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I know that there are approximately 2^88 diff erent permutations of 26 characters. The stream cipher A5/1, which generates a binary keystream using three irregularly clocked LFSR, and has a 64-bit key.

If we consider 1 000 characters of English text encrypted using either a substitution cipher or encoded in ASCII and encrypted using A5/1. From which ciphertext is it easiest to guess the plaintext?

In my opinion, the Substitution cipher is easy to guess. In the substitution cipher, it has repeated characters in the cipher-text make easier to figure out the text. Also, the substitution cipher has 26! possibilities. Can someone explain it to me in a more detail way?

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You're right.

For a substitution cipher the huge size of the keyspace is quite irrelevant in terms of security: it does not change the frequencies of letters, just maps them to other letters. So you you can go from the most common letter and map that, then the 2. most common, etc. And after maybe the 10 most common letters you can use a dictionary with English words and the partially mapped words to exclude other possibilities to get the remaining mappings. "Security" is barely better than a Caesar cipher.

For A5/1 it took over 10 years to be broken, so in contrast it is is not trivially broken by modern computers, like basically any classical cipher.

I guess the point of the exercise is: A large keyspace does not mean, the cipher is secure.

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    $\begingroup$ An alternative phrasing: there is a difference between attacking the cipher and attacking the key. A large key space doesn't matter if you are not attacking the key. $\endgroup$ – Jörg W Mittag Aug 16 '18 at 14:44

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