# Tag Info

6

No, that is impossible. The reason is simple: How would you decrypt this? If you input the ciphertext and the key into the decryption function, than you have to get exactly one output, not two. How would you decide which output is the correct one? The DES encryption and decryption functions are bijective under one given key. This means that for every ...

4

If you mean DES as block cipher without mode of operation then no, this is impossible. DES is a block cipher, and block ciphers are Pseudorandom Permutations (PRP). As permutations in turn are bijective functions of $\{0,1\}^n$ to $\{0,1\}^n$ there is always a one to one relationship between plaintext and ciphertext. If this wasn't the case then you would ...

3

Yes, it can; within the DES round function, two different 'right side' inputs can, after the sboxes, come up with the same value to xor into the 'left side'. This was a deliberate decision by the DES designers, who thought that this was an important property. I don't know their reasoning about why they thought it was important.

1

First, note that $192=3\cdot64$, so the real key length of 3DES is $192$ bits. However, since $8$ bits in each subkey are parity bits, this reduces to $3\cdot56=168$ bits of non-redundant key material. Now, the reason that 3DES' effective key length is usually classified as $2\cdot56=112$ bits is that 3DES is susceptible to a meet-in-the-middle attack: When ...

1

DES has been specified to take a 64 bit key, but only 56 of them are used. The remainder are parity bits. The key ostensibly consists of 64 bits; however, only 56 of these are actually used by the algorithm. Eight bits are used solely for checking parity, and are thereafter discarded. Hence the effective key length is 56 bits For 3DES the nominal key ...

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