rcgen is a Rust library for X.509 certificate generation, part of the rustls ecosystem. It provides a programmatic API to create self-signed certificates, CA-signed certificates, certificate signing requests (CSRs), and certificate revocation lists (CRLs) with full RFC 5280 compliance. The library supports multiple cryptographic backends (ring, aws-lc-rs), a wide range of signature algorithms (ECDSA, Ed25519, RSA, ML-DSA), and extensible extension handling. It is architected around traits like `SigningKey` and `PublicKeyData` for key abstraction, and uses `yasna` for DER encoding and `pki-types` for standard PKI types. The repository also includes a CLI tool `rustls-cert-gen` for quick certificate generation, and a verification test suite that validates generated certificates against OpenSSL, Botan, and webpki. The codebase demonstrates strong adherence to RFCs, careful error handling, and modular design.