Secure Shell Working Group J. Galbraith
Internet-Draft VanDyke Software
Expires: January 30, 2004 R. Thayer
The Tillerman Group
August 1, 2003
SSH Public Key File Format
draft-ietf-secsh-publickeyfile-04.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document formally documents the 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
3.4 Public Key File Body . . . . . . . . . . . . . . . . . . . . 6
3.5 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . 8
Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . 10
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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 [4].
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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 the existing public key file format, with few exceptions.
Where this document departs from current practice, it also suggests a
mechanism for backwards compatibility.
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3. Key File Format
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.
3.1 Line termination Characters
In order to achieve the goal of being able to exchange public key
files between servers, implementations are REQUIRED to read files
using any of the common line termination sequence, <CR>, <LF> or
<CR><LF>.
Implementations may generate files using which ever line termination
convention is most convenient
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 a 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. 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 appending the contents of
the next line.
The Header-tag MUST be US-ASCII. The Header-value MUST be encoded in
UTF-8. [2]
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A line that is not a continuation line that has no ':' in it is
assumed to be the first line of the base 64 encoded body (Section 8)
Compliant implementations MUST ignore unrecognized header fields.
Implementations SHOULD preserve unrecognized header fields when
manipulating the key file.
Existing implementations may not correctly handle unrecognized
fields. During a transition period, implementations SHOULD generate
key file headers that contain only a Subject field followed by a
Comment field.
3.3.1 Subject Header
This field currently is used to store the login-name that the key was
generated under. For example:
Subject: user
3.3.2 Comment Header
Contain a user specified comment which will 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@mycompany.com
Currently, common practice is to quote the Header-value of the
Comment, and some existing implementations fail if these quotes are
omitted.
Compliant implementations MUST function correctly if the quotes are
omitted.
During an interim period implementations MAY include the quotes. If
the first and last characters of the Header-value are matching
quotes, implementations SHOULD remove them before using the value.
3.4 Public Key File Body
The body of a public key file consists of the public key blob as
described in the SSH transport draft [1], section 4.6, "Public Key
Algorithms", encoded in base 64 as specified in RFC-2045, section
6.8, "Base64 Content-Transfer-Encoding". [5]
As with all other lines, each line in the body MUST NOT be longer
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than 72 characters.
3.5 Examples
The following are some example public key files that are compliant:
---- BEGIN SSH2 PUBLIC KEY ----
Comment: "1024-bit RSA, converted from OpenSSH by galb@test1"
AAAAB3NzaC1yc2EAAAABIwAAAIEA1on8gxCGJJWSRT4uOrR13mUaUk0hRf4RzxSZ1zRbYY
Fw8pfGesIFoEuVth4HKyF8k1y4mRUnYHP1XNMNMJl1JcEArC2asV8sHf6zSPVffozZ5TT4
SfsUu/iKy9lUcCfXzwre4WWZSXXcPff+EHtWshahu3WzBdnGxm5Xoi89zcE=
---- 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 galb@shimi Mon Jan 15 08:31:24 2001
AAAAB3NzaC1yc2EAAAABJQAAAIEAiPWx6WM4lhHNedGfBpPJNPpZ7yKu+dnn1SJejgt459
6k6YjzGGphH2TUxwKzxcKDKKezwkpfnxPkSMkuEspGRt/aZZ9wa++Oi7Qkr8prgHc4soW6
NUlfDzpvZK2H5E7eQaSeP3SAwGmQKUFHCddNaP0L+hM7zhFNzjFvpaMgJw0=
---- END SSH2 PUBLIC KEY ----
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4. 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.
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Normative References
[1] Rinne, T., Ylonen, T., Kivinen, T., Saarinen, M. and S.
Lehtinen, "SSH Protocol Transport Protocol", September 2002.
[2] Yergeau, F., "UTF-8, a Transformation Format of Unicode and ISO
10646", October 1996.
[3] Bradner, S., "The Internet Standards Process -- Revision 3",
October 1996.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", March 1997.
[5] Freed and Borenstein, "Multipurpose Internet Mail Extensions
(MIME) Part One: Format of Internet Message Bodies", November
1996.
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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