Crypto Backends

open62541 does not implement cryptography itself. It delegates to an external library through a thin plugin layer. The build option UA_ENABLE_ENCRYPTION selects which library is compiled in. No security policy from Supported Security Policies is available when this option is OFF.

Backend selection

CMake option	Effect
`UA_ENABLE_ENCRYPTION=OFF`	No encryption support. Default. Only the `None` policy is usable.
`UA_ENABLE_ENCRYPTION=MBEDTLS`	mbedTLS. All RSA profiles + the four NIST/Brainpool ECC curves.
`UA_ENABLE_ENCRYPTION=OPENSSL`	OpenSSL. All RSA profiles + every ECC curve, including Curve25519 and Curve448 (EdDSA).
`UA_ENABLE_ENCRYPTION=LIBRESSL`	LibreSSL. Experimental. Coverage is similar to OpenSSL.

The two boolean aliases UA_ENABLE_ENCRYPTION_OPENSSL and UA_ENABLE_ENCRYPTION_MBEDTLS are still accepted for backward compatibility. Prefer the unified UA_ENABLE_ENCRYPTION form in new projects.

Hardware-backed key storage (TPM2)

UA_ENABLE_ENCRYPTION_TPM2=ON adds TPM2-backed key storage on top of an already-selected UA_ENABLE_ENCRYPTION backend. It is not a standalone crypto library, so it must be combined with MBEDTLS or OPENSSL. When enabled, the private key used by the server lives on a TPM2 device instead of in a DER file. See the server_encryption_tpm_keystore and client_encryption_tpm_keystore examples, and the README_client_server_tpm_keystore.txt next to them for the required software stack.

Library version requirements

mbedTLS ≥ 3.0 is required for the four NIST/Brainpool ECC curves. mbedTLS does not support Curve25519 / Curve448.
OpenSSL ≥ 3.0 is required for full ECC support including EdDSA curves.
See Building open62541 for the exact flag set and the UA_ENABLE_ENCRYPTION_TPM2 option.

Registering policies

The backend is selected at build time, not at runtime. Once built with UA_ENABLE_ENCRYPTION=OPENSSL (or MBEDTLS), adding a policy to a server is one call per policy and the same call works with either backend:

UA_ServerConfig_addSecurityPolicyNone(config, &cert, &key);
UA_ServerConfig_addSecurityPolicyBasic256Sha256(config, &cert, &key);
UA_ServerConfig_addSecurityPolicyEccNistP256(config, &cert, &key);
/* ... or, in one shot: */
UA_ServerConfig_addAllSecurityPolicies(config, &cert, &key);

UA_ServerConfig_addAllSecurityPolicies() auto-detects the key type (RSA vs EC) and registers all policies matching the key. The ECC-specific helpers and the full set of addSecurityPolicy*() calls are listed in ECC Specifics.