DES works with 64 bit blocks of text using a 64 bit key (with 8 bits used for parity, so the effective key length is 56 bits).
Some people are getting the Key Size and the Block Size mixed up. The block size is usually a specific length. For example DES uses block size of 64 bits which results in 64
bits of encrypted data for each block. AES uses a block size of 128 bits, the block size on
AES can only be 128 as per the published standard FIPS-197.
A DES key consists of 64 binary digits ("0"s or "1"s) of which 56 bits are randomly
generated and used directly by the algorithm. The other 8 bits, which are not used by the
algorithm, may be used for error detection. The 8 error detecting bits are set to make the
parity of each 8-bit byte of the key odd, i.e., there is an odd number of "1"s in each 8-bit
byte1. Authorized users of encrypted computer data must have the key that was used to
encipher the data in order to decrypt it.
IN CONTRAST WITH AES
The input and output for the AES algorithm each consist of sequences of 128 bits (digits
with values of 0 or 1). These sequences will sometimes be referred to as blocks and the
number of bits they contain will be referred to as their length. The Cipher Key for the AES
algorithm is a sequence of 128, 192 or 256 bits. Other input, output and Cipher Key lengths
are not permitted by this standard.
The Advanced Encryption Standard (AES) specifies the Rijndael algorithm, a symmetric
block cipher that can process data blocks of 128 bits, using cipher keys with lengths of 128,
192, and 256 bits. Rijndael was designed to handle additional block sizes and key lengths,
however they are not adopted in the AES standard.
The AES algorithm may be used with the three different key lengths indicated above, and
therefore these different "flavors" may be referred to as "AES-128", "AES-192", and "AES-
256".
The other answers are not correct because:
"64 bits of data input results in 56 bits of encrypted output" is incorrect because while DES
does work with 64 bit block input, it results in 64 bit blocks of encrypted output.
"128 bit key with 8 bits used for parity" is incorrect because DES does not ever use a 128
bit key.
"56 bits of data input results in 56 bits of encrypted output" is incorrect because DES
always works with 64 bit blocks of input/output, not 56 bits.
Reference(s) used for this question:
Official ISC2 Guide to the CISSP CBK, Second Edition, page: 336-343
http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf
http://csrc.nist.gov/publications/fips/fips46-3/fips46-3.pdf