Negotiation of Extra Security Context Tokens for Kerberos V5 Generic Security Services Mechanism
draft-williams-kitten-krb5-extra-rt-03

Abstract

This Internet-Draft proposes an extension to the Kerberos V5 security mechanism for the Generic Security Services Application Programming Interface (GSS-API) for using extra security context tokens in order to recover from certain errors. Other benefits include: user-to-user authentication, authenticated errors, replay cache avoidance, and others.

Status of This Memo

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This Internet-Draft will expire on April 30, 2015.

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Table of Contents

• 1. Introduction
• 1.1. Conventions used in this document
• 2. Negotiation
• 2.1. Error Recovery
• 2.2. Number of Security Context Tokens
• 2.2.1. Security Token Sequences
• 2.3. PROT_READY
• 2.4. Per-Message Token Sequence Numbers
• 2.4.1. Negotiation Issues for User-to-User Authentication
• 3. ASN.1 for New Protocol Elements
• 4. Replay Cache Avoidance
• 5. User-to-User Authentication
• 6. Other Requirements, Recommendations, and Non-Requirements
• 7. Security Considerations
• 8. IANA Considerations
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Authors' Addresses

1. Introduction

The Kerberos V5 [RFC4120] AP protocol, and therefore the Kerberos V5 GSS-API [RFC2743] mechanism [RFC4121] security context token exchange, is a one-round trip protocol. Occasionally there are errors that the protocol could recover from by using an additional round trip, but until now there was no way to execute such an additional round trip. For many application protocols the failure of the Kerberos AP protocol is fatal, requiring closing TCP connections and starting over; often there is no automatic recovery. This document proposes a negotiation of additional security context tokens for automatic recovery from certain errors. This is done in a backwards-compatible way, thus retaining the existing mechanism OID for the Kerberos V5 GSS mechanism. Additionally we add support for user-to-user authentication and authenticated errors, and provide a way to avoid the need for replay caching.

1.1. Conventions used in this document

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 [RFC2119].

2. Negotiation

We introduce the following new protocol elements. A partial ASN.1 [CCITT.X680.2002] module (for inclusion in the base Kerberos ASN.1 module) is given in Section 3, and references to its contents are made below.

• a new ap-options flag for use in the clear-text part of AP-REQs to indicate the desire for an extra round trip if need be;
• a new Authorization-Data element for use in Authenticators for quoting back a challenge nonce from the acceptor;
• a new PDU: KRB-ERROR2, with additional fields and support for authenticated errors.

No new interface is needed for GSS-API applications to use this feature. In the following text “initiator” refers to the mechanism's initiator functionality (invoked via GSS_Init_sec_context()), and “acceptor” refers to the mechanism's acceptor functionality (invoked via GSS_Accept_sec_context()).

To use this feature, the Kerberos GSS mechanism MUST act as follows:

• To request this feature, initiators SHALL add the new ap-options flag to their AP-REQs.
• Acceptors that wish to request an additional security context token can only do so when initiators indicate support for it, and MUST do so by returning a KRB-ERROR2. The encrypted part of the KRB-ERROR2 SHALL be encrypted in one of the following keys: the sub-session key from the AP-REQ's Authenticator if it could be decrypted, else the session key from the Ticket, if it could be decrypted, else the null enc-type/key.
• The KRB-ERROR2 in this case SHALL have a the continue-needed e-flag set when the acceptor is willing to consume another security context token from the initiator; the acceptor SHALL also return GSS_S_CONTINUE_NEEDED to the application in this case.
• Initiators that request this feature and receive a KRB-ERROR2 SHOULD attempt to recover.
• Initiators that request this feature and receive a KRB-ERROR2 with the continue-needed e-flag set SHOULD attempt to recover and MAY produce a token to send to the acceptor: either a KRB-ERROR2 if the initiator failed to recover, or a new AP-REQ (with the traditional GSS-API pseudo-ASN.1 mechanism OID header).
  • In the successful recovery case the initiator MUST quote the nonce from the KRB-ERROR2 using an AD-CHALLENGE-RESPONSE-NONCE (see below) authorization data element.

• When it consumes a KRB-ERROR2, GSS_Init_sec_context() can return an error (GSS_S_FAILURE), or attempt recovery and output a new AP-REQ security context token.
  • When GSS_Init_sec_context() outputs a new AP-REQ security context token, it SHALL return GSS_S_CONTINUE_NEEDED if the application requested mutual authentication and the previous acceptor security context token was an error (rather than a request for one more AP-REQ for replay cache avoidance; see Section 4), else it SHALL return GSS_S_COMPLETE.
  • When GSS_Init_sec_context() returns an error and the acceptor is awaiting a security context token, GSS_Init_sec_context() MAY generate a KRB-ERROR to send to the acceptor.

• Acceptors MUST reject additional AP-REQs which do not have a challenge response nonce matching the one sent by the acceptor in the previous KRB-ERROR2.
• Acceptors MUST reject initial security context tokens that contain a challenge response nonce.
• When GSS_Accept_sec_context() returns an error and outputs an error token, the token MUST be either a KRB-ERROR or a KRB-ERROR2, with the latter having the continue-needed flag cleared.

[CREF1]Perhaps we should use a checksum 0x8003 [RFC4121] extension instead of authorization data in the authenticator.

2.1. Error Recovery

The following Kerberos errors can be recovered from automatically using this protocol:

• KRB_AP_ERR_TKT_EXPIRED: the initiator should get a new service ticket;
• KRB_AP_ERR_TKT_NYV: the initiator should get a new service ticket;
• KRB_AP_ERR_REPEAT: the initiator should build a new AP-REQ;
• KRB_AP_ERR_SKEW: the initiator should build a new AP-REQ with time corrected for the offset between the initiator's and acceptor's clocks;
• KRB_AP_ERR_BADKEYVER: the initiator should get a new service ticket;
• KRB_AP_PATH_NOT_ACCEPTED: the initiator should get a new service ticket using a different transit path;
• KRB_AP_ERR_INAPP_CKSUM: the initiator should try again with a different checksum type.

Error codes that denote PDU corruption (and/or an active attack) can also be recovered from by attempting a new AP-REQ:

• KRB_AP_ERR_BAD_INTEGRITY
• KRB_AP_ERR_BADVERSION
• KRB_AP_ERR_BADMATCH
• KRB_AP_ERR_MSG_TYPE
• KRB_AP_ERR_MODIFIED

Other error codes that may be recovered from:

• KRB_AP_ERR_BADADDR; the acceptor SHOULD include a list of one or more client network addresses as reported by the operating system, but if the acceptor does not then the continue-needed e-flag MUST NOT be included and the error must be final.

2.2. Number of Security Context Tokens

The first AP-REQ may well result in an error; the second should not. Therefore acceptors SHOULD return a fatal error when a second error results in one security context establishment attempt, except when the first error is that the initiator should use user-to-user authentication. This limits the maximum number of round trips to two (not user-to-user) or three (user-to-user).

Initiators and acceptors MUST impose some limit on the maximum number of security context tokens. For the time being that limit is six.

An initiator that rejects an additional round trip MUST respond with a KRB-ERROR2.

Note that in the user-to-user cases (see Section 5) it's possible to have up to three round trips under normal conditions if, for example, the acceptor wishes to avoid the use of replay caches (see Section 4), or if the initiator's clock is too skewed, for example.

2.2.1. Security Token Sequences

The following successful security context token exchange sequences are possible:

• one token (per-RFC4121).
• two tokens (per-RFC4121).
• three tokens: the initiator wanted mutual authentication and the acceptor wanted an extra AP-REQ for replay cache avoidance (see Section 4).
• four tokens: the initiator wanted mutual authentication and a recoverable error occurred, leading to the second round trip.
  • This case provides replay cache avoidance without no fifth token because the acceptor provides a challenge in its first (KRB-ERROR2) token and the initiator completes the challenges in its second token.

• six tokens: the initiator wanted mutual authentication, the acceptor wanted user-to-user authentication, and the initiator's second AP-REQ elicited a recoverable error

2.3. PROT_READY

It is REQUIRED that each AP-REQ in a security context token exchange assert an initiator sub-session key, possibly different from preceding AP-REQs in the same exchange. Each AP-REQ's initiator sub-session key in a security context token exchange MUST replace the sub-session key to be used for PROT_READY per-message tokens. This can conceivably cause failure-to-verify/unwrap errors for some applications (e.g., using datagram transports), but none that they shouldn't have been prepared to handle.

2.4. Per-Message Token Sequence Numbers

It is REQUIRED that each real AP-REQ in a single security token exchange specify the same start sequence number as preceding AP-REQs in the same security context token exchange.

2.4.1. Negotiation Issues for User-to-User Authentication

Initiator applications that can negotiate security mechanisms and which have available an existing user-to-user mechanism [I-D.swift-win2k-krb-user2user] as well as the Kerberos V5 GSS mechanism with the user-to-user extension defined here will have a problem: they may end up negotiating the use of the Kerberos V5 GSS mechanism and fail to establish a security context because the acceptor does not support the features defined in this document, but the application might have succeeded if it had selected the user-to-user mechanism.

[CREF2]Question: how should we address this? We could say “give priority to the user-to-user mechanism”, but in some cases that might require changes to the acceptor side.

3. ASN.1 for New Protocol Elements

A partial ASN.1 module appears below. This ASN.1 is to be used as if it were part of the base Kerberos ASN.1 module (see RFC4120), therefore the encoding rules to be used are the Distinguished Encoding Rules (DER) [CCITT.X690.2002], and the environment is one of explicit tagging.

 APOptions       ::= KerberosFlags
         -- reserved(0),
         -- use-session-key(1),
         -- mutual-required(2) 
         -- continue-needed-ok(TBD)
 
 ad-continue-nonce     Int32 ::= <TBD>
        -- ad-value is challenge nonce from KRB-ERROR2
 
 KrbErrorEncPartFlags ::= KerberosFlags
         -- reserved(0)  [XXX cargo cult!]
         -- use-initiator-subkey(1)
         -- use-ticket-session-key(2)
         -- use-null-enctype(3)
 
 KRB-ERROR2          ::= [APPLICATION <TBD>] SEQUENCE {
         pvno            [0] INTEGER (5),
         msg-type        [1] INTEGER (<TBD>),
         enc-part-key    [2] KrbErrorEncPartFlags,
         enc-part        [3] EncryptedData -- EncKRBErrorPart
 }
 
 ErrorFlags ::= KerberosFlags
         -- reserved(0)  [XXX sounds like cargo cult!]
         -- continue-needed(1)
 
 EncKRBErrorPart    ::= [APPLICATION <TBD>] SEQUENCE {
         challenge-nonce [0] OCTET STRING (16),
         stime           [1] KerberosTime,
         susec           [2] Microseconds,
         error-code      [3] Int32,
         e-flags         [4] ErrorFlags,
         e-text          [5] KerberosString OPTIONAL,
         e-data          [6] OCTET STRING OPTIONAL,
         e-typed-data    [7] TYPED-DATA OPTIONAL,
         tgt             [8] Ticket OPTIONAL, -- for user2user
         ...
 }