Network Working Group                                          T. Ylonen
INTERNET-DRAFT                                                T. Kivinen
draft-ietf-secsh-architecture-00.txt                         M. Saarinen
Expires in six months                                                SSH
                                                         14 October 1997

                       SSH Protocol Architecture

Status of This memo

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
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SSH is a protocol for secure remote login and other secure network ser-
vices over an insecure network.

This document describes the architecture of the SSH protocol, and 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: Transport layer
protocol provides server authentication, confidentiality, and integrity
with perfect forward secrecy. User authentication protocol authenticates
the client to the server. Connection protocol multiplexes the encrypted
tunnel into several logical channels. Details of these protocols are
described in separate documents.

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Table of Contents

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .  2
2.  Specification of Requirements   . . . . . . . . . . . . . . . . .  2
3.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
  3.1.  Host Keys   . . . . . . . . . . . . . . . . . . . . . . . . .  3
  3.2.  Extensibility   . . . . . . . . . . . . . . . . . . . . . . .  4
  3.3.  Policy Issues   . . . . . . . . . . . . . . . . . . . . . . .  4
  3.4.  Security Properties   . . . . . . . . . . . . . . . . . . . .  5
  3.5.  Packet Size and Overhead  . . . . . . . . . . . . . . . . . .  5
  3.6.  Localization and Character Set Support  . . . . . . . . . . .  6
4.  Data Type Representations Used in the SSH Protocols   . . . . . .  7
5.  Algorithm Naming  . . . . . . . . . . . . . . . . . . . . . . . .  8
6.  Message Numbers   . . . . . . . . . . . . . . . . . . . . . . . .  8
7.  IANA Considerations   . . . . . . . . . . . . . . . . . . . . . .  9
8.  Security Considerations   . . . . . . . . . . . . . . . . . . . .  9
9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.  Authors' Addresses   . . . . . . . . . . . . . . . . . . . . . . 10

1.  Introduction

SSH is a protocol for secure remote login and other secure network
services over an insecure network.  It consists of three major

o  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

o  User authentication protocol [SSH-USERAUTH] authenticates the client
   side user to the server.  It runs over the transport layer protocol.

o  Connection protocol [SSH-CONN] multiplexes the encrypted tunnel into
   several logical channels.  It runs over the user authentication

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.  Specification of Requirements

All of the documents related to the SSH protocols shall use the keywords

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NOT", "RECOMMENDED, "MAY", and "OPTIONAL" to describe requirements.
They are to be interpreted as described in [RFC-2119].

3.  Architecture

3.1.  Host Keys

Each server host MUST have a host key.  Hosts MAY have multiple host
keys using multiple different algorithms.  Multiple hosts MAY share the
same host key. Every host MUST have at least one key using each REQUIRED
public key algorithm (currently 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-
   key associations may become burdensome to maintain.

o  The host name - 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 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 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

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

3.2.  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

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'' for more

3.3.  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

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   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 from 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 it has been requested.
   Other issues, such as which TCP/IP ports can be forwarded and by
   whom, are clear local policy issues.  Many of these issues may
   involve traversing or bypassing firewalls, and are interrelated with
   the local security policy.

3.4.  Security Properties

The primary goal of the SSH protocols is improved security on the
Internet.  It attempts to do this in a way that is easy enough for users
to be taken into use, 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.

3.5.  Packet Size and Overhead

Some readers will worry about the increase in packet size due to new
headers, padding, and MAC.  The minimum packet size 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:

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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 by no more than 5 bytes.  When
   ethernet headers are considered, the increase is by less than 10

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
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 on the order
of 2ms, 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.

3.6.  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-2044].  When applicable, a field is also be
provided for a language tag [RFC-1766].

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
specifying the character set or encoding for terminal session data
directly.  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, 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 SHOULD 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.

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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 on using a numeric
code.  The remaining messages SHOULD be configurable.

4.  Data Type Representations Used in the SSH Protocols

      A byte represents an arbitrary 8-bit value (octet) [RFC1700].
      Fixed length data is sometimes represented as an array of bytes,
      written byte[n], where n is the number of bytes in the array.

      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.

      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.

      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. Terminating null character SHOULD
      normally not 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.

      Represents multiple precision integers in two's complement format,
      stored as a string, 8 bits per byte, MSB first.  Negative numbers
      have one in the most significant bit of the first byte of the data
      partition of.  If the most significant bit would be set for a
      positive number, the number MUST be preceded by a zero byte.
      Unnecessary leading zero or 255 bytes MUST NOT be included.  The

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      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.

      For example, the value 694531781388612263 (0x9a378f9b2e332a7) is
      represented as 00 00 00 08 09 a3 78 f9 b2 e3 32 a7.

5.  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
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 IANA (Internet Assigned Numbers Authority).  Examples include
   `3des-cbc', `sha-1', `hmac-sha1', and `zlib' (the quotes are not part
   of the name).  Additional names of this format may be registered with
   IANA; see Section ``IANA Considerations''.  Names of this format MUST
   NOT be used without first registering with IANA.  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. "".  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.

6.  Message Numbers

SSH packets have message numbers in the range 1-255. These numbers have
been allocated as follows:

  Transport layer protocol:

    1-19     Transport layer generic (e.g. disconnect, ignore, debug,
    20-29    Algorithm negotiation
    30-49    Key exchange method specific (numbers can be reused for
             different authentication methods)

  User authentication protocol:

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    50-59    User authentication generic
    60-79    User authentication method specific (numbers can be reused
             for different authentication methods)

  Connection protocol:

    80-89    Connection protocol generic
    90-127   Channel related messages

  Reserved for client protocols:

    128-191  Reserved

  Local extensions:

    192-255  Local extensions

7.  IANA Considerations

Allocation of the following types of names in the SSH protocols is
assigned to IANA:

o  encryption algorithm names,

o  MAC algorithm names,

o  public key algorithm names (public key algorithm also implies
   encoding and signature/encryption capability),

o  key exchange method names, and

o  protocol (service) names.

The IANA-allocated 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.

Each category of names listed above has a separate namespace.  However,
using the same name in multiple categories SHOULD be avoided to minimize

8.  Security Considerations

Special care should be taken to ensure that all of the random numbers
are of good quality. The random numbers SHOULD be produced with safe
mechanisms discussed in [RFC1750].

When displaying text, such as error or debug messages to the user, 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.

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Not using MAC or encryption SHOULD be avoided. The user authentication
protocol is subject to man-in-the-middle attacks if the encryption is
disabled. The SSH protocol does not protect against message alteration
if no MAC is used.

9.  References

[FIPS-186] Federal Information Processing Standards Publication (FIPS
PUB) 186, Digital Signature Standard, 18 May 1994.

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

[RFC-894] Hornig, C., "A Standard for the Transmission of IP Datagrams
over Ethernet Networks", April 1984.

[RFC-1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
November 1987.
[RFC-1134] Perkins, D., "The Point-to-Point Protocol: A Proposal for
Multi-Protocol Transmission o Datagrams Over Point-to-Point Links",
November 1989.

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

[RFC-1700] Reynolds, J. and Postel, J., "Assigned Numbers", October 1994
(also STD 2).

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

[RFC-1766] Alvestrand, H., "Tags for the Identification of Languages",
March 1995.

[RFC-2044] Yergeau, F., "UTF-8, a Transformation Format of Unicode and
ISO 10646", October 1996.

[RFC-2119] Bradner, S., "Key words for use in RFCs to indicate
Requirement Levels", March 1997

[Schneier] Schneier, B., "Applied Cryptography Second Edition", John
Wiley & Sons, New York, NY, 1995.

[SSH-TRANS] Ylonen, T., Kivinen, T, and Saarinen, M., "SSH Transport
Layer Protocol", Internet Draft, draft-ietf-secsh-transport-02.txt

[SSH-USERAUTH] Ylonen, T., Kivinen, T, and Saarinen, M., "SSH
Authentication Protocol", Internet Draft, draft-ietf-secsh-

[SSH-CONNECT] Ylonen, T., Kivinen, T, and Saarinen, M., "SSH Connection
Protocol", Internet Draft, draft-ietf-secsh-connect-02.txt

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10.  Authors' Addresses

    Tatu Ylonen
    SSH Communications Security Ltd.
    Tekniikantie 12
    FIN-02150 ESPOO

    Tero Kivinen
    SSH Communications Security Ltd.
    Tekniikantie 12
    FIN-02150 ESPOO

    Markku-Juhani O. Saarinen
    SSH Communications Security Ltd.
    Tekniikantie 12
    FIN-02150 ESPOO

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