AESKW

AES Key Wrap (AESKW) is a family of algorithms for securely wrapping cryptographic keys with a key-encryption key (KEK) using the AES block cipher. The design provides confidentiality and integrity protection for keys that are moved or stored. AESKW is defined in RFC 3394 (AES Key Wrap) and extended by RFC 5649 (AES Key Wrap with Padding).

How it works: The plaintext key to be wrapped is treated as a sequence of 64-bit blocks. An 8-byte initial value A, set to a fixed constant (0xA6A6A6A6A6A6A6A6), is combined with the data blocks and repeatedly encrypted under the KEK through six rounds of AES-based mixing. The wrapped key consists of the final A and the data blocks B1 through Bn. Unwrapping reverses the process and verifies integrity via the A value. The scheme provides both confidentiality of the wrapped key and an integrity check through the structure of A.

Variants and extensions: RFC 5649 introduces AES Key Wrap with Padding, which supports wrapping of keys whose length is not a multiple of 8 bytes by adding a padding mechanism and embedding length information in A. This makes AESKW applicable to a wider range of key material.

Usage and interoperability: AESKW is widely used in key management and secure storage systems to protect keys at rest or in transit. It is also employed in JSON Web Encryption (JWE) as the Akw algorithms (for example, A128KW, A256KW) to wrap content-encryption keys (CEKs).

Security considerations: The security of AESKW relies on the secrecy of the KEK and correct implementation of the standard. If the KEK is compromised, all wrapped keys are exposed. Proper key management, adherence to RFC 3394/5649, and use of appropriate KEK lengths (e.g., 128, 192, or 256 bits) are essential.