OpenSSL 4.0: What's New, Release History & EOL Status

What Is New in OpenSSL 4.0

What legacy protocols and engine support were removed in OpenSSL 4.0?

OpenSSL 4.0 drops support for SSLv2, SSLv3, and the engine framework.

• SSLv2 Client Hello is no longer recognized.
• SSLv3 has been removed entirely; it was already disabled by default since 1.1.0.
• The no-engine build option and OPENSSL_NO_ENGINE macro are now always present; all engine APIs are gone.
• Deprecated EC curves required by RFC 8422 are disabled at compile time unless explicitly enabled.

How does OpenSSL 4.0 improve security and compliance features?

The release adds Encrypted Client Hello, stricter X509 verification, and new post-quantum and Chinese cryptographic algorithms.

• ECH (RFC 9849) is now a first-class feature; see SSL_CTX_set_hello_cb() for integration.
• When X509_V_FLAG_X509_STRICT is set, AKID verification is performed automatically.
• CRL verification now includes additional checks for revoked certificates and distribution points.
• New algorithms: SM2/SM3 signatures, curveSM2, curveSM2MLKEM768, cSHAKE (SP 800-185), ML-DSA-MU, SNMP KDF, SRTP KDF.
• FIPS self-tests can be deferred with -defer_tests during openssl fipsinstall.

# Enable ECH on a server
openssl s_server -tls1_3 -ech -cert server.pem -key server.key -port 4433

What API changes should developers be aware of when migrating to OpenSSL 4.0?

Several APIs have been hardened with const qualifiers, opaque types, and removal of global cleanup via atexit.

• ASN1_STRING is now opaque; use accessor functions like ASN1_STRING_get0_data().
• Many X509-related functions now accept const pointers, reducing accidental mutation.
• OPENSSL_cleanup() is invoked automatically by a global destructor; manual calls are optional.
• Lower-bound checks for PKCS5_PBKDF2_HMAC are enforced when using the FIPS provider.
• Hex dump functions now produce fixed widths (24 bytes for signatures, 16 bytes otherwise).

# Optional explicit cleanup (usually unnecessary)
OPENSSL_cleanup();

Which tools and scripts were removed or replaced in OpenSSL 4.0?

The legacy c_rehash script is gone and the unreliable BIO_f_reliable implementation has been removed.

• c_rehash has been removed; use the new openssl rehash command instead.
• The broken BIO_f_reliable() implementation was dropped without replacement.
• The msie-hack option in openssl ca is no longer available.
• Engine loading code has been stripped, simplifying the build system.