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Merge pull request 'Add section on KDFs' (#83) from 03-key_derivation into draft

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Reviewed-on: https://codeberg.org/openpgp/notes/pulls/83
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Tammi L. Coles 2023-10-26 09:32:04 +00:00
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book/source/03-cryptography.md
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@ -21,6 +21,23 @@ 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.
## Message Authentication Codes
[Message Authentication Codes](https://en.wikipedia.org/wiki/Message_authentication_code) (MAC, also called authentication tags) are small pieces of information, which can be used to verify the integrity and authenticity of a message.
They are calculated over the original message using a (symmetric) secret key.
The recipient of a message containing a MAC, who is also in posession of the secret key can verify that the message has not been tampered with.
[HMAC](https://en.wikipedia.org/wiki/HMAC) is a hash-based message authentication code, which is used in the OpenPGP protocol.
### 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 a key derivation function based on the HMAC message authentication code.
More information on KDFs and their use in the OpenPGP protocol can be found in chapters [5](encrypted_secrets) and 10 (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.