Multiple decipherment and encipherment

CCA uses multiple encipherment and decipherment to protect and retrieve cryptographic keys and PIN data.

CCA uses multiple encipherment whenever it enciphers a key under a key-encrypting key such as the master key or the transport key using the WRAP-ECB key wrapping method, and in triple-DES encipherment for data privacy. Multiple encipherment is superior to single encipherment because multiple encipherment increases the work needed to break a key. CCA provides extra protection for a key by enciphering it under an enciphering key multiple times rather than once. The multiple encipherment method for keys enciphered under a key-encrypting key uses a double-length (128-bit) key split into two 64-bit halves. Like single encipherment, multiple encipherment uses a DES based on the electronic code book (ECB) mode of encipherment.

Keys can either be double-length or single-length depending on the installation and their cryptographic function. When a single-length key is encrypted under a double-length key, multiple encipherment is performed on the key. In the multiple encipherment method, the key is encrypted under the left half of the enciphering key. The result is then decrypted under the right half of the enciphering key. Finally, this result is encrypted under the left half of the enciphering key again.

When a double-length key is encrypted with multiple encipherment, the method is similar, except CCA uses two enciphering keys. One enciphering key encrypts each half of the double-length key. Double-length keys active on the system have two master key variants used when enciphering them.

Multiple encipherment and decipherment is not only used to protect or retrieve a cryptographic key, but they are also used to protect or retrieve 64-bit data in the area of PIN applications. For example, the following two sections use a double-length *KEK as an example to cipher a single-length key even though the same algorithms apply to cipher 64-bit data by a double-length PIN-related cryptographic key.

CCA also supports triple-DES encipherment for data privacy using double-length and triple-length DATA keys. For this procedure the data is first enciphered using the first DATA key. The result is then deciphered using the second DATA key. This second result is then enciphered using the third DATA key when a triple-length key is provided or reusing the first DATA key when a double-length key is provided.

Note that an asterisk (*) preceding the key means the key is double-length. Notations in this chapter have the following meaning:

eK(x), where x is enciphered under K
dK(y) represents plaintext, where K is the key and y is the ciphertext

Therefore, dK(eK(x)) equals x for any 64-bit key K and any 64-bit plaintext x.

When a key (*K) to be protected is double-length, two double-length *KEKs are used. One *KEK is used for protecting the left half of the key (*K); another is for the right half. Multiple encipherment is used with the appropriate *KEK for protecting each half of the key.