usually secret information used as input to various kinds of cryptographic algorithms, like encryption, signature, MAC, to select the concrete transformation done by the algorithm.

Asymmetric cryptosystems normally use pairs of one private and one public key, while symmetric cryptosystems have the same (secret) key for all participants.

Algorithms and protocols should be designed not to give out information about the (secret/private) key even if the attacker can control all other input and see all output.

Suitable questions for this tag could be about key generation, key size considerations, and key derivation (from other keys or from passwords) algorithms.

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