Block encryption algorithms are designed for encrypting and decrypting
data in multiple octet chunks. Their identifiers appear as the value of the
Algorithm attributes of EncryptionMethod elements that are children of EncryptedData.
Block encryption algorithms take, as implicit arguments, the data to be encrypted or decrypted, the keying material, and their direction of operation. For all of these algorithms specified below, an initialization vector (IV) is required that is encoded with the cipher text. For user specified block encryption algorithms, the IV, if any, could be specified as being with the cipher data, as an algorithm content element, or elsewhere.
The IV is encoded with and before the cipher text for the algorithms below for ease of availability to the decryption code and to emphasize its association with the cipher text. Good cryptographic practice requires that a different IV be used for every encryption.
Since the data being encrypted is an arbitrary number of octets, it may
not be a multiple of the block size. This is solved by padding the plain text
up to the block size before encryption and unpadding after decrytion.
(This us done after prepending the nonce for encryption.) The
padding algorithm is to calculate the smallest non-zero number of octets, say
N, that must be suffixed to the plain text to bring it up to a multiple
of the block size. We will assume the block size is B octets so N
is in the range of 1 to B. Pad by suffixing the plain text with N-1
arbitrary pad bytes and a final byte whose value is N. On decryption,
just take the last byte and, after sanity checking it, strip that many bytes
from the end of the decrypted cipher text.
For example, assume an 8 byte block size and plain text of 0x616263. The
padded plain text would then be 0x616263????????05 where the "??" bytes can be any value.
Similarly, plain text of 0x2122232425262728 would be padded to 0x2122232425262728??????????????08.
ANSI X9.52 [TRIPLEDES] specifies three sequential FIPS 46-3 [DES] operations. The XML Encryption TRIPLEDES consists of a DES encrypt, a DES decrypt, and a DES encrypt used in the Cipher Block Chaining (CBC) mode with 192 bits of key and a 64 bit Initialization Vector (IV). Of the key bits, the first 64 are used in the first DES operation, the second 64 bits in the middle DES operation, and the third 64 bits in the last DES operation.
Note: Each of these 64 bits of key contain 56 effective bits and 8 parity bits. Thus there are only 168 operational bits out of the 192 being transported for a TRIPLEDES key. (Depending on the criterion used for analysis, the effective strength of the key may thought to be 112 bits (due to meet in the middle attacks) or even less.)
The resulting cipher text is prefixed by the IV. If included in XML output, it is then base64 encoded. An example TRIPLEDES EncryptionMethod is as follows:
<EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#tripledes-cbc"/>
[AES] is used in the Cipher Block Chaining (CBC) mode with a 128 bit initialization vector (IV). The resulting cipher text is prefixed by the IV. If included in XML output, it is then base64 encoded. An example AES EncryptionMethod is as follows:
<EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#aes128-cbc"/>
[AES] is used in the Cipher Block Chaining (CBC) mode with a 128 bit initialization vector (IV). The resulting cipher text is prefixed by the ECB-encrypted IV. The "ECB-encrypted IV" is the IV which was encrypted using the content encryption key in electronic codebook mode (ECB). If included in XML output, it is then base64 encoded. An example AES EncryptionMethod is as follows:
<EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#aes128-cbc-encryptedIV"/>