P6R's Secure Shell Public Key Subsystem

Abstract

The Secure Shell (SSH) Public Key Subsystem protocol defines a key distribution protocol that is limited to provisioning an SSH server with a user's public keys. This document describes a new protocol that builds on the protocol defined in RFC 4819 to allow the provisioning of keys and certificates to a server using the SSH transport.

The new protocol allows the calling client to organize keys and certificates in different namespaces on a server. These namespaces can be used by the server to allow a client to configure any application running on the server (e.g., SSH, Key Management Interoperability Protocol (KMIP), Simple Network Management Protocol (SNMP)).

The new protocol provides a server-independent mechanism for clients to add public keys, remove public keys, add certificates, remove certificates, and list the current set of keys and certificates known by the server by namespace (e.g., list all public keys in the SSH namespace).

Rights to manage keys and certificates in a particular namespace are specific and limited to the authorized user and are defined as part of the server's implementation. The described protocol is backward compatible to version 2 defined by RFC 4819.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7076.

Copyright Notice

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Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. Overview of Extensions to the Public Key Subsystem ..............3
      3.1. Extended Status Codes ......................................4
      3.2. The Version Packet .........................................4
      3.3. The Namespace Attribute ....................................4
   4. New Operations ..................................................5
      4.1. Adding a Certificate .......................................5
      4.2. Removing a Certificate .....................................6
      4.3. Listing Certificates .......................................6
      4.4. Listing Namespaces .........................................7
   5. Extending Public Key Operations .................................8
      5.1. Adding a Public Key ........................................8
      5.2. Removing a Public Key ......................................8
      5.3. Listing Public Keys ........................................9
   6. Security Considerations .........................................9
   7. IANA Considerations ............................................10
   8. References .....................................................10
      8.1. Normative References ......................................10
      8.2. Informative References ....................................10

1. Introduction

This document describes a new protocol that builds on the protocol defined in RFC 4819 that can be used to configure public keys and certificates in an implementation-independent fashion. The concept of a namespace is added to the protocol's operations; it allows the client to organize keys and certificates by application or organizational structure.

P6R's Secure Shell Public Key Subsystem has been designed to run on top of the Secure Shell transport layer [3] and user authentication protocols [4]. It provides a simple mechanism for the client to manage the public keys and certificates on the server related to that client. These keys and certificates are normally used for authentication of the client to a service, but they can be used for encrypting results back to the client as well. Uploaded keys and certificates are meant to be able to configure all protocols running on a server (e.g., SSH, SSL, KMIP [8]) that use keys and certificates, as well as the applications that run on a server.

This document should be read only after reading the Secure Shell Public Key Subsystem [1] document. The new protocol described in this document builds on and is meant to be backwards compatible with the protocol described in [1].

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 [2].

3. Overview of Extensions to the Public Key Subsystem

The Public Key Subsystem provides a server-independent mechanism for clients to add public keys, remove public keys, list the current public keys known by the server, add certificates, remove certificates, and list the current set of certificates known by the server. This secure key distribution mechanism is implemented by a new SSH subsystem with the name of "publickey@p6r.com".

3.1. Extended 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_CERTIFICATE_NOT_FOUND        192
        SSH_PUBLICKEY_CERTIFICATE_NOT_SUPPORTED    193
        SSH_PUBLICKEY_CERTIFICATE_ALREADY_PRESENT  194
        SSH_PUBLICKEY_ACTION_NOT_AUTHORIZED        195
        SSH_PUBLICKEY_CANNOT_CREATE_NAMESPACE      196

The meaning of the failure codes is as implied by their names. See Security Considerations for the use of the failure code: SSH_PUBLICKEY_ACTION_NOT_AUTHORIZED.

3.2. 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 defines version 3 of the new protocol. We are using version 3 so that it can be backward compatible with the protocol defined by RFC 4819 [1].

3.3. The Namespace Attribute

The "namespace" attribute is added as an extension to what was described in RFC 4819. The purpose of this attribute is to be able to organize the uploaded keys and certificates into groups where each group represents an application or organization structure. This attribute is a string that should not be longer than 300 characters and MUST be specified in UTF-8 format [5].

This new protocol uses the "ssh" namespace for the manipulation of public keys in an SSH server and should be considered as the default namespace when none is provided.

As a convention, namespaces used for protocols are lowercase strings of the protocol's standard abbreviation. For example, "ssl" should be the namespace used for the Secure Sockets Layer protocol. Namespaces for applications should contain the product and vendor's name. To help determine what namespaces already exist on a server, a new operation "list-namespaces" is defined in Section 4.

4. New Operations

P6R's Public Key Subsystem extends the functionality defined in RFC 4819 with the following operations: add-certificate, remove-certificate, list-certificates, and list-namespaces.

4.1. Adding a Certificate

If the client wishes to add a certificate, the client sends:

        string    "add-certificate"
        string    certificate format name
        string    certificate blob
        boolean   overwrite
        uint32    attribute-count
         string    attrib-name
         string    attrib-value
         bool      critical
       repeated attribute-count times

This request MUST include at least the "namespace" attribute so that the server knows where to save the certificate. Only one namespace attribute can be used per an add-certificate request. It is possible for the same user to save the same certificate into multiple namespaces, but this must be done with several separate add-certificate requests.

If the namespace appearing in an add-certificate request does not already exist on a server, then it is created by this operation. However, if the user is not authorized to create a namespace, the server MUST return SSH_PUBLICKEY_CANNOT_CREATE_NAMESPACE.

If the overwrite field is false and the specified certificate already exists in the given namespace, the server MUST return SSH_PUBLICKEY_CERTIFICATE_ALREADY_PRESENT. If the server returns this, the client SHOULD provide an option to the user to overwrite the certificate. If the overwrite field is true and the specified key already exists in the given namespace but cannot be overwritten, the server MUST return SSH_PUBLICKEY_ACCESS_DENIED.

However, a user may not be authorized to add a certificate to the specified namespace. If the user does not have permission to add a certificate, then the server MUST return SSH_PUBLICKEY_ACTION_NOT_AUTHORIZED.

Examples of possible "certificate format names" are: "X509", "pgp-sign-rsa", and "pgp-sign-dss". The format of the public key and certificate blobs are detailed in Section 6.6, "Public Key

Algorithms", of the SSH Transport Protocol document [3], where X.509 certificates are to be encoded using a DER format [6] [7] in a certificate blob.

4.2. Removing a Certificate

If the client wishes to remove a certificate, the client sends:

        string    "remove-certificate"
        string    certificate format name
        string    certificate blob
        uint32    attribute-count
         string    attrib-name
         string    attrib-value
        repeated attribute-count times

This request MUST include at least the "namespace" attribute so that the server knows from where to delete the certificate. Only one namespace attribute can be used per remove-certificate request. The server MUST attempt to remove the certificate from the appropriate location.

However, a user may not be authorized to remove a certificate from the specified namespace. If the user does not have permission to remove the certificate, then the server MUST return SSH_PUBLICKEY_ACTION_NOT_AUTHORIZED.

Examples of possible "certificate format names" are: "X509", "pgp-sign-rsa", and "pgp-sign-dss".

4.3. Listing Certificates

If the client wishes to list the known certificates, the client sends:

        string    "list-certificates"

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

        string    "certificate"
        string    certificate format name
        string    certificate 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.

This response MUST include at least the "namespace" attribute so that a client can tell in which namespace the certificate resides. Only one namespace attribute can be used per list-certificate request.

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

4.4. Listing Namespaces

If the client wishes to know existing namespaces on the server, it sends:

        string    "list-namespaces"

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

        string    "namespace"
        string    namespace name

It is possible that not all namespaces will be visible to every authenticated user. In this case, the responding server will return a subset of existing namespaces. See Security Considerations below.

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

5. Extending Public Key Operations

In addition to adding new operations, this document describes extensions to the operations defined in RFC 4819.

5.1. 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

This request MAY include one "namespace" attribute so that a client can save the public key into a specific namespace. It is possible for the same user to save the same key into multiple namespaces, but this requires multiple add requests.

If the namespace appearing in an add public key request does not already exist on a server, then it is created by this operation. However, if the user is not authorized to create a namespace the server MUST return SSH_PUBLICKEY_CANNOT_CREATE_NAMESPACE,

5.2. 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
        uint32    attribute-count
         string    attrib-name
         string    attrib-value
         bool      critical
        repeated attribute-count times

This extension allows attributes to be added to a remove request. This request MAY include one "namespace" attribute so that a client can remove the public key from a specific namespace.

5.3. Listing Public Keys

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

        string    "list"
        uint32    attribute-count
         string    attrib-name
         string    attrib-value
         bool      critical
        repeated attribute-count times

This extension allows attributes to be added to a list request. This request MAY include one "namespace" attribute so that a client can list the public keys from a specific namespace.

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

This response MAY include the "namespace" attribute so that a client can tell in which namespace the key resides.

6. 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 key and certificate material to be uploaded and manipulated into a server application. It is the responsibility of the server implementation to enforce access controls that may be required to limit any particular user's access to the data in a namespace. For example, one user may be allowed to list only the contents of a namespace but not add or remove keys or certificates to/from it. The server MUST return SSH_PUBLICKEY_ACTION_NOT_AUTHORIZED when a user's action goes against its defined access controls.

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 and certificates for access they were not intended to have, or to apply fewer restrictions than were intended.

7. IANA Considerations

Although Section 3.1 defines four new status codes, these are in the 'Private Use' range of IANA's Publickey Subsystem Status Codes registry as defined by Section 6.6.1 ("Conventions") in [1]. No IANA actions are required for this document.

8. References

8.1. Normative References

   [1] Galbraith, J., Van Dyke, J., and J. Bright, "Secure Shell Public
       Key Subsystem", RFC 4819, March 2007.
   
   [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
   
   [3] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Transport
       Layer Protocol", RFC 4253, January 2006.
   
   [4] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
       Authentication Protocol", RFC 4252, January 2006.
   
   [5] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
       63, RFC 3629, November 2003.
   
   [6] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R.,
       and W. Polk, "Internet X.509 Public Key Infrastructure
       Certificate and Certificate Revocation List (CRL) Profile", RFC
       5280, May 2008.
   
   [7] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
       Information technology -- ASN.1 encoding rules: Specification of
       Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and
       Distinguished Encoding Rules (DER).

8.2. Informative References

[8] OASIS, "Key Management Interoperability Protocol (KMIP) 1.1",

       January 2013, <http://docs.oasis-open.org/kmip/spec/v1.1/os/
       kmip-spec-v1.1-os.html>.

Authors' Addresses

Mark Joseph, PhD
P6R, Inc
1840 41st Ave
Suite 102-139
Capitola, CA 95010
US

   Phone: +1 888 452 2580 (x702)
   EMail: mark@p6r.com

Jim Susoy
P6R, Inc
1840 41st Ave
Suite 102-139
Capitola, CA 95010
US

   Phone: +1 888 452 2580 (x701)
   EMail: jim@p6r.com