A PFS-preserving protocol for LURK
draft-erb-lurk-rsalg-00

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Network Working Group                                             S. Erb
Internet-Draft                                                   R. Salz
Intended status: Standards Track                     Akamai Technologies
Expires: September 19, 2016                               March 18, 2016

                   A PFS-preserving protocol for LURK
                        draft-erb-lurk-rsalg-00

Abstract

   This document defines a protocol between a content provider and an
   external key owner that enables the provider to act as a TLS
   termination end-point for the key owner, without having the key
   actually being provisioned at the provider.

   The protocol between the two preserves forward secrecy, and is also
   designed to prevent the use of the key owner as a general-purpose
   signing oracle which would make it complicit in attacks against uses
   of the very keys it is trying to protect.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 19, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Goals and Non-Goals . . . . . . . . . . . . . . . . . . . . .   3
   3.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Server Key Exchange . . . . . . . . . . . . . . . . . . .   3
     3.2.  RSALG . . . . . . . . . . . . . . . . . . . . . . . . . .   3
       3.2.1.  Implementation Note - Modified Bleichenbacher Attack    4
       3.2.2.  Implementation Note - Hash Calculation  . . . . . . .   4
   4.  LURK Message Formats  . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Request Message . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Response Message  . . . . . . . . . . . . . . . . . . . .   6
   5.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  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 [RFC2119].

   Three entities are involved in this protocol, although only two
   actually

   participate in the protocol exchanges:

         Client    <----->    Server    <---->    KeyOwner

   The "KeyOwner" is an entity holding a Certificate and associated
   private Key, typically bound to an identity such as a DNS name.

   The "server" acts on behalf of the KeyOwner, such as terminating TLS
   connections.  From external appearances, such as TLS peer name
   verification, the server is indistinguishable from the KeyOwner.

   The "client" is the end-entity that initiates a connection to the
   server.

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2.  Goals and Non-Goals

   It is not a goal to protect against an active attacker who can
   decrypt or actively MiTM any of the traffic.

   It is not a goal to protect Client-Server traffic in the event of a
   full compromise of a KeyOwner private key.

   This protocol can support Client-Server communications from SSLv3 up
   through TLS 1.2.  (TLS 1.3 will have to be evaluated at a later
   date.)

   Past Client-Server communications must remain private in the event
   that access to the KeyOwner is compromised (Perfect Forward Secrecy).
   For Server Key Exchange signing requests, this is not an issue.  For
   RSA decryption requests used by the TLS_RSA_* cipher suites, the
   "RSALG" message exchanges described below provide PFS protection.

   The protocol should not become a generic signing oracle, even if it
   is suboptimal with regard to network bandwidth utilization.  This is
   done by not simply signing values, but by computing the full
   signature hash at the KeyOwner.

3.  Protocol Overview

   Communication between the Server and KeyOwner MUST be over a
   mutually-authenticated TLS connection that uses PFS key exchange.
   TLS 1.2 or later SHOULD be used.

3.1.  Server Key Exchange

   To be provided.

3.2.  RSALG

   The basic premise of RSALG is that in the TLS_RSA_* handshakes:

   o  The KeyOwner will not decrypt the PMS and provide it back to the
      Server.  Instead, the KeyOwner will full compute the Master Secret
      (via the PRF function) and provide that.

   o  The Server will choose a random ephemeral value, N, and provide a
      cryptographically-hashed value of (such as SHA256(N)) as its
      Server Random value.  The Server sends N to KeyOwner which then
      computes the same hashed value and uses that hash as its input to
      the PRF.

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   An attacker who later gains access to KeyOwner would be unable to
   derive the same Master Secret.  This attacker would be able to see
   the Client Random, Server Random and encrypted PMS, but would be
   unable to replay this to KeyOwner unless they could reverse the
   cryptographic hash function used to compute the server random.

3.2.1.  Implementation Note - Modified Bleichenbacher Attack

   If an attacker can gain access to the KeyOwner, they could mount a
   Bleichenbacher attack against it (REF NEEDED).  The standard SSL/TLS
   defense against the Bleichenbacher attack (generating a string of
   random bytes) is not effective here, since an attacker could generate
   two requests with identical inputs and learn information about the
   validity of the padding by seeing whether it gets a consistent output
   in both cases.  This is possible because the attacker also controls
   (the input to) the server random.

   To avoid this variation on the Bleichenbacher attack, KeyOwner should
   compute the HMAC-SHA-384 over the PRF inputs as its "invalid"
   response, using a private key as the hash key, to ensure that the
   output is a deterministic function of the input and cannot be
   calculated by the attacker.  This private key must be globally unique
   per keypair, therefore the RSA private key being used to decrypt the
   PMS is an obvious choice.

3.2.2.  Implementation Note - Hash Calculation

   In TLS 1.2 and earlier, the first four bytes of a server random value
   are actually a timestamp.  An implementation must use those four
   bytes as an input to the hash function as described above, then
   overwrite them as input to the PRF calculated by the KeyOwner and the
   Server Random value provided to the Client.

   Example:

       server_random = N
       server_random[0..3] = get_time()

   Server communicates server_random to KeyOwner

   Both Server and KeyOwner compute the following:

       saved_time = server_random[0..3]
       server_random = sha256(server_random)
       server_random[0..3] = saved_time

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4.  LURK Message Formats

   The formats below are described using the TLS Presentation Language.

   The following message header appears at the start of every message:

       enum {
           one(1), (255)
       } Version
       enum {
           request(0), response(1), (255)
       } Type
       struct {
           Version  version;
           Type  type;
           uint16 length;
       } lurk_msg_header;

   version  The version of this protocol.

   type  The message type, either request or response.  Details defined
      below.

   length  Length of the entire message, including header, in bytes.

4.1.  Request Message

   A request message looks like this:

       enum {
           rsalg(0), server_kx(1), (255)
       } ReqType
       struct {
           lurk_msg_header  header;
           uint64           id;
           ReqType          op_type;
           uint8            cert<32>;
           uint16           client_version;
           uint16           server_version;
           uint8            client_random<32>;
           uint8            server_random<32>;
           SignatureAndHashAlgorithm sig_hash_alg;
           PRFHashAlgorithm          prf_hash_alg;
           opaque           data<0..2^16-1>;
       } lurk_request;

   id A unique identifier to allow pipelining and match requests and
      responses.

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   cert  The identifier for the keypair to be used in this request.
      This SHOULD be the SHA256 value of the public key.

   client_version  The TLS Version Number provided by the Client in the
      clientHello message.  Note that for RSALG requests, the value must
      be verified (see RFC5264, section 7.4.7.1)

   server_version  The TLS Version Number provided by the Server in the
      serverHello message.  Note that for RSALG requests, the value must
      be verified (see RFC5264, section 7.4.7.1)

   client_random  The TLS Client Random provided by the clientHello
      message.

   server_random  The TLS Server Random provided by the serverHello
      message.  Note that for RSALG requests, this is actually the
      digested value of N.

   sig_hash_alg  For server_kx requests, this is the signature hash
      value that the Server will use (see RFC5246, section 7.4.1.4.1).
      For rsalg requests, this field is ignored and SHOULD be NULL.

   prf_hash_alg  For rsalg requests, this identifies the PRF function to
      use.  For server_kx requests, this field is ignored and SHOULD be
      NULL.

   TODO: this likely should follow the same format as the first byte of
   sighashalgo above, also need md5/sha1 combo value here.

   data  For rsalg requests, this contains the encrypted PRF.  For
      server_kx signing reqeusts, this contains the key parameters to
      sign.

4.2.  Response Message

   A response message looks like this:

       enum {
           success(0), invalidParameters(1), certUnavailable(2),
           permissionDenied(3), insufficentResources(4), (255)
       } ResponseStatus
       struct {
           lurk_msg_header  header;
           RseponseStatus   status;
           uint64           id;
           opaque           data<0..2^16-1>;
       } lurk_response;

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   id The request id for which this is the response.

   data  For any status other than success, the data is ignored and MUST
      be NULL.  For rsalg requests, the data contains the master secret.
      For server_kx requests, the data contains the signed hash.

5.  Open Issues

   The KeyOwner could choose the TLS server random.  This makes RSALG
   even less likely to be useful as an oracle, but has turned out to be
   difficult to integrate into existing TLS/SSL libraries.

   Should the lurk_request and lurk_response messages be padded out to
   eight-byte alignment?

6.  Acknowledgements

   We acknowledge the cooperation of Charlie Gero and Phil Lisiecki of
   Akamai Technologies, and their disclosure of US Patent Application
   20150106624, "Providing forward secrecy in a terminating TLS
   connection proxy."

7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

Authors' Addresses

   Samuel Erb
   Akamai Technologies

   Email: serb@akamai.com

   Rich Salz
   Akamai Technologies

   Email: rsalz@akamai.com

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