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]. Weis Expires February, 2004 1 The Use of RSA Signatures with ESP and AH August, 2003 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. Weis Expires February, 2004 2 The Use of RSA Signatures with ESP and AH August, 2003 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 Weis Expires February, 2004 3 The Use of RSA Signatures with ESP and AH August, 2003 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. Weis Expires February, 2004 4 The Use of RSA Signatures with ESP and AH August, 2003 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. Weis Expires February, 2004 5 The Use of RSA Signatures with ESP and AH August, 2003 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 Weis Expires February, 2004 6 The Use of RSA Signatures with ESP and AH August, 2003 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 Weis Expires February, 2004 7