RFC 4819 PROPOSED STANDARD SSH · KEY MANAGEMENT · AUTHENTICATION
SSH Public Key Subsystem: Secure Shell Public Key Subsystem
A small SSH subsystem that lets a logged-in user add, remove and list their own public keys on a server.
RFC EDITOR
Galbraith, J., Van Dyke, J., and J. Bright, "Secure Shell Public Key Subsystem", RFC 4819, DOI 10.17487/RFC4819, March 2007, <https://www.rfc-editor.org/info/rfc4819>.
BIBTEX
@misc{rfc4819,
series = {Request for Comments},
number = 4819,
howpublished = {RFC 4819},
publisher = {RFC Editor},
doi = {10.17487/RFC4819},
url = {https://www.rfc-editor.org/info/rfc4819},
author = {J. Galbraith and J. Van Dyke and J. Bright},
title = {{Secure Shell Public Key Subsystem}},
pagetotal = 17,
year = 2007,
month = mar,
} RFC text
Contents
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.
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
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".
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.
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
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
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].
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.
"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.
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"
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.
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 "our-attribute@example.com" (without the double quotes).
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.
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
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.
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: galb@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
Jon Bright
Silicon Circus
24 Jubilee Road
Chichester, West Sussex PO19 7XB
UK
Phone: +49 172 524 0521
EMail: jon@siliconcircus.com
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
PROBLEMSSH can authenticate users by public key but defines no way to get keys onto the server, so every implementation does it differently.
MECHANISMA subsystem named publickey runs over an authenticated SSH session and offers four operations (add, remove, list, listattributes), with optional attributes that restrict how a key may be used.
WHY IT MATTERSClient software can manage a user's keys the same way on any server that implements it. The RFC has since been updated by RFC 9519.
§1Why a Public Key Subsystem
Secure Shell defines public-key user authentication but no mechanism for key distribution. Common practice is to log in once with a password and transfer the public key to the server, yet no two implementations use the same way to configure a key.
This RFC describes a subsystem that configures public keys in an implementation-independent way, so client software can take on the burden. It is designed for extreme simplicity and is explicitly not a replacement for PKIX. It runs on top of the SSH transport and user authentication protocols and is used from the SSH Connection Protocol as a subsystem named publickey. The user must already be able to authenticate; if password authentication is used, servers SHOULD offer an option to disable it after the first public key is added.
Everyone used to hand over keys in their own odd way. This gives one common way, like all shops agreeing on how to hand out spare keys.
§3Opening the Subsystem
The subsystem lets clients add, remove and list public keys. The keys it handles are specific to, and limited to, the authenticated user. Requests go to the server as packets, and the server sends response packets that indicate success or failure plus any data.
To start it, the client sends an SSH_MSG_CHANNEL_REQUEST with request type subsystem and the name publickey on an existing session channel. If the client asked for a reply, the server MUST answer SSH_MSG_CHANNEL_SUCCESS or SSH_MSG_CHANNEL_FAILURE. The server SHOULD fail it if the user is not allowed access, for example because they authenticated with a restricted public key. Clients are RECOMMENDED to request and check the reply.
You knock on a special door marked "publickey" inside the building you already got into. If you are not allowed in that room, the door stays shut.
§4Packets, Status and Version
Every request and response has the form uint32 length, string name, then data specific to that name. The client MUST receive acknowledgement of each request before sending the next. A request is acknowledged by a status packet holding a status code, a description and a language tag; if there is response data, the status comes after it. A status MUST be sent for unrecognized packets.
Both sides MUST begin by sending a version packet. This RFC describes version 2 (version 1 was used by an early draft). Each side sends the highest version it implements and the lower number is used. If a side cannot support it, it SHOULD send status SSH_PUBLICKEY_VERSION_NOT_SUPPORTED and close the subsystem. This is the only time a client sends a status message. The version packet MUST NOT be sent again.
First both sides say which rulebook edition they know and agree to use the older one. After that, every question gets an answer saying "done" or what went wrong.
- 0SUCCESS — The request completed.
- 1ACCESS_DENIED — Not permitted.
- 2STORAGE_EXCEEDED — No room for the key.
- 3VERSION_NOT_SUPPORTED — Only sent during the version exchange.
- 4KEY_NOT_FOUND — The key is not known.
- 5KEY_NOT_SUPPORTED — Key type not supported.
- 6KEY_ALREADY_PRESENT — Key exists and overwrite was false.
- 7GENERAL_FAILURE — Other failure.
- 8REQUEST_NOT_SUPPORTED — Request not implemented.
- 9ATTRIBUTE_NOT_SUPPORTED — A critical attribute is not implemented.
§5Add, Remove, List and Listattributes
The subsystem defines four operations. add sends the key algorithm name, the key blob, an overwrite flag and a list of attributes (name, value, critical flag). The server MUST store the key so it can be used for later authentication. If overwrite is false and the key exists, it MUST return SSH_PUBLICKEY_KEY_ALREADY_PRESENT; if overwrite is true but the key cannot be overwritten, SSH_PUBLICKEY_ACCESS_DENIED. If the server does not implement a critical attribute it MUST fail the add with SSH_PUBLICKEY_ATTRIBUTE_NOT_SUPPORTED; merely storing it is not enough.
remove sends the algorithm name and key blob, and the server MUST remove that key so it cannot authenticate later. list returns zero or more publickey responses in no particular order, followed by a status packet; implementations SHOULD support it. listattributes returns zero or more attribute responses, each with a compulsory flag showing whether the server applies that attribute to all added keys because of administrative settings, then a status packet. Supporting it is optional.
You can hand in a new key, take one back, ask to see all your keys, or ask what special rules the door understands.
ClientServer
- version 2Both sides send their highest version first.
- version 2
- addAlgorithm, key blob, overwrite flag, attributes.
- status SSH_PUBLICKEY_SUCCESS
§6Key Attributes
Attributes attach restrictions or notes to a key. comment holds user text in UTF-8 that the server must not act upon, and comment-language must immediately follow a comment. Neither should be critical.
Restrictions, mostly marked SHOULD be critical, include command-override (run a fixed command instead of any requested shell or command; empty denies both), subsystem (a comma-separated list of allowed subsystems), x11, shell, exec, agent and env (deny that kind of request), from (a comma-separated list of hosts the key may be used from; others MUST be denied), port-forward (only listed hosts for direct-tcpip) and reverse-forward (only listed ports for tcpip-forward).
Servers MAY also let administrators enforce certain attributes compulsorily. Names can be extended locally with the form name@domainname.
A key can come with rules attached, such as "only works from this computer" or "can only run this one task". The server must either follow a rule or refuse the key.
§7Security Considerations
The protocol assumes it runs over a secure channel with authenticated endpoints, and so is externally protected from network-level attacks. Because it lets authentication data be uploaded and changed, the server must enforce access controls. Users can overwrite an existing key with fewer restrictions than before, so servers should take care that clients cannot override presets set by the administrator.
Clients must also trust the server to implement attributes correctly; a server bug could authorize a key for more access than intended or apply fewer restrictions.
Because this lets you change who may enter, the server must double-check what you are allowed to change, and you have to trust it to follow your rules.
§8IANA Registrations
The RFC registers the subsystem name publickey and creates registries for request names, response names, attribute names and status codes. Names must be printable US-ASCII, must not contain at-sign, comma, whitespace or control characters, are case-sensitive and at most 64 characters. New names need IETF Consensus.
Status codes range from 0 to 255. New codes in the range 0 to 191 need Standards Action, and 192 to 255 is private use. The initial codes run from SSH_PUBLICKEY_SUCCESS (0) to SSH_PUBLICKEY_ATTRIBUTE_NOT_SUPPORTED (9).
There is an official list of the words this protocol uses so nobody invents clashing ones, plus a private corner for personal extras.
§8References
[1]RFC 4251Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol Architecture", RFC 4251, January 2006.January 2006[2]RFC 4253Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, January 2006.January 2006[3]RFC 4252Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Authentication Protocol", RFC 4252, January 2006.January 2006[4]RFC 4254Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Connection Protocol", RFC 4254, January 2006.January 2006[5]RFC 2119Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.March 1997[8]RFC 4250Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH) Protocol Assigned Numbers", RFC 4250, January 2006.January 2006