Secure Shell Working Group J. Galbraith Internet-Draft J. Van Dyke Expires:
March 15,October 1, 2004 B. McClure VanDyke Software J. Bright Silicon Circus September 15, 2003April 2, 2004 Secure Shell Public-Key Subsystem draft-ietf-secsh-publickey-subsystem-00.txtdraft-ietf-secsh-publickey-subsystem-01.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 15,October 1, 2004. Copyright Notice Copyright (C) The Internet Society (2003).(2004). 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  as a subsystem, as described in Section ``Starting a Shell or a Command''. The subsystem name used with this protocol is "publickey". 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. 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 . . . . . . . . . . . . . . . . . . . 10 3.4 Listing public keys . . . . . . . . . . . . . . . . . . . . 10 3.5 Listing server capabilities . . . . . . . . . . . . . . . . 10 4. Security Considerations . . . . . . . . . . . . . . . . . . 12 Normative References . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 13 Intellectual Property and Copyright Statements . . . . . . . 15 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  and user authentication protocols . 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  and SECSH connection  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. 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". 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 . 2.1 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  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 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. 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 acknowledgement of each request prior to sending a new request. All requests described in Section 3 are a description of the 'request-name' and 'data' portion 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. 18.104.22.168 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 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 2. 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. 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 . 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 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 . 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 . 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-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 authorisation checks the server may perform). This attribute SHOULD be mandatory. "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 mandatory. If the value is empty, no subsystems may be started. "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 mandatory. "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 mandatory. "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 mandatory. "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 mandatory. "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 mandatory. "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 authorisation purposes, the authorisation attempt MUST be denied. The server SHOULD make a log entry regarding this. "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 specified by the client, the server MAY provide a method for administrators to compulsorily enforce certain attributes. 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. 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 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. 4. 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 ). 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 authorise keys for access they were not intended to have, or to apply fewer restrictions than were intended. Normative References  Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S. Lehtinen, "SSH Protocol Architecture", draft-ietf-secsh-architecture-13 (work in progress), January 2002.  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.  Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S. Lehtinen, "SSH Authentication Protocol", draft-ietf-secsh-userauth-16 (work in progress), February 2002.  Ylonen, T., Kivinen, T., Saarinen, M., Rinne, T. and S. Lehtinen, "SSH Connection Protocol", draft-ietf-secsh-connect-16 (work in progress), January 2002.  Alvestrand, H., "Tags for the Identification of Languages", RFC 1766, March 1995.  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: firstname.lastname@example.org Jeff P. Van Dyke VanDyke Software 4848 Tramway Ridge Blvd Suite 101 Albuquerque, NM 87111 US Phone: +1 505 332 5700 EMail: email@example.com Brent McClure VanDyke Software 4848 Tramway Ridge Blvd Suite 101 Albuquerque, NM 87111 US Phone: +1 505 332 5700 EMail: firstname.lastname@example.org Jon Bright Silicon Circus 24 Jubilee Road Chichester, West Sussex PO19 7XB UK Phone: +49 172 524 0521 EMail: email@example.com Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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