OpenSSL 3.x vs 1.1 on Windows — Breaking Changes and Migration Guide

OpenSSL 1.1.1 reached end-of-life in September 2023 and no longer receives security updates. Upgrading to 3.x or 4.x is strongly recommended.

DLL filenames

Changed DLL names on Windows

This is the most common Windows-specific breaking change. Apps that bundle or link against OpenSSL DLLs by name will break:

File	OpenSSL 1.1.1	OpenSSL 3.x / 4.x
Crypto DLL	libcrypto-1_1-x64.dll	libcrypto-3-x64.dll
SSL DLL	libssl-1_1-x64.dll	libssl-3-x64.dll
Import lib	libcrypto.lib / libssl.lib	libcrypto.lib / libssl.lib (same)

If you bundle DLLs next to your application: replace the 1.x DLLs with the 3.x versions. See Fix libcrypto.dll if you get errors.

Provider model

New provider model in 3.x

OpenSSL 3.0 replaced the old ENGINE system with a provider model. This is the biggest architectural change:

Algorithms are now implemented in loadable providers: default, legacy, fips, base.
FIPS_mode_set(1) no longer exists. FIPS now requires loading the FIPS provider via config. See FIPS Notes.
The old ENGINE API is deprecated and may be removed in a future release.
Some older algorithms (MD4, RC4, DES, Blowfish) moved to the legacy provider and are not available by default.

cmd.exe

# Check loaded providers:

C:\> openssl list -providers

Providers:

default

name: OpenSSL Default Provider

status: active

Deprecated algorithms

Algorithms removed from default in 3.x

These algorithms still exist in the legacy provider but are not loaded by default. Code that uses them will fail with errors like unknown message digest algorithm:

MD4 — legacy provider only
RC4 — legacy provider only
DES / 3DES — legacy provider only
Blowfish, CAST, SEED, IDEA — legacy provider only
MDC2, RIPEMD-160 — legacy provider only

To enable legacy algorithms, load the legacy provider in your openssl.cnf or programmatically via OSSL_PROVIDER_load(NULL, "legacy").

API changes

Key API changes for developers

EVP functions are now mandatory

Many low-level algorithm APIs (RSA_*, DSA_*, EC_KEY_*) are deprecated in 3.x and removed in 4.x. Use the high-level EVP functions (EVP_PKEY_*, EVP_DigestSign*) instead. The EVP API has been available since OpenSSL 1.0 so code using it already works across versions.

X509 and ASN.1 struct members are now opaque

Direct struct member access (e.g. cert->cert_info) no longer compiles. Use the accessor functions instead. This was already recommended practice in 1.1.x but is now enforced.

RAND_pseudo_bytes() removed

RAND_pseudo_bytes() is removed. Replace all calls with RAND_bytes() which is cryptographically secure.

SSL_CTX_set_tmp_rsa_callback() removed

This callback for ephemeral RSA key exchange is removed — ephemeral RSA is no longer supported in TLS. Remove the callback and ensure your server is configured for ECDHE or DHE key exchange instead.

Migration

Migrating from 1.1.1 to 3.x or 4.x on Windows

1
Update the DLL files
Replace libcrypto-1_1-x64.dll and libssl-1_1-x64.dll with libcrypto-3-x64.dll and libssl-3-x64.dll from the new installer. See Update OpenSSL on Windows.
2
Test for deprecated API usage
Build your project with OpenSSL 3.x. Compiler warnings about deprecated functions indicate code that needs updating. Focus on low-level RSA/EC/DSA direct struct access first.
3
Check for legacy algorithm usage
If your code uses MD4, RC4, DES or other legacy algorithms, either migrate to modern equivalents (SHA-256, AES) or explicitly load the legacy provider in your OpenSSL config.
4
Test FIPS configuration if applicable
The old FIPS_mode_set(1) is gone. See FIPS Notes for the new provider-based approach.