*Kiddle Encyclopedia.*

# Key space facts for kids

In cryptography, the **key space** of an algorithm refers to the set of all possible keys that can be used to initialize the cryptographic algorithm. 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 bits; i.e. we have key space of size 2^{10} = 1024.

To avoid attackers from guessing the key using a brute-force attack, the key space is usually designed to be very large. Another needed attribute is that the key space designed to 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 about 2^{256} which is over 1.1579 × 10^{77} 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 2^{56}.

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