Block Cipher

Tags

cryptography

Is a [see page 3, kind] of Cipher that reversibly maps an entire fixed-length block of bits, using some key, to another fixed-length block of bits. Without knowledge of the key the affect of a block-cipher should look like a random association.

Such ciphers can take advantage of permutation or substitution, unlike stream ciphers, because they can see adjacent bits in the block when deciding how to manipulate them. Stream ciphers only have access to the current plaintext and key bit and thus must perform some operation using those directly.

Block ciphers are preferred for general cryptography because their comparatively fast (to Public Key Cryptography) and bulk encryption is often carried out with block ciphers. There are [see page 6, many] block ciphers, including:

• DES - Data Encryption Standard
• LUCIFER - Precursor to DES
• IDEA - Used in PGP
• AES - A replacement for DES

Initial Block Generation

Many block-cipher algorithms use a initial-block for encryption/decryption. Usage of the same initial block across different messages can make it clear when they share the same initial-sequence (because the algorithm will produce the same ciphertext).

Initial blocks [see page 28, should] never be repeated between multiple messages. This can be done by using a random initial block or incrementing from an initial-block every time we start encrypting a new message.