Network Working Group
Request for Comments: 4716
Category: Informational

J. Galbraith
VanDyke Software
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

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.

   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

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>.

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 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

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:

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

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

There is no Cyclic Redundancy Check (CRC) at the end of the base64-encoded block.

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.)

   ---- 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 ----
   
   ---- 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: 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].

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.

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.

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

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