AES implementations

There are various implementations of the Advanced Encryption Standard, also known as Rijndael. There are various implementations of the Advanced Encryption Standard, also known as Rijndael. Rijndael is free for any use public or private, commercial or non-commercial. The authors of Rijndael used to provide a homepage for the algorithm. Care should be taken when implementing AES in software. Like most encryption algorithms, Rijndael was designed on big-endian systems. For this reason, little-endian systems, which include the common PC, return correct test vector results only through swapping bytes of the input and output words. The algorithm operates on plaintext blocks of 16 bytes. Encryption of shorter blocks is possible only by padding the source bytes, usually with null bytes. This can be accomplished via several methods, the simplest of which assumes that the final byte of the cipher identifies the number of null bytes of padding added. Careful choice must be made in selecting the mode of operation of the cipher. The simplest mode encrypts and decrypts each 128-bit block separately. In this mode, called electronic code book (ECB), blocks that are identical will be encrypted identically; this is entirely insecure. It makes some of the plaintext structure visible in the ciphertext. Selecting other modes, such as using a sequential counter over the block prior to encryption (i.e., CTR mode) and removing it after decryption avoids this problem.