Glossary

Essential terms for understanding quantum threats and post-quantum cryptography

This glossary defines technical terms used throughout the site. Terms are organized alphabetically with cross-references to related concepts. For deeper explanations, follow the links to full articles.

A B C D E F G H I K L M N O P Q R S T U V Z

A

Algorithm: A step-by-step procedure for solving a problem or performing a computation. In cryptography, algorithms define how encryption, decryption, and signature generation work. Examples include ECDSA (current) and Dilithium (post-quantum).
Asymmetric Cryptography: Encryption system using two keys: a public key (shared openly) and a private key (kept secret). Also called public-key cryptography. Cryptocurrency addresses are public keys; spending funds requires the corresponding private key. Vulnerable to quantum computers running Shor’s algorithm.

B

Blockchain: Distributed ledger technology where transactions are grouped into blocks and cryptographically linked in chronological order. Security relies on cryptographic signatures (vulnerable to quantum computers) and hash functions (quantum-resistant).
Bridge: Protocol connecting two blockchains, enabling asset transfers between them. In Layer-2 context, bridges connect rollups or sidechains to their base layer. Bridge security often depends on multi-signature schemes vulnerable to quantum attacks.

C

Classical Cryptography: Cryptographic systems not designed to resist quantum computer attacks. Includes RSA, ECDSA, and Diffie-Hellman. Secure against classical computers but vulnerable to quantum algorithms like Shor’s.
Coherence Time: Duration a qubit maintains its quantum state before decoherence destroys the information. Longer coherence times allow more quantum operations. Current systems achieve milliseconds to seconds; cryptographically useful quantum computers require minutes or error correction.
CRQC (Cryptographically Relevant Quantum Computer): Quantum computer powerful enough to break current cryptographic systems. Requires approximately 1,500 logical qubits to break 256-bit ECDSA used in most cryptocurrencies. Current systems have achieved ~5-10 logical qubits.

D

Decoherence: Loss of quantum properties when a qubit interacts with its environment. Primary challenge in building quantum computers. Causes computational errors that must be corrected through quantum error correction.
Dilithium: NIST-standardized post-quantum signature algorithm based on lattice cryptography. Produces signatures around 2,420 bytes (38× larger than ECDSA). Primary candidate for cryptocurrency migration due to practical performance characteristics.

E

ECDSA (Elliptic Curve Digital Signature Algorithm): Current signature algorithm used by Bitcoin, Ethereum, and most cryptocurrencies. Provides 128-bit security with 256-bit keys. Vulnerable to Shor’s algorithm—quantum computers could derive private keys from public keys or signatures.
Error Correction (Quantum): Techniques for detecting and fixing errors in quantum computations caused by decoherence. Requires multiple physical qubits to create one reliable logical qubit. The efficiency of error correction determines how many physical qubits are needed for CRQC.

F

FALCON: NIST-standardized post-quantum signature algorithm. Produces smaller signatures than Dilithium (~600 bytes) but is more complex to implement securely. Alternative option for cryptocurrency migration.
Fault Tolerance: Ability to perform reliable quantum computations despite errors. Achieved through quantum error correction when error rates fall below specific thresholds. Essential for building CRQC.
Fraud Proof: Cryptographic proof that a transaction or state transition in an optimistic rollup is invalid. Used to challenge incorrect rollup operations. Must support post-quantum signature verification after migration.

G

Gate Fidelity: Accuracy of quantum gate operations. Measured as percentage of successful operations. Current systems achieve 99-99.9%; fault-tolerant quantum computing requires 99.99%+. Higher fidelity reduces error correction overhead.
Governance: Process by which blockchain protocol changes are proposed, debated, and implemented. Critical bottleneck for post-quantum migration—requires community consensus, typically taking 1-3 years even for urgent changes.
Grover’s Algorithm: Quantum algorithm providing quadratic speedup for unstructured search. Reduces effective security of symmetric cryptography and hash functions by half. SHA-256 retains 128-bit security against quantum attacks—still sufficient.

H

Hard Fork: Blockchain protocol change incompatible with previous versions, requiring all nodes to upgrade. Post-quantum migration likely requires hard forks, creating coordination challenges and potential chain splits.
Hash Function: One-way cryptographic function converting arbitrary input into fixed-size output. Quantum-resistant—Grover’s algorithm provides only quadratic speedup. Blockchain security (proof-of-work mining, Merkle trees) relies on hash functions and remains safe.
Hash-Based Signatures: Signature schemes relying solely on hash function security. Include SPHINCS+, XMSS, and LMS. Provably quantum-resistant but produce very large signatures (7-50 KB). Suitable for low-frequency, high-security operations like governance.
Hybrid Cryptography: Using both classical and post-quantum algorithms simultaneously. Protects against quantum attacks while maintaining backward compatibility. Transitional strategy allowing gradual migration. Example: ECDSA + Dilithium dual signatures.

I

Ion Trap: Quantum computing technology using electrically trapped ions as qubits. Advantages: high fidelity, long coherence times. Disadvantages: slower gates, harder to scale. Leading company: IonQ.

K

Kyber: NIST-standardized post-quantum key encapsulation mechanism (KEM) based on lattice cryptography. Used for establishing shared secrets in encrypted communications. Less relevant for blockchain (which uses signatures, not key exchange) but important for TLS and node communications.

L

Lattice-Based Cryptography: Post-quantum cryptographic approach based on hard problems in high-dimensional lattices. Includes Dilithium, Kyber, FALCON. Offers practical performance with reasonable key and signature sizes. Foundation for most NIST-selected PQC algorithms.
Layer-2 (L2): Scaling solution built on top of blockchain base layer. Includes rollups, sidechains, and state channels. Can potentially migrate to post-quantum cryptography independently and faster than Layer-1, but bridges remain vulnerable if base layer isn’t quantum-safe.
Logical Qubit: Error-corrected qubit created from multiple physical qubits through quantum error correction. Reliable enough for meaningful computation. Current systems: ~5-10 logical qubits. Needed for CRQC: ~1,500 logical qubits.

M

Merkle Tree: Data structure using hash functions to create compact cryptographic commitments to large datasets. Foundation for blockchain efficiency and hash-based signatures. Quantum-resistant because it relies on hash functions, not public-key cryptography.
Multi-Signature (Multisig): Wallet requiring multiple private keys to authorize transactions. Common for treasury management and governance. Post-quantum migration must update signature verification while maintaining threshold security properties.

N

NIST (National Institute of Standards and Technology): US government agency that conducted global competition to select post-quantum cryptography standards. Selected Kyber, Dilithium, SPHINCS+, and FALCON (2022). Published final specifications (2024) as FIPS 203, 204, 205.
NISQ (Noisy Intermediate-Scale Quantum): Current era of quantum computing: 50-1000 physical qubits with significant error rates. Useful for research but insufficient for breaking cryptography. Next milestone: fault-tolerant quantum computing with logical qubits.

O

Optimistic Rollup: Layer-2 scaling solution assuming transactions are valid unless challenged via fraud proofs. Examples: Arbitrum, Optimism, Base. Easier to migrate to post-quantum cryptography than ZK-rollups—update signature schemes and fraud proof verification.

P

Physical Qubit: Raw, noisy qubit before error correction. Current systems achieve 1,000+ physical qubits but high error rates make them unreliable. Multiple physical qubits (currently 100-1,000) required to create one logical qubit.
Post-Quantum Cryptography (PQC): Cryptographic algorithms designed to resist attacks by quantum computers. Based on mathematical problems believed hard for both classical and quantum computers. Includes lattice-based, hash-based, code-based, and isogeny-based approaches.
Private Key: Secret cryptographic key used to sign transactions and prove ownership of funds. Must never be shared. Quantum computers running Shor’s algorithm can derive private keys from public keys or signatures, compromising wallets.
Public Key: Cryptographic key derived from a private key, shared openly as a wallet address. In classical cryptography, computationally infeasible to reverse. Quantum computers can derive private keys from public keys using Shor’s algorithm.

Q

Q-Day: The day cryptographically relevant quantum computers (CRQC) become available, breaking current cryptography. Consensus estimate: 2030-2035. Critical deadline for cryptocurrency migration—projects need 5-7 years for full deployment, meaning migration must start now.
QKD (Quantum Key Distribution): Method for secure key exchange using quantum properties. Often confused with post-quantum cryptography but serves different purpose. Requires specialized hardware, point-to-point connections. Not practical for blockchain—PQC is the correct solution.
Qubit: Quantum bit—fundamental unit of quantum information. Unlike classical bits (0 or 1), qubits can exist in superposition of both states simultaneously. Multiple qubits can be entangled, enabling quantum algorithms to solve certain problems exponentially faster than classical computers.
Quantum Error Correction: Techniques using redundant physical qubits to protect against decoherence and create reliable logical qubits. Current overhead: 100-1,000 physical qubits per logical qubit. Breakthroughs reducing this ratio would dramatically accelerate Q-Day timeline.
Quantum Supremacy / Quantum Advantage: Milestone when quantum computers solve specific problems faster than classical computers. Google claimed this in 2019 with 53-qubit Sycamore. Controversial milestone—doesn’t mean quantum computers are useful for breaking cryptography yet.

R

Rollup: Layer-2 scaling solution executing transactions off-chain while posting data or proofs to Layer-1. Two types: optimistic (fraud proofs) and ZK (validity proofs). Can potentially migrate to PQC independently of base layer.
RSA: Classical public-key cryptography algorithm. Widely used for TLS/SSL and secure communications but not commonly used in cryptocurrency (most use ECDSA). Also vulnerable to Shor’s algorithm.

S

Sequencer: Operator ordering and executing transactions in a Layer-2 rollup. Often centralized in current implementations. Uses cryptographic signatures to commit to transaction batches—must migrate to post-quantum signatures.
SHA-256: Cryptographic hash function used extensively in Bitcoin and other blockchains. Quantum-resistant—Grover’s algorithm reduces security from 256 bits to 128 bits, which remains more than sufficient. Mining and Merkle trees remain secure.
Shor’s Algorithm: Quantum algorithm efficiently solving integer factorization and discrete logarithm problems. Breaks RSA and ECDSA. Foundation of quantum threat to cryptocurrency—enables deriving private keys from public keys. Requires ~1,500 logical qubits to break 256-bit ECDSA.
Sidechain: Independent blockchain with bridge to main chain. Examples: Polygon PoS, Gnosis Chain. Can migrate to post-quantum cryptography independently like any Layer-1, but bridge security depends on both chains being quantum-safe.
Signature: Cryptographic proof that transaction was authorized by the holder of the private key. Current blockchains use ECDSA signatures (~64 bytes). Post-quantum alternatives: Dilithium (~2,420 bytes), FALCON (~600 bytes), SPHINCS+ (7-50 KB).
SPHINCS+: NIST-standardized hash-based signature algorithm. Provably secure against quantum attacks but produces very large signatures (7-50 KB). Suitable for governance keys and treasury multisig, not everyday transactions. Stateless—no key reuse risks.
State Channel: Layer-2 solution where participants conduct transactions off-chain and periodically settle to Layer-1. Example: Lightning Network. Requires updating channel contracts and signature schemes for post-quantum migration.
Store Now, Decrypt Later: Attack where adversaries collect encrypted data today, storing it until quantum computers become available to decrypt it. Particularly concerning for cryptocurrency transactions containing sensitive information or long-term value.
Superconducting Qubit: Quantum computing technology using superconducting circuits as qubits. Advantages: fast gates, proven scalability. Disadvantages: requires extreme cooling (~0.01 Kelvin). Leading companies: IBM, Google, Rigetti.
Surface Code: Leading quantum error correction approach arranging physical qubits in 2D grid. Heavy overhead (hundreds of physical qubits per logical qubit) but well-understood and implementable with current technology.

T

Testnet: Separate blockchain environment for testing protocol changes without risking real value. Essential step before mainnet deployment. Several cryptocurrency projects have deployed post-quantum cryptography to testnets (Cardano, Ethereum proposals).
Threshold Signature: Signature scheme requiring a minimum number of participants (threshold) from a larger group to authorize transactions. Example: 3-of-5 multisig. Post-quantum migration must preserve threshold properties while updating cryptographic primitives.

U

UTXO (Unspent Transaction Output): Bitcoin’s transaction model where funds exist as discrete outputs from previous transactions. Quantum vulnerability: if public keys are exposed (addresses reused or outputs spent), quantum computers could steal remaining funds before transactions confirm.

V

Validator: Node operator participating in proof-of-stake consensus by attesting to block validity. Uses cryptographic signatures to vote—must migrate to post-quantum signatures. Large validator sets increase coordination complexity for protocol upgrades.

Z

Zero-Knowledge Proof: Cryptographic method proving statement is true without revealing why. Used in privacy coins and ZK-rollups. Current implementations (SNARKs, STARKs) rely on elliptic curves vulnerable to quantum attacks. Post-quantum zero-knowledge proofs are active research area.
ZK-Rollup: Layer-2 scaling solution using zero-knowledge proofs to validate transactions. Examples: zkSync, StarkNet, Polygon zkEVM. Face unique quantum challenges—both signatures AND proof systems need quantum-resistant replacements. Migration timeline: 2-5 years (research dependent).

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