(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"). ## 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 referred to as "session keys." ### 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. Public-key cryptography doesn't require participants to pre-arrange a shared secret. ### Asymmetric cryptographic key pairs In many places, we'll deal with asymmetric cryptographic key pairs: ```{figure} diag/cryptographic_keys.png --- --- 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)