Introduction

As part of Google summer of Code work in 2017 described in , RFC-7427 Digital Signature Authentication was implemented with support for RSA. This work is an extension to support ECDSA. Implementation of ECDSA requires the modification of the existing Libreswan public key code to fix the RSA only parts so that it is able to accept different new types of keys in the future ( not just limited to ECDSA ). This will ensure compliance to RFC-7427 and RFC-8247.

As per RFC-4754, ECDSA signatures are smaller than RSA signatures of similar cryptographic strength. ECDSA public keys (and certificates) are smaller than similar strength DSA keys, resulting in improved communications efficiency. Furthermore, on many platforms, ECDSA operations can be computed more quickly than similar strength RSA or DSA operations for a security analysis of key sizes across public key algorithms.

Implementation

To make Libreswan RFC 7427 and RFC 8247 compliant, the following items have been implemented :

1. Fixing the RSA only public key code

Major code changes were done in ikev2_keys.c , x509.c and secrets.c. New structures are defined for Private and public ECDSA key parameters. Most functions that had a specific check for rsa are now able to also handle ecdsa as public key algorithm. This involves checking if a certificate is of type ecdsa and extracting the public key from it and storing it in the NSS Database. Code changes are made to obtain the keyid and ckaid of the ECDSA public keys. These IDs are used to retrieve the public key from pluto secrets and extract the private key from the certificate respectively. New public key algorithm and policy , PUBKEY_ALG_ECDSA and POLICY_ECDSA are introduced respectively.

2. Signature Verification

To verify a signature from the peer, the public key of the peer has to be retrieved from pluto_pubkeys and check if the keyed matches. Inorder to verify the hash received by the peer, we also compute our own hash using the PRF(SK_d,ID[ir]). The computed hash, received Signature and the retrieved Public are then used by NSS API : ECDSA_VerifyDigest to verify the Signature. The ECDSA Signature is DER encoded and is as follows :

Ecdsa-Sig-Value ::= SEQUENCE {

          r     INTEGER,           s     INTEGER  }

Therefore before using the ECDSA_VerifyDigest API, the Signature must be DER decoded to obtain the integers r and s. EC_FillParams is used to fill the ecParams of the public key.

3. Signature generation

Private key is retrieved using the API PK11_FindKeyByKeyID or PK11_FindKeyByAnyCert through the CKAID. Signature generation is performed by using the PK11_Sign API

4. Test Suite changes

The Test Suite was extended by adding test cases to verify feature functionality and perform interoperability tests with strong swan.

Configuring ECDSA in Libreswan

Support for configuring authby=ecdsa

Possible options for setting ecdsa as the public key to be used for Digital Signature Authentication with appropriate SHA2 hash algorithm are as follows :

authby = ecdsa/ecdsa-sha2_256, ecdsa-sha2_384, ecdsa-sha2_512

Issues encountered

NSS looks for specific x509v3 certificate extensions in the end certificates and It is unclear which one is exactly missing in the certificates being used. This error has be be debugged further (as there is little information from the nss logs): SECERR: 35 (0x23): Certificate extension not found. It was found that this error was set and not cleared by NSS. It was solved by checking for errors only when private key retrieved was not equal to NULL.

Future work

Interoperability with Apple and Elvis Plus

Source code

https://github.com/libreswan/libreswan/commit/12f2f1a03de214e1e3ecf5cfa84950f09a8d35c4

https://github.com/libreswan/libreswan/commit/c6a711c091974b323feb61b3ea5c86713b80ea63

This project work was sponsored by Google as part of the Google Summer of Code 2018 Program. The implementation for this project is done by Sahana Prasad (sahana.prasad07@gmail.com) under the tutelage of Paul Wouters and Andrew Cagney

Additional Work during GSoC 2018

Implementation of RSA-PSS and support for SHA2 and it’s variants. The work was started before GSoC 2018 but was fully completed and tested during the GSoC 2018 period. Work is described in this project page RSA-PSS Support in compliance with RFC 7427 and RFC 8247.