Introduction

This project aims to extend the RFC-7427 Digital signature authentication scheme to support EdDSA as per RFC 8420. Since Libreswan relies on the NSS library for cryptographic operations, modifications in the NSS code are required to add the implementation of EdDSA. The mechanisms for generating Ed25519 keys, generating and verifying Ed25519 signatures need to be added to the NSS library.

As per RFC 8420, a new value “Identity” for the IKEv2 SIGNATURE_HASH_ALGORITHMS notification registry must be sent to indicate this support. Sending this new value in the IKE_AUTH Message Exchange indicates that the peer supports at least one signature algorithm that accepts messages of an arbitrary size such as Ed25519 or Ed448.

Implementation

Implementation of EdDSA required code changes in the NSS crypto library as well as in libreswan itself.

NSS Implementation

The freebl is the base cryptographic layer where the primitives are implemented. The formally verified implementation (HACL* Library) of Ed25519 was integrated and EDDSA_SignDigest, EDDSA_VerifyDigest(), and ED_NewKey() were added to in the freebl layer. The Softoken code is the PKCS#11 plumbing layer. The current PKCS#11 spec defines the CKM_EDDSA mechanism which is needed to add support for EdDSA. This mechanism was implemented and the functions EDDSA_SignDigest() and EDDSA_VerifyDigest() were hooked into NSC_SignInit() and NSC_VerifyInit() using the stub functions nsc_EDDSASignStub() and nsc_EDDSAVerifyStub(). This code was submitted to NSS upstream for inclusion. Good feedback was received and the code is being merged.

The test suite of the gtests (freebl_gtest and pk11_gtest) was extended and test vectors from RFC 8032 were included. The files Hacl_Ed25519.c, Hacl_Ed25519.h, ed25519_unittest.cc, pk11_eddsa_unittest.cc, pk11_eddsa_vectors.h were added. Modifications were made to pk11_signature_test.h as EdDSA does not require prehashing of the messages to generate and verify signatures. Other modifications were made in blapi.h, pk11table.c, ec.c, loader.c, loader.h, pk11mech.c, pkcs11c.c.

Libreswan implementation

Changes were made in confread.c to parse the new eddsa option. Connection policy, ASN.1 Object and structures for the public and private keys were added to libreswan. Libreswan already supported ECDSA. To avoid too much code duplication, functions were renamed and code was generalized to EC that can handle both ECDSA and EDDSA. Modifications were made in secrets.c to generalize the code. RFC-8420 specifies that EdDSA versions that do not require prehashing of the message SHOULD be used. To signal that no hashing needs to be done, the standard defines a new value “Identity” in the SIGNATURE HASH ALGORITHMS IANA Registry. This new hash algorithm IKEv2_HASH_ALGORITHM_IDENTITY was added and its structure was defined in crypt_hash.c. To generate and verify signatures, ikev2_eddsa.c was added and changes were made to v2_calculate_sighash(), authsig_and_log_using_pubkey() to include support for AUTHBY_EDDSA.

To test the new code, dest_certs.py and strongswan-ec-gen.sh were extended to generate Ed25519 certificates and new test cases were added to verify feature functionality and to perform interoperability tests with strongswan to verify proper implementation.

To use EdDSA, the user must set the “authby” value of the connection in the ipsec.conf to EdDSA (authby=eddsa).

Issues encountered and Future Work

Currently, the NSS library supports curve25519 and uses it for Key Exchange but the public keys generated can not be used for the Ed25519 signature scheme even though both of these generate keys using the same curve. This is caused by NSS’s public key generation code that currently uses the encoding of the “x” coordinate of a point on the Curve25519 curve. Whereas an Ed25519 public key is encoded using the “x” and “y” coordinates of a point on the Curve25519 curve.

A separate SECOID needs to be added to support Ed25519 x509 certificates. Changes need to be made to secsign.c and secvfy.c to recognize and pass around the new OID value added for Ed25519. The new OID has to be added to nss/lib/util/secoid*. The oid table in the secoid.c will link the new SECOID to the CKM_EDDSA mechanism. With this functionality in place, libreswan can use the EDDSA algorithm as specified in RFC 8420.

Source code

NSS modifications: https://github.com/Rishabh-Kumar-07/nss/commit/ad50771b411fdf27251be9c5842180f2153e7e4a

libreswan modifications:

1. https://github.com/Rishabh-Kumar-07/libreswan/commit/245dd9726bcf7ebd6efb81dc76c55837d25c42d8

2. https://github.com/Rishabh-Kumar-07/libreswan/commit/14b0e0704fbc7b2afecae8e0f02390495bd69b25

3. https://github.com/Rishabh-Kumar-07/libreswan/commit/0469e43d7df35413de7c654b0c745e7c54469e18

This project work was sponsored by Google as part of the Google Summer of Code 2021 Program. The implementation for this project is done by Rishabh Kumar (cs19mtech11026@iith.ac.in) under the mentorship of Paul Wouters and Andrew Cagney.

Note: The commits had been accepted by the libreswan and will be included in the release once support for EdDSA has been added to the NSS library.