Subject: draft-ietf-secsh-architecture-15.txt

From: Darren J Moffat <[email-masked]>
Date: Sun, 26 Oct 2003 18:49:05 -0800 (PST)

Network Working Group                                          T. Ylonen
Internet-Draft                          SSH Communications Security Corp
Expires: March 31, 2004                                   D. Moffat, Ed.
                                                   Sun Microsystems, Inc
                                                                Oct 2003

                       SSH Protocol Architecture
                  draft-ietf-secsh-architecture-15.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on March 31, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   SSH 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

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   documents.

Table of Contents

   1.    Contributors . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.    Specification of Requirements  . . . . . . . . . . . . . . .  3
   4.    Architecture . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.1   Host Keys  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.2   Extensibility  . . . . . . . . . . . . . . . . . . . . . . .  5
   4.3   Policy Issues  . . . . . . . . . . . . . . . . . . . . . . .  5
   4.4   Security Properties  . . . . . . . . . . . . . . . . . . . .  6
   4.5   Packet Size and Overhead . . . . . . . . . . . . . . . . . .  6
   4.6   Localization and Character Set Support . . . . . . . . . . .  7
   5.    Data Type Representations Used in the SSH Protocols  . . . .  8
   6.    Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . 10
   7.    Message Numbers  . . . . . . . . . . . . . . . . . . . . . . 11
   8.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 11
   9.    Security Considerations  . . . . . . . . . . . . . . . . . . 12
   9.1   Pseudo-Random Number Generation  . . . . . . . . . . . . . . 12
   9.2   Transport  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   9.2.1 Confidentiality  . . . . . . . . . . . . . . . . . . . . . . 13
   9.2.2 Data Integrity . . . . . . . . . . . . . . . . . . . . . . . 16
   9.2.3 Replay . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   9.2.4 Man-in-the-middle  . . . . . . . . . . . . . . . . . . . . . 17
   9.2.5 Denial-of-service  . . . . . . . . . . . . . . . . . . . . . 19
   9.2.6 Covert Channels  . . . . . . . . . . . . . . . . . . . . . . 19
   9.2.7 Forward Secrecy  . . . . . . . . . . . . . . . . . . . . . . 20
   9.3   Authentication Protocol  . . . . . . . . . . . . . . . . . . 20
   9.3.1 Weak Transport . . . . . . . . . . . . . . . . . . . . . . . 21
   9.3.2 Debug messages . . . . . . . . . . . . . . . . . . . . . . . 21
   9.3.3 Local security policy  . . . . . . . . . . . . . . . . . . . 21
   9.3.4 Public key authentication  . . . . . . . . . . . . . . . . . 22
   9.3.5 Password authentication  . . . . . . . . . . . . . . . . . . 22
   9.3.6 Host based authentication  . . . . . . . . . . . . . . . . . 23
   9.4   Connection protocol  . . . . . . . . . . . . . . . . . . . . 23
   9.4.1 End point security . . . . . . . . . . . . . . . . . . . . . 23
   9.4.2 Proxy forwarding . . . . . . . . . . . . . . . . . . . . . . 23
   9.4.3 X11 forwarding . . . . . . . . . . . . . . . . . . . . . . . 24
         Normative References . . . . . . . . . . . . . . . . . . . . 24
         Informative References . . . . . . . . . . . . . . . . . . . 25
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 27
         Intellectual Property and Copyright Statements . . . . . . . 28

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1. Contributors

   The major original contributors of this document were: Tatu Ylonen,
   Tero Kivinen, Timo J. Rinne, Sami Lehtinen (all of SSH Communications
   Security Corp), and Markku-Juhani O. Saarinen (University of
   Jyvaskyla)

   The document editor is: [email-masked].  Comments on this
   internet draft should be sent to the IETF SECSH working group,
   details at: http://ietf.org/html.charters/secsh-charter.html

2. Introduction

   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.

3. Specification of Requirements

   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.  They are to be interpreted as described in [RFC2119].

4. Architecture

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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 using each REQUIRED public key algorithm (DSS
   [FIPS-186]).

   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
      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 some trusted
      certification authority.  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 yet, 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 [RFC-854] and rlogin
   [RFC-1282]).

   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

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   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 of the public key. Such fingerprints can easily be
   verified by using telephone or other external communication channels.

   All implementations SHOULD provide an option to not accept host keys
   that cannot be verified.

   We 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 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 necessary 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 drafts.  See Section Algorithm Naming (Section
   6) 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

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   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.
   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 where the user is trying to log in from.
   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 improved 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 wide-spread 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 Packet Size and Overhead

   Some readers will worry about the increase in packet size due to new

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   headers, padding, and MAC.  The minimum packet size is in the order
   of 28 bytes (depending on negotiated algorithms).  The increase is
   negligible for large packets, but very significant for one-byte
   packets (telnet-type sessions).  There are, however, several factors
   that make this a non-issue in almost all cases:
   o  The minimum size of a TCP/IP header is 32 bytes.  Thus, the
      increase is actually from 33 to 51 bytes (roughly).
   o  The minimum size of the data field of an Ethernet packet is 46
      bytes [RFC-894]. Thus, the increase is no more than 5 bytes. When
      Ethernet headers are considered, the increase is less than 10
      percent.
   o  The total fraction of telnet-type data in the Internet is
      negligible, even with increased packet sizes.

   The only environment where the packet size increase is likely to have
   a significant effect is PPP [RFC-1134] over slow modem lines (PPP
   compresses the TCP/IP headers, emphasizing the increase in packet
   size). However, with modern modems, the time needed to transfer is in
   the order of 2 milliseconds, which is a lot faster than people can
   type.

   There are also issues related to the maximum packet size.  To
   minimize delays in screen updates, one does not want excessively
   large packets for interactive sessions.  The maximum packet size is
   negotiated separately for each channel.

4.6 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 the
   data MUST be explicitly specified. In most places, ISO 10646 with
   UTF-8 encoding is used [RFC-2279]. When applicable, a field is also
   provided for a language tag [RFC-3066].

   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

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   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) [RFC-1700].
      Fixed length data is sometimes represented as an array of bytes,
      written byte[n], where n is the number of bytes in the array.

   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).

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   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 UTF8 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.

       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

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      names. A name MUST be non-zero length, and it MUST NOT contain a
      comma (','). Context may impose additional restrictions on the
      names; for example, the names in a list may have to be valid
      algorithm identifier (see Algorithm Naming below), or [RFC-3066]
      language tags. The order of the names in a list may or may not be
      significant, also depending on the context where the list is is
      used. Terminating NUL characters are not used, neither for the
      individual names, nor for the list as a whole.

       Examples:
       value              representation (hex)
       ---------------------------------------
       (), the empty 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 Naming

   The SSH protocols refer to particular hash, encryption, integrity,
   compression, and key exchange algorithms or protocols by names.
   There are some standard algorithms that all implementations MUST
   support. There are also algorithms that are defined in the protocol
   specification but are OPTIONAL.  Furthermore, it is expected that
   some organizations will want to use their own algorithms.

   In this protocol, all algorithm identifiers MUST be printable
   US-ASCII non-empty strings no longer than 64 characters. Names MUST
   be case-sensitive.

   There are two formats for algorithm names:
   o  Names that do not contain an at-sign (@) are reserved to be
      assigned by IETF consensus (RFCs).  Examples include `3des-cbc',
      `sha-1', `hmac-sha1', and `zlib' (the quotes are not part of the
      name).  Names of this format MUST NOT be used without first
      registering them.  Registered names MUST NOT contain an at-sign
      (@) or a comma (,).
   o  Anyone can define additional algorithms by using names in the
      format name@domainname, e.g. "[email-masked]". The
      format of the part preceding the at sign is not specified; it MUST
      consist of US-ASCII characters except at-sign and comma. The part
      following the at-sign MUST be a valid fully qualified internet
      domain name [RFC-1034] controlled by the person or organization
      defining the name. It is up to each domain how it manages its
      local namespace.

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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)

     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

   The initial state of the IANA registry is detailed in [SSH-NUMBERS].

   Allocation of the following types of names in the SSH protocols is
   assigned by IETF consensus:
   o  SSH encryption algorithm names,
   o  SSH MAC algorithm names,
   o  SSH public key algorithm names (public key algorithm also implies
      encoding and signature/encryption capability),
   o  SSH key exchange method names, and
   o  SSH protocol (service) names.

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   These names MUST be printable US-ASCII strings, and MUST NOT contain
   the characters at-sign ('@'), comma (','), or whitespace or control
   characters (ASCII codes 32 or less).  Names are case-sensitive, and
   MUST NOT be longer than 64 characters.

   Names with the at-sign ('@') in them are allocated by the owner of
   DNS name after the at-sign (hierarchical allocation in [RFC-2343]),
   otherwise the same restrictions as above.

   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 Message Numbers (Section 7)) in the
   range of 0..191 are allocated via IETF consensus; message numbers in
   the 192..255 range (the "Local extensions" set) are reserved for
   private use.

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 [1] 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 [2] provides a suite of mechanisms which
   can be used to authenticate the client user to the server.
   Individual mechanisms specified in the in 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 [3] 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., DH

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   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.  RFC 1750 [1750] offers suggestions for sources of random
   numbers and entropy.  Implementors should note the importance of
   entropy and the well-meant, anecdotal warning about the difficulty in
   properly implementing pseudo-random number generating functions.

   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 Transport

9.2.1 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
   which have been established and accepted within the industry.  At the
   time of this writing, ciphers commonly in use include 3DES, ARCFOUR,
   twofish, serpent and blowfish.  AES has been accepted by The
   published as a US Federal Information Processing Standards [FIPS-197]
   and the cryptographic community as being acceptable for this purpose
   as well has accepted AES.  As always, implementors and users should
   check current literature to ensure that no recent vulnerabilities
   have been found in ciphers used within products.  Implementors 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.  It's cryptographic properties are
   sufficiently described in RFC 2410, 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,PERLMAN,SPECINER].  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

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   is still deemed difficult and not considered fully practicable
   especially if relatively longer block sizes are used.

   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,KOHNO,NAMPREMPRE]) to work, the attacker
   would need to know the 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 implementor 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. Implementors may wish to
   check to see if there are any unsent packets awaiting transmission,
   but unfortunately it is not normally easy to obtain this information
   from the kernel or buffers.  If there are not, 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 successfull.

   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, since it gets ACKed.

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   3.  Yet, the attack is possible because Record 1 has already been
       seen.

   As this example indicates, it's totally unsafe to use the existence
   of unflushed data in the TCP buffers proper as a guide to whether you
   need an empty packet, since when you do the second write(), the
   buffers will contain the un-ACKed Record 1.

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   On the other hand, it's 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 second SSH Record is fixed after the data for
      the Data packet is determined -i.e. you do:
           read from user
           encrypt null packet
           encrypt data packet

9.2.2 Data Integrity

   This protocol does allow the Data Integrity mechanism to be disabled.
   Implementors 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 [1] 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.

9.2.3 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
   replay of data from prior sessions. For example, the authentication
   protocol 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.

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   Note that the session ID can be made public without harming the
   security of the protocol.

   If two session happen to 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 so 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 [RFC1510].  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 as 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) then the calculated MAC will not match the
   MAC received with the packet.

9.2.4 Man-in-the-middle

   This protocol makes no assumptions nor 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 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

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   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 3.1 of [ARCH], 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 they 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 3.1
   of [SSH-ARCH] and may also include secured Web pages, physical pieces
   of paper, etc. Implementors 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

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   available through a secure DNS lookup, or using kerberos over gssapi
   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 the Replay part of this section, a
   successful attack of this nature is very improbable.  As in the
   Replay section, 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 RFC 2104.  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 that, implementors
   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.  It may however be necessary in
   non-security critical environments, and will still provide protection
   against passive attacks.  Implementors of protocols and applications
   running on top of this protocol should keep this possibility in mind.

9.2.5 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.  Implementors SHOULD provide features that make this
   more difficult.  For example, only allowing connections from a subset
   of IPs known to have valid users.

9.2.6 Covert Channels

   The protocol was not designed to eliminate covert channels.  For

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   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 to verify whether such information
   is being sent.

9.2.7 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]  SSHv2 sessions resulting from a key exchange using
   diffie-hellman-group1-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, SSHv2 does have
   PFS.  It is hoped that all other key exchange mechanisms proposed and
   used in the future will also provide PFS.  This property is not
   commuted to any of the applications or protocols using SSH as a
   transport however.  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 Authentication Protocol

   The purpose of this protocol is to perform client user
   authentication. It assumes that this run 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.

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9.3.1 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 as stated above that the Authentication Protocol only
   run 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-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.

9.3.2 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 messages 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.

9.3.3 Local security policy

   Implementer MUST ensure that the credentials provided validate the
   professed user and also MUST ensure that the local policy of the

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   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 or 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.

   Implementors are encouraged to provide a default local policy and
   make its parameters known to administrators and users.  At the
   discretion of the implementors, 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
   which case the administrators will have to actively make changes to
   this policy 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.

9.3.4 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 section on password authentication below.)

9.3.5 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

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   authentication.  For example, public-key authentication makes no
   assumptions about security on the server.

9.3.6 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.

9.4 Connection protocol

9.4.1 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 end point 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.  Implementors 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 target in 'port-forwarding' operations, which users are allowed to
   use interactive shell facilities, or which users are allowed to use
   exposed subsystems.

9.4.2 Proxy forwarding

   The SSH connection protocol allows for proxy forwarding of other
   protocols such as SNMP, 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.  Implementors 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

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   all data passed over proxy forwarding.

9.4.3 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.  Implementors, administrators and
   users who wish to further explore the security mechanisms of X11 are
   invited to read [SCHEIFLER] and analyze previously reported 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 SSHv2 (or other, secure protocols), combined
   with actual and pseudo-displays which accept connections only over
   local IPC mechanisms authorized by permissions or 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 allowing
   display opens only over local IPC.  It is RECOMMENDED that SSHv2
   server implementations that support X11 forwarding default to
   allowing display opens only over local IPC.  On single-user systems
   it might be reasonable to default to allowing local display opens
   over TCP/IP.

   Implementors 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.

Normative References

   [SSH-ARCH]
              Ylonen, T., "SSH Protocol Architecture", I-D
              draft-ietf-architecture-15.txt, Oct 2003.

   [SSH-TRANS]
              Ylonen, T., "SSH Transport Layer Protocol", I-D
              draft-ietf-transport-17.txt, Oct 2003.

   [SSH-USERAUTH]
              Ylonen, T., "SSH Authentication Protocol", I-D
              draft-ietf-userauth-18.txt, Oct 2003.

   [SSH-CONNECT]

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              Ylonen, T., "SSH Connection Protocol", I-D
              draft-ietf-connect-18.txt, Oct 2003.

   [SSH-NUMBERS]
              Lehtinen, S. and D. Moffat, "SSH Protocol Assigned
              Numbers", I-D draft-ietf-secsh-assignednumbers-05.txt, Oct
              2003.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

Informative References

   [FIPS-186]
              Federal Information Processing Standards Publication,
              "FIPS PUB 186, Digital Signature Standard", May 1994.

   [FIPS-197]
              National Institue of Standards and Technology, "FIPS 197,
              Specification for the Advanced Encryption Standard",
              November 2001.

   [ANSI T1.523-2001]
              American National Standards Insitute, Inc., "Telecom
              Glossary 2000", February 2001.

   [SCHEIFLER]
              Scheifler, R., "X Window System : The Complete Reference
              to Xlib, X Protocol, Icccm, Xlfd, 3rd edition.", Digital
              Press ISBN 1555580882, Feburary 1992.

   [RFC0854]  Postel, J. and J. Reynolds, "Telnet Protocol
              Specification", STD 8, RFC 854, May 1983.

   [RFC0894]  Hornig, C., "Standard for the transmission of IP datagrams
              over Ethernet networks", STD 41, RFC 894, April 1984.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1134]  Perkins, D., "Point-to-Point Protocol: A proposal for
              multi-protocol transmission of datagrams over
              Point-to-Point links", RFC 1134, November 1989.

   [RFC1282]  Kantor, B., "BSD Rlogin", RFC 1282, December 1991.

   [RFC1510]  Kohl, J. and B. Neuman, "The Kerberos Network
              Authentication Service (V5)", RFC 1510, September 1993.

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   [RFC1700]  Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1700,
              October 1994.

   [RFC1750]  Eastlake, D., Crocker, S. and J. Schiller, "Randomness
              Recommendations for Security", RFC 1750, December 1994.

   [RFC3066]  Alvestrand, H., "Tags for the Identification of
              Languages", BCP 47, RFC 3066, January 2001.

   [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., Allen, C., Treese, W., Karlton, P., Freier, A.
              and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246,
              January 1999.

   [RFC2279]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 2279, January 1998.

   [RFC2410]  Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
              Its Use With IPsec", RFC 2410, November 1998.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [SCHNEIER]
              Schneier, B., "Applied Cryptography Second Edition:
              protocols algorithms and source in code in C", 1996.

   [KAUFMAN,PERLMAN,SPECINER]
              Kaufman, C., Perlman, R. and M. Speciner, "Network
              Security: PRIVATE Communication in a PUBLIC World", 1995.

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              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 [email-masked] ftp://
              ftp.ietf.org/ietf-mail-archive/secsh/2002-02.mail, Feb
              2002.

   [BELLARE,KOHNO,NAMPREMPRE]
              Bellaire, M., Kohno, T. and C. Namprempre, "Authenticated
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Authors' Addresses

   Tatu Ylonen
   SSH Communications Security Corp
   Fredrikinkatu 42
   HELSINKI  FIN-00100
   Finland

   EMail: [email-masked]

   Darren J. Moffat (editor)
   Sun Microsystems, Inc
   17 Network Circle
   Menlo Park  CA 94025
   USA

   EMail: [email-masked]

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--
Darren J Moffat