Introduction

Prior to RFC 7427, the Internet Key Exchange version 2 (IKEv2) signature based authentication is signaled per algorithm i.e., there is one Authentication Method for RSA digital signatures, one Authentication Method for DSS digital signatures (using SHA-1), and three separate Authentication Methods for the different ECDSA curves with each variant tied to exactly one hash algorithm. This leads to two problems:

• The sending and receiving parties do not know which Hash Algorithm is used to generate a signature.
• Each time there is a new Signature Algorithm, a new Authentication Method is required, which required writing a new RFC specification.

Therefore this design is cumbersome when more signature algorithms, hash algorithms, and elliptic curves need to be supported.

RFC 7427 solves these problems by generalising the IKEv2 signature support to allow any signature method supported by PKIX and also adds a signature hash algorithm negotiation. RFC 7427 defines a new “Digital Signature Authentication” method. This method is flexible to include all current signature methods (RSA, DSA, ECDSA, RSASSA-PSS, etc.) and add new methods (ECGDSA, ElGamal, etc.) in the future.

The libreswan implementation of RFC 7427 is included in version 3.22. RFC 7427 based Digital Signatures is automatically enabled if the “authby” parameter in ipsec.conf is configured to use RSA and it has received SHA-1 in the Hash Algorithm notification payload from the peer indicating that the remote peer supports this method as well.

Implementation

To make Libreswan RFC 7427 compliant, the following items have been implemented :

1. Hash Algorithm Notification

Notify payload of type SIGNATURE_HASH_ALGORITHMS is sent inside the IKE_SA_INIT exchange. The supported hash algorithms like SHA1, SHA2 and IDENTITY are exchanged by the initiator and responder in this notify payload. The negotiated hash algorithm is the one that is supported by both parties. This hash algorithm is used to generate the signature sent in the Authentication payload. The decision of sending the Hash Algorithm notification when Libreswan is the initiator is based on checking which Signature algorithm is configured in the “authby” parameter in ipsec.conf. When Libreswan is the responder, it responds with a hash algorithm notification only if it has received one and SHA-1 was sent in it.

2. Authentication payload

A new Authentication Method called Digital Signature is sent in the IKE_AUTH message exchange. Earlier, the Authentication Data field inside the Authentication payload contained only the signature value. Now, 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 algorithm used for calculating the signature. IANA registry specifies the values to be used. The initial Digital Signature Authentication method implementation for Libreswan only includes support for the RSA with SHA-1 hash algorithm.

The following items are not yet implemented :

• Support for Signature Algorithms ECDSA and RSASSA-PSS
• Support for Hash Algorithm SHA-2

3. Test Suite changes

The Test Suite was extended by adding test cases to verify feature functionality and perform interoperability tests. Negative tests cases were added using impairments that emulate clients that do not support the Digital Signature Authentication scheme as specified by the RFC 7427, to confirm connectivity with clients not supporting the new Authentication Method.

Issues encountered

The RFC 4307bis mandates the usage of RSASSA-PSS along with Digital Signature Authentication. However the older flavour PKCS v1.5 may still be supported. But a way to indicate to the peer, which flavour of RSA should be used is not yet described. Since no other client supports RSASSA-PSS, interoperability tests cannot be performed.

Future work

• Support for Signature algorithms ECDSA and RSASSA-PSS

Implementation of ECDSA requires the extension of the Libreswan’s public key code to remove the hardwiring for RSA. Implementation of RSASSA-PSS would have to use different NSS library method call. The implementation is waiting for RFC 4307bis to clarify the usage of PSS.

• Support for Hash algorithm SHA-2.

SHA2 needs an extended parser for the authby = keyword, in ipsec.conf.

Source code

Feature implementation

Addition and modification of test cases

This project work was sponsored by Google as part of the Google Summer of Code 2017 Program. The implementation for this project is done by Sahana Prasad (sahana.prasad07@gmail.com) under the tutelage of Paul Wouters.