Cryptography Reference
In-Depth Information
Because of the more complex
InvMixColumns
operation, the times for
decryption and encryption can diverge, depending on the implementation,
though this effect can be completely compensated by using the tables described
previously. Of course, the times depend on, in addition to the key length, the block
length and the number of rounds (see Table 11-4). For comparison, on a Pentium
III/200 MHz, throughput of about 8 MByte per second for a key of length 128 bits,
about 7 Mbyte per second for 192-bit keys, and about 6 MByte per second for
256-bit keys for blocks of length 128 bits is achievable in both directions. On the
same platform, the DES in C can encrypt and decrypt about 3.8 MByte per second
11.11 Modes of Operation
The classical operating modes Electronic Code Book (ECB), Cipher Block
Chaining (CBC), Cipher Feedback (CFB), and Output Feedback (OFB) for block
ciphers were updated by NIST for use with AES and provided with appropriate
test vectors (see [FI81, N38A]). Consideration of additional operating modes,
which had begun already in the framework of standardization of AES and which
relates to the use of modes of operation in Internet communication, has resulted
in the following operating modes:
•
Counter Mode (CTR):
A block keystream is generated and joined to the
plain text blocks using XOR.
•
CCM Mode:
To ensure the reliability and integrity of a message, the counter
mode is combined with a message authentication code (MAC) based on
cipher block chaining (see [N38C]).
•
RMAC:
Using a randomized message authentication code, which is still in
development, the validity of a message can be checked with respect to both
its content and its source (see [N38B]).
For further details, investigations into security and cryptanalysis, computa-
tional times, and current information on AES and Rijndael the reader is referred to
the literature cited above as well as the Internet sites of NIST and Vincent Rijmen,
which in turn contain many links to further sources of information:
In the downloadable source code to this topic there is an implementation
of AES in the file
aes.c
, which can be used to deepen an understanding of the
procedure and to do some experimentation.
