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rfc:rfc4716

Network Working Group J. Galbraith Request for Comments: 4716 VanDyke Software Category: Informational R. Thayer

                                                        Canola & Jones
                                                         November 2006
           The Secure Shell (SSH) Public Key File Format

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The IETF Trust (2006).

Abstract

 This document formally documents an existing public key file format
 in use for exchanging public keys between different Secure Shell
 (SSH) implementations.
 In addition, this document defines a standard textual representation
 for SSH public key fingerprints.

Table of Contents

 1. Introduction ....................................................2
 2. Conventions Used in This Document ...............................2
 3. Key File Format .................................................2
    3.1. Line Termination Characters ................................2
    3.2. Begin and End Markers ......................................3
    3.3. Key File Header ............................................3
         3.3.1. Subject Header ......................................3
         3.3.2. Comment Header ......................................4
         3.3.3. Private Use Headers .................................4
    3.4. Public Key File Body .......................................4
    3.5. Differences with RFC 1421 PEM Formats ......................4
    3.6. Examples ...................................................5
 4. Public Key Fingerprints .........................................6
 5. IANA Considerations .............................................6
 6. Security Considerations .........................................7
 7. References ......................................................8
    7.1. Normative References .......................................8
    7.2. Informative References .....................................8

Galbraith & Thayer Informational [Page 1] RFC 4716 SSH Public Key File Format November 2006

1. Introduction

 The SSH protocol supports the use of public/private key pairs in
 order to perform authentication based on public key cryptography.
 However, in order to use public key authentication in the SSH
 protocol, public keys must first be exchanged between client and
 server.
 This document formally describes an existing public key file format
 that can be used with any of the common existing file transfer
 mechanisms in order to exchange public keys.
 The SSH protocol also uses public/private key pairs to authenticate
 the server.  In this scenario, it is important to verify that the
 public key provided by the server is indeed the server's public key.
 This document describes a mechanism for creating a short text string
 that uniquely represents a particular public key, called
 fingerprinting.

2. 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].

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 8-bit 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>.

Galbraith & Thayer Informational [Page 2] RFC 4716 SSH Public Key File Format November 2006

3.2. Begin and End Markers

 The first line of a conforming key file MUST be a begin marker, which
 is the literal text:
  1. — BEGIN SSH2 PUBLIC KEY —-
 The last line of a conforming key file MUST be an end marker, which
 is the literal text:
  1. — 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 8-bit bytes and is case-
 insensitive.  The Header-value MUST NOT be more than 1024 8-bit
 bytes.  Each line in the header MUST NOT be more than 72 8-bit 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 are 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
 encoded in UTF-8 [RFC3629].
 A line that is not a continuation line that has no ':' in it is the
 first line of the base64-encoded body.  (See Section 3.4.)
 The space of header-tags is managed as described in Section 5.
 Compliant implementations MUST ignore headers with unrecognized
 header-tags.  Implementations SHOULD preserve such unrecognized
 headers 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

Galbraith & Thayer Informational [Page 3] RFC 4716 SSH Public Key File Format November 2006

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. Private Use Headers

 Headers with header-tags beginning with "x-" are reserved for private
 use.

3.4. Public Key File Body

 The body of a public key file is the base64 encoded ([RFC2045])
 public key data as specified by [RFC4253], Section 6.6:
       string    certificate or public key format identifier
       byte[n]   key/certificate data
 As with all other lines, each line in the body MUST NOT be longer
 than 72 8-bit bytes excluding line termination characters.

3.5. Differences with RFC 1421 PEM Formats

 Implementers 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 delimiters (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.

Galbraith & Thayer Informational [Page 4] RFC 4716 SSH Public Key File Format November 2006

 o  There is no Cyclic Redundancy Check (CRC) at the end of the
    base64-encoded block.
 o  Header continuation uses a backslash at the end of the continued
    line rather than whitespace at the start of the next line.

3.6. Examples

 The following are some examples of public key files that are
 compliant (note that the examples all wrap before 72 bytes to meet
 IETF document requirements; however, they are still compliant.)
  1. — BEGIN SSH2 PUBLIC KEY —-

Comment: "1024-bit RSA, converted from OpenSSH by me@example.com"

 x-command: /home/me/bin/lock-in-guest.sh
 AAAAB3NzaC1yc2EAAAABIwAAAIEA1on8gxCGJJWSRT4uOrR13mUaUk0hRf4RzxSZ1zRb
 YYFw8pfGesIFoEuVth4HKyF8k1y4mRUnYHP1XNMNMJl1JcEArC2asV8sHf6zSPVffozZ
 5TT4SfsUu/iKy9lUcCfXzwre4WWZSXXcPff+EHtWshahu3WzBdnGxm5Xoi89zcE=
 ---- END SSH2 PUBLIC KEY ----
  1. — 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 ----
  1. — 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 ----

Galbraith & Thayer Informational [Page 5] RFC 4716 SSH Public Key File Format November 2006

  1. — BEGIN SSH2 PUBLIC KEY —-

Subject: me

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

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
 Public Key Infrastructure (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 data as specified by [RFC4253].  (This is the same
 data that is base64 encoded to form the body of the public key file.)
 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"

5. IANA Considerations

 Section 3.3 defines a new namespace of "Header-tags".  These are
 US-ASCII strings of maximum length 64 characters and are
 case-insensitive.
 IANA has created and maintains a registry of these header-tags.  The
 registry maps each header-tag to a reference defining the header.
 The initial contents of the registry are as follows:
    subject defined in Section 3.3.1
    comment defined in Section 3.3.2
 Header-tags beginning with "x-" are reserved for private use, as
 defined in [RFC2434].

Galbraith & Thayer Informational [Page 6] RFC 4716 SSH Public Key File Format November 2006

 All other allocations are to be made by IETF consensus, as defined in
 [RFC2434].

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 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
 one-way hash function, which is known to have certain weaknesses
 regarding its collision resistance; however, the particular use made
 of MD5 here depends solely on its 2nd-preimage resistance, not on its
 collision resistance.
 MD5 is used here for historical reasons.

Galbraith & Thayer Informational [Page 7] RFC 4716 SSH Public Key File Format November 2006

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.
 [RFC4253]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
            Transport Layer Protocol", RFC 4253, January 2006.
 [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 2434,
            October 1998.

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.

Galbraith & Thayer Informational [Page 8] RFC 4716 SSH Public Key File Format November 2006

Authors' Addresses

 Joseph Galbraith
 VanDyke Software
 4848 Tramway Ridge Blvd
 Suite 101
 Albuquerque, NM  87111
 US
 Phone: +1 505 332 5700
 EMail: galb@vandyke.com
 Rodney Thayer
 Canola & Jones
 650 Castro Street Suite 120-205
 Mountain View CA 94041
 US
 Phone: +1 650 704 8389
 EMail: rodney@canola-jones.com

Galbraith & Thayer Informational [Page 9] RFC 4716 SSH Public Key File Format November 2006

Full Copyright Statement

 Copyright (C) The IETF Trust (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST,
 AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
 EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
 THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY
 IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
 PURPOSE.

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 Intellectual Property Rights or other rights that might be claimed to
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 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
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 attempt made to obtain a general license or permission for the use of
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 The IETF invites any interested party to bring to its attention any
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Galbraith & Thayer Informational [Page 10]

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