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Versions: (draft-ietf-secsh-fingerprint) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 4716 Draft is active
In: Informational
Secure Shell Working Group                                  J. Galbraith
Internet-Draft                                          VanDyke Software
Expires: February 23, 2006                                     R. Thayer
                                                     The Tillerman Group
                                                         August 22, 2005


                       SSH Public Key File Format
                 draft-ietf-secsh-publickeyfile-09.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on February 23, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document formally documents an existing public key file format
   in use for exchanging public keys between different SSH
   implementations.







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Table of Contents

   1.  Conventions used in this document  . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Key File Format  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Line Termination Characters  . . . . . . . . . . . . . . .  5
     3.2.  Begin and End Markers  . . . . . . . . . . . . . . . . . .  5
     3.3.  Key File Header  . . . . . . . . . . . . . . . . . . . . .  5
       3.3.1.  Subject Header . . . . . . . . . . . . . . . . . . . .  6
       3.3.2.  Comment Header . . . . . . . . . . . . . . . . . . . .  6
       3.3.3.  New Headers  . . . . . . . . . . . . . . . . . . . . .  6
     3.4.  Public Key File Body . . . . . . . . . . . . . . . . . . .  6
     3.5.  Differences with RFC1421 PEM formats . . . . . . . . . . .  7
     3.6.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Public Key Fingerprints  . . . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14





























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1.  Conventions used in this document

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














































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2.  Introduction

   In order to use public key authentication, public keys must be
   exchanged between client and server.  This document formally
   describes an existing public key file format.














































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3.  Key File Format

   In order to implement public key authentication, SSH implementations
   must share public key files between the client and the server in
   order to interoperate.

   A key file is a text file, containing a sequence of lines.  Each line
   in the file MUST NOT be longer than 72 bytes excluding line
   termination characters.

3.1.  Line Termination Characters

   Implementations SHOULD generate public key files using their system's
   local text file representation.

   In the event that public key files are not transferred as text files,
   implementations SHOULD be prepared to read files using any of the
   common line termination sequence, <CR>, <LF> or <CR><LF>.

3.2.  Begin and End Markers

   The first line of a conforming key file MUST be a begin marker, which
   is the literal text:

   ---- BEGIN SSH2 PUBLIC KEY ----

   The last line of a conforming key file MUST be an end marker, which
   is the literal text:

   ---- END SSH2 PUBLIC KEY ----

3.3.  Key File Header

   The key file header section consists of multiple RFC822-style header
   fields.  Each field is a line of the following format:

   Header-tag ':' ' ' Header-value

   The Header-tag MUST NOT be more than 64 bytes, and is case-
   insensitive.  The Header-value MUST NOT be more than 1024 bytes.
   Each line in the header MUST NOT be more than 72 bytes.

   A line is continued if the last character in the line is a '\'.  If
   the last character of a line is a '\', then the logical contents of
   the line is formed by removing the '\' and the line termination
   characters, and appending the contents of the next line.

   The Header-tag MUST be encoded in US-ASCII.  The Header-value MUST be



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   encoded in UTF-8.  [RFC3629]

   A line that is not a continuation line that has no ':' in it is the
   first line of the base 64 encoded body (See Section 3.4.)

   Compliant implementations MUST ignore unrecognized header fields.
   Implementations SHOULD preserve unrecognized header fields when
   manipulating the key file.

3.3.1.  Subject Header

   This field is used to store the login-name that the key was generated
   under.  For example:

   Subject: user

3.3.2.  Comment Header

   The comment header contains a user specified comment.  The comment
   SHOULD be displayed when using the key.

   It is suggested that this field default to user@hostname for the user
   and machine used to generate the key.  For example:

   Comment: user@example.com

   Currently, common practice is to quote the Header-value of the
   Comment by prefixing and suffixing it with '"' characters, and some
   existing implementations fail if these quotation marks are omitted

   Compliant implementations MUST function correctly if the quotation
   marks are omitted.

   Implementations MAY include the quotation marks.  If the first and
   last characters of the Header-value are matching quotation marks,
   implementations SHOULD remove them before using the value.

3.3.3.  New Headers

   New headers that are of the from "x-" are considered experimental,
   and may be used without IETF consensus.

   All other headers are reserved for use only by IETF consensus.

3.4.  Public Key File Body

   The body of a public key file consists of the public key blob as
   described in [I-D.ietf-secsh-transport], section 4.6, "Public Key



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   Algorithms", encoded in base 64 as specified in [RFC2045], section
   6.8, "Base64 Content-Transfer-Encoding".

   As with all other lines, each line in the body MUST NOT be longer
   than 72 characters excluding line termination characters.

3.5.  Differences with RFC1421 PEM formats

   Implemetors should take care to notice that while the format is
   superficially similar to those specified by PEM [RFC1421] and OpenPGP
   [RFC2440], it is not identical; most notably:

   o  The other specifications use different BEGIN/END delimeters (five
      dashes, no space rather than four dashes and a space).

   o  There is no blank line before the start of the base64-encoded
      contents.

   o  There is no CRC at the end of the base64-encoded block.

   o  Header continuation uses a backslash at the end of the continued
      line rather then whitespace at the start of the next line.

3.6.  Examples

   The following are some example public key files that are compliant
   (note that the examples all wrap before 72 lines to meet ietf
   document requirements; however, they are still compliant.)

   ---- BEGIN SSH2 PUBLIC KEY ----
   Comment: "1024-bit RSA, converted from OpenSSH by me@myhost"
   x-command: /home/galb/bin/lock-in-guest.sh
   AAAAB3NzaC1yc2EAAAABIwAAAIEA1on8gxCGJJWSRT4uOrR13mUaUk0hRf4RzxSZ1zRb
   YYFw8pfGesIFoEuVth4HKyF8k1y4mRUnYHP1XNMNMJl1JcEArC2asV8sHf6zSPVffozZ
   5TT4SfsUu/iKy9lUcCfXzwre4WWZSXXcPff+EHtWshahu3WzBdnGxm5Xoi89zcE=
   ---- END SSH2 PUBLIC KEY ----















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   ---- BEGIN SSH2 PUBLIC KEY ----
   Comment: This is my public key for use on \
   servers which I don't like.
   AAAAB3NzaC1kc3MAAACBAPY8ZOHY2yFSJA6XYC9HRwNHxaehvx5wOJ0rzZdzoSOXxbET
   W6ToHv8D1UJ/z+zHo9Fiko5XybZnDIaBDHtblQ+Yp7StxyltHnXF1YLfKD1G4T6JYrdH
   YI14Om1eg9e4NnCRleaqoZPF3UGfZia6bXrGTQf3gJq2e7Yisk/gF+1VAAAAFQDb8D5c
   vwHWTZDPfX0D2s9Rd7NBvQAAAIEAlN92+Bb7D4KLYk3IwRbXblwXdkPggA4pfdtW9vGf
   J0/RHd+NjB4eo1D+0dix6tXwYGN7PKS5R/FXPNwxHPapcj9uL1Jn2AWQ2dsknf+i/FAA
   vioUPkmdMc0zuWoSOEsSNhVDtX3WdvVcGcBq9cetzrtOKWOocJmJ80qadxTRHtUAAACB
   AN7CY+KKv1gHpRzFwdQm7HK9bb1LAo2KwaoXnadFgeptNBQeSXG1vO+JsvphVMBJc9HS
   n24VYtYtsMu74qXviYjziVucWKjjKEb11juqnF0GDlB3VVmxHLmxnAz643WK42Z7dLM5
   sY29ouezv4Xz2PuMch5VGPP+CDqzCM4loWgV
   ---- END SSH2 PUBLIC KEY ----


   ---- BEGIN SSH2 PUBLIC KEY ----
   Comment: DSA Public Key for use with MyIsp
   AAAAB3NzaC1kc3MAAACBAPY8ZOHY2yFSJA6XYC9HRwNHxaehvx5wOJ0rzZdzoSOXxbET
   W6ToHv8D1UJ/z+zHo9Fiko5XybZnDIaBDHtblQ+Yp7StxyltHnXF1YLfKD1G4T6JYrdH
   YI14Om1eg9e4NnCRleaqoZPF3UGfZia6bXrGTQf3gJq2e7Yisk/gF+1VAAAAFQDb8D5c
   vwHWTZDPfX0D2s9Rd7NBvQAAAIEAlN92+Bb7D4KLYk3IwRbXblwXdkPggA4pfdtW9vGf
   J0/RHd+NjB4eo1D+0dix6tXwYGN7PKS5R/FXPNwxHPapcj9uL1Jn2AWQ2dsknf+i/FAA
   vioUPkmdMc0zuWoSOEsSNhVDtX3WdvVcGcBq9cetzrtOKWOocJmJ80qadxTRHtUAAACB
   AN7CY+KKv1gHpRzFwdQm7HK9bb1LAo2KwaoXnadFgeptNBQeSXG1vO+JsvphVMBJc9HS
   n24VYtYtsMu74qXviYjziVucWKjjKEb11juqnF0GDlB3VVmxHLmxnAz643WK42Z7dLM5
   sY29ouezv4Xz2PuMch5VGPP+CDqzCM4loWgV
   ---- END SSH2 PUBLIC KEY ----


   ---- BEGIN SSH2 PUBLIC KEY ----
   Subject: galb
   Comment: 1024-bit rsa, created by me@myhost Mon Jan 15 08:31:24 2001
   AAAAB3NzaC1yc2EAAAABJQAAAIEAiPWx6WM4lhHNedGfBpPJNPpZ7yKu+dnn1SJejgt4
   596k6YjzGGphH2TUxwKzxcKDKKezwkpfnxPkSMkuEspGRt/aZZ9wa++Oi7Qkr8prgHc4
   soW6NUlfDzpvZK2H5E7eQaSeP3SAwGmQKUFHCddNaP0L+hM7zhFNzjFvpaMgJw0=
   ---- END SSH2 PUBLIC KEY ----















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4.  Public Key Fingerprints

   The security of the SSH protocols relies on the verification of
   public host keys.  Since public keys tend to be very large, it is
   difficult for a human to verify an entire host key.  Even with a PKI
   in place, it is useful to have a standard for exchanging short
   fingerprints of public keys.

   This section formally describes the method of generating public key
   fingerprints that is in common use in the SSH community.

   The fingerprint of a public key consists of the output of the MD5
   message-digest algorithm [RFC1321].  The input to the algorithm is
   the public key blob as described in [I-D.ietf-secsh-transport].  The
   output of the algorithm is presented to the user as a sequence of 16
   octets printed as hexadecimal with lowercase letters and separated by
   colons.

   For example: "c1:b1:30:29:d7:b8:de:6c:97:77:10:d7:46:41:63:87"
































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5.  IANA Considerations

   There are no IANA registries or other considerations associated with
   this document.















































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6.  Security Considerations

   The file format described by this document provides no mechanism to
   verify the integrity or otherwise detect tampering with the data
   stored in such files.  Given the potential of an adversarial
   tampering with this data, system-specific measures (e.g.  Access
   Control Lists, UNIX permissions, other Discretionary and/or Mandatory
   Access Controls) SHOULD be used to protect these files.  Also, if the
   contents of these files are transferred it SHOULD be done over a
   trusted channel.

   The header data allowed by this file format could contain an
   unlimited range of information.  While in many environments the
   information conveyed by this header data may be considered innocuous
   public information, it may constitute a channel through which
   information about a user, a key or its use may be disclosed
   intentionally or otherwise (e.g "Comment: Mary E. Jones, 123 Main St,
   Home Phone:...").  The presence and use of this header data SHOULD be
   reviewed by sites that deploy this file format.

   The public-key fingerprint method presented here relies on the MD5
   hash, which is known to have certain weaknesses.  The use of it here
   is for historical purposes, and the particular use made of it depends
   solely one it's 2nd-preimage resistance, not on it's collision-
   resistance.


























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7.  References

7.1.  Normative References

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

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

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [I-D.ietf-secsh-transport]
              Lonvick, C., "SSH Transport Layer Protocol",
              draft-ietf-secsh-transport-24 (work in progress),
              March 2005.

7.2.  Informative References

   [RFC1421]  Linn, J., "Privacy Enhancement for Internet Electronic
              Mail: Part I: Message Encryption and Authentication
              Procedures", RFC 1421, February 1993.

   [RFC2440]  Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
              "OpenPGP Message Format", RFC 2440, November 1998.





















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Authors' Addresses

   Joseph Galbraith
   VanDyke Software
   4848 Tramway Ridge Blvd
   Suite 101
   Albuquerque, NM  87111
   US

   Phone: +1 505 332 5700
   Email: galb-list@vandyke.com


   Rodney Thayer
   The Tillerman Group
   370 Altair Way, PMB 321
   Sunnyvale, CA  94086

   Phone: +1 408 757 9693
   Email: rodney@tillerman.to































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Intellectual Property Statement

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   Internet Society.




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