Glossary

The words used on the maps and the box pages, in plain language. New to all this? Start here.

A

Bits of security: A measure of effort: n-bit security means about 2^n operations to break. 128 bits is far beyond any computer.
Blob: A chunk of rollup data attached to an Ethereum block. Cheaper than regular data, and deleted after about 18 days.
BSI: Germany's federal cybersecurity agency.

Certificate authority: An organization your browser trusts to vouch that a public key really belongs to a website.
Collision: Two different inputs that produce the same hash fingerprint.
Commitment: A cryptographic sealed envelope: it binds you to a value now and lets you reveal it later.

Diffie-Hellman: The 1976 method for two parties to agree on a secret in public. CDH and DDH are the puzzles that keep it safe.
Digital signature: Proof, made with a private key, that a message came from the key's owner and wasn't changed. Anyone can check it with the public key.
Discrete logarithm: Given g and g to the power x, find x. Easy forwards, believed hard backwards in the right number systems.

EIP: Ethereum Improvement Proposal: the documents that specify protocol changes.
Elliptic curve: A set of points with a special "addition" rule. The basis of compact modern public-key cryptography.
EOA: Externally owned account: an ordinary Ethereum account controlled by a single private key.
Erasure coding: Adding redundancy to data so the whole thing can be rebuilt from any large enough part of it.
Error-correcting code: A way to add redundancy to data so errors can be detected and fixed.

Factoring: Splitting a number into the primes that multiply to give it.
Fiat-Shamir: A trick that turns an interactive proof into a non-interactive one by using a hash function to generate the verifier's challenges. Proven safe only for an idealized hash.
Finality: The point after which a block can't be reverted without destroying a huge amount of stake.
FIPS: US federal information processing standards. FIPS 203, 204 and 205 are the first post-quantum ones. FIPS 206 is in progress.

Generic group model: Proof shortcuts that assume attackers can only use basic operations on curve points.
Grover's algorithm: A quantum search algorithm that only gives a square-root speedup. Doubling key lengths compensates.

Hardness assumption: A bet that a math problem can't be solved efficiently. Unproven, but backed by decades of failed attempts.
Hash function: Turns any data into a short, fixed-size fingerprint. Change the input even slightly and the fingerprint changes completely.
Homomorphic encryption: Encryption that lets someone compute on encrypted data without decrypting it.
Hybrid: Using a classical and a post-quantum scheme together, so an attacker has to break both.

KEM: Key encapsulation mechanism: a packaged form of key exchange where one side sends the other a freshly encrypted random key.
Key derivation: Turning one secret into one or more strong keys, for example a separate key for each direction of a connection.
Key exchange: A way for two parties to agree on a shared secret key while others are listening.

Lattice: A regular grid of points in many dimensions. Some problems on lattices are believed hard even for quantum computers.
LWE: Learning with errors: solving equations that each carry a little random noise.

MAC: Message authentication code: a short tag made with a shared secret key that reveals any tampering.
Merkle tree: A tree of hashes whose single root fingerprints all the data. Short proofs show that any item is included.

NIST: The US standards agency running the post-quantum cryptography standardization.
Nonce: A number that must only ever be used once. Reusing one can leak secrets in some schemes.

Oblivious transfer: A sender offers two messages. The receiver gets one of their choice. Neither learns anything more.
One-way function: Easy to compute, practically impossible to reverse.

Pairing: A special operation on some elliptic curves that combines two points. It enables short signatures and compact proofs.
Post-quantum: Designed to stay secure even against large quantum computers.
Precompile: A built-in Ethereum function for heavy cryptography that smart contracts can call cheaply.
Preimage: An input that produces a given hash output.
Private key: The secret half of a key pair. Whoever has it can sign or decrypt as you.
Pseudorandom: Looks completely random to anyone who doesn't have the key, even though it's computed by a fixed recipe.
Public-key cryptography: Cryptography with a key pair: a public key you can share with anyone, and a private key you keep secret.

Quantum computer: A computer that uses quantum effects. A large one would break today's public-key cryptography.

Random oracle model: A proof shortcut that pretends a hash function is a perfectly random function. Widely used, not literally true.
Reduction: A proof that breaking one thing would let you solve another, so the first is at least as hard.
Rollup: A separate chain that bundles transactions and settles them on Ethereum.
RSA: The classic public-key system from 1977, based on factoring.

Secret sharing: Splitting a secret into pieces so that only enough pieces together can rebuild it.
Secure multiparty computation: Several parties computing a result from their private inputs without revealing those inputs.
Seed phrase: The 12 or 24 words that generate every key in a wallet.
Shor's algorithm: A quantum algorithm (1994) that factors numbers and computes discrete logarithms efficiently.
Side channel: A leak from how a system physically runs (timing, power use, light) rather than from the math.
Slashing: A penalty that destroys part of a validator's stake for provably dishonest behavior.
Slot and epoch: A slot is a 12-second window for one block. An epoch is 32 slots, about 6.4 minutes.
SNARK: A tiny, quick-to-check proof that a computation was done correctly.
STARK: A proof of correct computation built only from hash functions, needing no trusted setup.
Strawmap: The Ethereum Foundation's draft long-term roadmap, at strawmap.org.
Symmetric encryption: Encryption where the same secret key both locks and unlocks.

Trapdoor: Secret information that makes a hard-to-invert function easy to invert, giving back the whole input (like RSA's private exponent). Public-key encryption only needs the message back, so ElGamal has a secret key without being a trapdoor function.
Trusted setup: A one-time generation of public parameters from a secret that must be destroyed afterwards.

Unconditional security: Secure even against an attacker with unlimited computing power.

Validator: A participant who locks up ETH (stake) to propose and vote on Ethereum blocks.

Weak subjectivity: In proof of stake, the need for a new node to start from a recent checkpoint it gets from a trusted source.

Zero-knowledge proof: A proof that a statement is true which reveals nothing beyond that fact, not even the secret inputs.