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
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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.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---- END SSH2 PUBLIC KEY ----
---- BEGIN SSH2 PUBLIC KEY ----
Comment: DSA Public Key for use with MyIsp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---- 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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