Key space

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In cryptography, an algorithm's key space refers to all possible keys that can be used to initialize it. Put in its most simplistic terms, the possibilities in the series A,B,C...Z represent a much smaller key space than AAA,AAB,AAC...ZZZ. A well-designed cryptographic algorithm should be highly computationally expensive when trying to brute-force through all possible key values.

[edit] Example

Rijndael/AES, the flagship of symmetric ciphers, allows a key of up to 256 bits, over 1.1579 × 10⁷⁷ possible keys. This makes it computationally infeasible to check each possible key by brute force.