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Add section on KDFs

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Paul Schaub 2023-10-23 12:40:15 +02:00
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@ -16,6 +16,15 @@ Here are two important properties of cryptographic hash functions:
- ["Pre-image resistance"](https://en.wikipedia.org/wiki/Preimage_attack): Given a hash value, it should be very difficult to determine the original data it represents.
- ["Collision resistance"](https://en.wikipedia.org/wiki/Collision_resistance): It should be very difficult to find two distinct pieces of data that map to the same hash value.
### Key Derivation Functions
Hash functions can also be used to construct [key derivation functions](https://en.wikipedia.org/wiki/Key_derivation_function) (KDF).
Those can for instance be used to derive symmetric key material from a password by repeatedly feeding it through a hash function.
A prominent example of a KDF which is relevant for the OpenPGP specification is the so-called [HKDF](https://en.wikipedia.org/wiki/HKDF) which is based on the [HMAC](https://en.wikipedia.org/wiki/HMAC) message authentication code.
More information on KDFs and their use in the OpenPGP protocol can be found in chapters XXX (S2K) and YYY (SEIPDv2).
## Symmetric-key cryptography
[Symmetric-key cryptography](https://en.wikipedia.org/wiki/Symmetric-key_algorithm) uses the same cryptographic key for both encryption and decryption, unlike asymmetric cryptography where a pair of keys is used: a public key for encryption and a corresponding private key for decryption. Symmetric-key cryptographic systems support *encryption/decryption* operations.