OpenSSL 4.0.0 has been officially released, bringing significant new features and breaking changes. Key additions include Encrypted Client Hello (ECH, RFC 9849), post-quantum cryptography support (ML-DSA-MU, curveSM2MLKEM768), cSHAKE function support, and FIPS improvements. Notable removals include SSLv2/SSLv3 support, the engine subsystem, deprecated elliptic curves in TLS, and several deprecated API functions. API signatures have been updated with const qualifiers, ASN1_STRING is now opaque, and OPENSSL_cleanup() behavior has changed. OpenSSL 4.0 will be supported until May 14, 2027.
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