Indicate T (True) or F (false):

1. DES is a block cipher symmetrical encryption;
2. DES is a Feistel cipher with 16 rounds;
3. DES has a 56-bit ock length;
4. DES uses a 64-bit key;
5. Each round of DES uses a 48-bit subkey and each subkey consists of a 48-bit subset of the 56-bit key;
6. A popular variant of DES known as triple DES, or 3DES;
7. The existence of a permutation box, or P-Box, in a round function in the DES scheme serves no security purpose. One plausible explanation is that the designers wanted to make DES more difficult to implement in software since the original design called for special hardware.