Hash (fingerprint, digest): evenly and randomly maps variable length data into a smaller fixed size such that it's "one-way" (hard to find a data object for a given hash result) and "collision-free" (hard to find two data objects with the same hash result).

Secret Key Cryptography (symmetric): the key used for processing is kept as a secret between the parties.

Public Key Cryptography (asymmetric): a private/public key pair (inverse of each other) are used to sign (via the private key) and encrypt (via the public key).

Signature: a private key is applied to some data (or its hash)

- authenticity (a specific key was used),
- integrity (the document has been changed),
- and non-repudiation (possessor of the key can not deny it was used.)

Encryption: One often uses a public key (easy to obtain) to send a symmetric key (efficient) for a "session" of communication.

Key Management: How to obtain the *real* key of the person with
whom you want to communicate. This typically involves chains of signatures on
a key that must be checked/validated.

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