Add post-quantum cryptography (PQC) support to SSLConfig

[junaruga]

This PR is related to #52.

Proof of concept

I prepared proof-of-concept scripts for this PR.

The document (drb/README.md) is here. A list of the sections in the drb/README.md is below. The sections without "(development)" test with the current master branch. These work. The sections with "(development)" test with this PR.

$ grep ^## drb/README.md
## druby (non-SSL)
## drbssl (SSL) auto-generated RSA cert
## drbssl (SSL) pre-generated RSA cert
## drbssl (SSL) pre-generated ML-DSA-65 cert
## drbssl (SSL) pre-generated RSA cert with client cert
## drbssl (SSL) pre-generated ML-DSA-65 cert with client cert
## drbssl (SSL) auto-generated ML-DSA-65 cert (development)
## drbssl (SSL) auto-generated ML-DSA-65/RSA multi cert (development)
## drbssl (SSL) pre-generated ML-DSA-65/RSA multi cert (development)
## drbssl (SSL) pre-generated ML-DSA-65/RSA multi cert with client cert (development)

The script is here. The CI result is here.

Commit message

DRb::DRbSSLSocket::SSLConfig already works
with pre-generated ML-DSA cert/key by
:SSLCertificate and :SSLPrivateKey config options.

This commit adds the following features in PQC use cases to #setup_certificate and #setup_ssl_context.

• Add :SSLCertificates config option accepting an Array of [certificate, private_key] pairs for dual/multiple certificate support via OpenSSL::SSL::SSLContext#add_certificate changing from OpenSSL::SSL::SSLContext#cert and #key. Because an SSL server that accepts clients with either ML-DSA-NN or RSA certificates is useful in the use case of PQC migration from RSA (non-PQC) to ML-DSA (PQC). :SSLCertificate and :SSLPrivateKey are available for backward compatibility. But use OpenSSL::SSL::SSLContext#add_certificate internally. This is a behavior change.
• Add :SSLPrivateKeyAlgorithms option accepting an Array of key algorithms (RSA, ML-DSA-44, ML-DSA-65, ML-DSA-87) for multi-certificate generation, defaulting to ["RSA"] for backward compatibility.
• Add :SSLGroups option to control key exchange group such as ML-KEM in PQC.
• Add :SSLSignatureAlgorithms, :SSLClientSignatureAlgorithms to control signature algorithms such as ML-DSA-NN in PQC, and RSA in non-PQC.
• Add tool/create_certs.sh to generate test/drb/fixtures/*.{crt,key} to test with pre-generated certs/keys and add DRbTests::Fixtures module to read the certs/keys.
• Add test/drb/test_ssl.rb to test lib/drb/ssl.rb as an unit test level. This approach aligns with test/drb/test_acl.rb to test lib/drb/acl.rb as an unit test level.
• Add TestDRbSSLPQC, TestDRbSSLPQCMultiCertMLDSA65, TestDRbSSLPQCMultiCertRSA in test/drb/test_drbssl.rb. TestDRbSSLPQC is to test single PQC ML-KEM/ML-DSA server via test/drb/ut_drb_drbssl_pqc.rb. The testing class is inspired by TestDRbSSLCore. TestDRbSSLPQCMultiCertMLDSA65 and TestDRbSSLPQCMultiCertRSA are to test ML-DSA-65 (PQC) and RSA (non-PQC) dual (multiple) certificate server with client certificate via test/drb/ut_drb_drbssl_pqc_multi_cert.rb. The test is designed for a high-security use case with :SSLVerifyMode OpenSSL::SSL::VERIFY_PEER | OpenSSL::SSL::VERIFY_FAIL_IF_NO_PEER_CERT.

Assisted-by: Claude:claude-opus-4-6[1m]

Notes

CI cases supporting PQC

PQC (ML-KEM/ML-DSA) works in OpenSSL >= 3.5. OpenSSL 3.5 added the feature. And works in Ruby OpenSSL >= 4.0. Ruby OpenSSL 4.0.0 added some functions in PQC use cases, and fixed a bug.

The .github/workflows/test.yml uses runner ubuntu-latest, macos-latest, windows-latest.

According to https://github.com/actions/runner-images, ubuntu-latest is ubuntu-24.04, macos-latest is macOS-26-arm64, windows-latest is windows-2025-vs2026.

The ubuntu-24.04 uses OpenSSL 3.0.13 which doesn't support PQC. The windows-2025-vs2026 uses OpenSSL 3.6.3 which supports PQC. The macos-26 uses OpenSSL 3.6.2 which supports PQC. The new PQC tests should be executed on the macos-latest and windows-latest CI cases.

test/drb/test_ssl.rb testing lib/drb/ssl.rb as an unit test level

I checked all the existing test/drb/test_*.rb files.

• test/drb/test_acl.rb tests drb/acl.rb as an unit test level.
• test/drb/test_drbobject.rb tests DRbObject class as an unit test level.
• test/drb/test_drb.rb tests druby protocol as an integration test level.
• test/drb/test_drbssl.rb tests drbssl protocol as an integration test level.
• test/drb/test_drbunix.rb is tests drbunix protocol as an integration test level.

So, I created test/drb/test_ssl.rb to test DRb::DRbSSLSocket::SSLConfig in lib/drb/ssl.rb as an unit test level.

tool/create_certs.sh design decisions

The tool/create_certs.sh is inspired by ruby/rubygems#9678. The directory test/drb/fixtures to manage SSL key/cert files is inspired by ruby/openssl managing SSL keys in test/openssl/fixtures/pkey directory. ruby/openssl manages only key files, not certificate files. ruby/openssl is generating testing certificates from the keys in tests.

Testing in RubyCI servers

I plan to test this PR with RHEL 9 server (OpenSSL version is >= 3.5, but OpenSSL config disables PQC by default), which is the case of support_pqc_handshake? is false ,and OpenBSD server as I saw the following logic in DRbSSLService, test/drb/test_drbssl.rb.

    if RUBY_PLATFORM.match?(/openbsd/)
      config[:SSLMinVersion] = OpenSSL::SSL::TLS1_2_VERSION
      config[:SSLMaxVersion] = OpenSSL::SSL::TLS1_2_VERSION
    end

[junaruga]

I see the following CI failure. Let me fix it.

https://github.com/ruby/drb/actions/runs/30444292244/job/90550763624?pr=53

[junaruga]

The reason for this change is because OpenSSL::PKey::PKey class for ML-DSA-NN doesn't have #public_key unlike OpenSSL::PKey::RSA. So, I picked up the following OpenSSL::PKey::RSA#public_key internal logic.

https://github.com/ruby/openssl/blob/9796ee8f47003ec78fd8e052bc8dd6d1fcb0ae2b/lib/openssl/pkey.rb#L353-L355