Key space

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In cryptography, an algorithm's key space refers to the set of all possible keys that can be used to initialize it. For example, if an algorithm works using a key that is a string of 10 bits, then its key space is the set of all binary strings of length 10.

To avoid adversaries from guessing the key using a brute-force attack, the key space is usually designed to be extremely large. Another desirable attribute is that the key space be flat, having no or very few weak keys.

Examples

The block cipher Rijndael/AES uses a key of up to 256 bits, resulting in a key space containing over 1.1579 × 10⁷⁷ keys. This makes it computationally infeasible to check each possible key by brute force.

In the DES block cipher, a 56-bit key is used, resulting in a relatively small key space of size 2⁵⁶.

The ROT13 cipher is only intended to prevent people from accidentally reading messages (e.g. movie plot details). As there is no key, the key space is therefore empty.