Network Working Group
Request for Comments: 4252
Category: Standards Track
T. Ylonen
SSH Communications Security Corp
C. Lonvick, Ed.
Cisco Systems, Inc.
January 2006

The Secure Shell (SSH) Authentication Protocol

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 © The Internet Society (2006).


The Secure Shell Protocol (SSH) is a protocol for secure remote login and other secure network services over an insecure network. This document describes the SSH authentication protocol framework and public key, password, and host-based client authentication methods. Additional authentication methods are described in separate documents. The SSH authentication protocol runs on top of the SSH transport layer protocol and provides a single authenticated tunnel for the SSH connection protocol.

Table of Contents

   1. Introduction ....................................................2
   2. Contributors ....................................................3
   3. Conventions Used in This Document ...............................3
   4. The Authentication Protocol Framework ...........................4
   5. Authentication Requests .........................................4
      5.1. Responses to Authentication Requests .......................5
      5.2. The "none" Authentication Request ..........................7
      5.3. Completion of User Authentication ..........................7
      5.4. Banner Message .............................................7
   6. Authentication Protocol Message Numbers .........................8
   7. Public Key Authentication Method: "publickey" ...................8
   8. Password Authentication Method: "password" .....................10
   9. Host-Based Authentication: "hostbased" .........................12
   10. IANA Considerations ...........................................14
   11. Security Considerations .......................................14
   12. References ....................................................15
      12.1. Normative References .....................................15
      12.2. Informative References ...................................15
   Authors' Addresses ................................................16
   Trademark Notice ..................................................16

1. Introduction

The SSH authentication protocol is a general-purpose user authentication protocol. It is intended to be run over the SSH transport layer protocol [SSH-TRANS]. This protocol assumes that the underlying protocols provide integrity and confidentiality protection.

This document should be read only after reading the SSH architecture document [SSH-ARCH]. This document freely uses terminology and notation from the architecture document without reference or further explanation.

The 'service name' for this protocol is "ssh-userauth".

When this protocol starts, it receives the session identifier from the lower-level protocol (this is the exchange hash H from the first key exchange). The session identifier uniquely identifies this session and is suitable for signing in order to prove ownership of a private key. This protocol also needs to know whether the lower- level protocol provides confidentiality protection.

2. Contributors

The major original contributors of this set of documents have been: Tatu Ylonen, Tero Kivinen, Timo J. Rinne, Sami Lehtinen (all of SSH Communications Security Corp), and Markku-Juhani O. Saarinen (University of Jyvaskyla). Darren Moffat was the original editor of this set of documents and also made very substantial contributions.

Many people contributed to the development of this document over the years. People who should be acknowledged include Mats Andersson, Ben Harris, Bill Sommerfeld, Brent McClure, Niels Moller, Damien Miller, Derek Fawcus, Frank Cusack, Heikki Nousiainen, Jakob Schlyter, Jeff Van Dyke, Jeffrey Altman, Jeffrey Hutzelman, Jon Bright, Joseph Galbraith, Ken Hornstein, Markus Friedl, Martin Forssen, Nicolas Williams, Niels Provos, Perry Metzger, Peter Gutmann, Simon Josefsson, Simon Tatham, Wei Dai, Denis Bider, der Mouse, and Tadayoshi Kohno. Listing their names here does not mean that they endorse this document, but that they have contributed to it.

3. Conventions Used in This Document

All documents related to the SSH protocols shall use the keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" to describe requirements. These keywords are to be interpreted as described in [RFC2119].

The keywords "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in this document when used to describe namespace allocation are to be interpreted as described in [RFC2434].

Protocol fields and possible values to fill them are defined in this set of documents. Protocol fields will be defined in the message definitions. As an example, SSH_MSG_CHANNEL_DATA is defined as follows.

      byte      SSH_MSG_CHANNEL_DATA
      uint32    recipient channel
      string    data

Throughout these documents, when the fields are referenced, they will appear within single quotes. When values to fill those fields are referenced, they will appear within double quotes. Using the above example, possible values for 'data' are "foo" and "bar".

4. The Authentication Protocol Framework

The server drives the authentication by telling the client which authentication methods can be used to continue the exchange at any given time. The client has the freedom to try the methods listed by the server in any order. This gives the server complete control over the authentication process if desired, but also gives enough flexibility for the client to use the methods it supports or that are most convenient for the user, when multiple methods are offered by the server.

Authentication methods are identified by their name, as defined in [SSH-ARCH]. The "none" method is reserved, and MUST NOT be listed as supported. However, it MAY be sent by the client. The server MUST always reject this request, unless the client is to be granted access without any authentication, in which case, the server MUST accept this request. The main purpose of sending this request is to get the list of supported methods from the server.

The server SHOULD have a timeout for authentication and disconnect if the authentication has not been accepted within the timeout period. The RECOMMENDED timeout period is 10 minutes. Additionally, the implementation SHOULD limit the number of failed authentication attempts a client may perform in a single session (the RECOMMENDED limit is 20 attempts). If the threshold is exceeded, the server SHOULD disconnect.

Additional thoughts about authentication timeouts and retries may be found in [ssh-1.2.30].

5. Authentication Requests

All authentication requests MUST use the following message format. Only the first few fields are defined; the remaining fields depend on the authentication method.

      string    user name in ISO-10646 UTF-8 encoding [RFC3629]
      string    service name in US-ASCII
      string    method name in US-ASCII
      ....      method specific fields

The 'user name' and 'service name' are repeated in every new authentication attempt, and MAY change. The server implementation MUST carefully check them in every message, and MUST flush any accumulated authentication states if they change. If it is unable to flush an authentication state, it MUST disconnect if the 'user name' or 'service name' changes.

The 'service name' specifies the service to start after authentication. There may be several different authenticated services provided. If the requested service is not available, the server MAY disconnect immediately or at any later time. Sending a proper disconnect message is RECOMMENDED. In any case, if the service does not exist, authentication MUST NOT be accepted.

If the requested 'user name' does not exist, the server MAY disconnect, or MAY send a bogus list of acceptable authentication 'method name' values, but never accept any. This makes it possible for the server to avoid disclosing information on which accounts exist. In any case, if the 'user name' does not exist, the authentication request MUST NOT be accepted.

While there is usually little point for clients to send requests that the server does not list as acceptable, sending such requests is not an error, and the server SHOULD simply reject requests that it does not recognize.

An authentication request MAY result in a further exchange of messages. All such messages depend on the authentication 'method name' used, and the client MAY at any time continue with a new SSH_MSG_USERAUTH_REQUEST message, in which case the server MUST abandon the previous authentication attempt and continue with the new one.

The following 'method name' values are defined.

      "publickey"             REQUIRED
      "password"              OPTIONAL
      "hostbased"             OPTIONAL
      "none"                  NOT RECOMMENDED

Additional 'method name' values may be defined as specified in [SSH-ARCH] and [SSH-NUMBERS].

5.1. Responses to Authentication Requests

If the server rejects the authentication request, it MUST respond with the following:

      byte         SSH_MSG_USERAUTH_FAILURE
      name-list    authentications that can continue
      boolean      partial success

The 'authentications that can continue' is a comma-separated name- list of authentication 'method name' values that may productively continue the authentication dialog.

It is RECOMMENDED that servers only include those 'method name' values in the name-list that are actually useful. However, it is not illegal to include 'method name' values that cannot be used to authenticate the user.

Already successfully completed authentications SHOULD NOT be included in the name-list, unless they should be performed again for some reason.

The value of 'partial success' MUST be TRUE if the authentication request to which this is a response was successful. It MUST be FALSE if the request was not successfully processed.

When the server accepts authentication, it MUST respond with the following:


Note that this is not sent after each step in a multi-method authentication sequence, but only when the authentication is complete.

The client MAY send several authentication requests without waiting for responses from previous requests. The server MUST process each request completely and acknowledge any failed requests with a SSH_MSG_USERAUTH_FAILURE message before processing the next request.

A request that requires further messages to be exchanged will be aborted by a subsequent request. A client MUST NOT send a subsequent request if it has not received a response from the server for a previous request. A SSH_MSG_USERAUTH_FAILURE message MUST NOT be sent for an aborted method.

SSH_MSG_USERAUTH_SUCCESS MUST be sent only once. When SSH_MSG_USERAUTH_SUCCESS has been sent, any further authentication requests received after that SHOULD be silently ignored.

Any non-authentication messages sent by the client after the request that resulted in SSH_MSG_USERAUTH_SUCCESS being sent MUST be passed to the service being run on top of this protocol. Such messages can be identified by their message numbers (see Section 6).

5.2. The "none" Authentication Request

A client may request a list of authentication 'method name' values that may continue by using the "none" authentication 'method name'.

If no authentication is needed for the user, the server MUST return SSH_MSG_USERAUTH_SUCCESS. Otherwise, the server MUST return SSH_MSG_USERAUTH_FAILURE and MAY return with it a list of methods that may continue in its 'authentications that can continue' value.

This 'method name' MUST NOT be listed as supported by the server.

5.3. Completion of User Authentication

Authentication is complete when the server has responded with SSH_MSG_USERAUTH_SUCCESS. All authentication related messages received after sending this message SHOULD be silently ignored.

After sending SSH_MSG_USERAUTH_SUCCESS, the server starts the requested service.

5.4. Banner Message

In some jurisdictions, sending a warning message before authentication may be relevant for getting legal protection. Many UNIX machines, for example, normally display text from /etc/issue, use TCP wrappers, or similar software to display a banner before issuing a login prompt.

The SSH server may send an SSH_MSG_USERAUTH_BANNER message at any time after this authentication protocol starts and before authentication is successful. This message contains text to be displayed to the client user before authentication is attempted. The format is as follows:

      string    message in ISO-10646 UTF-8 encoding [RFC3629]
      string    language tag [RFC3066]

By default, the client SHOULD display the 'message' on the screen. However, since the 'message' is likely to be sent for every login attempt, and since some client software will need to open a separate window for this warning, the client software may allow the user to explicitly disable the display of banners from the server. The 'message' may consist of multiple lines, with line breaks indicated by CRLF pairs.

If the 'message' string is displayed, control character filtering, discussed in [SSH-ARCH], SHOULD be used to avoid attacks by sending terminal control characters.

6. Authentication Protocol Message Numbers

All message numbers used by this authentication protocol are in the range from 50 to 79, which is part of the range reserved for protocols running on top of the SSH transport layer protocol.

Message numbers of 80 and higher are reserved for protocols running after this authentication protocol, so receiving one of them before authentication is complete is an error, to which the server MUST respond by disconnecting, preferably with a proper disconnect message sent to ease troubleshooting.

After successful authentication, such messages are passed to the higher-level service.

These are the general authentication message codes:

      SSH_MSG_USERAUTH_REQUEST            50
      SSH_MSG_USERAUTH_FAILURE            51
      SSH_MSG_USERAUTH_SUCCESS            52
      SSH_MSG_USERAUTH_BANNER             53

In addition to the above, there is a range of message numbers (60 to 79) reserved for method-specific messages. These messages are only sent by the server (client sends only SSH_MSG_USERAUTH_REQUEST messages). Different authentication methods reuse the same message numbers.

7. Public Key Authentication Method: "publickey"

The only REQUIRED authentication 'method name' is "publickey" authentication. All implementations MUST support this method; however, not all users need to have public keys, and most local policies are not likely to require public key authentication for all users in the near future.

With this method, the possession of a private key serves as authentication. This method works by sending a signature created with a private key of the user. The server MUST check that the key is a valid authenticator for the user, and MUST check that the signature is valid. If both hold, the authentication request MUST be accepted; otherwise, it MUST be rejected. Note that the server MAY require additional authentications after successful authentication.

Private keys are often stored in an encrypted form at the client host, and the user must supply a passphrase before the signature can be generated. Even if they are not, the signing operation involves some expensive computation. To avoid unnecessary processing and user interaction, the following message is provided for querying whether authentication using the "publickey" method would be acceptable.

      string    user name in ISO-10646 UTF-8 encoding [RFC3629]
      string    service name in US-ASCII
      string    "publickey"
      boolean   FALSE
      string    public key algorithm name
      string    public key blob

Public key algorithms are defined in the transport layer specification [SSH-TRANS]. The 'public key blob' may contain certificates.

Any public key algorithm may be offered for use in authentication. In particular, the list is not constrained by what was negotiated during key exchange. If the server does not support some algorithm, it MUST simply reject the request.

The server MUST respond to this message with either SSH_MSG_USERAUTH_FAILURE or with the following:

      byte      SSH_MSG_USERAUTH_PK_OK
      string    public key algorithm name from the request
      string    public key blob from the request

To perform actual authentication, the client MAY then send a signature generated using the private key. The client MAY send the signature directly without first verifying whether the key is acceptable. The signature is sent using the following packet:

      string    user name
      string    service name
      string    "publickey"
      boolean   TRUE
      string    public key algorithm name
      string    public key to be used for authentication
      string    signature

The value of 'signature' is a signature by the corresponding private key over the following data, in the following order:

      string    session identifier
      string    user name
      string    service name
      string    "publickey"
      boolean   TRUE
      string    public key algorithm name
      string    public key to be used for authentication

When the server receives this message, it MUST check whether the supplied key is acceptable for authentication, and if so, it MUST check whether the signature is correct.

If both checks succeed, this method is successful. Note that the server may require additional authentications. The server MUST respond with SSH_MSG_USERAUTH_SUCCESS (if no more authentications are needed), or SSH_MSG_USERAUTH_FAILURE (if the request failed, or more authentications are needed).

The following method-specific message numbers are used by the "publickey" authentication method.

      SSH_MSG_USERAUTH_PK_OK              60

8. Password Authentication Method: "password"

Password authentication uses the following packets. Note that a server MAY request that a user change the password. All implementations SHOULD support password authentication.

      string    user name
      string    service name
      string    "password"
      boolean   FALSE
      string    plaintext password in ISO-10646 UTF-8 encoding [RFC3629]

Note that the 'plaintext password' value is encoded in ISO-10646 UTF-8. It is up to the server how to interpret the password and validate it against the password database. However, if the client reads the password in some other encoding (e.g., ISO 8859-1 - ISO Latin1), it MUST convert the password to ISO-10646 UTF-8 before transmitting, and the server MUST convert the password to the encoding used on that system for passwords.

From an internationalization standpoint, it is desired that if a user enters their password, the authentication process will work regardless of what OS and client software the user is using. Doing so requires normalization. Systems supporting non-ASCII passwords SHOULD always normalize passwords and user names whenever they are added to the database, or compared (with or without hashing) to existing entries in the database. SSH implementations that both store the passwords and compare them SHOULD use [RFC4013] for normalization.

Note that even though the cleartext password is transmitted in the packet, the entire packet is encrypted by the transport layer. Both the server and the client should check whether the underlying transport layer provides confidentiality (i.e., if encryption is being used). If no confidentiality is provided ("none" cipher), password authentication SHOULD be disabled. If there is no confidentiality or no MAC, password change SHOULD be disabled.

Normally, the server responds to this message with success or failure. However, if the password has expired, the server SHOULD indicate this by responding with SSH_MSG_USERAUTH_PASSWD_CHANGEREQ. In any case, the server MUST NOT allow an expired password to be used for authentication.

      string    prompt in ISO-10646 UTF-8 encoding [RFC3629]
      string    language tag [RFC3066]

In this case, the client MAY continue with a different authentication method, or request a new password from the user and retry password authentication using the following message. The client MAY also send this message instead of the normal password authentication request without the server asking for it.

      string    user name
      string    service name
      string    "password"
      boolean   TRUE
      string    plaintext old password in ISO-10646 UTF-8 encoding
      string    plaintext new password in ISO-10646 UTF-8 encoding

The server must reply to each request message with SSH_MSG_USERAUTH_SUCCESS, SSH_MSG_USERAUTH_FAILURE, or another SSH_MSG_USERAUTH_PASSWD_CHANGEREQ. The meaning of these is as follows:

SSH_MSG_USERAUTH_SUCCESS - The password has been changed, and authentication has been successfully completed.

SSH_MSG_USERAUTH_FAILURE with partial success - The password has been changed, but more authentications are needed.

SSH_MSG_USERAUTH_FAILURE without partial success - The password has not been changed. Either password changing was not supported, or the old password was bad. Note that if the server has already sent SSH_MSG_USERAUTH_PASSWD_CHANGEREQ, we know that it supports changing the password.

SSH_MSG_USERAUTH_CHANGEREQ - The password was not changed because the new password was not acceptable (e.g., too easy to guess).

The following method-specific message numbers are used by the password authentication method.


9. Host-Based Authentication: "hostbased"

Some sites wish to allow authentication based on the host that the user is coming from and the user name on the remote host. While this form of authentication is not suitable for high-security sites, it can be very convenient in many environments. This form of authentication is OPTIONAL. When used, special care SHOULD be taken to prevent a regular user from obtaining the private host key.

The client requests this form of authentication by sending the following message. It is similar to the UNIX "rhosts" and "hosts.equiv" styles of authentication, except that the identity of the client host is checked more rigorously.

This method works by having the client send a signature created with the private key of the client host, which the server checks with that host's public key. Once the client host's identity is established, authorization (but no further authentication) is performed based on the user names on the server and the client, and the client host name.

      string    user name
      string    service name
      string    "hostbased"
      string    public key algorithm for host key
      string    public host key and certificates for client host
      string    client host name expressed as the FQDN in US-ASCII
      string    user name on the client host in ISO-10646 UTF-8 encoding
      string    signature

Public key algorithm names for use in 'public key algorithm for host key' are defined in the transport layer specification [SSH-TRANS]. The 'public host key and certificates for client host' may include certificates.

The value of 'signature' is a signature with the private host key of the following data, in this order:

      string    session identifier
      string    user name
      string    service name
      string    "hostbased"
      string    public key algorithm for host key
      string    public host key and certificates for client host
      string    client host name expressed as the FQDN in US-ASCII
      string    user name on the client host in ISO-10646 UTF-8 encoding

The server MUST verify that the host key actually belongs to the client host named in the message, that the given user on that host is allowed to log in, and that the 'signature' value is a valid signature on the appropriate value by the given host key. The server MAY ignore the client 'user name', if it wants to authenticate only the client host.

Whenever possible, it is RECOMMENDED that the server perform additional checks to verify that the network address obtained from the (untrusted) network matches the given client host name. This makes exploiting compromised host keys more difficult. Note that this may require special handling for connections coming through a firewall.

10. IANA Considerations

   This document is part of a set.  The IANA considerations for the SSH
   protocol, as defined in [SSH-ARCH], [SSH-TRANS], [SSH-CONNECT], and
   this document, are detailed in [SSH-NUMBERS].

11. Security Considerations

The purpose of this protocol is to perform client user authentication. It assumed that this runs over a secure transport layer protocol, which has already authenticated the server machine, established an encrypted communications channel, and computed a unique session identifier for this session. The transport layer provides forward secrecy for password authentication and other methods that rely on secret data.

Full security considerations for this protocol are provided in [SSH-ARCH].

12. References

12.1. Normative References

   [SSH-ARCH]    Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
                 Protocol Architecture", RFC 4251, January 2006.
   [SSH-CONNECT] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
                 Connection Protocol", RFC 4254, January 2006.
   [SSH-TRANS]   Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
                 Transport Layer Protocol", RFC 4253, January 2006.
   [SSH-NUMBERS] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell
                 (SSH) Protocol Assigned Numbers", RFC 4250, January
   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.
   [RFC2434]     Narten, T. and H. Alvestrand, "Guidelines for Writing
                 an IANA Considerations Section in RFCs", BCP 26, RFC
                 2434, October 1998.
   [RFC3066]     Alvestrand, H., "Tags for the Identification of
                 Languages", BCP 47, RFC 3066, January 2001.
   [RFC3629]     Yergeau, F., "UTF-8, a transformation format of ISO
                 10646", STD 63, RFC 3629, November 2003.
   [RFC4013]     Zeilenga, K., "SASLprep: Stringprep Profile for User
                 Names and Passwords", RFC 4013, February 2005.

12.2. Informative References

   [ssh-1.2.30]  Ylonen, T., "ssh-1.2.30/RFC", File within compressed
                 ssh/ssh-1.2.30.tar.gz, November 1995.

Authors' Addresses

Tatu Ylonen
SSH Communications Security Corp
Valimotie 17
00380 Helsinki


   Chris Lonvick (editor)
   Cisco Systems, Inc.
   12515 Research Blvd.
   Austin  78759


Trademark Notice

"ssh" is a registered trademark in the United States and/or other countries.

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Copyright © The Internet Society (2006).

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