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129 lines
7.6 KiB
Markdown
129 lines
7.6 KiB
Markdown
(cyrptography_chapter)=
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# Cryptographic concepts and terms
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```{admonition} VISUAL
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:class: warning
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- Introduce visualizations for cryptographic primitives
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- Show example visualizations for operations? (encrypt/decrypt and signing/verification - only if we're going to reuse the visual primitives later)
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```
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## (Cryptographic) hash functions
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[(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").
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Hash functions are used in cryptography to produce shorthand "placeholders" for data. Two properties of cryptographic hash functions are particularly important:
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- ["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.
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- ["Collision resistance"](https://en.wikipedia.org/wiki/Collision_resistance): It should be hard to find two messages that map to the same hash value.
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## Symmetric-key cryptography
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[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.
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Participants in symmetric-key operations need to exchange the shared secret over a secure channel.
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```{admonition} VISUAL
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:class: warning
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- visualization? (maybe a black key icon, following wikipedia's example?)
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```
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### Benefits and downsides
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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[^postquantum].
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[^postquantum]: Daniel J. Bernstein (2009). ["Introduction to post-quantum cryptography" (PDF)](http://www.pqcrypto.org/www.springer.com/cda/content/document/cda_downloaddocument/9783540887010-c1.pdf) states that: "many important classes of cryptographic systems", including secret-key cryptographic mechanisms like AES "[..] are believed to resist classical computers and quantum computers." (pages 1, 2).
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However, exchanging the required shared secret is a problem that needs to be solved separately.
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[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).
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### Symmetric-key cryptography in OpenPGP
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Symmetric-key cryptography is used in OpenPGP in three contexts:
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- most prominently, as part of a hybrid cryptosystem to encrypt and decrypt data,
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- to encrypt [password-protected private key material](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-secret-key-encryption), and
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- for [password-protected data encryption](https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-10.html#name-symmetric-key-encrypted-ses) (a less commonly used feature of the standard).
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Where symmetric keys are used in OpenPGP for data encryption, they are called either "message keys" or "session keys[^sessionkey]."
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[^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.
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### Authenticated encryption with associated data (AEAD)
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[Authenticated encryption](https://en.wikipedia.org/wiki/Authenticated_encryption) is a class of cryptographic schemes that gives additional guarantees besides confidentiality.
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In OpenPGP version 6, AEAD was introduced as a successor to the MDC[^MDC] mechanism. AEAD is a common mechanism to solve the problem of "malleability": In past versions of the OpenPGP protocol, some malicious changes to ciphertext were undetectable. AEAD protects against undetected changes of ciphertext.
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[^MDC]: In OpenPGP version 4, a mechanism called MDC (Modification Detection Code) was introduced to serve a comparable purpose as AEAD. While MDC is a non-standard mechanism, as of this writing, there are no known attacks against the scheme.
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Protecting against malleability counters a variation of the EFAIL[^efail] attack.
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[^efail]: A variation of the [EFAIL](https://en.wikipedia.org/wiki/EFAIL) attack can be prevented by both the MDC and AEAD mechanisms. Also see ["No, PGP is not broken, not even with the Efail vulnerabilities"](https://proton.me/blog/pgp-vulnerability-efail), especially the section "Malleability Gadget Exfiltration Channel Attack".
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## Public-key, or asymmetric cryptography
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[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* as well as *digital signature* operations.
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Unlike symmetric cryptography, public-key cryptography doesn't require participants to pre-arrange a shared secret. Instead, with public-key cryptography, the public parts of the key material can be shared openly and then used for cryptographic operations.
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(asymmetric_key_pair)=
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### Asymmetric cryptographic key pairs
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In many places, we'll deal with asymmetric cryptographic key pairs:
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```{figure} diag/cryptographic_keypair.png
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---
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---
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An asymmetric cryptographic key pair
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```
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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.
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Note that in many contexts, only the public part is present (more on that later):
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```{figure} diag/keypair_pub.png
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---
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---
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Only the public part of an asymmetric key pair
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```
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### Public-key cryptography in OpenPGP
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OpenPGP makes heavy use of public-key cryptography, both for encryption and signing operations.
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```{admonition} Terminology
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:class: note
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For historical reasons, the OpenPGP RFC and other documentation often use the non-standard term "secret key" instead of the more common "private key."
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So in OpenPGP, the pair of terms "public/secret key" is sometimes used instead of the more common "public/private key."
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```
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### Cryptographic digital signatures
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[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.
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The signer uses digital signatures to make statements about the message. Third parties can then inspect these statements.
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```{admonition} VISUAL
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:class: warning
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- add visualization showing: message + private key + sign = signature -> message + signature + public key + verify = ok?
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```
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In OpenPGP, digital signatures are used in two different contexts:
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- [Certification statements](certifications_chapter)
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- [Signatures over data](signing_data)
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(hybrid_cryptosystems)=
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## Hybrid cryptosystems
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[Hybrid cryptosystems](https://en.wikipedia.org/wiki/Hybrid_cryptosystem) combine two cryptosystems and make use of their respective advantages:
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- A public-key cryptosystem is used to safely handle shared secrets over insecure channels (in OpenPGP: so-called "session keys")
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- A symmetric-key cryptosystem is used to efficiently encrypt and decrypt long messages (using an OpenPGP "session key" as the shared secret)
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