OpenSSL 4.0.0 Released

OpenSSL 4.0.0 has been released. This release is a major turning point including numerous new features and breaking changes.

The key changes are as follows.

• Built-in ECH (Encrypted Client Hello, RFC 9849) support means a separate implementation is no longer needed to protect TLS client privacy.
• SSLv2 Client Hello, SSLv3, and engine code have been completely removed, finalizing the break with legacy protocols.
• RFC 8998-based SM2 signatures (sm2sig_sm3), key exchange (curveSM2), and post-quantum group curveSM2MLKEM768 support have been added.
• New cryptographic features have been introduced, including cSHAKE, ML-DSA-MU digest, SNMP KDF, and SRTP KDF.
• RFC 7919-based FFDHE key exchange negotiation is now possible in TLS 1.2.

There are also significant changes on the security and operations side.

• When installing the FIPS module, the -defer_tests option now allows deferred execution of FIPS self-tests.
• A lower bound check is now enforced when using the FIPS provider with PKCS5_PBKDF2_HMAC.
• Additional checks have been reinforced for CRL validation as well.

The API and build system have also been modernized.

• const qualifiers have been added to the signatures of numerous API functions.
• ASN1_STRING opacification has been carried out.
• For deprecated functions such as X509_cmp_time(), using X509_check_certificate_times() is recommended.
• A large amount of legacy functionality has been cleaned up, including deprecated custom EVP_CIPHER, EVP_MD, EVP_PKEY methods, fixed SSL/TLS version functions, c_rehash, and BIO_f_reliable().
• Old Apple build targets such as darwin-i386 and darwin-ppc have also been removed.
• On Windows, choosing between static/dynamic VC runtime linking is now supported.

Overall, this release marks a turning point that realigns OpenSSL around strengthened security, standards conformance, and legacy cleanup.