What could be the keylength of the DES encryption if we have an output of 64 bytes?


DES has a 56-bit key in 8-bytes. The length of the output depends on the length of the input, mode and padding, not the key.

Note: The LSb (least significant bit) of each key byte is reserved for parity, depending on the DES implementation parity may or not be checked.

See: Data Encryption Standard.

  • 2
    $\begingroup$ Note that I have seen DES being used as synonym for 2 key or 3 key triple DES (which one is chosen during key configuration). That's of course not a correct use of the DES acronym (as there are other schemes based on DES), but I think it may be interesting for real world scenarios. $\endgroup$
    – Maarten Bodewes
    Sep 16 '17 at 14:21

DES is the block cipher with the

  • fixed key length of 56 bits, and
  • the fixed block size of 64 bits.

It is co called cryptographic primitive (something as building block for constructing real ciphers).

To cipher some message it must be first divided / padded to the integer number of 64-bits blocks.

So the key length must be 56-bits.

BTW., DES is now considered not be safe (mainly because of that short key length). Triple DES (3DES) is now (relatively simple) substitution for it (with the 3 times longer, i. e. 168-bits key).

  • 1
    $\begingroup$ 1. CTR mode does not require padding. 2. There is also a lot of 2TDEA with a 112 bit key and that has been deprecated. 3. 3DES provides is only 112 bits of security due to a meet-in-the-middle attack. 4. Arguably a better upgrade for DES is AES. $\endgroup$
    – zaph
    Sep 15 '17 at 23:30

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