AES Key Wrapping

NIST specifies two mechanisms for wrapping (encrypting) symmetric keys using another key. The first (and older, more widely supported) methd requres the input be a multiple of 8 bytes long. The other allows any length input, though only up to 2**32 bytes.

These algorithms are described in NIST SP 800-38F, and RFCs 3394 and 5694.

This API, defined in nist_keywrap.h, first became available in version 2.4.0

These functions take an arbitrary 128-bit block cipher object, which must already have been keyed with the key encryption key. NIST only allows these functions with AES, but any 128-bit cipher will do and some other implementations (such as in OpenSSL) do also allow other ciphers. Use AES for best interop.

std::vector<uint8_t> nist_key_wrap(const uint8_t input[], size_t input_len, const BlockCipher &bc)

This performs KW (key wrap) mode. The input must be a multiple of 8 bytes long.

secure_vector<uint8_t> nist_key_unwrap(const uint8_t input[], size_t input_len, const BlockCipher &bc)

This unwraps the result of nist_key_wrap, or throw Integrity_Failure on error.

std::vector<uint8_t> nist_key_wrap_padded(const uint8_t input[], size_t input_len, const BlockCipher &bc)

This performs KWP (key wrap with padding) mode. The input can be any length.

secure_vector<uint8_t> nist_key_unwrap_padded(const uint8_t input[], size_t input_len, const BlockCipher &bc)

This unwraps the result of nist_key_wrap_padded, or throws Integrity_Failure on error.

RFC 3394 Interface

This is an older interface that was first available (with slight changes) in 1.10, and available in its current form since 2.0 release. It uses a 128-bit, 192-bit, or 256-bit key to encrypt an input key. AES is always used. The input must be a multiple of 8 bytes; if not an exception is thrown.

This interface is defined in rfc3394.h.

secure_vector<uint8_t> rfc3394_keywrap(const secure_vector<uint8_t> &key, const SymmetricKey &kek)

Wrap the input key using kek (the key encryption key), and return the result. It will be 8 bytes longer than the input key.

secure_vector<uint8_t> rfc3394_keyunwrap(const secure_vector<uint8_t> &key, const SymmetricKey &kek)

Unwrap a key wrapped wtih rfc3394_keywrap.