mirror of
https://codeberg.org/openpgp/notes.git
synced 2024-11-23 16:12:05 +01:00
119 lines
6 KiB
Markdown
119 lines
6 KiB
Markdown
(cyrptography_chapter)=
|
|
# Cryptographic concepts and terms
|
|
|
|
```{admonition} VISUAL
|
|
:class: warning
|
|
|
|
- Introduce visualizations for cryptographic primitives
|
|
- Show example visualizations for operations? (encrypt/decrypt and signing/verification - only if we're going to reuse the visual primitives later)
|
|
```
|
|
|
|
## (Cryptographic) hash functions
|
|
|
|
[(Cryptographic) hash functions](https://en.wikipedia.org/wiki/Cryptographic_hash_function) map binary data of arbitrary length to a fixed size "hash" (hashes are also sometimes called "digests").
|
|
|
|
Hash functions are used in cryptography to produce shorthand "placeholders" for data. Two properties of cryptographic hash functions are particularly important:
|
|
|
|
- ["Pre-image resistance"](https://en.wikipedia.org/wiki/Preimage_attack): Given a hash value, it should be hard to find a message that maps to that hash value.
|
|
- ["Collision resistance"](https://en.wikipedia.org/wiki/Collision_resistance): It should be hard to find two messages that map to the same hash value.
|
|
|
|
## Symmetric-key cryptography
|
|
|
|
[Symmetric-key cryptography](https://en.wikipedia.org/wiki/Symmetric-key_algorithm) uses the same cryptographic key for both encryption and decryption. Symmetric-key cryptographic systems support *encryption/decryption* operations.
|
|
|
|
Participants in symmetric-key operations need to exchange the shared secret over a secure channel.
|
|
|
|
```{admonition} VISUAL
|
|
:class: warning
|
|
|
|
- visualization? (maybe a black key icon, following wikipedia's example?)
|
|
```
|
|
|
|
### Benefits and downsides
|
|
|
|
Symmetric-key cryptography has major benefits: it is much faster than public-key cryptography (see below). Also, most current symmetric cryptographic algorithms are considered quantum-resistant.
|
|
|
|
However, exchanging the required shared secret is a problem that needs to be solved separately.
|
|
|
|
[Hybrid cryptosystems](hybrid_cryptosystems) (see below) are one common approach to leverage the benefits of symmetric-key cryptography, while handling the shared secret with a separate mechanism (using public-key cryptography).
|
|
|
|
### Symmetric-key cryptography in OpenPGP
|
|
|
|
Symmetric cryptography is used in OpenPGP as part of a hybrid cryptosystem.
|
|
|
|
Where symmetric keys are used in OpenPGP, they are called either "message keys" or "session keys[^sessionkey]."
|
|
|
|
[^sessionkey]: In OpenPGP version 6, when using the ["Version 2 Symmetrically Encrypted Integrity Protected Data Packet Format"](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-version-2-symmetrically-enc), a "message key" is derived from a "session key". Previously (up to OpenPGP version 4, and in version 6 when using ["Version 1 Symmetrically Encrypted Integrity Protected Data Packet Format"](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-version-1-symmetrically-enc)), the "session key" was used directly as a symmetric encryption key.
|
|
|
|
### Authenticated encryption with associated data (AEAD)
|
|
|
|
[Authenticated encryption](https://en.wikipedia.org/wiki/Authenticated_encryption) is a class of cryptographic schemes that gives additional guarantees besides confidentiality.
|
|
|
|
In OpenPGP version 6, AEAD is used to solve the problem of "malleability": In past versions of the OpenPGP protocol, some malicious changes to ciphertext were undetectable. With AEAD, undetected changes of ciphertext are not possible.
|
|
|
|
## Public-key, or asymmetric cryptography
|
|
|
|
[Public-key cryptography](https://en.wikipedia.org/wiki/Public-key_cryptography) systems use asymmetric pairs of related keys. Public-key cryptographic systems support *encryption/decryption* and *digital signature* operations.
|
|
|
|
Unlike symmetric cryptography, public-key cryptography doesn't require participants to pre-arrange a shared secret.
|
|
|
|
(asymmetric_key_pair)=
|
|
### Asymmetric cryptographic key pairs
|
|
|
|
In many places, we'll deal with asymmetric cryptographic key pairs:
|
|
|
|
```{figure} diag/cryptographic_keypair.svg
|
|
---
|
|
---
|
|
An asymmetric cryptographic key pair
|
|
```
|
|
|
|
```{admonition} VISUAL
|
|
:class: warning
|
|
|
|
- Wiktor notes: red-green color-blindness affects 8,5% of the population.
|
|
- Heiko: maybe use colors + distinct shapes for the two key halves?
|
|
```
|
|
|
|
An asymmetric cryptographic key pair consists of a public and a private part. In this document, we'll show the public part of a key pair in green, and the private part in red.
|
|
|
|
Note that in many contexts, only the public part is present (more on that later):
|
|
|
|
```{figure} diag/keypair_pub.png
|
|
---
|
|
---
|
|
Only the public part of an asymmetric key pair
|
|
```
|
|
|
|
### Public-key cryptography in OpenPGP
|
|
|
|
OpenPGP makes heavy use of public-key cryptography, both for encryption and signing operations.
|
|
|
|
Note that, for historical reasons, the OpenPGP RFC and other documentation often use the non-standard term "secret key" instead of the more common "private key."
|
|
|
|
So in OpenPGP, the pair of terms "public/secret key" is sometimes used instead of the more common "public/private key."
|
|
|
|
### Cryptographic digital signatures
|
|
|
|
[Digital signatures](https://en.wikipedia.org/wiki/Digital_signature) are a mechanism that is based on asymmetric cryptography. With this mechanism, one actor can make a signature over a digital message, and another actor can check the validity of that signature.
|
|
|
|
The signer uses digital signatures to make statements about the message. Third parties can then inspect these statements.
|
|
|
|
```{admonition} VISUAL
|
|
:class: warning
|
|
|
|
- add visualization showing: message + private key + sign = signature -> message + signature + public key + verify = ok?
|
|
```
|
|
|
|
In OpenPGP, digital signatures are used in two different contexts:
|
|
|
|
- [Certification statements](certifications_chapter)
|
|
- [Signatures over data](signing_data)
|
|
|
|
(hybrid_cryptosystems)=
|
|
## Hybrid cryptosystems
|
|
|
|
[Hybrid cryptosystems](https://en.wikipedia.org/wiki/Hybrid_cryptosystem) combine two cryptosystems and make use of their respective advantages:
|
|
|
|
- A public-key cryptosystem is used to safely handle shared secrets over insecure channels (in OpenPGP: so-called "session keys")
|
|
- A symmetric-key cryptosystem is used to efficiently encrypt and decrypt long messages (using an OpenPGP "session key" as the shared secret)
|