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Versions: 00 01 draft-ietf-msec-ipsec-signatures

   Internet Engineering Task Force                                   Brian Weis
   INTERNET-DRAFT                                                 Cisco Systems
   Document: draft-bew-ipsec-signatures-01.txt                     August, 2003
   Expires: February, 2004


                The Use of RSA Signatures within ESP and AH

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.

Abstract

   This memo describes the use of the RSA Signature algorithm [RSA] as
   an authentication algorithm within the revised IPSEC Encapsulating
   Security Payload [ESP] and the revised IPSEC Authentication Header
   [AH]. The use of a digital signature algorithm such as RSA provides
   origin authentication, even when ESP and AH are used to secure group
   data flows.

   Further information on the other components necessary for ESP and AH
   implementations is provided by [ROADMAP].










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Table of Contents

1.0 Introduction......................................................2
2.0 Algorithm and Mode................................................3
3.0 Performance.......................................................3
4.0 Interaction with the ESP Cipher Mechanism.........................4
5.0 Security Association Considerations for Group Traffic.............4
6.0 Key Management Considerations.....................................4
7.0 Security Considerations...........................................5
  7.1 Eavesdropping...................................................5
  7.2 Replay..........................................................5
  7.3 Message Insertion...............................................5
  7.4 Deletion........................................................5
  7.5 Modification....................................................6
  7.6 Man in the middle...............................................6
  7.7 Denial of Service...............................................6
8.0 IANA Considerations...............................................6
9.0 Acknowledgements..................................................6
10.0 References.......................................................7
  10.1 Normative References...........................................7
  10.2 Informative References.........................................7
Authors Addresses.....................................................7

1.0 Introduction

   AH and ESP payloads can be used to protect unicast traffic, or group
   traffic, such as IPv4 and IPv6 multicast traffic. When unicast
   traffic is protected between a pair of entities, HMAC transforms
   (such as [HMAC-SHA] and [HMAC-MD5]) are sufficient to prove source
   authentication. An HMAC is strong enough in that scenario because
   only one other entity knows the key. However when AH and ESP protect
   group traffic, this property is no longer valid -- all group members
   share the single HMAC key and thus can spoof one another.

   Some multicast applications require true origin authentication, where
   one group member cannot be spoofed by another group member without
   detection. The use of asymmetric encryption algorithms, such as RSA,
   can provide true origin authentication. The sender generates a pair
   of keys, one of which is never shared (called the "private key") and
   one of which is distributed to other group members (called the
   "public key").

   When an asymmetric encryption algorithm is used to encrypt the output
   of a cryptographic hash algorithm, the cipher text is called a
   "digital signature". A receiver of the digital signature uses the
   public key to decrypt the hash.

   This memo describes how RSA digital signatures can be applied as an
   authentication mechanism to AH and ESP payloads to provide origin
   authentication.

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   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].

2.0 Algorithm and Mode

   The RSA Public Key Algorithm [RSA] is a widely deployed public key
   algorithm commonly used for digital signatures. Compared to other
   public key algorithms, signature verification is relatively quick.
   This property is useful for groups where receivers may have limited
   processing capabilities. The RSA Algorithm is commonly supported in
   hardware, and is no longer encumbered by intellectual property
   claims.

   Several schemes for the RSA Algorithm are described in [RSA]. One
   scheme exists specifically for digital signatures. However, that
   scheme is not the most efficient for this application. In addition,
   the signature scheme encodes the hash type into the signature data
   block, and this is not necessary because the hash algorithm is pre-
   determined.

   The RSAES-OAEP raw RSA scheme [RSA, Section 7.1] MUST be used as the
   encryption scheme. As recommended in [RSA, Section B.1], SHA-1 MUST
   be used as the signature hash algorithm both as the message to be
   encrypted by the RSA Algorithm, and as the encoding parameter for the
   OAEP encoding. The value of parameter string P MUST be the default,
   which is the hash of an empty string. The mask generation function
   MUST be MGF1 as defined in [RSA, Section B.2.1].

   The size of the RSA modulus used can vary, but MUST be at least 496
   bits. This restriction is a function of the size of the SHA-1 hash
   and the number of bits needed for OAEP encapsulation. (For more
   information, see [RSA, Section 7.1].)

3.0 Performance

   The RSA asymmetric key algorithm is very costly in terms of
   processing time compared to the HMAC algorithms. However, processing
   cost is decreasing over time. Faster general-purpose processors are
   being deployed, faster software implementations are being developed,
   and hardware acceleration support for the algorithm is becoming more
   prevalent. However, care should always be taken that RSA signatures
   are not used for applications that expect to have bandwidth
   requirements that would be adversely affected.

   The RSA asymmetric key algorithm is best suited to protect network
   traffic for network traffic for which:

   o  the sender has a substantial amount of processing power, whereas
   receivers are not guaranteed to have substantial processing power,
   and


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   o the network traffic is small enough that adding a relatively large
   authentication tag (in the range of 61 to 256 bytes) does not cause
   packet fragmentation.

   Both key pair generation and encryption (or signing) are
   substantially more expensive operations, but these are isolated to
   the sender.

   The size of the RSA modulus can affect the processing required to
   create and verify RSA digital signatures. Care should be taken to
   determine what the size of key is needed for the application.
   Smaller key sizes may be chosen as long as the network traffic
   protected by the private key flows for less time than it is estimated
   that an attacker would take to discover the private key. This
   lifetime is considerably smaller than most public key applications,
   so a key that is normally considered weak may be satisfactory. (If
   the RSA public key algorithm were used to encrypt the packet, then
   the lifetime would be substantially longer.)

   The addition of a digital signature as an authentication tag adds a
   significant number of bytes to the packet. This increases the
   likelihood that the packet encapsulated in AH or ESP may be
   fragmented.

4.0 Interaction with the ESP Cipher Mechanism

   As of this writing, there are no known issues that preclude the use
   of the RSA signatures algorithm with any specific cipher algorithm.

5.0 Security Association Considerations for Group Traffic

   When RSA Signatures are used to protect group traffic, they MUST be
   in accordance with the restrictions set forth in the IPSec
   Architecture [RFC2401] to ensure that security associations remain
   unique. That is, one of the following two cases:

       o Single sender groups where only one source IPv4 or IPV6
   address is sending packets to the IP multicast destination address.

       o Multiple sender groups where a single group controller is
   choosing SPI values for AH and ESP security associations for all
   group members sending packets to the same IP multicast destination
   address.

6.0 Key Management Considerations

   The key management mechanism that negotiates the use of RSA
   Signatures MUST include the length of the RSA modulus during policy
   negotiation in order to give a device the opportunity to decline.
   This is especially important for devices with constrained processors
   that might not be able to handle the verification of signatures using
   larger key sizes.


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   A receiver must have the RSA public key in order to decrypt the
   packet. When used with a group key management system such as GDOI
   [RFC3547], the public key SHOULD be to sent as part of the key
   download policy. If the group has multiple senders, the public key of
   each sender SHOULD be sent as part of the key download policy.

   When used with a pair-wise key management system such as IKE
   [RFC2409] the public key could be passed as an SA attribute. However,
   given the above performance considerations, the value of using this
   transform for a pair-wise connection is limited.

7.0 Security Considerations

   This document provides a method of authentication for AH and ESP
   using digital signatures. This feature provides the following
   protections:

       o Message modification integrity. The digital signature allows
   the receiver of the message to verify that it was exactly the same as
   when the sender signed it.

       o Host authentication. The asymmetric nature of the RSA public
   key algorithm allows the sender to be uniquely verified, even when
   the message is sent to a group.

   As suggested by [RFC3552], the following sections describe various
   attacks that could be deployed against using RSA signatures as a
   means of authentication.

7.1 Eavesdropping

   This document does not address confidentiality. That function, if
   desired, must be addressed by an ESP cipher that is used with the
   RSA Signatures authentication method. The RSA signature itself does
   not need to be protected in any way.

7.2 Replay

   This document does not address replay attacks. That function, if
   desired, is addressed through use of AH and ESP sequence numbers as
   defined in [AH] and [ESP].

7.3 Message Insertion

   This document directly addresses message insertion attacks. Inserted
   messages will fail authentication and be dropped by the receiver.

7.4 Deletion

   This document does not address deletion attacks. It is only
   concerned with validating the legitimacy of messages that are not
   deleted.


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7.5 Modification

   This document directly addresses message modification attacks.
   Modified messages will fail authentication and be dropped by the
   receiver.

7.6 Man in the middle

   As long as a receiver is given the sender RSA public key in a
   trusted manner, it will be able to verify that the digital signature
   is correct. A man in the middle will not be able to spoof the actual
   sender unless it acquires the RSA private key through some means.

   RSA modulus sizes must be chosen carefully to ensure that the time it
   takes a man in the middle to determine the RSA private key through
   cryptanalysis is longer than the amount of time that packets are
   signed with that private key.

7.7 Denial of Service

   A receiver of an ESP and AH packet starts by looking up the Security
   Association in the SADB. If one is found, the ESP or AH sequence
   number in the packet is verified. If the sequence number falls
   within the window, the authentication step is performed.

   An attacker that sends an ESP or AH packet which matches a valid SA
   on the system and which also has a valid sequence number will cause
   the receiver to perform the AH or ESP authentication step. Because
   the process of verifying an RSA digital signature consumes
   relatively large amounts of processing, this could lead to a denial
   of service attack on the receiver if many such packets were sent.

   If the message was sent to an IPv4 or IPv6 multicast group all group
   members that received the packet would be under attack
   simultaneously.

   Further investigation is needed to better estimate the actual
   effects of this attack, and how it can be mitigated.

8.0 IANA Considerations

   An assigned number is required in the IPSec Authentication Algorithm
   name space in the ISAKMP registry [ISAKMP-REG]. The mnemonic should
   be SIG-RSA.

   A new IPSEC Security Association Attribute is required in the ISAKMP
   registry to pass the RSA modulus size. The attribute class should be
   called Authentication Key Length, and it should a Variable type.

9.0 Acknowledgements

   TBD


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10.0 References

10.1 Normative References

   [AH] Kent, S., and R. Atkinson, "IP Authentication Header", RFC 2402,
   November 1998.

   [ESP] Kent, S., and R. Atkinson, "IP Encapsulating Security Payload",
   RFC 2406, November 1998.

   [ISAKMP-REG] http://www.iana.org/assignments/isakmp-registry

   [RFC2401] Kent, S., R. Atkinson, "Security Architecture for the
   Internet Protocol", November 1998

   [ROADMAP] Thayer, R., Doraswamy, N., and R. Glenn, "IP Security
   Document Roadmap", RFC 2411, November 1998.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Level", BCP 14, RFC 2119, March 1997.

   [RFC3552] E. Rescorla, et. al., Guidelines for Writing RFC Text on
   Security Considerations, RFC 3552, July 2003.

   [RSA] Jonsson, J., B. Kaliski,  "PKCS #1: RSA Cryptography
   Specifications Version 2.1", RFC 3447, February 2003.

10.2 Informative References

   [HMAC-MD5] Madson, C., and R. Glenn, "The Use of HMAC-MD5-96 within
   ESP and AH"", RFC 2403, November, 1998.

   [HMAC-SHA] Madson, C., and R. Glenn, " The Use of HMAC-SHA-1-96
   within ESP and AH", RFC 2404, November, 1998.

   [RFC3547] M. Baugher, B. Weis, T. Hardjono, H. Harney, , The Group
   Domain of Interpretation, RFC 3547, December 2002.



Authors Addresses

   Brian Weis
   Cisco Systems
   170 W. Tasman Drive,
   San Jose, CA 95134-1706, USA
   (408) 526-4796
   bew@cisco.com

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