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Network Working Group                                            B. WeisRequest for Comments: 4359                                 Cisco SystemsCategory: Standards Track                                   January 2006The Use of RSA/SHA-1 Signatures withinEncapsulating Security Payload (ESP) and Authentication Header (AH)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 (C) The Internet Society (2006).Abstract   This memo describes the use of the RSA digital signature algorithm as   an authentication algorithm within the revised IP Encapsulating   Security Payload (ESP) as described inRFC 4303 and the revised IP   Authentication Header (AH) as described inRFC 4302.  The use of a   digital signature algorithm, such as RSA, provides data origin   authentication in applications when a secret key method (e.g., HMAC)   does not provide this property.  One example is the use of ESP and AH   to authenticate the sender of an IP multicast packet.Weis                        Standards Track                     [Page 1]

RFC 4359         RSA/SHA-1 Signatures within ESP and AH     January 2006Table of Contents1. Introduction ....................................................22. Algorithm and Mode ..............................................32.1. Key Size Discussion ........................................43. Performance .....................................................54. Interaction with the ESP Cipher Mechanism .......................65. Key Management Considerations ...................................66. Security Considerations .........................................76.1. Eavesdropping ..............................................76.2. Replay .....................................................76.3. Message Insertion ..........................................86.4. Deletion ...................................................86.5. Modification ...............................................86.6. Man in the Middle ..........................................86.7. Denial of Service ..........................................87. IANA Considerations .............................................98. Acknowledgements ...............................................109. References .....................................................109.1. Normative References ......................................109.2. Informative References ....................................101.  Introduction   Encapsulating Security Payload  (ESP) [ESP] and Authentication Header   (AH) [AH] headers can be used to protect both unicast traffic and   group (e.g., IPv4 and IPv6 multicast) traffic.  When unicast traffic   is protected between a pair of entities, HMAC transforms (such as   [HMAC-SHA]) are sufficient to prove data origin authentication.  An   HMAC is sufficient protection in that scenario because only the two   entities involved in the communication have access to the key, and   proof-of-possession of the key in the HMAC construct authenticates   the sender.  However, when ESP and AH authenticate group traffic,   this property no longer holds because all group members share the   single HMAC key.  In the group case, the identity of the sender is   not uniquely established, since any of the key holders has the   ability to form the HMAC transform.  Although the HMAC transform   establishes a group-level security property, data origin   authentication is not achieved.   Some group applications require true data origin authentication,   where one group member cannot successfully impersonate another group   member.  The use of asymmetric digital signature algorithms, such as   RSA, can provide true data origin authentication.   With asymmetric algorithms, 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").Weis                        Standards Track                     [Page 2]

RFC 4359         RSA/SHA-1 Signatures within ESP and AH     January 2006   When the private key is used to sign the output of a cryptographic   hash algorithm, the result is called a "digital signature".  A   receiver of the digital signature uses the public key, the signature   value, and an independently computed hash to determine whether or not   the claimed origin of the packet is correct.   This memo describes how RSA digital signatures can be applied as an   ESP and AH authentication mechanism to provide data origin   authentication.   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.  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   efficient.  This property is useful for groups where receivers may   have limited processing capabilities.  The RSA algorithm is commonly   supported in hardware.   Two digital signature encoding methods are supported in [RSA].   RSASSA-PKCS1-v1_5 MUST be supported by a conforming implementation.   RSASSA-PSS is generally believed to be more secure, but at the time   of this writing is not ubiquitous.  RSASSA-PSS SHOULD be used   whenever it is available.  SHA-1 [SHA] MUST be used as the signature   hash algorithm used by the RSA digital signature algorithm.   When specified for ESP, the Integrity Check Value (ICV) is equal in   size to the RSA modulus, unless the RSA modulus is not a multiple of   8 bits.  In this case, the ICV MUST be prepended with between 1 and 7   bits set to zero such that the ICV is a multiple of 8 bits.  This   specification matches the output S [RSA,Section 8.1.1] (RSASSA-PSS)   and [RSA,Section 8.2.1] (RSASSA-PKCS1-v1_5) when the RSA modulus is   not a multiple of 8 bits.  No implicit ESP ICV Padding bits are   necessary.   When specified for AH, the ICV is equal in size of the RSA modulus,   unless the RSA modulus is not a multiple of 32 bits (IPv4) or 64 bits   (IPv6) [AH,Section 2.6].  In this case, explicit ICV Padding bits   are necessary to create a suitably sized ICV [AH,Section 3.3.3.2.1].   The distribution mechanism of the RSA public key and its replacement   interval are a group policy matter.  The use of an ephemeral key pair   with a lifetime of the ESP or AH Security Association (SA) is   RECOMMENDED.  This recommended policy reduces the exposure of the RSAWeis                        Standards Track                     [Page 3]

RFC 4359         RSA/SHA-1 Signatures within ESP and AH     January 2006   private key to the lifetime of the data being signed by the private   key.  Also, this obviates the need to revoke or transmit the validity   period of the key pair.   Digital signature generation is performed as described in [RSA,Section 8.1.1] (RSASSA-PSS) and [RSA,Section 8.2.1](RSASSA-PKCS1-   v1_5).  The authenticated portion of the AH or ESP packet ([AH,Section 3.3.3], [ESP,Section 3.3.2]) is used as the message M, which   is passed to the signature generation function.  The signer's RSA   private key is passed as K.  Summarizing, the signature generation   process computes a SHA-1 hash of the authenticated packet bytes,   signs the SHA-1 hash using the private key, and encodes the result   with the specified RSA encoding type.  This process results in a   value S, which is known as the ICV in AH and ESP.   Digital signature verification is performed as described in [RSA,Section 8.1.2] (RSASSA-PSS) and [RSA,Section 8.2.2]   (RSASSA-PKCS1-v1_5).  Upon receipt, the ICV is passed to the   verification function as S.  The authenticated portion of the AH or   ESP packet is used as the message M, and the RSA public key is passed   as (n, e).  In summary, the verification function computes a SHA-1   hash of the authenticated packet bytes, decrypts the SHA-1 hash in   the ICV, and validates that the appropriate encoding was applied and   was correct.  The two SHA-1 hashes are compared, and if they are   identical the validation is successful.2.1.  Key Size Discussion   The choice of RSA modulus size must be made carefully.  If too small   of a modulus size is chosen, an attacker may be able to reconstruct   the private key used to sign packets before the key is no longer used   by the sender to sign packets.  This order of events may result in   the data origin authentication property being compromised.  However,   choosing a modulus size larger than necessary will result in an   unnecessarily high cost of CPU cycles for the sender and all   receivers of the packet.   A conforming implementation MUST support a modulus size of 1024 bits.   Recent guidance [TWIRL,RSA-TR] on key sizes makes estimates as to   the amount of effort an attacker would need to expend in order to   reconstruct an RSA private key.  Table 1 summarizes the maximum   length of time that selected modulus sizes should be used.  Note that   these recommendations are based on factors such as the cost of   processing and memory, as well as cryptographic analysis methods,   which were current at the time these documents were published.  As   those factors change, choices of key lifetimes should take them into   account.Weis                        Standards Track                     [Page 4]

RFC 4359         RSA/SHA-1 Signatures within ESP and AH     January 2006                    Number of     Recommended Maximum                   Modulus Bits         Lifetime                   ------------    -------------------                       768               1 week                       1024              1 year             Table 1.  RSA Key Use Lifetime Recommendations3.  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.   Care should be taken that RSA signatures are not used for   applications when potential receivers are known to lack sufficient   processing power to verify the signature.  It is also important to   use this scheme judiciously when any receiver may be battery powered.   The RSA asymmetric key algorithm is best suited to protect network   traffic for which:    o The sender has a substantial amount of processing power, and    o The network traffic is small enough that adding a relatively large      authentication tag (in the range of 62 to 256 bytes) does not      cause packet fragmentation.   RSA key pair generation and signing are substantially more expensive   operations than signature verification, but these are isolated to the   sender.   The size of the RSA modulus affects the processing required to create   and verify RSA digital signatures.  Care should be taken to determine   the size of modulus needed for the application.  Smaller modulus   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 that store the   signed data for a period of time.  But since the digital signature is   used only for sender verification purposes, a modulus that is   considered weak in another context may be satisfactory.Weis                        Standards Track                     [Page 5]

RFC 4359         RSA/SHA-1 Signatures within ESP and AH     January 2006   The size of the RSA public exponent can affect the processing   required to verify RSA digital signatures.  Low-exponent RSA   signatures may result in a lower verification processing cost.  At   the time of this writing, no attacks are known against low-exponent   RSA signatures that would allow an attacker to create a valid   signature using the RSAES-OAEP scheme.   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 ESP or AH may be   fragmented.4.  Interaction with the ESP Cipher Mechanism   The RSA signature algorithm cannot be used with an ESP Combined Mode   algorithm that includes an explicit ICV.  The Combined Mode algorithm   will add the ESP ICV field, which does not allow use of a separate   authentication algorithm to add the ESP ICV field.  One example of   such an algorithm is the ESP Galois/Counter Mode algorithm [AES-GCM].5.  Key Management Considerations   Key management mechanisms negotiating the use of RSA signatures MUST   include the length of the RSA modulus during policy negotiation using   the Authentication Key Length SA Attribute.  This gives a device the   opportunity to decline use of the algorithm.  This is especially   important for devices with constrained processors that might not be   able to verify signatures using larger key sizes.   Key management mechanisms negotiating the use of RSA signatures also   MUST include the encoding method during policy negotiation using the   Signature Encoding Algorithm SA Attribute.   A receiver must have the RSA public key in order to verify integrity   of the packet.  When used with a group key management system (e.g.,RFC 3547 [GDOI]), the public key SHOULD be 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.   Use of this transform to obtain data origin authentication for   pairwise SAs is NOT RECOMMENDED.  In the case of pairwise SAs (such   as negotiated by the Internet Key Exchange [IKEV2]), data origin   authentication can be achieved with an HMAC transform.  Because the   performance impact of an RSA signature is typically greater than an   HMAC, the value of using this transform for a pairwise connection is   limited.Weis                        Standards Track                     [Page 6]

RFC 4359         RSA/SHA-1 Signatures within ESP and AH     January 20066.  Security Considerations   This document provides a method of authentication for ESP and AH   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.   Non-repudiation is not claimed as a property of this transform.  At   times, the property of non-repudiation may be applied to digital   signatures on application-level objects (e.g., electronic mail).   However, this document describes a means of authenticating network-   level objects (i.e., IP packets), which are ephemeral and not   directly correlated to any application.  Non-repudiation is not   applicable to network-level objects (i.e., IP packets).   A number of attacks are suggested by [RFC3552].  The following   sections describe the risks those attacks present when RSA signatures   are used for ESP and AH packet authentication.   SHA-1 has been scheduled to be phased out in 2010, due to the steady   advances in technology by which an adversary can double its computing   power in roughly eighteen months.  Recent attacks on SHA-1 underscore   the importance of replacing SHA-1, but have not motivated replacing   it before that date [SHA-COMMENTS].  The use of this transform after   that date SHOULD be preceded by an analysis as to its continued   suitability.6.1.  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 from an eavesdropper.6.2.  Replay   This document does not address replay attacks.  That function, if   desired, is addressed through use of ESP and AH sequence numbers as   defined in [ESP] and [AH].Weis                        Standards Track                     [Page 7]

RFC 4359         RSA/SHA-1 Signatures within ESP and AH     January 20066.3.  Message Insertion   This document directly addresses message insertion attacks.  Inserted   messages will fail authentication and be dropped by the receiver.6.4.  Deletion   This document does not address deletion attacks.  It is concerned   only with validating the legitimacy of messages that are not deleted.6.5.  Modification   This document directly addresses message modification attacks.   Modified messages will fail authentication and be dropped by the   receiver.6.6.  Man in the Middle   As long as a receiver is given the sender RSA public key in a trusted   manner (e.g., by a key management protocol), 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.   The RSA modulus size must be chosen carefully to ensure that the time   a man in the middle needs to determine the RSA private key through   cryptanalysis is longer than the amount of time that packets are   signed with that private key.6.7.  Denial of Service   According to IPsec processing rules, a receiver of an ESP and AH   packet begins by looking up the Security Association in the SA   database.  If one is found, the ESP or AH sequence number in the   packet is verified.  No further processing will be applied to packets   with an invalid sequence number.   An attacker that sends an ESP or AH packet matching a valid SA on the   system and also having a valid sequence number will cause the   receiver to perform the ESP or AH authentication step.  Because the   process of verifying an RSA digital signature consumes relatively   large amounts of processing, many such packets could lead to a denial   of service (DoS) attack on the receiver.   If the message was sent to an IPv4 or IPv6 multicast group, all group   members that received the packet would be under attack   simultaneously.Weis                        Standards Track                     [Page 8]

RFC 4359         RSA/SHA-1 Signatures within ESP and AH     January 2006   This attack can be mitigated against most attackers by encapsulating   ESP or AH using an RSA signature for authentication within ESP or AH   using an HMAC transform for authentication.  In this case, the HMAC   transform would be validated first, and as long as the attacker does   not possess the HMAC key no digital signatures would be evaluated on   the attacker packets.  However, if the attacker does possess the HMAC   key (e.g., the attacker is a legitimate member of the group using the   SA), then the DoS attack cannot be mitigated.7.  IANA Considerations   An assigned number is required in the "IPSec Authentication   Algorithm" name space in the Internet Security Association and Key   Management Protocol (ISAKMP) registry [ISAKMP-REG].  The mnemonic   should be "SIG-RSA".   An assigned number is also required in the "IPSEC AH Transform   Identifiers" name space in the ISAKMP registry.  Its mnemonic should   be "AH_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 be a   Variable type.   A second "IPSEC Security Association Attribute" is required in the   ISAKMP registry to pass the RSA signature encoding type.  The   attribute class should be called "Signature Encoding Algorithm", and   it should be a Basic type.  The following rules apply to define the   values of the attribute:                 Name                Value                 ----                -----                 Reserved            0                 RSASSA-PKCS1-v1_5   1                 RSASSA-PSS          2   Values 3-61439 are reserved to IANA.  New values MUST be added due to   a Standards Action as defined in [RFC2434].  Values 61440-65535 are   for private use and may be allocated by implementations for their own   purposes.Weis                        Standards Track                     [Page 9]

RFC 4359         RSA/SHA-1 Signatures within ESP and AH     January 20068.  Acknowledgements   Scott Fluhrer and David McGrew provided advice regarding applicable   key sizes.  Scott Fluhrer also provided advice regarding key   lifetimes.  Ian Jackson, Steve Kent, and Ran Canetti provided many   helpful comments.  Sam Hartman, Russ Housley, and Lakshminth Dondeti   provided valuable guidance in the development of this document.9.  References9.1.  Normative References   [AH]           Kent, S., "IP Authentication Header",RFC 4302,                  December 2005.   [ESP]          Kent, S., "IP Encapsulating Security Payload (ESP)",RFC 4303, December 2005.   [ISAKMP-REG]http://www.iana.org/assignments/isakmp-registry   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate                  Requirement Level",BCP 14,RFC 2119, March 1997.   [RFC3552]      Rescorla, E. and B. Korver, "Guidelines for Writing                  RFC Text on Security Considerations",BCP 72,RFC3552, July 2003.   [RSA]          Jonsson, J. and B. Kaliski,  "Public-Key Cryptography                  Standard (PKCS) #1: RSA Cryptography Specifications                  Version 2.1",RFC 3447, February 2003.   [SHA]          FIPS PUB 180-2: Specifications for the Secure Hash                  Standard, August 2002.http://csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf.9.2.  Informative References   [AES-GCM]      Viega, J. and D. McGrew, "The Use of Galois/Counter                  Mode (GCM) in IPsec Encapsulating Security Payload                  (ESP)",RFC 4106, June 2005.   [GDOI]         Baugher, M., Weis, B., Hardjono, T., and H. Harney,                  "The Group Domain of Interpretation",RFC 3547,                  December 2002.   [HMAC-SHA]     Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96                  within ESP and AH",RFC 2404, November 1998.Weis                        Standards Track                    [Page 10]

RFC 4359         RSA/SHA-1 Signatures within ESP and AH     January 2006   [IKEV2]        Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",RFC 4306, December 2005.   [RFC2434]      Narten, T. and H. Alvestrand, "Guidelines for Writing                  an IANA Considerations Section in RFCs",BCP 26,RFC2434, October 1998.   [RSA-TR]       B. Kaliski, "TWIRL and RSA Key Size", RSA Laboratories                  Technical Note,http://www.rsasecurity.com/rsalabs/node.asp?id=2004, May 6, 2003.   [SHA-COMMENTS] NIST Brief Comments on Recent Cryptanalytic Attacks on                  Secure Hashing Functions and the Continued Security                  Provided by SHA-1, August, 2004.http://csrc.nist.gov/hash_standards_comments.pdf.   [TWIRL]        Shamir, A., and E. Tromer, "Factoring Large Numbers                  with the TwIRL Device", Work in Progress, February 9,                  2003.Author's Address   Brian Weis   Cisco Systems   170 W. Tasman Drive,   San Jose, CA 95134-1706, USA   Phone: (408) 526-4796   EMail: bew@cisco.comWeis                        Standards Track                    [Page 11]

RFC 4359         RSA/SHA-1 Signatures within ESP and AH     January 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Weis                        Standards Track                    [Page 12]