rekeying SHOULD happen after 2**28 packets at the very most.

9.3.3. Replay

 The use of a MAC other than "none" provides integrity and
 authentication.  In addition, the transport protocol provides a
 unique session identifier (bound in part to pseudo-random data that
 is part of the algorithm and key exchange process) that can be used
 by higher level protocols to bind data to a given session and prevent

Ylonen & Lonvick Standards Track [Page 16] RFC 4251 SSH Protocol Architecture January 2006

 replay of data from prior sessions.  For example, the authentication
 protocol ([SSH-USERAUTH]) uses this to prevent replay of signatures
 from previous sessions.  Because public key authentication exchanges
 are cryptographically bound to the session (i.e., to the initial key
 exchange), they cannot be successfully replayed in other sessions.
 Note that the session id can be made public without harming the
 security of the protocol.
 If two sessions have the same session id (hash of key exchanges),
 then packets from one can be replayed against the other.  It must be
 stressed that the chances of such an occurrence are, needless to say,
 minimal when using modern cryptographic methods.  This is all the
 more true when specifying larger hash function outputs and DH
 parameters.
 Replay detection using monotonically increasing sequence numbers as
 input to the MAC, or HMAC in some cases, is described in [RFC2085],
 [RFC2246], [RFC2743], [RFC1964], [RFC2025], and [RFC4120].  The
 underlying construct is discussed in [RFC2104].  Essentially, a
 different sequence number in each packet ensures that at least this
 one input to the MAC function will be unique and will provide a
 nonrecurring MAC output that is not predictable to an attacker.  If
 the session stays active long enough, however, this sequence number
 will wrap.  This event may provide an attacker an opportunity to
 replay a previously recorded packet with an identical sequence number
 but only if the peers have not rekeyed since the transmission of the
 first packet with that sequence number.  If the peers have rekeyed,
 then the replay will be detected since the MAC check will fail.  For
 this reason, it must be emphasized that peers MUST rekey before a
 wrap of the sequence numbers.  Naturally, if an attacker does attempt
 to replay a captured packet before the peers have rekeyed, then the
 receiver of the duplicate packet will not be able to validate the MAC
 and it will be discarded.  The reason that the MAC will fail is
 because the receiver will formulate a MAC based upon the packet
 contents, the shared secret, and the expected sequence number.  Since
 the replayed packet will not be using that expected sequence number
 (the sequence number of the replayed packet will have already been
 passed by the receiver), the calculated MAC will not match the MAC
 received with the packet.

9.3.4. Man-in-the-middle

 This protocol makes no assumptions or provisions for an
 infrastructure or means for distributing the public keys of hosts.
 It is expected that this protocol will sometimes be used without
 first verifying the association between the server host key and the
 server host name.  Such usage is vulnerable to man-in-the-middle
 attacks.  This section describes this and encourages administrators

Ylonen & Lonvick Standards Track [Page 17] RFC 4251 SSH Protocol Architecture January 2006

 and users to understand the importance of verifying this association
 before any session is initiated.
 There are three cases of man-in-the-middle attacks to consider.  The
 first is where an attacker places a device between the client and the
 server before the session is initiated.  In this case, the attack
 device is trying to mimic the legitimate server and will offer its
 public key to the client when the client initiates a session.  If it
 were to offer the public key of the server, then it would not be able
 to decrypt or sign the transmissions between the legitimate server
 and the client unless it also had access to the private key of the
 host.  The attack device will also, simultaneously to this, initiate
 a session to the legitimate server, masquerading itself as the
 client.  If the public key of the server had been securely
 distributed to the client prior to that session initiation, the key
 offered to the client by the attack device will not match the key
 stored on the client.  In that case, the user SHOULD be given a
 warning that the offered host key does not match the host key cached
 on the client.  As described in Section 4.1, the user may be free to
 accept the new key and continue the session.  It is RECOMMENDED that
 the warning provide sufficient information to the user of the client
 device so the user may make an informed decision.  If the user
 chooses to continue the session with the stored public key of the
 server (not the public key offered at the start of the session), then
 the session-specific data between the attacker and server will be
 different between the client-to-attacker session and the attacker-
 to-server sessions due to the randomness discussed above.  From this,
 the attacker will not be able to make this attack work since the
 attacker will not be able to correctly sign packets containing this
 session-specific data from the server, since he does not have the
 private key of that server.
 The second case that should be considered is similar to the first
 case in that it also happens at the time of connection, but this case
 points out the need for the secure distribution of server public
 keys.  If the server public keys are not securely distributed, then
 the client cannot know if it is talking to the intended server.  An
 attacker may use social engineering techniques to pass off server
 keys to unsuspecting users and may then place a man-in-the-middle
 attack device between the legitimate server and the clients.  If this
 is allowed to happen, then the clients will form client-to-attacker
 sessions, and the attacker will form attacker-to-server sessions and
 will be able to monitor and manipulate all of the traffic between the
 clients and the legitimate servers.  Server administrators are
 encouraged to make host key fingerprints available for checking by
 some means whose security does not rely on the integrity of the
 actual host keys.  Possible mechanisms are discussed in Section 4.1
 and may also include secured Web pages, physical pieces of paper,

Ylonen & Lonvick Standards Track [Page 18] RFC 4251 SSH Protocol Architecture January 2006

 etc.  Implementers SHOULD provide recommendations on how best to do
 this with their implementation.  Because the protocol is extensible,
 future extensions to the protocol may provide better mechanisms for
 dealing with the need to know the server's host key before
 connecting.  For example, making the host key fingerprint available
 through a secure DNS lookup, or using Kerberos ([RFC4120]) over
 GSS-API ([RFC1964]) during key exchange to authenticate the server
 are possibilities.
 In the third man-in-the-middle case, attackers may attempt to
 manipulate packets in transit between peers after the session has
 been established.  As described in Section 9.3.3, a successful attack
 of this nature is very improbable.  As in Section 9.3.3, this
 reasoning does assume that the MAC is secure and that it is
 infeasible to construct inputs to a MAC algorithm to give a known
 output.  This is discussed in much greater detail in Section 6 of
 [RFC2104].  If the MAC algorithm has a vulnerability or is weak
 enough, then the attacker may be able to specify certain inputs to
 yield a known MAC.  With that, they may be able to alter the contents
 of a packet in transit.  Alternatively, the attacker may be able to
 exploit the algorithm vulnerability or weakness to find the shared
 secret by reviewing the MACs from captured packets.  In either of
 those cases, an attacker could construct a packet or packets that
 could be inserted into an SSH stream.  To prevent this, implementers
 are encouraged to utilize commonly accepted MAC algorithms, and
 administrators are encouraged to watch current literature and
 discussions of cryptography to ensure that they are not using a MAC
 algorithm that has a recently found vulnerability or weakness.
 In summary, the use of this protocol without a reliable association
 of the binding between a host and its host keys is inherently
 insecure and is NOT RECOMMENDED.  However, it may be necessary in
 non-security-critical environments, and will still provide protection
 against passive attacks.  Implementers of protocols and applications
 running on top of this protocol should keep this possibility in mind.

9.3.5. Denial of Service

 This protocol is designed to be used over a reliable transport.  If
 transmission errors or message manipulation occur, the connection is
 closed.  The connection SHOULD be re-established if this occurs.
 Denial of service attacks of this type (wire cutter) are almost
 impossible to avoid.
 In addition, this protocol is vulnerable to denial of service attacks
 because an attacker can force the server to go through the CPU and
 memory intensive tasks of connection setup and key exchange without
 authenticating.  Implementers SHOULD provide features that make this

Ylonen & Lonvick Standards Track [Page 19] RFC 4251 SSH Protocol Architecture January 2006

 more difficult, for example, only allowing connections from a subset
 of clients known to have valid users.

9.3.6. Covert Channels

 The protocol was not designed to eliminate covert channels.  For
 example, the padding, SSH_MSG_IGNORE messages, and several other
 places in the protocol can be used to pass covert information, and
 the recipient has no reliable way of verifying whether such
 information is being sent.

9.3.7. Forward Secrecy

 It should be noted that the Diffie-Hellman key exchanges may provide
 perfect forward secrecy (PFS).  PFS is essentially defined as the
 cryptographic property of a key-establishment protocol in which the
 compromise of a session key or long-term private key after a given
 session does not cause the compromise of any earlier session
 [ANSI-T1.523-2001].  SSH sessions resulting from a key exchange using
 the diffie-hellman methods described in the section Diffie-Hellman
 Key Exchange of [SSH-TRANS] (including "diffie-hellman-group1-sha1"
 and "diffie-hellman-group14-sha1") are secure even if private
 keying/authentication material is later revealed, but not if the
 session keys are revealed.  So, given this definition of PFS, SSH
 does have PFS.  However, this property is not commuted to any of the
 applications or protocols using SSH as a transport.  The transport
 layer of SSH provides confidentiality for password authentication and
 other methods that rely on secret data.
 Of course, if the DH private parameters for the client and server are
 revealed, then the session key is revealed, but these items can be
 thrown away after the key exchange completes.  It's worth pointing
 out that these items should not be allowed to end up on swap space
 and that they should be erased from memory as soon as the key
 exchange completes.

9.3.8. Ordering of Key Exchange Methods

 As stated in the section on Algorithm Negotiation of [SSH-TRANS],
 each device will send a list of preferred methods for key exchange.
 The most-preferred method is the first in the list.  It is
 RECOMMENDED that the algorithms be sorted by cryptographic strength,
 strongest first.  Some additional guidance for this is given in
 [RFC3766].

Ylonen & Lonvick Standards Track [Page 20] RFC 4251 SSH Protocol Architecture January 2006

9.3.9. Traffic Analysis

 Passive monitoring of any protocol may give an attacker some
 information about the session, the user, or protocol specific
 information that they would otherwise not be able to garner.  For
 example, it has been shown that traffic analysis of an SSH session
 can yield information about the length of the password - [Openwall]
 and [USENIX].  Implementers should use the SSH_MSG_IGNORE packet,
 along with the inclusion of random lengths of padding, to thwart
 attempts at traffic analysis.  Other methods may also be found and
 implemented.

9.4. Authentication Protocol

 The purpose of this protocol is to perform client user
 authentication.  It assumes that this runs over a secure transport
 layer protocol, which has already authenticated the server machine,
 established an encrypted communications channel, and computed a
 unique session identifier for this session.
 Several authentication methods with different security
 characteristics are allowed.  It is up to the server's local policy
 to decide which methods (or combinations of methods) it is willing to
 accept for each user.  Authentication is no stronger than the weakest
 combination allowed.
 The server may go into a sleep period after repeated unsuccessful
 authentication attempts to make key search more difficult for
 attackers.  Care should be taken so that this doesn't become a self-
 denial of service vector.

9.4.1. Weak Transport

 If the transport layer does not provide confidentiality,
 authentication methods that rely on secret data SHOULD be disabled.
 If it does not provide strong integrity protection, requests to
 change authentication data (e.g., a password change) SHOULD be
 disabled to prevent an attacker from modifying the ciphertext without
 being noticed, or rendering the new authentication data unusable
 (denial of service).
 The assumption stated above, that the Authentication Protocol only
 runs over a secure transport that has previously authenticated the
 server, is very important to note.  People deploying SSH are reminded
 of the consequences of man-in-the-middle attacks if the client does
 not have a very strong a priori association of the server with the
 host key of that server.  Specifically, for the case of the
 Authentication Protocol, the client may form a session to a man-in-

Ylonen & Lonvick Standards Track [Page 21] RFC 4251 SSH Protocol Architecture January 2006

 the-middle attack device and divulge user credentials such as their
 username and password.  Even in the cases of authentication where no
 user credentials are divulged, an attacker may still gain information
 they shouldn't have by capturing key-strokes in much the same way
 that a honeypot works.

9.4.2. Debug Messages

 Special care should be taken when designing debug messages.  These
 messages may reveal surprising amounts of information about the host
 if not properly designed.  Debug messages can be disabled (during
 user authentication phase) if high security is required.
 Administrators of host machines should make all attempts to
 compartmentalize all event notification messages and protect them
 from unwarranted observation.  Developers should be aware of the
 sensitive nature of some of the normal event and debug messages, and
 may want to provide guidance to administrators on ways to keep this
 information away from unauthorized people.  Developers should
 consider minimizing the amount of sensitive information obtainable by
 users during the authentication phase, in accordance with the local
 policies.  For this reason, it is RECOMMENDED that debug messages be
 initially disabled at the time of deployment and require an active
 decision by an administrator to allow them to be enabled.  It is also
 RECOMMENDED that a message expressing this concern be presented to
 the administrator of a system when the action is taken to enable
 debugging messages.

9.4.3. Local Security Policy

 The implementer MUST ensure that the credentials provided validate
 the professed user and also MUST ensure that the local policy of the
 server permits the user the access requested.  In particular, because
 of the flexible nature of the SSH connection protocol, it may not be
 possible to determine the local security policy, if any, that should
 apply at the time of authentication because the kind of service being
 requested is not clear at that instant.  For example, local policy
 might allow a user to access files on the server, but not start an
 interactive shell.  However, during the authentication protocol, it
 is not known whether the user will be accessing files, attempting to
 use an interactive shell, or even both.  In any event, where local
 security policy for the server host exists, it MUST be applied and
 enforced correctly.
 Implementers are encouraged to provide a default local policy and
 make its parameters known to administrators and users.  At the
 discretion of the implementers, this default policy may be along the
 lines of anything-goes where there are no restrictions placed upon
 users, or it may be along the lines of excessively-restrictive, in

Ylonen & Lonvick Standards Track [Page 22] RFC 4251 SSH Protocol Architecture January 2006

 which case, the administrators will have to actively make changes to
 the initial default parameters to meet their needs.  Alternatively,
 it may be some attempt at providing something practical and
 immediately useful to the administrators of the system so they don't
 have to put in much effort to get SSH working.  Whatever choice is
 made must be applied and enforced as required above.

9.4.4 Public Key Authentication

 The use of public key authentication assumes that the client host has
 not been compromised.  It also assumes that the private key of the
 server host has not been compromised.
 This risk can be mitigated by the use of passphrases on private keys;
 however, this is not an enforceable policy.  The use of smartcards,
 or other technology to make passphrases an enforceable policy is
 suggested.
 The server could require both password and public key authentication;
 however, this requires the client to expose its password to the
 server (see the section on Password Authentication below.)

9.4.5. Password Authentication

 The password mechanism, as specified in the authentication protocol,
 assumes that the server has not been compromised.  If the server has
 been compromised, using password authentication will reveal a valid
 username/password combination to the attacker, which may lead to
 further compromises.
 This vulnerability can be mitigated by using an alternative form of
 authentication.  For example, public key authentication makes no
 assumptions about security on the server.

9.4.6. Host-Based Authentication

 Host-based authentication assumes that the client has not been
 compromised.  There are no mitigating strategies, other than to use
 host-based authentication in combination with another authentication
 method.

Ylonen & Lonvick Standards Track [Page 23] RFC 4251 SSH Protocol Architecture January 2006

9.5. Connection Protocol

9.5.1. End Point Security

 End point security is assumed by the connection protocol.  If the
 server has been compromised, any terminal sessions, port forwarding,
 or systems accessed on the host are compromised.  There are no
 mitigating factors for this.
 If the client has been compromised, and the server fails to stop the
 attacker at the authentication protocol, all services exposed (either
 as subsystems or through forwarding) will be vulnerable to attack.
 Implementers SHOULD provide mechanisms for administrators to control
 which services are exposed to limit the vulnerability of other
 services.  These controls might include controlling which machines
 and ports can be targeted in port-forwarding operations, which users
 are allowed to use interactive shell facilities, or which users are
 allowed to use exposed subsystems.

9.5.2. Proxy Forwarding

 The SSH connection protocol allows for proxy forwarding of other
 protocols such as SMTP, POP3, and HTTP.  This may be a concern for
 network administrators who wish to control the access of certain
 applications by users located outside of their physical location.
 Essentially, the forwarding of these protocols may violate site-
 specific security policies, as they may be undetectably tunneled
 through a firewall.  Implementers SHOULD provide an administrative
 mechanism to control the proxy forwarding functionality so that
 site-specific security policies may be upheld.
 In addition, a reverse proxy forwarding functionality is available,
 which, again, can be used to bypass firewall controls.
 As indicated above, end-point security is assumed during proxy
 forwarding operations.  Failure of end-point security will compromise
 all data passed over proxy forwarding.

9.5.3. X11 Forwarding

 Another form of proxy forwarding provided by the SSH connection
 protocol is the forwarding of the X11 protocol.  If end-point
 security has been compromised, X11 forwarding may allow attacks
 against the X11 server.  Users and administrators should, as a matter
 of course, use appropriate X11 security mechanisms to prevent
 unauthorized use of the X11 server.  Implementers, administrators,
 and users who wish to further explore the security mechanisms of X11
 are invited to read [SCHEIFLER] and analyze previously reported

Ylonen & Lonvick Standards Track [Page 24] RFC 4251 SSH Protocol Architecture January 2006

 problems with the interactions between SSH forwarding and X11 in CERT
 vulnerabilities VU#363181 and VU#118892 [CERT].
 X11 display forwarding with SSH, by itself, is not sufficient to
 correct well known problems with X11 security [VENEMA].  However, X11
 display forwarding in SSH (or other secure protocols), combined with
 actual and pseudo-displays that accept connections only over local
 IPC mechanisms authorized by permissions or access control lists
 (ACLs), does correct many X11 security problems, as long as the
 "none" MAC is not used.  It is RECOMMENDED that X11 display
 implementations default to allow the display to open only over local
 IPC.  It is RECOMMENDED that SSH server implementations that support
 X11 forwarding default to allow the display to open only over local
 IPC.  On single-user systems, it might be reasonable to default to
 allow the local display to open over TCP/IP.
 Implementers of the X11 forwarding protocol SHOULD implement the
 magic cookie access-checking spoofing mechanism, as described in
 [SSH-CONNECT], as an additional mechanism to prevent unauthorized use
 of the proxy.

Ylonen & Lonvick Standards Track [Page 25] RFC 4251 SSH Protocol Architecture January 2006

10. References

10.1. Normative References

 [SSH-TRANS]        Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
                    (SSH) Transport Layer Protocol", RFC 4253, January
                    2006.
 [SSH-USERAUTH]     Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
                    (SSH) Authentication Protocol", RFC 4252, January
                    2006.
 [SSH-CONNECT]      Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
                    (SSH) Connection Protocol", RFC 4254, January
                    2006.
 [SSH-NUMBERS]      Lehtinen, S. and C. Lonvick, Ed., "The Secure
                    Shell (SSH) Protocol Assigned Numbers", RFC 4250,
                    January 2006.
 [RFC2119]          Bradner, S., "Key words for use in RFCs to
                    Indicate Requirement Levels", BCP 14, RFC 2119,
                    March 1997.
 [RFC2434]          Narten, T. and H. Alvestrand, "Guidelines for
                    Writing an IANA Considerations Section in RFCs",
                    BCP 26, RFC 2434, October 1998.
 [RFC3066]          Alvestrand, H., "Tags for the Identification of
                    Languages", BCP 47, RFC 3066, January 2001.
 [RFC3629]          Yergeau, F., "UTF-8, a transformation format of
                    ISO 10646", STD 63, RFC 3629, November 2003.

10.2. Informative References

 [RFC0822]          Crocker, D., "Standard for the format of ARPA
                    Internet text messages", STD 11, RFC 822, August
                    1982.
 [RFC0854]          Postel, J. and J. Reynolds, "Telnet Protocol
                    Specification", STD 8, RFC 854, May 1983.
 [RFC1034]          Mockapetris, P., "Domain names - concepts and
                    facilities", STD 13, RFC 1034, November 1987.

Ylonen & Lonvick Standards Track [Page 26] RFC 4251 SSH Protocol Architecture January 2006

 [RFC1282]          Kantor, B., "BSD Rlogin", RFC 1282, December 1991.
 [RFC4120]          Neuman, C., Yu, T., Hartman, S., and K. Raeburn,
                    "The Kerberos Network Authentication Service
                    (V5)", RFC 4120, July 2005.
 [RFC1964]          Linn, J., "The Kerberos Version 5 GSS-API
                    Mechanism", RFC 1964, June 1996.
 [RFC2025]          Adams, C., "The Simple Public-Key GSS-API
                    Mechanism (SPKM)", RFC 2025, October 1996.
 [RFC2085]          Oehler, M. and R. Glenn, "HMAC-MD5 IP
                    Authentication with Replay Prevention", RFC 2085,
                    February 1997.
 [RFC2104]          Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
                    Keyed-Hashing for Message Authentication", RFC
                    2104, February 1997.
 [RFC2246]          Dierks, T. and C. Allen, "The TLS Protocol Version
                    1.0", RFC 2246, January 1999.
 [RFC2410]          Glenn, R. and S. Kent, "The NULL Encryption
                    Algorithm and Its Use With IPsec", RFC 2410,
                    November 1998.
 [RFC2743]          Linn, J., "Generic Security Service Application
                    Program Interface Version 2, Update 1", RFC 2743,
                    January 2000.
 [RFC3766]          Orman, H. and P. Hoffman, "Determining Strengths
                    For Public Keys Used For Exchanging Symmetric
                    Keys", BCP 86, RFC 3766, April 2004.
 [RFC4086]          Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                    "Randomness Requirements for Security", BCP 106,
                    RFC 4086, June 2005.
 [FIPS-180-2]       US National Institute of Standards and Technology,
                    "Secure Hash Standard (SHS)", Federal Information
                    Processing Standards Publication 180-2, August
                    2002.
 [FIPS-186-2]       US National Institute of Standards and Technology,
                    "Digital Signature Standard (DSS)", Federal
                    Information Processing Standards Publication 186-
                    2, January 2000.

Ylonen & Lonvick Standards Track [Page 27] RFC 4251 SSH Protocol Architecture January 2006

 [FIPS-197]         US National Institute of Standards and Technology,
                    "Advanced Encryption Standard (AES)", Federal
                    Information Processing Standards Publication 197,
                    November 2001.
 [ANSI-T1.523-2001] American National Standards Institute, Inc.,
                    "Telecom Glossary 2000", ANSI T1.523-2001,
                    February 2001.
 [SCHNEIER]         Schneier, B., "Applied Cryptography Second
                    Edition:  protocols algorithms and source in code
                    in C", John Wiley and Sons, New York, NY, 1996.
 [SCHEIFLER]        Scheifler, R., "X Window System : The Complete
                    Reference to Xlib, X Protocol, Icccm, Xlfd, 3rd
                    edition.", Digital Press, ISBN 1555580882,
                    February 1992.
 [KAUFMAN]          Kaufman, C., Perlman, R., and M. Speciner,
                    "Network Security: PRIVATE Communication in a
                    PUBLIC World", Prentice Hall Publisher, 1995.
 [CERT]             CERT Coordination Center, The.,
                    "http://www.cert.org/nav/index_red.html".
 [VENEMA]           Venema, W., "Murphy's Law and Computer Security",
                    Proceedings of 6th USENIX Security Symposium, San
                    Jose CA
                    http://www.usenix.org/publications/library/
                    proceedings/sec96/venema.html, July 1996.
 [ROGAWAY]          Rogaway, P., "Problems with Proposed IP
                    Cryptography", Unpublished paper
                    http://www.cs.ucdavis.edu/~rogaway/ papers/draft-
                    rogaway-ipsec-comments-00.txt, 1996.
 [DAI]              Dai, W., "An attack against SSH2 protocol", Email
                    to the SECSH Working Group ietf-ssh@netbsd.org
                    ftp:// ftp.ietf.org/ietf-mail-archive/secsh/2002-
                    02.mail, Feb 2002.
 [BELLARE]          Bellaire, M., Kohno, T., and C. Namprempre,
                    "Authenticated Encryption in SSH: Fixing the SSH
                    Binary Packet Protocol", Proceedings of the 9th
                    ACM Conference on Computer and Communications
                    Security, Sept 2002.

Ylonen & Lonvick Standards Track [Page 28] RFC 4251 SSH Protocol Architecture January 2006

 [Openwall]         Solar Designer and D. Song, "SSH Traffic Analysis
                    Attacks", Presentation given at HAL2001 and
                    NordU2002 Conferences, Sept 2001.
 [USENIX]           Song, X.D., Wagner, D., and X. Tian, "Timing
                    Analysis of Keystrokes and SSH Timing Attacks",
                    Paper given at 10th USENIX Security Symposium,
                    2001.

Authors' Addresses

 Tatu Ylonen
 SSH Communications Security Corp
 Valimotie 17
 00380 Helsinki
 Finland
 EMail: ylo@ssh.com
 Chris Lonvick (editor)
 Cisco Systems, Inc.
 12515 Research Blvd.
 Austin  78759
 USA
 EMail: clonvick@cisco.com

Trademark Notice

 "ssh" is a registered trademark in the United States and/or other
 countries.

Ylonen & Lonvick Standards Track [Page 29] RFC 4251 SSH Protocol Architecture January 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Ylonen & Lonvick Standards Track [Page 30]

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