![]() This is actually reasonably long it's weak by modern standards, but in the 1980s and 1990s would have been fine. That's why you'd look at the smallest you could possibly make the key, which is 89 bits. ![]() But letters are not bits if each letter is encoded in 5 bits, this is a 130-bit key, but very few of the 130-bit keys are valid. Expressed another way, it has a 26-letter key, but not all 26-letter keys are valid keys (AA.AA isn't, for instance). If you compress the key as much as possible, you can fit it in 89 bits ($2^$), so if you want to directly compare key length with other symmetric algorithms (to talk about brute-forcing) you'd say it has an 89-bit key. With monoalphabetic substitution ciphers, you have $26!$ keys, but key length depends on how you're expressing the key. With asymmetric ciphers, it's more complicated. With symmetric ciphers, where any string of $n$ bits is a valid key, the key space has $2^n$ elements. The key space is the set of all possible keys. With algorithms like that, there's generally some standard thing whose length is the key length (with RSA, it's the modulus). With RSA, it's more complicated - the key has a bunch of elements, and can be written in various formats or with more or less info. With digital symmetric ciphers, it's fairly simple, because those tend to have a key that's just a string of some number of bits, and any string of that length is a valid key. ![]() It's a term whose meaning has evolved over time these days, it typically means length in bits.
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