Block size (cryptography)

In modern cryptography, symmetric key ciphers are generally divided into stream ciphers and block ciphers. Block ciphers operate on a fixed length string of bits. The length of this bit string is the block size. Both the input (plaintext) and output (ciphertext) are the same length; the output cannot be shorter than the input — this follows logically from the Pigeonhole principle and the fact that the cipher must be reversible — and it is undesirable for the output to be longer than the input.

Until the announcement of NIST's AES contest, the majority of block ciphers followed the example of the DES in using a block size of 64 bits (8 bytes). However the Birthday paradox tells us that after accumulating a number of blocks equal to the square root of the total number possible, there will be an approximately 50% chance of two or more being the same, which would start to leak information about the message contents. Thus even when used with a proper encryption mode (e.g. CBC or OFB), only 2³² x 8 B = 32 GB of data can be safely sent under one key. In practice a greater margin of security is desired, restricting a single key to the encryption of much less data - say a few hundred megabytes. Once that seemed like a fair amount of data, but today it is easily exceeded. If the cipher mode does not properly randomise the input, the limit is even lower.

Block size

Block size can refer to:

Block (data storage)

In computing (specifically data transmission and data storage), a block, sometimes called a physical record, is a sequence of bytes or bits, usually containing some whole number of records, having a maximum length, a block size. Data thus structured are said to be blocked. The process of putting data into blocks is called blocking, while deblocking is the process of extracting data from blocks. Blocked data is normally stored in a data buffer and read or written a whole block at a time. Blocking reduces the overhead and speeds up the handling of the data-stream. For some devices such as magnetic tape and CKD disk devices blocking reduces the amount of external storage required for the data. Blocking is almost universally employed when storing data to 9-track magnetic tape, to NAND flash memory, and to rotating media such as floppy disks, hard disks, and optical discs.

Most file systems are based on a block device, which is a level of abstraction for the hardware responsible for storing and retrieving specified blocks of data, though the block size in file systems may be a multiple of the physical block size. This leads to space inefficiency due to internal fragmentation, since file lengths are often not integer multiples of block size, and thus the last block of a file may remain partially empty. This will create slack space, which averages half a block per file. Some newer file systems attempt to solve this through techniques called block suballocation and tail merging.