<cryptography> The practise and study of encryption and
decryption - encoding data so that it can only be decoded by
specific individuals. A system for encrypting and decrypting
data is a cryptosystem. These usually involve an algorithm
for combining the original data ("plaintext") with one or
more "keys" - numbers or strings of characters known only to
the sender and/or recipient. The resulting output is known as
"ciphertext".

The security of a cryptosystem usually depends on the secrecy
of (some of) the keys rather than with the supposed secrecy of
the algorithm. A strong cryptosystem has a large range of
possible keys so that it is not possible to just try all
possible keys (a "brute force" approach). A strong
cryptosystem will produce ciphertext which appears random to
all standard statistical tests. A strong cryptosystem will
resist all known previous methods for breaking codes
("cryptanalysis").