1
$\begingroup$

Question: If a cipher’s key size is $k$ bits and its block size is $b$ bits, how many possible keys are there, how many possible plaintext blocks are there, and how many possible ciphertext blocks are there?

Studying for an exam and this popped up in a past paper without an answer, is the solution e.g. for key size $2^k$ bits? and similarly for the others will be $2^b$ bits? My friend thinks its $b^k$ so were unsure and clearly don't understand the logic behind it.

$\endgroup$
1
$\begingroup$

The answers very are very easy if you know about binary representations.

$k$-bit means the binary representation contains $k$ binary digits.

xx......xx : k binary digits

For each position can be 0 and 1 so, a $k$-bit can take $2^k$ distinct values.

You can similarly argue for $b$-bit block size, if you know that block size is the input size for block ciphers.

Since block-ciphers are keyed permutations the input space equal to output space.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.