Internet Engineering Task Force (IETF) T. Kivinen Request for Comments: 7427 INSIDE Secure Updates: 7296 J. Snyder Category: Standards Track Opus One ISSN: 2070-1721 January 2015
Signature Authentication in the Internet Key Exchange Version 2 (IKEv2)
Abstract
The Internet Key Exchange Version 2 (IKEv2) protocol has limited support for the Elliptic Curve Digital Signature Algorithm (ECDSA). The current version only includes support for three Elliptic Curve groups, and there is a fixed hash algorithm tied to each group. This document generalizes IKEv2 signature support to allow any signature method supported by PKIX and also adds signature hash algorithm negotiation. This is a generic mechanism and is not limited to ECDSA; it can also be used with other signature algorithms.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7427.
Copyright Notice
Copyright (c) 2015 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 Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect 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.
Kivinen & Snyder Standards Track [Page 1]
RFC 7427 Signature Authentication in IKEv2 January 2015
This document adds a new IKEv2 [RFC7296] authentication method to support signature methods in a more general way. The current signature-based authentication methods in IKEv2 are per algorithm, i.e., there is one for RSA digital signatures, one for DSS digital signatures (using SHA-1), and three for different ECDSA curves, each tied to exactly one hash algorithm. This design is cumbersome when more signature algorithms, hash algorithms, and elliptic curves need to be supported:
Kivinen & Snyder Standards Track [Page 2]
RFC 7427 Signature Authentication in IKEv2 January 2015
o In IKEv2, authentication using RSA digital signatures calls for padding based on RSASSA-PKCS1-v1_5, although the newer RSASSA-PSS padding method is now recommended. (See Section 5 of "Additional Algorithms and Identifiers for RSA Cryptography for use in PKIX Profile" [RFC4055].)
o With ECDSA and the Digital Signature Standard (DSS), there is no way to extract the hash algorithm from the signature. Thus, for each new hash function to be supported with ECDSA or DSA, new authentication methods would be needed. Support for new hash functions is particularly needed for DSS, because the current restriction to SHA-1 limits its security, meaning there is no point of using long keys with SHA-1.
o The tying of ECDSA authentication methods to particular elliptic curve groups requires definition of additional methods for each new group. The combination of new ECDSA groups and hash functions will cause the number of required authentication methods to become unmanageable. Furthermore, the restriction of ECDSA authentication to a specific group is inconsistent with the approach taken with DSS.
With the selection of SHA-3, it might be possible that a signature method can be used with either SHA-3 or SHA-2. This means that a new mechanism for negotiating the hash algorithm for a signature algorithm is needed.
This document specifies two things:
1. A new authentication method that includes enough information inside the Authentication payload data so the signature hash algorithm can be extracted (see Section 3).
2. A method to indicate supported signature hash algorithms (see Section 4). This allows the peer to know which hash algorithms are supported by the other end and use one of them (provided one is allowed by policy). There is no requirement to actually negotiate one common hash algorithm, as different hash algorithms can be used in different directions if needed.
The new digital signature method is flexible enough to include all current signature methods (RSA, DSA, ECDSA, RSASSA-PSS, etc.) and add new methods (ECGDSA, ElGamal, etc.) in the future. To support this flexibility, the signature algorithm is specified in the same way that PKIX [RFC5280] specifies the signature of the Digital Certificate, by placing a simple ASN.1 object before the actual signature data. This ASN.1 object contains an OID specifying the algorithm and associated parameters. When an IKEv2 implementation
Kivinen & Snyder Standards Track [Page 3]
RFC 7427 Signature Authentication in IKEv2 January 2015
supports a fixed set of signature methods with commonly used parameters, it is acceptable for the implementation to treat the ASN.1 object as a binary blob that can be compared against the fixed set of known values. IKEv2 implementations can also parse the ASN.1 and extract the signature algorithm and associated parameters.
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].
This document specifies a new "Digital Signature" authentication method. This method can be used with any type of signature. As the authentication methods are not negotiated in IKEv2, the peer is only allowed to use this authentication method if the Notify payload of type SIGNATURE_HASH_ALGORITHMS has been sent and received by each peer.
In this authentication method, the Authentication Data field inside the Authentication payload does not just include the signature value, as do other existing IKEv2 Authentication payloads. Instead, the signature value is prefixed with an ASN.1 object indicating the algorithm used to generate the signature. The ASN.1 object contains the algorithm identification OID, which identifies both the signature algorithm and the hash used when calculating the signature. In addition to the OID, the ASN.1 object can contain optional parameters that might be needed for algorithms such as RSASSA-PSS (see Section 8.1 of [RFC3447]).
To make implementations easier, the ASN.1 object is prefixed by the 8-bit length field. This length field allows simple implementations to know the length of the ASN.1 object without the need to parse it, so they can use it as a binary blob to be compared against known signature algorithm ASN.1 objects. Thus, simple implementations may not need to be able to parse or generate ASN.1 objects. See Appendix A for commonly used ASN.1 objects.
The ASN.1 used here is the same ASN.1 used in the AlgorithmIdentifier of PKIX (see Section 4.1.1.2 of [RFC5280]), encoded using distinguished encoding rules (DER) [CCITT.X690.2002]. The algorithm OID inside the ASN.1 specifies the signature algorithm and the hash function, both of which are needed for signature verification.
Currently, only the RSASSA-PSS signature algorithm uses the optional parameters. For other signature algorithms, the parameters are
Kivinen & Snyder Standards Track [Page 4]
RFC 7427 Signature Authentication in IKEv2 January 2015
either NULL or missing. Note that for some algorithms there are two possible ASN.1 encodings, one with optional parameters included but set to NULL and the other where the optional parameters are omitted. These dual encodings exist because of the way those algorithms are specified. When encoding the ASN.1, implementations SHOULD use the preferred format called for by the algorithm specification. If the algorithm specification says "preferredPresent", then the parameters object needs to be present, although it will be NULL if no parameters are specified. If the algorithm specification says "preferredAbsent", then the entire optional parameters object is missing.
The Authentication payload is defined in IKEv2 as follows:
o Auth Method (1 octet) - Specifies the method of authentication used.
Mechanism Value ----------------------------------------------------------------- Digital Signature 14
Computed as specified in Section 2.15 of [RFC7296] using a private key associated with the public key sent in the Certificate payload and using one of the hash algorithms sent by the other end in the Notify payload of type SIGNATURE_HASH_ALGORITHMS. If both ends send and receive SIGNATURE_HASH_ALGORITHMS Notify payloads, and signature authentication is to be used, then the authentication method specified in this Authentication payload MUST be used. The format of the Authentication Data field is different from other Authentication methods and is specified below.
o Authentication Data (variable length) - See Section 2.15 of [RFC7296]. For "Digital Signature" format, the Authentication Data is formatted as follows:
Kivinen & Snyder Standards Track [Page 5]
RFC 7427 Signature Authentication in IKEv2 January 2015
* ASN.1 Length (1 octet) - This field contains the length of the ASN.1-encoded AlgorithmIdentifier object.
* Algorithm Identifier (variable length) - This field contains the AlgorithmIdentifier ASN.1 object.
* Signature Value (variable length) - This field contains the actual signature value.
There is no padding between the ASN.1 object and the signature value. For hash truncation, the method specified in ANSI X9.62:2005 [X9.62] MUST be used.
The supported hash algorithms that can be used for the signature algorithms are indicated with a Notify payload of type SIGNATURE_HASH_ALGORITHMS sent inside the IKE_SA_INIT exchange.
This notification also implicitly indicates support of the new "Digital Signature" algorithm method, as well as the list of hash functions supported by the sending peer.
Both ends send their list of supported hash algorithms. When calculating the digital signature, a peer MUST pick one algorithm sent by the other peer. Note that different algorithms can be used in different directions. The algorithm OID indicating the selected hash algorithm (and signature algorithm) used when calculating the signature is sent inside the Authentication Data field of the Authentication payload (with Auth Method of "Digital Signature" as defined above).
Kivinen & Snyder Standards Track [Page 6]
RFC 7427 Signature Authentication in IKEv2 January 2015
The Notify payload format is defined in Section 3.10 of [RFC7296]. When a Notify payload of type SIGNATURE_HASH_ALGORITHMS is sent, the Protocol ID field is set to 0, the SPI Size is set to 0, and the Notify Message Type is set to 16431.
The Notification Data field contains the list of 16-bit hash algorithm identifiers from the Hash Algorithm Identifiers of IANA's "Internet Key Exchange Version 2 (IKEv2) Parameters" registry. There is no padding between the hash algorithm identifiers.
This specification does not provide a way for the peers to indicate the public/private key pair types they have. This raises the question of how the responder selects a public/private key pair type that the initiator supports. This information can be found by several methods.
One method to signal the key the initiator wants the responder to use is to indicate that in the IDr (Identification - Responder) payload of the IKE_AUTH request sent by the initiator. In this case, the initiator indicates that it wants the responder to use a particular public/private key pair by sending an IDr payload that indicates that information. In this case, the responder has different identities configured, with each of those identities associated to a public/ private key or key type.
Another method to ascertain the key the initiator wants the responder to use is through a Certificate Request payload sent by the initiator. For example, the initiator could indicate in the
Kivinen & Snyder Standards Track [Page 7]
RFC 7427 Signature Authentication in IKEv2 January 2015
Certificate Request payload that it trusts a certificate authority certificate signed by an ECDSA key. This indication implies that the initiator can process ECDSA signatures, which means that the responder can safely use ECDSA keys when authenticating.
A third method is for the responder to check the key type used by the initiator and use the same key type that the initiator used. This method does not work if the initiator is using shared secret or Extensible Authentication Protocol (EAP) authentication (i.e., is not using public keys). If the initiator is using public key authentication, this method is the best way for the responder to ascertain the type of key the initiator supports.
If the initiator uses a public key type that the responder does not support, the responder replies with a Notify message with error type AUTHENTICATION_FAILED. If the initiator has multiple different keys, it may try a different key (and perhaps a different key type) until it finds a key that the other end accepts. The initiator can also use the Certificate Request payload sent by the responder to help decide which public key should be tried. In normal cases, when the initiator has multiple public keys, out-of-band configuration is used to select a public key for each connection.
Tables 2 and 3 of the "Recommendations for Key Management" [NIST800-57] give recommendations for how to select suitable hash functions for the signature.
This new digital signature method does not tie the Elliptic Curve to a specific hash function, which was done in the old IKEv2 ECDSA methods. This means it is possible to mix different security levels. For example, it is possible to use a 512-bit Elliptic Curve with SHA1. This means that the security of the authentication method is the security of the weakest component (signature algorithm, hash algorithm, or curve). This complicates the security analysis of the system.
IKEv2 peers have a series of policy databases (see Section 4.4 of [RFC4301]) that define which security algorithms and methods should be used during establishment of security associations. To help end users select the desired security levels for communications protected by IPsec, implementers may wish to provide a mechanism in the IKE policy databases to limit the mixing of security levels or to restrict combinations of protocols.
Security downgrade attacks, where more secure methods are deleted or modified from a payload by a man-in-the-middle to force lower levels
Kivinen & Snyder Standards Track [Page 8]
RFC 7427 Signature Authentication in IKEv2 January 2015
of security, are not a significant concern in IKEv2 Authentication payloads, as discussed in this RFC. This is because a modified AUTH payload will be detected when the peer computes a signature over the IKE messages.
One specific class of downgrade attacks requires selection of catastrophically weak ciphers. In this type of attack, the man-in- the-middle attacker is able to "break" the cryptography in real time. This type of downgrade attack should be blocked by policy regarding cipher algorithm selection, as discussed above.
The hash algorithm registry does not include MD5 as a supported hash algorithm, as it is not considered safe enough for signature use [WY05].
The current IKEv2 protocol uses RSASSA-PKCS1-v1_5, which has known security vulnerabilities [KA08] [ME01] and does not allow using newer padding methods such as RSASSA-PSS. The new method described in this RFC allows the use of other padding methods.
The current IKEv2 protocol only allows use of normal DSA with SHA-1, which means the security of the authentication is limited to the security of SHA-1. This new method allows using longer keys and longer hashes with DSA.
MD5 is not included in the hash algorithm list, as it is not considered safe enough for signature hash uses.
Values 5-1023 are Unassigned. Values 1024-65535 are reserved for Private Use among mutually consenting parties.
Kivinen & Snyder Standards Track [Page 9]
RFC 7427 Signature Authentication in IKEv2 January 2015
This specification also adds a new value for SIGNATURE_HASH_ALGORITHMS (16431) to the "IKEv2 Notify Message Types - Status Types" registry and adds a new value for Digital Signature (14) to the "IKEv2 Authentication Method" registry.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008, <http://www.rfc-editor.org/info/rfc5280>.
[CCITT.X690.2002] International Telephone and Telegraph Consultative Committee, "ASN.1 encoding rules: Specification of basic encoding Rules (BER), Canonical encoding rules (CER) and Distinguished encoding rules (DER)", CCITT Recommendation X.690, July 2002.
[KA08] Kuehn, U., Pyshkin, A., Tews, E., and R. Weinmann, "Variants of Bleichenbacher's Low-Exponent Attack on PKCS#1 RSA Signatures", Proceedings of Sicherheit 2008, pp.97-109, 2008.
[ME01] Menezes, A., "Evaluation of Security Level of Cryptography: RSA-OAEP, RSA-PSS, RSA Signature", December 2001.
[NIST800-57] Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid, "Recommendation for Key Management - Part 1: General (Revised)", NIST Special Publication 800-57, March 2007.
Kivinen & Snyder Standards Track [Page 10]
RFC 7427 Signature Authentication in IKEv2 January 2015
[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3279, April 2002, <http://www.rfc-editor.org/info/rfc3279>.
[RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional Algorithms and Identifiers for RSA Cryptography for use in the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 4055, June 2005, <http://www.rfc-editor.org/info/rfc4055>.
[RFC5758] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T. Polk, "Internet X.509 Public Key Infrastructure: Additional Algorithms and Identifiers for DSA and ECDSA", RFC 5758, January 2010, <http://www.rfc-editor.org/info/rfc5758>.
[WY05] Wang, X. and H. Yu, "How to break MD5 and other hash functions", Proceedings of EuroCrypt 2005, Lecture Notes in Computer Science Vol. 3494, 2005.
[X9.62] American National Standards Institute, "Public Key Cryptography for the Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)", ANSI X9.62, November 2005.
Kivinen & Snyder Standards Track [Page 11]
RFC 7427 Signature Authentication in IKEv2 January 2015
This section lists commonly used ASN.1 objects in binary form. This section is not normative, and these values should only be used as examples. If the ASN.1 object listed in Appendix A and the ASN.1 object specified by the algorithm differ, then the algorithm specification must be used. These values are taken from "New ASN.1 Modules for the Public Key Infrastructure Using X.509 (PKIX)" [RFC5912].
The algorithm identifiers here include several different ASN.1 objects with different hash algorithms. This document only includes the commonly used ones, i.e., the ones using SHA-1 or SHA-2 as the hash function. Some other algorithms (such as MD2 and MD5) are not safe enough to be used as signature hash algorithms and are omitted. The IANA registry does not have code points for these other algorithms with RSA Encryption. Note that there are no optional parameters in any of these algorithm identifiers, but all included here need NULL optional parameters present in the ASN.1.
See "Algorithms and Identifiers for PKIX Profile" [RFC3279] and "Additional Algorithms and Identifiers for RSA Cryptography for use in the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile" [RFC4055] for more information.
With DSA algorithms, optional parameters are always omitted. Only algorithm combinations for DSA that are listed in the IANA registry are included.
See "Algorithms and Identifiers for PKIX Profile" [RFC3279] and "PKIX Additional Algorithms and Identifiers for DSA and ECDSA" [RFC5758] for more information.
With ECDSA algorithms, the optional parameters are always omitted. Only algorithm combinations for the ECDSA listed in the IANA registry are included.
See "Elliptic Curve Cryptography Subject Public Key Information" [RFC5480], "Algorithms and Identifiers for PKIX Profile" [RFC3279], and "PKIX Additional Algorithms and Identifiers for DSA and ECDSA" [RFC5758] for more information.
With RSASSA-PSS, the algorithm object identifier must always be id-RSASSA-PSS, and the hash function and padding parameters are conveyed in the parameters (which are not optional in this case). See Additional RSA Algorithms and Identifiers [RFC4055] for more information.
Here the parameters are present and contain the default parameters, i.e., hashAlgorithm of SHA-1, maskGenAlgorithm of mgf1SHA1, saltLength of 20, and trailerField of 1.
Where the NN will be the next payload type (i.e., the value depends on the next payload after this Authentication payload), the LL will be the length of this payload, and after the sha1WithRSAEncryption ASN.1 block (15 bytes) there will be the actual signature, which is omitted here.
Kivinen & Snyder Standards Track [Page 17]
RFC 7427 Signature Authentication in IKEv2 January 2015
Acknowledgements
Most of this work was based on the work done in the IPsecME design team for the ECDSA. The design team members were: Dan Harkins, Johannes Merkle, Tero Kivinen, David McGrew, and Yoav Nir.
Authors' Addresses
Tero Kivinen INSIDE Secure Eerikinkatu 28 Helsinki FI-00180 Finland
EMail: kivinen@iki.fi
Joel Snyder Opus One 1404 East Lind Road Tucson, AZ 85719