Key space (cryptography)

The key space is a term used in cryptography and denotes the set of all possible for a encryption key. The size of the key space puts the total number of keys and is just referred to as the key number. A logarithmic measure of the size of the key space is the key length that is specified in bits.

The security of encryption is dependent on inter alia the size of the key space. This must be at least so large that the correct key can not be found by exhaustive ( exhausting ) search using a brute-force attack.

An adequate key space size is an important and necessary condition for the security of a cryptographic procedure. However, it alone does not have sufficient protection against unauthorized decryption ( crack ). In cryptographically weak encryption methods, it may be possible for an attacker to discover shortcuts that enable him to find the key, without having to try to all possible keys. An example of this is the pattern search as a cryptanalytic attack vector to monoalphabetically encrypted cipher text. Although the key space in this case 26! (Faculty ) of which are about 4.1026 possible keys, the succeed in deciphering monoalphabetically encrypted texts with relative ease and without exhaustion.