The AES key wrap of RFC 3394, and AES key wrap with padding of RFC 5649.

CMS uses them to encrypt a content-encryption key with another key: the key-encryption key of a KEKRecipientInfo, or the key that a KeyAgreeRecipientInfo derives. They are written in Elixir over the AES block cipher of :crypto.

The key-encryption key is 16, 24 or 32 bytes (AES-128, AES-192 or AES-256). wrap/2 takes key data of 16 bytes or more, in multiples of 8; wrap_with_padding/2 takes any length from 1 byte. Unwrapping checks the integrity value and returns :error when it does not match, which happens with a wrong key-encryption key as well as with a changed ciphertext; nothing tells the two apart.

iex> kek = Base.decode16!("000102030405060708090A0B0C0D0E0F")
iex> key = Base.decode16!("00112233445566778899AABBCCDDEEFF")
iex> wrapped = Cmsex.KeyWrap.wrap(kek, key)
iex> Base.encode16(wrapped)
"1FA68B0A8112B447AEF34BD8FB5A7B829D3E862371D2CFE5"
iex> Cmsex.KeyWrap.unwrap(kek, wrapped)
{:ok, key}