Glossary

Short, precise definitions of the terms used across the wiki.

67 entries

AES (Advanced Encryption Standard)AES is the standardized symmetric block cipher; Grover's algorithm gives only a quadratic speedup, so a 256-bit key preserves the post-quantum margin.
Bech32mBech32m is the checksummed address encoding defined in BIP 350; it fixes a bech32 weakness and encodes taproot addresses as well as BTX addresses.
Bloch sphereThe Bloch sphere is the geometric representation of a single qubit's pure state as a point on the surface of a unit sphere in three dimensions.
Constant-time implementationA constant-time implementation runs in time independent of secret data, the standard defense against timing and other side-channel attacks on cryptography.
Copenhagen interpretationThe Copenhagen interpretation is the traditional view of quantum mechanics in which measurement collapses the wave function to a single definite outcome.
Cryptographic agilityCryptographic agility is the ability to swap algorithms and parameters without redesigning a system, a first-class requirement of the post-quantum era.
Cryptographic bill of materials (CBOM)A cryptographic bill of materials (CBOM) is a machine-readable inventory of the algorithms, keys, certificates, and libraries a system uses for cryptography.
Cryptographic hash functionA cryptographic hash function maps input of any length to a fixed-size digest and must resist preimage, second preimage, and collision attacks.
Cryptographic inventoryCryptographic inventory is the process of discovering and cataloging where cryptography is used in an organization, the first step of post-quantum migration.
Cryptographically relevant quantum computer (CRQC)A cryptographically relevant quantum computer (CRQC) is a machine capable enough to break the public-key cryptography, such as RSA and ECC, in wide use today.
DecoherenceDecoherence is the loss of a quantum system's coherent state through interaction with its environment, the reason quantum error correction is needed.
Diffie-HellmanDiffie-Hellman is the 1976 key-exchange protocol that lets two parties derive a shared secret over a public channel, with security that Shor's algorithm breaks.
Digital signature schemeA digital signature scheme binds a message to a private key holder so that anyone can verify its origin and integrity with the matching public key.
Dilithium (CRYSTALS-Dilithium)Dilithium, formally CRYSTALS-Dilithium, is the original name of the lattice-based signature scheme NIST renamed ML-DSA and standardized in FIPS 204.
Discrete logarithm problemThe discrete logarithm problem asks for the exponent in a modular power, underpinning Diffie-Hellman and ECDSA and solved efficiently by Shor's algorithm.
ECDSAECDSA is the elliptic-curve digital signature used by Bitcoin and TLS; its discrete logarithm security falls to Shor's algorithm on a quantum computer.
Ed25519Ed25519 is a fast, modern EdDSA signature scheme over Curve25519 used in SSH and TLS; it is classical and vulnerable to Shor's algorithm on a quantum computer.
EigenstateAn eigenstate is a quantum state with a definite value, the eigenvalue, of a given observable, and measuring that observable returns that value.
Elliptic-curve cryptography (ECC)Elliptic-curve cryptography gives public-key security with smaller keys than RSA, on a discrete logarithm problem that Shor's algorithm solves efficiently.
EntanglementEntanglement is a quantum correlation linking two or more qubits so that their states cannot be described independently of one another.
EUF-CMAEUF-CMA is existential unforgeability under chosen-message attack, the standard security target a digital signature scheme must meet to be considered secure.
Fiat-Shamir transformThe Fiat-Shamir transform turns an interactive identification protocol into a non-interactive digital signature by computing the challenge as a hash.
FORS (Forest of Random Subsets)FORS (Forest of Random Subsets) is the few-time hash-based signature that signs message digests inside the stateless standard SLH-DSA, formerly SPHINCS+.
Forward secrecyForward secrecy keeps past session keys safe if long-term keys leak later, but it does not protect recorded traffic from future quantum decryption.
Fujisaki-Okamoto transformThe Fujisaki-Okamoto transform turns a weakly secure encryption scheme into an IND-CCA2 secure key encapsulation mechanism, the method behind ML-KEM.
HamiltonianThe Hamiltonian is the operator representing a quantum system's total energy, and it generates the system's time evolution in the Schrodinger equation.
IND-CCA2IND-CCA2 is indistinguishability under adaptive chosen-ciphertext attack, the standard security target that post-quantum key encapsulation mechanisms must meet.
Integer factorizationInteger factorization is the problem of splitting a number into prime factors, the hardness assumption behind RSA and a target of Shor's quantum algorithm.
KEM combinerA KEM combiner merges a classical and a post-quantum shared secret into one key that stays secure if either mechanism holds, the core of hybrid cryptography.
Key encapsulation mechanism (KEM)A key encapsulation mechanism (KEM) uses a public key to establish a shared secret, the primitive that replaces Diffie-Hellman in post-quantum protocols.
Key exchangeKey exchange lets two parties establish a shared secret over a public channel; quantum-vulnerable Diffie-Hellman variants are giving way to KEMs.
Kyber (CRYSTALS-Kyber)Kyber, formally CRYSTALS-Kyber, is the original name of the lattice-based KEM that NIST renamed ML-KEM and standardized in FIPS 203 in August 2024.
Learning With Errors (LWE)Learning With Errors (LWE) is the lattice problem of solving noisy linear equations, the hardness assumption underpinning most post-quantum encryption.
LMS (Leighton-Micali Signature)LMS (Leighton-Micali Signature) is a stateful hash-based signature scheme defined in RFC 8554 and approved by NIST SP 800-208 for firmware signing.
Logical qubitA logical qubit is an error-corrected qubit encoded across many physical qubits, the unit in which cryptographically relevant quantum computers are sized.
Many-worlds interpretationThe many-worlds interpretation is Everett's proposal that every quantum measurement outcome is realized in a branching universe, with no wave-function collapse.
Merkle treeA Merkle tree is a hash tree whose root commits to an entire data set, enabling compact membership proofs in hash-based signatures and blockchains.
Module Learning With Errors (Module-LWE)Module-LWE is the structured lattice assumption behind ML-KEM and ML-DSA, tuning algebraic structure by module rank to reach each NIST security level.
Mosca's theoremMosca's theorem is an inequality that judges post-quantum migration urgency by comparing data secrecy time, migration time, and time to a quantum computer.
NIST security levelsNIST security levels are five strength categories for post-quantum algorithms, each anchored to the cost of attacking AES or SHA-2 at a given size.
NTRUNTRU is the oldest practical lattice-based cryptosystem, a polynomial-ring public-key scheme from 1998 and the basis of the FN-DSA (Falcon) signature.
ObservableAn observable is a measurable physical quantity in quantum mechanics, represented by an operator whose eigenvalues are the possible measurement outcomes.
PhotonA photon is the quantum of the electromagnetic field, a massless particle of light that carries energy proportional to its frequency and spin one.
Planck's constant (h)Planck's constant (h) is the fundamental constant of quantum theory, relating a quantum of energy to its frequency through the relation E = h nu.
Public-key cryptographyPublic-key cryptography uses key pairs for key exchange, encryption, and signatures; the deployed schemes rest on problems Shor's algorithm breaks.
Quantum annealerA quantum annealer is a special-purpose quantum device that finds low-energy configurations of a problem through the process of quantum annealing.
Quantum numberA quantum number is one of the discrete labels, such as n, l, m, and spin, that specify the allowed state of a quantum system like an atomic electron.
Quantum stateA quantum state is the complete description of a quantum system, given by a state vector for pure states or a density operator for mixed states.
Quantum supremacyQuantum supremacy is the milestone where a quantum computer beats every classical computer at one contrived task, distinct from breaking real cryptography.
Quantum tunnelingQuantum tunneling is the effect by which a particle passes through a potential energy barrier that classical physics says it cannot cross.
QubitA qubit is the basic unit of quantum information, a two-level quantum system that can hold superpositions and become entangled with other qubits.
Ring Learning With Errors (Ring-LWE)Ring-LWE is the polynomial-ring variant of Learning With Errors, trading extra algebraic structure for the compact keys and fast arithmetic PQ schemes need.
RSARSA is a classical public-key cryptosystem based on integer factoring, securing encryption and signatures but broken by Shor's algorithm on a quantum computer.
SHA-2SHA-2 is the widely deployed family of NIST cryptographic hash functions, including SHA-256, weakened only quadratically by Grover's algorithm.
SHA-3SHA-3 is the Keccak-based NIST hash standard whose SHAKE extendable-output functions are used inside the ML-KEM and SLH-DSA post-quantum schemes.
Short Integer Solution (SIS) problemThe Short Integer Solution (SIS) problem seeks a short vector a random matrix maps to zero, the dual of LWE that underlies lattice-based signatures.
SoulboundSoulbound describes blockchain tokens or artifacts bound to one holder and not designed to be transferred, a term popularized by Vitalik Buterin in 2022.
SPHINCS+ (SLH-DSA)SPHINCS+ is the original name of the stateless hash-based signature scheme that NIST renamed SLH-DSA and standardized in FIPS 205 in August 2024.
SpinSpin is the intrinsic, quantized angular momentum carried by particles, dividing matter into half-integer fermions and integer-spin bosons.
SuperpositionSuperposition is the quantum property by which a qubit holds a weighted combination of 0 and 1 until measurement collapses it to one outcome.
Surface codeThe surface code is the leading quantum error-correcting code, encoding one logical qubit in a two-dimensional grid of physical qubits.
Symmetric cryptographySymmetric cryptography uses one shared key, as in AES; quantum attacks give only a quadratic speedup, so larger keys preserve the security margin.
UTXOA UTXO is an unspent transaction output; the UTXO ledger model limits public key exposure per output, which shapes quantum risk on Bitcoin-style chains.
Wave functionA wave function is the mathematical object encoding a quantum system's state, whose squared magnitude gives the probability of each measurement outcome.
Winternitz one-time signature (WOTS)A Winternitz one-time signature (WOTS) signs one message from a hash-based key pair and is the building block of larger hash-based signature schemes.
X25519X25519 is the Curve25519 Diffie-Hellman function; TLS now pairs it with ML-KEM in hybrid groups such as X25519MLKEM768 for post-quantum key exchange.
XMSS (eXtended Merkle Signature Scheme)XMSS is a stateful hash-based signature scheme standardized in RFC 8391 that arranges one-time keys under a Merkle tree for many signatures per key pair.