Secure Shell Working Group J. Galbraith
Internet-Draft J. Van Dyke
Expires: December 22, 2003 B. McClure
VanDyke Software
June 23, 2003
Secure Shell Public-Key Subsystem
draft-galb-secsh-publickey-subsystem-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
SECSH defines an 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.
This protocol is intended to be used from the Secure Shell Connection
Protocol [4] as a subsystem, as described in Section ``Starting a
Shell or a Command''. The subsystem name used with this protocol is
"publickey@vandyke.com".
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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 a mandatory command.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Public-Key Subsystem Overview . . . . . . . . . . . . . . . 4
2.1 Opening the Public-Key Subsystem . . . . . . . . . . . . . . 4
2.2 Requests . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 Responses . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3.1 The Status Response . . . . . . . . . . . . . . . . . . . . 5
3. Public-Key Subsystem Operations . . . . . . . . . . . . . . 7
3.1 Version Packet . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Adding a public key . . . . . . . . . . . . . . . . . . . . 7
3.3 Removing a public key . . . . . . . . . . . . . . . . . . . 7
3.4 Listing public keys . . . . . . . . . . . . . . . . . . . . 8
3.5 Associate public key with a mandatory command . . . . . . . 8
References . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . 11
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1. Introduction
SECSH is a protocol for secure remote login and other secure network
services over an insecure network. SECSH defines an 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 PKIX replacement.
The Secure Shell Public-Key subsystem has been designed to run on top
of the SECSH 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 SECSH
architecture [1] and SECSH connection [4] documents.
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.
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2. 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@vandyke.com".
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 the SSH Transport
Protocol document [2].
2.1 Opening the Public-Key Subsystem
The public-key subsystem is opened when the clients sends a
SSH_MSG_CHANNEL_REQUEST over an existing session.
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@vandyke.com"
Client implementations SHOULD reject this request; it is normally
only sent 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
authenticated with a restricted public key that does not allow access
to the publickey subsystem.
It is RECOMMENDED that clients request and check the reply for this
request.
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2.2 Requests
All public-key subsystem requests are sent in the following form:
uint32 length
string request-name
... request specific data follows
The length field describes the length of the request-name field and
the request-specific data, but not of the length field itself. The
client MUST receive a response to each request prior to sending a
new request.
All requests described in Section 3 are a description of the
'request-name' and 'data' portions of the packet.
2.3 Responses
All public-key subsystem responses are sent in the following form:
uint32 length
string response-name
... response specific data follows
2.3.1 The Status Response
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 [RFC-2279]
string language tag [RFC-1766]
A status message MUST be sent for any unrecognized packets and the
request SHOULD NOT close the subsystem.
2.3.1.1 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_REQUEST_NOT_SUPPORTED 3
SSH_PUBLICKEY_KEY_NOT_FOUND 4
SSH_PUBLICKEY_KEY_NOT_SUPPORTED 5
SSH_PUBLICKEY_KEY_ALREADY_PRESENT 6
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SSH_PUBLICKEY_GENERAL_FAILURE 7
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3. Public-Key Subsystem Operations
The public-key subsystem currently defines four operations: add,
remove, list, and command.
3.1 Version Packet
Both sides MUST start by sending a version packet that indicates the
version of the protocol they are using.
string "version"
uint32 protocol-version-number
The version of the protocol described by this document is version 1.
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.
Both sides MUST wait to receive this version before continuing.
3.2 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 mandatory
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 [SSH-ARCH] (section 5). If the server does not
implement a mandatory attribute, it MUST fail the add. For the
purposes of a mandatory attribute, storage of the attribute is not
sufficient, but requires that the server understand and implement
the intent of the attribute.
The following attributes are currently defined:
"comment"
The comment field 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 a list operation.
The server MUST NOT attempt to interpret or act upon the content
of the comment field in any way.
The comment field is useful so the user can identify the key
without resorting to comparing its fingerprint.
This attribute SHOULD NOT be mandatory.
"comment-language"
If this attribute is specified, it MUST immediately follow a
"comment" attribute and specifies the language for that attribute
[RFC1766]. The client MAY specify more than comment if it
additionally specifies a different language for each of those
comments. The server SHOULD attempt to store each comment,
together with that comment's lanuage attribute.
This attribute SHOULD NOT be mandatory.
"command"
"command" bypasses the session channel "exec" and "shell" requests
by always executing the specified command (as if it had been
executed using an "exec" request).
This attribute SHOULD be mandatory. This attribute MUST NOT be
specified if the "subsystem" attribute is specified.
"subsystem"
"subsystem" specifies that the specified subsystem should be started
when this key is used (as if it had been started using a "subsystem"
request.
This attribute SHOULD be mandatory. This attribute MUST NOT be
specified if the "command" attribute is specified.
"restrict"
The value of this attribute contains server functions that may
not be performed when this key is used. It is a comma seperated
list. Element names are specified in the same way as attribute
names, above. The following restrictions are currently defined:
Currently defined restrictions are:
"x11"
"shell"
"exec"
"agent"
"env"
"subsystem"
The "x11" restriction specifies that X11 forwarding may not be
performed when this key is in use. The "shell" restriction
specifies that session channel "shell" requests should be denied
when this key is in use. The "exec" restriction specifies that
session channel "exec" requests should be denied when this key
is in use. The "agent" restriction specifies that session channel
"auth-agent-req" requests should be denied when this key is in use.
The "env" restriction specifies that session channel "env" requests
should be denied when this key is in use. The "subsystem"
restriction specifies that subsystems may not be started when this
public key is in use (if the "subsystem" attribute is also specified,
the subsystem specified in that attribute is exempted from this
restriction).
This attribute SHOULD be mandatory.
"port-forward"
"port-forward" specifies that no "direct-tcpip" requests should be
accepted, except to those 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 mandatory.
"reverse-forward"
"reverse-forward" specifies that no "tcpip-forward" requests should
be accepted, accept 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 mandatory.
In addition to the attributes and restrictions specified by the client,
the server MAY provide a method for administrators to compulsorily enforce
certain attributes or restrictions.
3.3 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.
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3.4 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
Following the last "publickey" response, a status packet MUST be
sent.
An implementation MAY choose not to support this request.
3.5 Listing server capabilities
If the client wishes to know which restrictions the server supports,
it sends:
string "listattributes"
The server will respond with zero or more of the following responses:
string "attribute"
string attribute name
boolean compulsory
The server will then respond with zero or more of the following
responses:
string "restriction"
string restriction name
boolean compulsory
The server MAY include "restrict" in the list of attributes it supports.
The client SHOULD NOT require the server to do so in order to accept
that the server supports the list of restrictions returned by the
server.
Following the last "restriction" response, a status packet MUST be
sent.
An implementation MAY choose not to support this request.
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References
[1] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Protocol Architecture",
draft-ietf-secsh-architecture-13 (work in progress), January
2002.
[2] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Transport Layer Protocol",
draft-ietf-secsh-transport-15 (work in progress), March 2002.
[3] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Authentication Protocol",
draft-ietf-secsh-userauth-16 (work in progress), February 2002.
[4] Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S.
Lehtinen, "SSH Connection Protocol", draft-ietf-secsh-connect-16
(work in progress), January 2002.
[5] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.
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
Jeff P. Van Dyke
VanDyke Software
4848 Tramway Ridge Blvd
Suite 101
Albuquerque, NM 87111
US
Phone: +1 505 332 5700
EMail: jpv@vandyke.com
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Brent McClure
VanDyke Software
4848 Tramway Ridge Blvd
Suite 101
Albuquerque, NM 87111
US
Phone: +1 505 332 5700
EMail: bdm@vandyke.com
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