Symmetric ciphers (CIPHER)

The OpenSSL engine interface provides an opaque EVP_CIPHER type for supporting symmetric cipher implementations other than the built-ins. Via this mechanism, the BCrypt EVP engine provides authenticated encryption (AE) and authenticated encryption with associated data (AEAD) to simultaneously assure the confidentiality and authenticity of data. This is achieved by means of AES in GCM mode of operation, which is supports for cryptographic keys of 128, 192, and 256 bits. In OpenSSL terms, this corresponds to the ciphers with the identifiers NID_aes_128_gcm, NID_aes_192_gcm and NID_aes_256_gcm.

The OpenSSL man page for the EVP cipher routines explains its usage. For GCM specifically, see the section called AEAD Interface. Tests for this algorithm are found in the file test_bcrypt_aes_gcm.cpp, showing example usage as well.

Control commands

Of the GCM-specific control commands documented in the aforementioned man page and section, only EVP_CTRL_AEAD_GET_TAG and EVP_CTRL_AEAD_SET_TAG are currently supported. Alternative identifiers for the same commands are EVP_CTRL_GCM_GET_TAG and EVP_CTRL_GCM_SET_TAG.

Setting the IV size via the EVP_CTRL_AEAD_SET_IVLEN command, or its alternative EVP_CTRL_GCM_SET_IVLEN, is currently not supported.

Known issues or limitations

The functional tests for this algorithm are currently limited to 256 bits keys and they verify authenticated encryption and decryption only.