Roadmap towards PQC support (ML-DSA)

Yubico has recently demo'ed a Yubikey with ML-DSA support for FIDO usage. Time to start thinking what it takes to make the code fit for PQC support.

• Keys are extracted using the library web-auth/cose-lib. That library does not currently support ML-DSA keys (https://github.com/web-auth/cose-lib/blob/4.6.x/src/Key/Key.php)
• actual signature checks are done with openssl_verify and the public key loaded with openssl_pkey_get_public. Currently unknown whether recent PHP supports handling of ML-DSA keys.
• underlying openssl library in version 3.5.0+ DOES support ML-DSA keys.

I will run some tests with openssl and the openssl_* functions in PHP to determine how far we can get on that front.

web-auth/cose-lib の src/Key/Key.php におけるキー処理と、モジュールによる openssl_verify および openssl_pkey_get_public の使用から始めます。PHP が OpenSSL 3.5.0 以降で ML-DSA キーを処理できるかテストし、その後、互換性の調査結果とサポートに必要な具体的な作業を文書化してください。