Network Working Group J. Galbraith

Network Working Group J. Galbraith
Request for Comments: 4819 J. Van Dyke
Category: Standards Track VanDyke Software
J. Bright
Silicon Circus
March 2007

Secure Shell Public Key Subsystem

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The IETF Trust (2007).

Abstract

Secure Shell defines a user authentication mechanism that is based on
public keys, but does not define any mechanism for key distribution.
No common key management solution exists in current implementations.
This document describes a protocol that can be used to configure
public keys in an implementation-independent fashion, allowing client
software to take on the burden of this configuration.

The Public Key Subsystem provides a server-independent mechanism for
clients to add public keys, remove public keys, and list the current
public keys known by the server. Rights to manage public keys are
specific and limited to the authenticated user.

A public key may also be associated with various restrictions,
including a mandatory command or subsystem.

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RFC 4819 Secure Shell Public Key Subsystem March 2007

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Public Key Subsystem Overview . . . . . . . . . . . . . . . . 3
3.1. Opening the Public Key Subsystem . . . . . . . . . . . . . 4
3.2. Requests and Responses . . . . . . . . . . . . . . . . . . 5
3.3. The Status Message . . . . . . . . . . . . . . . . . . . . 5
3.3.1. Status Codes . . . . . . . . . . . . . . . . . . . . . 5
3.4. The Version Packet . . . . . . . . . . . . . . . . . . . . 6
4. Public Key Subsystem Operations . . . . . . . . . . . . . . . 7
4.1. Adding a Public Key . . . . . . . . . . . . . . . . . . . 7
4.2. Removing a Public Key . . . . . . . . . . . . . . . . . . 10
4.3. Listing Public Keys . . . . . . . . . . . . . . . . . . . 10
4.4. Listing Server Capabilities . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6.1. Registrations . . . . . . . . . . . . . . . . . . . . . . 12
6.2. Names . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.2.1. Conventions for Names . . . . . . . . . . . . . . . . 12
6.2.2. Future Assignments of Names . . . . . . . . . . . . . 13
6.3. Public Key Subsystem Request Names . . . . . . . . . . . . 13
6.4. Public Key Subsystem Response Names . . . . . . . . . . . 13
6.5. Public Key Subsystem Attribute Names . . . . . . . . . . . 13
6.6. Public Key Subsystem Status Codes . . . . . . . . . . . . 14
6.6.1. Conventions . . . . . . . . . . . . . . . . . . . . . 14
6.6.2. Initial Assignments . . . . . . . . . . . . . . . . . 14
6.6.3. Future Assignments . . . . . . . . . . . . . . . . . . 15
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. Normative References . . . . . . . . . . . . . . . . . . . 15
7.2. Informative References . . . . . . . . . . . . . . . . . . 15
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16

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RFC 4819 Secure Shell Public Key Subsystem March 2007

1. Introduction

Secure Shell (SSH) is a protocol for secure remote login and other
secure network services over an insecure network. Secure Shell
defines a user authentication mechanism that is based on public keys,
but does not define any mechanism for key distribution. Common
practice is to authenticate once with password authentication and
transfer the public key to the server. However, to date no two
implementations use the same mechanism to configure a public key for
use.

This document describes a subsystem that can be used to configure
public keys in an implementation-independent fashion. This approach
allows client software to take on the burden of this configuration.
The Public Key Subsystem protocol is designed for extreme simplicity
in implementation. It is not intended as a Public Key Infrastructure
for X.509 Certificates (PKIX) replacement.

The Secure Shell Public Key Subsystem has been designed to run on top
of the Secure Shell transport layer [2] and user authentication
protocols [3]. It provides a simple mechanism for the client to
manage public keys on the server.

This document should be read only after reading the Secure Shell
architecture [1] and Secure Shell connection [4] documents.

This protocol is intended to be used from the Secure Shell Connection
Protocol [4] as a subsystem, as described in the section "Starting a
Shell or a Command". The subsystem name used with this protocol is
"publickey".

This protocol requires that the user be able to authenticate in some
fashion before it can be used. If password authentication is used,
servers SHOULD provide a configuration option to disable the use of
password authentication after the first public key is added.

2. Terminology

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 RFC 2119 [5].

3. Public Key Subsystem Overview

The Public Key Subsystem provides a server-independent mechanism for
clients to add public keys, remove public keys, and list the current
public keys known by the server. The subsystem name is "publickey".

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The public keys added, removed, and listed using this protocol are
specific and limited to those of the authenticated user.

The operations to add, remove, and list the authenticated user's
public keys are performed as request packets sent to the server. The
server sends response packets that indicate success or failure as
well as provide specific response data.

The format of public key blobs are detailed in Section 6.6, "Public
Key Algorithms" of the SSH Transport Protocol document [2].

3.1. Opening the Public Key Subsystem

The Public Key Subsystem is started by a client sending an
SSH_MSG_CHANNEL_REQUEST over an existing session's channel.

The details of how a session is opened are described in the SSH
Connection Protocol document [4] in the section "Opening a Session".

To open the Public Key Subsystem, the client sends:

byte SSH_MSG_CHANNEL_REQUEST
uint32 recipient channel
string "subsystem"
boolean want reply
string "publickey"

Client implementations SHOULD reject this request; it is normally
sent only by the client.

If want reply is TRUE, the server MUST respond with
SSH_MSG_CHANNEL_SUCCESS if the Public Key Subsystem was successfully
started, or SSH_MSG_CHANNEL_FAILURE if the server failed to start or
does not support the Public Key Subsystem.

The server SHOULD respond with SSH_MSG_CHANNEL_FAILURE if the user is
not allowed access to the Public Key Subsystem (for example, because
the user authenticated with a restricted public key).

It is RECOMMENDED that clients request and check the reply for this
request.

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3.2. Requests and Responses

All Public Key Subsystem requests and responses are sent in the
following form:

uint32 length
string name
... request/response specific data follows

The length field describes the length of the name field and of the
request/response-specific data, but does not include the length of
the length field itself. The client MUST receive acknowledgement of
each request prior to sending a new request.

The version packet, as well as all requests and responses described
in Section 4, are a description of the 'name' field and the data part
of the packet.

3.3. The Status Message

A request is acknowledged by sending a status packet. If there is
data in response to the request, the status packet is sent after all
data has been sent.

string "status"
uint32 status code
string description [7]
string language tag [6]

A status message MUST be sent for any unrecognized packets, and the
request SHOULD NOT close the subsystem.

3.3.1. Status Codes

The status code gives the status in a more machine-readable format
(suitable for localization), and can have the following values:

SSH_PUBLICKEY_SUCCESS 0
SSH_PUBLICKEY_ACCESS_DENIED 1
SSH_PUBLICKEY_STORAGE_EXCEEDED 2
SSH_PUBLICKEY_VERSION_NOT_SUPPORTED 3
SSH_PUBLICKEY_KEY_NOT_FOUND 4
SSH_PUBLICKEY_KEY_NOT_SUPPORTED 5
SSH_PUBLICKEY_KEY_ALREADY_PRESENT 6
SSH_PUBLICKEY_GENERAL_FAILURE 7
SSH_PUBLICKEY_REQUEST_NOT_SUPPORTED 8
SSH_PUBLICKEY_ATTRIBUTE_NOT_SUPPORTED 9

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If a request completed successfully, the server MUST send the status
code SSH_PUBLICKEY_SUCCESS. The meaning of the failure codes is as
implied by their names.

3.4. The Version Packet

Both sides MUST start a connection by sending a version packet that
indicates the version of the protocol they are using.

string "version"
uint32 protocol-version-number

This document describes version 2 of the protocol. Version 1 was
used by an early draft of this document. The version number was
incremented after changes in the handling of status packets.

Both sides send the highest version that they implement. The lower
of the version numbers is the version of the protocol to use. If
either side can't support the lower version, it should close the
subsystem and notify the other side by sending an
SSH_MSG_CHANNEL_CLOSE message. Before closing the subsystem, a
status message with the status SSH_PUBLICKEY_VERSION_NOT_SUPPORTED
SHOULD be sent. Note that, normally, status messages are only sent
by the server (in response to requests from the client). This is the
only occasion on which the client sends a status message.

Both sides MUST wait to receive this version before continuing. The
"version" packet MUST NOT be sent again after this initial exchange.
The SSH_PUBLICKEY_VERSION_NOT_SUPPORTED status code must not be sent
in response to any other request.

Implementations MAY use the first 15 bytes of the version packet as a
"magic cookie" to avoid processing spurious output from the user's
shell (as described in Section 6.5 of [4]). These bytes will always
be:

0x00 0x00 0x00 0x0F 0x00 0x00 0x00 0x07 0x76 0x65 0x72 0x73 0x69 0x6F
0x6E

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4. Public Key Subsystem Operations

The Public Key Subsystem currently defines four operations: add,
remove, list, and listattributes.

4.1. Adding a Public Key

If the client wishes to add a public key, the client sends:

string "add"
string public key algorithm name
string public key blob
boolean overwrite
uint32 attribute-count
string attrib-name
string attrib-value
bool critical
repeated attribute-count times

The server MUST attempt to store the public key for the user in the
appropriate location so the public key can be used for subsequent
public key authentications. If the overwrite field is false and the
specified key already exists, the server MUST return
SSH_PUBLICKEY_KEY_ALREADY_PRESENT. If the server returns this, the
client SHOULD provide an option to the user to overwrite the key. If
the overwrite field is true and the specified key already exists, but
cannot be overwritten, the server MUST return
SSH_PUBLICKEY_ACCESS_DENIED.

Attribute names are defined following the same scheme laid out for
algorithm names in [1]. If the server does not implement a critical
attribute, it MUST fail the add, with the status code
SSH_PUBLICKEY_ATTRIBUTE_NOT_SUPPORTED. For the purposes of a
critical attribute, mere storage of the attribute is not sufficient
-- rather, the server must understand and implement the intent of the
attribute.

The following attributes are currently defined:

"comment"

The value of the comment attribute contains user-specified text about
the public key. The server SHOULD make every effort to preserve this
value and return it with the key during any subsequent list
operation. The server MUST NOT attempt to interpret or act upon the
content of the comment field in any way. The comment attribute must
be specified in UTF-8 format [7].

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The comment field is useful so the user can identify the key without
resorting to comparing its fingerprint. This attribute SHOULD NOT be
critical.

"comment-language"

If this attribute is specified, it MUST immediately follow a
"comment" attribute and specify the language for that attribute [6].
The client MAY specify more than one comment if it additionally
specifies a different language for each of those comments. The
server SHOULD attempt to store each comment with its language
attribute. This attribute SHOULD NOT be critical.

"command-override"

"command-override" specifies a command to be executed when this key
is in use. The command should be executed by the server when it
receives an "exec" or "shell" request from the client, in place of
the command or shell which would otherwise have been executed as a
result of that request. If the command string is empty, both "exec"
and "shell" requests should be denied. If no "command-override"
attribute is specified, all "exec" and "shell" requests should be
permitted (as long as they satisfy other security or authorization
checks the server may perform). This attribute SHOULD be critical.

"subsystem"

"subsystem" specifies a comma-separated list of subsystems that may
be started (using a "subsystem" request) when this key is in use.
This attribute SHOULD be critical. If the value is empty, no
subsystems may be started. If the "subsystem" attribute is not
specified, no restrictions are placed on which subsystems may be
started when authenticated using this key.

"x11"

"x11" specifies that X11 forwarding may not be performed when this
key is in use. The attribute-value field SHOULD be empty for this
attribute. This attribute SHOULD be critical.

"shell"

"shell" specifies that session channel "shell" requests should be
denied when this key is in use. The attribute-value field SHOULD be
empty for this attribute. This attribute SHOULD be critical.

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RFC 4819 Secure Shell Public Key Subsystem March 2007

"exec"

"exec" specifies that session channel "exec" requests should be
denied when this key is in use. The attribute-value field SHOULD be
empty for this attribute. This attribute SHOULD be critical.

"agent"

"agent" specifies that session channel "auth-agent-req" requests
should be denied when this key is in use. The attribute-value field
SHOULD be empty for this attribute. This attribute SHOULD be
critical.

"env"

"env" specifies that session channel "env" requests should be denied
when this key is in use. The attribute-value field SHOULD be empty
for this attribute. This attribute SHOULD be critical.

"from"

"from" specifies a comma-separated list of hosts from which the key
may be used. If a host not in this list attempts to use this key for
authorization purposes, the authorization attempt MUST be denied.
The server SHOULD make a log entry regarding this. The server MAY
provide a method for administrators to disallow the appearance of a
host in this list. The server should use whatever method is
appropriate for its platform to identify the host -- e.g., for IP-
based networks, checking the IP address or performing a reverse DNS
lookup. For IP-based networks, it is anticipated that each element
of the "from" parameter will take the form of a specific IP address
or hostname.

"port-forward"

"port-forward" specifies that no "direct-tcpip" requests should be
accepted, except those to hosts specified in the comma-separated list
supplied as a value to this attribute. If the value of this
attribute is empty, all "direct-tcpip" requests should be refused
when using this key. This attribute SHOULD be critical.

"reverse-forward"

"reverse-forward" specifies that no "tcpip-forward" requests should
be accepted, except for the port numbers in the comma-separated list
supplied as a value to this attribute. If the value of this
attribute is empty, all "tcpip-forward" requests should be refused
when using this key. This attribute SHOULD be critical.

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In addition to the attributes specified by the client, the server MAY
provide a method for administrators to enforce certain attributes
compulsorily.

4.2. Removing a Public Key

If the client wishes to remove a public key, the client sends:

string "remove"
string public key algorithm name
string public key blob

The server MUST attempt to remove the public key for the user from
the appropriate location, so that the public key cannot be used for
subsequent authentications.

4.3. Listing Public Keys

If the client wishes to list the known public keys, the client sends:

string "list"

The server will respond with zero or more of the following responses:

string "publickey"
string public key algorithm name
string public key blob
uint32 attribute-count
string attrib-name
string attrib-value
repeated attribute-count times

There is no requirement that the responses be in any particular
order. Whilst some server implementations may send the responses in
some order, client implementations should not rely on responses being
in any order.

Following the last "publickey" response, a status packet MUST be
sent.

Implementations SHOULD support this request.

4.4. Listing Server Capabilities

If the client wishes to know which key attributes the server
supports, it sends:

string "listattributes"

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RFC 4819 Secure Shell Public Key Subsystem March 2007

The server will respond with zero or more of the following responses:

string "attribute"
string attribute name
boolean compulsory

The "compulsory" field indicates whether this attribute will be
compulsorily applied to any added keys (irrespective of whether the
attribute has been specified by the client) due to administrative
settings on the server. If the server does not support
administrative settings of this nature, it MUST return false in the
compulsory field. An example of use of the "compulsory" attribute
would be a server with a configuration file specifying that the user
is not permitted shell access. Given this, the server would return
the "shell" attribute, with "compulsory" marked true. Whatever
attributes the user subsequently asked the server to apply to their
key, the server would also apply the "shell" attribute, rendering it
impossible for the user to use a shell.

Following the last "attribute" response, a status packet MUST be
sent.

An implementation MAY choose not to support this request.

5. Security Considerations

This protocol assumes that it is run over a secure channel and that
the endpoints of the channel have been authenticated. Thus, this
protocol assumes that it is externally protected from network-level
attacks.

This protocol provides a mechanism that allows client authentication
data to be uploaded and manipulated. It is the responsibility of the
server implementation to enforce any access controls that may be
required to limit the access allowed for any particular user (the
user being authenticated externally to this protocol, typically using
the SSH User Authentication Protocol [3]). In particular, it is
possible for users to overwrite an existing key on the server with
this protocol, whilst at the same time specifying fewer restrictions
for the new key than were previously present. Servers should take
care that when doing this, clients are not able to override presets
from the server's administrator.

This protocol requires the client to assume that the server will
correctly implement and observe attributes applied to keys.
Implementation errors in the server could cause clients to authorize
keys for access they were not intended to have, or to apply fewer
restrictions than were intended.

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RFC 4819 Secure Shell Public Key Subsystem March 2007

6. IANA Considerations

This section contains conventions used in naming the namespaces, the
initial state of the registry, and instructions for future
assignments.

6.1. Registrations

Consistent with Section 4.9.5 of [8], this document makes the
following registration:

The subsystem name "publickey".

6.2. Names

In the following sections, the values for the namespaces are textual.
The conventions and instructions to the IANA for future assignments
are given in this section. The initial assignments are given in
their respective sections.

6.2.1. Conventions for Names

All names registered by the IANA in the following sections MUST be
printable US-ASCII strings, and MUST NOT contain the characters
at-sign ("@"), comma (","), or whitespace or control characters
(ASCII codes 32 or less). Names are case-sensitive, and MUST NOT be
longer than 64 characters.

A provision is made here for locally extensible names. The IANA will
not register and will not control names with the at-sign in them.
Names with the at-sign in them will have the format of
"name@domainname" (without the double quotes) where the part
preceding the at-sign is the name. The format of the part preceding
the at-sign is not specified; however, these names MUST be printable
US-ASCII strings, and MUST NOT contain the comma character (","), or
whitespace, or control characters (ASCII codes 32 or less). The part
following the at-sign MUST be a valid, fully qualified Internet
domain name [10] controlled by the person or organization defining
the name. Names are case-sensitive, and MUST NOT be longer than 64
characters. It is up to each domain how it manages its local
namespace. It has been noted that these names resemble STD 11 [9]
email addresses. This is purely coincidental and actually has
nothing to do with STD 11 [9]. An example of a locally defined name
is "[email protected]" (without the double quotes).

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RFC 4819 Secure Shell Public Key Subsystem March 2007

6.2.2. Future Assignments of Names

Requests for assignments of new Names MUST be done through the IETF
Consensus method as described in [11].

6.3. Public Key Subsystem Request Names

The following table lists the initial assignments of Public Key
Subsystem Request names.

Request Name
-------------
version
add
remove
list
listattributes

6.4. Public Key Subsystem Response Names

The following table lists the initial assignments of Public Key
Subsystem Response names.

Response Name
--------------
version
status
publickey
attribute

6.5. Public Key Subsystem Attribute Names

Attributes are used to define properties or restrictions for public
keys. The following table lists the initial assignments of Public
Key Subsystem Attribute names.

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RFC 4819 Secure Shell Public Key Subsystem March 2007

Attribute Name
---------------
comment
comment-language
command-override
subsystem
x11
shell
exec
agent
env
from
port-forward
reverse-forward

6.6. Public Key Subsystem Status Codes

The status code is a byte value, describing the status of a request.

6.6.1. Conventions

Status responses have status codes in the range 0 to 255. These
numbers are allocated as follows. Of these, the range 192 to 255 is
reserved for use by local, private extensions.

6.6.2. Initial Assignments

The following table identifies the initial assignments of the Public
Key Subsystem status code values.

Status code Value Reference
------------ ----- ---------
SSH_PUBLICKEY_SUCCESS 0
SSH_PUBLICKEY_ACCESS_DENIED 1
SSH_PUBLICKEY_STORAGE_EXCEEDED 2
SSH_PUBLICKEY_VERSION_NOT_SUPPORTED 3
SSH_PUBLICKEY_KEY_NOT_FOUND 4
SSH_PUBLICKEY_KEY_NOT_SUPPORTED 5
SSH_PUBLICKEY_KEY_ALREADY_PRESENT 6
SSH_PUBLICKEY_GENERAL_FAILURE 7
SSH_PUBLICKEY_REQUEST_NOT_SUPPORTED 8
SSH_PUBLICKEY_ATTRIBUTE_NOT_SUPPORTED 9

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RFC 4819 Secure Shell Public Key Subsystem March 2007

6.6.3. Future Assignments

Requests for assignments of new status codes in the range of 0 to 191
MUST be done through the Standards Action method as described in
[11].

The IANA will not control the status code range of 192 through 255.
This range is for private use.

7. References

7.1. Normative References

[1] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol
Architecture", RFC 4251, January 2006.

[2] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Transport
Layer Protocol", RFC 4253, January 2006.

[3] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, January 2006.

[4] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Connection
Protocol", RFC 4254, January 2006.

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

[6] Phillips, A. and M. Davis, "Tags for Identifying Languages",
BCP 47, RFC 4646, September 2006.

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

7.2. Informative References

[8] Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH) Protocol
Assigned Numbers", RFC 4250, January 2006.

[9] Crocker, D., "Standard for the format of ARPA Internet text
messages", STD 11, RFC 822, August 1982.

[10] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.

[11] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.

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8. Acknowledgements

Brent McClure contributed to the writing of this document.

Authors' Addresses

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

Phone: +1 505 332 5700
EMail: [email protected]

Jeff P. Van Dyke
VanDyke Software
4848 Tramway Ridge Blvd
Suite 101
Albuquerque, NM 87111
US

Phone: +1 505 332 5700
EMail: [email protected]

Jon Bright
Silicon Circus
24 Jubilee Road
Chichester, West Sussex PO19 7XB
UK

Phone: +49 172 524 0521
EMail: [email protected]

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RFC 4819 Secure Shell Public Key Subsystem March 2007

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