Compliance of RFC 7427 - Signature Authentication in IKEv2

Introduction

Currently in the Internet Key Exchange version 2 (IKEv2), signature based authentication is 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 leads to 2 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.

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 recommends a new digital signature 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.

on Libreswan, RFC 7427 is automatically enabled if the "authby" parameter in ipsec.conf has RSA configured and we receive SHA-1 in the hash algorithm notification.

Implementation

To make Libreswan RFC 7427 compliant, the following items are 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. The negotiated hash algorithm is the one that is supported by both the parties. This hash algorithm is further 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 ipse.conf.

2. Authentication payload

A new authentication method called Digital Signature which is sent in the IKE_AUTH message exchange. Earlier, the Authentication Data field inside the Authentication payload contained only the signature value. Now, 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 while calculating the signature. IANA registry specifies the values to be used. Currently on Libreswan, the Digital Signature Authentication method only includes support for RSA with SHA-1 being negotiated through the hash algorithm notification.

Following items are not yet implemented :

• Support for Signature algorithms ECDSA and RSASSA-PSS
• Support for Hash algorithm SHA-2

3. Test Suite changes

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.

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.