draft-ietf-secsh-architecture-15.txt

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      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. "[e-mail]". 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