Glossary
Every term, in one place
125 terms used across this site, defined plainly and cross-linked back to the module that covers them in depth.
125 of 125 terms
A
ACME (Automatic Certificate Management Environment)
The protocol behind Let's Encrypt (RFC 8555) that automates domain-control validation and certificate issuance/renewal entirely without human involvement, via HTTP-01, DNS-01, or TLS-ALPN-01 challenges — the reason short-lived certificates are operationally viable at scale.
AEAD (Authenticated Encryption with Associated Data)
An encryption mode that provides confidentiality and integrity together in one pass, and can also authenticate extra data that isn't encrypted (like a packet header). AES-GCM and ChaCha20-Poly1305 are both AEAD ciphers.
AES (Advanced Encryption Standard)
The current standard symmetric-key block cipher, operating on 128-bit blocks with 128-, 192-, or 256-bit keys, selected by NIST in 2001 after a public competition (winning design: Rijndael).
AES Key Wrap (RFC 3394)
A dedicated construction for encrypting key material specifically — deterministic (no IV/nonce needed, since keys are already high-entropy), with a built-in 8-byte integrity check that makes unwrapping with the wrong KEK fail immediately rather than silently.
AES-NI
Dedicated AES instructions built into modern CPUs, executing in constant time regardless of data — closing the cache-timing side channel that plagued early software lookup-table implementations.
Algorithm confusion
A JWT attack where a verifier that trusts the algorithm named in a token's own header can be tricked into verifying an RS256 token as HS256, using the (public) RSA public key as the HMAC secret.
Asymmetric encryption
Encryption using a mathematically related key pair — a public key for encrypting or verifying, and a private key for decrypting or signing — so two parties never need to share a secret in advance. RSA and ECC are the two families covered in this catalog.
Authentication tag
The short value an AEAD cipher (like AES-GCM) appends to a ciphertext, letting the receiver detect any tampering with the ciphertext or its associated data before trusting the decrypted plaintext.
Avalanche effect
The property that changing a single input bit to a hash function or cipher flips roughly half the output bits, unpredictably — making outputs for similar inputs look completely unrelated.
B
Birthday bound
The reason an n-bit hash offers only n/2-bit collision resistance: thanks to the birthday paradox, finding any two colliding inputs takes roughly 2^(n/2) attempts, far fewer than the 2^n needed to reverse one specific digest.
Bleichenbacher attack
A padding-oracle attack against RSA PKCS#1 v1.5 encryption: by observing whether a server treats a modified ciphertext's padding as valid, an attacker can gradually decrypt it without ever learning the private key.
Block cipher
A symmetric cipher that encrypts data in fixed-size chunks (blocks) — AES operates on 128-bit blocks. A mode of operation (GCM, CBC, CTR) defines how a block cipher handles messages longer than one block.
C
Cache-timing attack
A side-channel attack that infers secret data (like an AES key) by observing which CPU cache lines an implementation accesses, exploitable when attacker and victim code share a physical machine.
CAVV (Cardholder Authentication Verification Value)
The cryptogram a 3-D Secure authentication produces as proof a card's issuer actually authenticated the cardholder. Unlike the PIN block, CVV, and ARQC algorithms, the real CAVV construction has never been publicly disclosed by any card network — it's issuer/network-proprietary by design.
CBC (Cipher Block Chaining)
An older block cipher mode that XORs each plaintext block with the previous ciphertext block before encrypting. Vulnerable to padding-oracle attacks (like Lucky Thirteen) unless paired with a separate MAC.
Certificate Revocation List (CRL)
A downloadable, CA-signed list of revoked certificate serial numbers, checked by clients to detect certificates that should no longer be trusted before their expiry date.
Certificate Transparency (CT)
A system requiring newly issued certificates to be logged in public, append-only logs, so domain owners can detect certificates fraudulently issued in their name by a compromised or misbehaving CA.
ChaCha20-Poly1305
An AEAD cipher combining the ChaCha20 stream cipher with the Poly1305 MAC, widely used as a fast, side-channel-resistant alternative to AES-GCM, especially on devices without AES hardware acceleration.
Chaining value
The running intermediate state passed from one compression-function step to the next inside the Merkle-Damgård hash construction; the final chaining value, after the last block, is the digest itself.
Collision resistance
The property that it should be computationally infeasible to find two different inputs producing the same hash digest — one of the three core guarantees of a cryptographic hash function.
Constant-time code
Code written so its execution time, memory access pattern, and power draw never depend on secret data — the general defense against timing and cache side-channel attacks.
CSPRNG (Cryptographically Secure Pseudorandom Number Generator)
A random number generator whose output is unpredictable even to an attacker who observes part of it — required for every key, nonce, and IV. Ordinary statistical PRNGs (like the Mersenne Twister) are unsafe for this purpose.
CTF (Capture the Flag)
A hands-on security competition format where solving a puzzle — breaking a cipher, exploiting a bug — recovers a hidden "flag" string as proof of the solve. This site's Challenges section, and reference sites like CryptoHack and Cryptopals, follow this convention.
Curveball (CVE-2020-0601)
A 2020 Windows CryptoAPI vulnerability that let an attacker forge a trusted ECDSA certificate by substituting a crafted elliptic-curve base point, without ever learning the real private key — the vulnerable verifier checked the public key's value but not that the curve's generator matched the standard one.
CVV / CVC (Card Verification Value / Code)
The 3-digit code derived from the PAN, expiry date, and service code via a two-block 3DES-and-decimalize algorithm under a Card Verification Key. CVV1 (magstripe), CVV2 (printed, service code 000), and iCVV (chip, service code 999) use the identical formula with only the service code changed, which is why they deliberately never cross-validate.
D
DEK / KEK (Data-Encryption Key / Key-Encryption Key)
The two-key pattern behind envelope encryption: a DEK does the actual bulk encryption locally and is never stored in plaintext; a KEK — held by a KMS/HSM and never exported — encrypts ("wraps") the DEK, so only the KMS can turn a stored wrapped DEK back into something usable.
DES (Data Encryption Standard)
The first publicly standardized, thoroughly analyzed symmetric cipher (1977), using a 16-round Feistel network on 64-bit blocks with a 56-bit effective key — retired not for a design flaw, but because that key became exhaustively searchable.
Diceware
A method for generating a memorable, measurably strong passphrase by rolling physical dice to pick whole words from a published 7,776-word list — entropy adds up predictably at log₂(7776) ≈ 12.925 bits per word, unlike the entropy of a human-invented password, which is notoriously hard to estimate honestly.
Diffie-Hellman (DH)
A key-exchange protocol letting two parties derive a shared secret over a public channel without ever transmitting it, based on the difficulty of the discrete logarithm problem.
Digest
The fixed-size output of a hash function — for SHA-256, always 256 bits, regardless of the input's length.
Digital signature
A cryptographic proof that a message came from the holder of a specific private key and wasn't altered since signing, produced by transforming a hash of the message with that private key (hash-then-sign).
Discrete logarithm problem
The problem of finding the exponent k such that g^k = h in a mathematical group, given g and h — believed computationally hard classically, and the trapdoor underlying Diffie-Hellman and (in its elliptic-curve form) ECC.
Double Ratchet
The Signal Protocol algorithm that derives a fresh key for every message by combining a symmetric-key ratchet (a one-way chain) with a Diffie-Hellman ratchet, giving forward secrecy and post-compromise security.
E
ECC (Elliptic Curve Cryptography)
A public-key family based on the algebraic structure of points on an elliptic curve, offering equivalent security to RSA at much smaller key sizes because the elliptic curve discrete log problem has no known sub-exponential classical attack.
ECDH (Elliptic Curve Diffie-Hellman)
Diffie-Hellman key exchange performed using elliptic curve point multiplication instead of modular exponentiation — the key-exchange method behind TLS 1.3, SSH, and Signal's X3DH.
ECDSA (Elliptic Curve Digital Signature Algorithm)
The elliptic-curve digital signature scheme used in TLS certificates, SSH, and Bitcoin/Ethereum transactions. Catastrophically breaks if the same per-signature random nonce is ever reused.
ElGamal cryptosystem
A public-key encryption scheme built directly on Diffie-Hellman's discrete-log math, letting a sender encrypt to an already-published public key with no live handshake. Ciphertexts are randomized (a fresh one-time secret every message) and always twice the length of the plaintext.
EMVCo
The organization that maintains the EMV chip-card specifications, EMV 3-D Secure, and the EMVCo Payment Tokenisation specification — owned outright in equal shares by six card networks (Visa, Mastercard, American Express, Discover, JCB, UnionPay) rather than run as a government agency or open consortium.
Entropy
A measure of genuine unpredictability, in bits. CSPRNGs are seeded from physical entropy sources (hardware interrupt timing, dedicated RNG chips) rather than anything an attacker could predict or replay. Cryptography sizes key material against min-entropy specifically, not the looser Shannon-entropy average.
Envelope encryption
The pattern every major cloud KMS uses: a fast, local Data-Encryption Key (DEK) does the actual bulk encryption, while the KMS's master key (KEK) only ever wraps that small DEK — never the data itself. Rotating the KEK then never requires re-encrypting stored data.
Ephemeral key
A key pair generated fresh for a single session or exchange and discarded afterward — the mechanism behind forward secrecy in ECDHE key exchange.
F
Feistel network
A block cipher structure that splits each block in half and repeatedly transforms one half using the other, keyed by a round function — decryption reuses the identical network with round keys applied in reverse, regardless of the round function's complexity. DES is the canonical example; AES uses a different structure instead.
FIPS (Federal Information Processing Standard)
A NIST publication that's mandatory for US federal systems — where algorithms like AES (FIPS 197) and the post-quantum standards (FIPS 203/204/205) actually live, as opposed to NIST's SP 800 series, which is guidance rather than a hard specification.
Flag (challenge)
The proof-of-solve string a CTF-style challenge asks for, conventionally wrapped in a fixed prefix and braces — cct{...} on this site — so a solver can tell at a glance whether their recovered plaintext or key is actually right.
Forward secrecy
The property that compromising a long-term private key later doesn't expose past session keys, achieved by using fresh ephemeral keys for each session's key exchange instead of reusing a static one.
Frequency analysis
The oldest documented cryptanalysis technique (Al-Kindi, c. 850 CE): exploiting the fact that letters occur at predictable frequencies in natural language to break simple substitution ciphers like the Caesar cipher.
G
GCM (Galois/Counter Mode)
The dominant AEAD mode for AES, combining CTR-mode encryption with GHASH-based authentication in a single pass, producing both ciphertext and an authentication tag.
Grover's algorithm
A quantum algorithm giving a quadratic speedup for unstructured search, which roughly halves the effective security level of symmetric ciphers and hash functions — countered simply by doubling key/output sizes, unlike Shor's algorithm's total break of public-key crypto.
H
Harvest now, decrypt later
A passive attack strategy: an adversary records RSA/ECDH-protected traffic today and stores it, waiting for a future quantum computer to decrypt the key exchange — meaning long-lived confidential data is at risk today, not just after quantum computers arrive.
Hash-based signatures
A family of digital signature schemes whose security rests only on hash function properties rather than factoring or discrete logs — making them quantum-resistant. SLH-DSA (SPHINCS+) is NIST's standardized hash-based PQC signature scheme.
HKDF (HMAC-based Key Derivation Function)
A fast key derivation function used to derive multiple cryptographic keys from a single high-entropy shared secret (e.g. inside TLS or Signal handshakes) — distinct from password-hashing KDFs, which are deliberately slow.
HMAC (Hash-based Message Authentication Code)
A keyed hash construction proving a message wasn't tampered with, verifiable by anyone holding the shared key. Its nested double-hash design specifically defeats length-extension attacks that a naive H(key‖message) would be vulnerable to.
HSM (Hardware Security Module)
A dedicated, tamper-resistant hardware device that generates and stores private keys and performs cryptographic operations without ever exposing the key material to the host system.
I
ICC Master Key
A chip card's own long-term key, derived on demand from the issuer's master key (IMK) plus the card's PAN and PAN Sequence Number — the issuer never stores a separate key per card. Each transaction then derives a fresh, one-time session key from the ICC Master Key and the transaction counter (ATC).
IETF (Internet Engineering Task Force)
The open, individual-participation body behind the RFC series — TLS, JWT, X.509/PKIX, SSH, OAuth2 — developed by rough consensus and running code rather than formal membership voting.
ISO 9564 (PIN Security)
The standard defining how a PIN is packed into a fixed-width PIN block before encryption — Format 0/ISO-0 (PAN-XOR'd, fixed 0xF padding) and Format 3/ISO-3 (random padding, closing a guessing-attack weakness in Format 0). Published by ISO's financial-services committee (TC 68), a different technical committee from the IT-security one (JTC 1/SC 27) behind ISO/IEC 27001 and the other ISO/IEC entries on this site.
ISO/IEC
A federation of national standards bodies (ANSI, BSI, DIN, and others) that jointly publishes paywalled international standards — ISO/IEC 27001 (ISMS certification), 18033 (encryption algorithms), and 19790 (the international counterpart to FIPS 140-3), among others.
IV (Initialization Vector)
A value used to randomize the output of a cipher mode so that encrypting the same plaintext twice with the same key doesn't produce the same ciphertext. In AES-GCM this is usually called a nonce; it must never repeat for a given key.
J
JWT (JSON Web Token)
A compact, signed token format — header, payload, and signature, each base64url-encoded and dot-separated — commonly used for API authentication. The payload is encoded, not encrypted, so it's readable by anyone who has the token.
K
Kasiski examination
The technique (Babbage, c. 1854; Kasiski, 1863) that broke the Vigenère cipher: find repeated sequences in the ciphertext, and the distances between them reveal the keyword's length via their greatest common divisor — after which each resulting stream is just an ordinary Caesar shift.
KDF (Key Derivation Function)
An umbrella term for functions that derive one or more cryptographic keys from input material — covering both fast KDFs like HKDF (stretching high-entropy secrets) and slow, deliberately expensive password-hashing KDFs like PBKDF2 and Argon2.
Key ceremony
The scripted, audited, multi-party process used to generate a root CA's private key — the HSM generates it internally, it never exists outside the hardware boundary, and sensitive operations require multiple trusted individuals to jointly authorize, echoing threshold secret sharing.
L
Length-extension attack
An attack exploiting the Merkle-Damgård construction: given only H(message) and its length, an attacker can compute H(message ‖ extra) for attacker-chosen data, without ever knowing the original message.
Logjam attack
A 2015 downgrade attack that forced TLS connections onto weak, 512-bit "export-grade" Diffie-Hellman parameters left over from 1990s US cryptography export restrictions, making the resulting discrete log breakable via precomputation against a small number of widely reused primes.
Luhn algorithm
The public checksum formula every real card PAN's last digit satisfies — not a security mechanism, just an error-detecting check digit. A network token has to be Luhn-valid too, so it passes through existing card-rail systems exactly like a real PAN.
M
MAC (Message Authentication Code)
A short value proving both the integrity and authenticity of a message to anyone holding a shared secret key. HMAC is the most common hash-based MAC construction.
Meet-in-the-middle attack
A cryptanalysis technique that trades memory for time: precompute and store one direction of a two-step operation, then check the other direction against that table. Famously reduces naive double-DES's expected 2^112 security to roughly 2^57.
ML-DSA
NIST's standardized post-quantum digital signature algorithm (based on the Dilithium scheme, standardized as FIPS 204), replacing RSA and ECDSA signatures against quantum attack.
ML-KEM
NIST's standardized post-quantum key encapsulation mechanism (based on the Kyber scheme, standardized as FIPS 203), replacing RSA and Diffie-Hellman key exchange against quantum attack — often deployed in hybrid with classical ECDH during migration.
Min-entropy
The stricter, worst-case cousin of Shannon entropy: H∞ = −log₂(max pᵢ), driven entirely by a source's single most probable outcome rather than its average. Cryptographic key material is sized against this figure, since an attacker's best strategy is always to guess the most likely value, not an average one.
Modular arithmetic
Arithmetic that "wraps around" after reaching a fixed modulus, like a clock face — the mathematical foundation underlying RSA, Diffie-Hellman, and the discrete logarithm problem.
Montgomery's ladder
A constant-time algorithm for elliptic-curve scalar multiplication that performs one doubling and one addition every step regardless of the scalar's bits, closing the timing side channel that double-and-add leaves open. The algorithm underlying X25519.
MD5 / SHA-1
Older, now-broken hash functions. Both have had practical collisions publicly demonstrated and should never be used for security purposes today — only for non-adversarial checksums.
mTLS (Mutual TLS)
A TLS variant where both the client and the server present certificates, each verified by the other — common for service-to-service authentication and zero-trust network architectures.
N
Network tokenization (EMVCo)
Replacing a PAN with a surrogate token of the same length and format, bound to a specific Token Requestor ID so it only authorizes within the domain it was issued for. De-tokenization back to the real PAN happens only inside the network's own token vault — a merchant that's breached only ever exposes tokens, never the underlying card number.
NIST (National Institute of Standards and Technology)
A US Department of Commerce agency whose Computer Security Division publishes FIPS and SP 800 documents — AES, SHA-2/3, and the post-quantum standards all originate here, usually selected via open, multi-year public competitions.
NIST SP 800-90B
The NIST standard defining how to statistically assess the min-entropy of a real-world entropy source — ten different estimators in its non-IID track alone, with the final assessed entropy taken as the minimum across every estimator that runs. Three of the ten (Most Common Value, Collision, Markov) are implemented, ported from NIST's own reference code, in this site's Entropy Testing Playground tool.
Nonce
A value used only once — most critically, the per-encryption value in AES-GCM (which must never repeat under the same key) and the per-signature random value in ECDSA (which, if reused, directly exposes the private key).
O
OAEP (Optimal Asymmetric Encryption Padding)
The modern, randomized padding scheme for RSA encryption, using a double-masking (MGF1-based) construction to defeat the padding-oracle attacks that plagued the older PKCS#1 v1.5 padding.
OASIS (Organization for the Advancement of Structured Information Standards)
A member-driven nonprofit consortium — not a government body — that ratifies specifications through open Technical Committees, including SAML 2.0, KMIP, and PKCS#11 (whose stewardship RSA Laboratories transferred to OASIS in the early 2010s).
OAuth2
A delegated authorization framework — it lets an application act on a user's behalf with a specific, revocable scope of access, and by itself says nothing about who the user actually is. That identity gap is exactly what OIDC adds.
OCSP (Online Certificate Status Protocol)
A protocol letting a client ask a CA in real time whether a specific certificate is still valid, as an alternative to downloading a full CRL. OCSP stapling moves this query to the server to avoid a per-client round trip and metadata leak.
OIDC (OpenID Connect)
An authentication layer built directly on top of OAuth2, adding the one thing OAuth2 leaves out: the ID token, a signed JWT with standardized identity claims (sub, iss, aud, exp) proving who the user is.
One-way function
A function that's easy to compute in one direction but computationally infeasible to reverse — the conceptual foundation of hash functions, and (in trapdoor form) of public-key cryptography.
P
PAN (Primary Account Number)
The card number itself — the value a PIN block's PAN-field XOR binds to, a CVV is derived from, and a network token stands in for everywhere except inside the issuer's and network's own systems.
Passkey
A public-key credential replacing a password: the private key is generated on and never leaves a user's device, the server stores only the matching public key, and the browser binds every key pair to its exact origin, making it structurally phishing-resistant. Built on the WebAuthn standard.
PBKDF2
A key derivation function that deliberately slows password hashing by applying HMAC repeatedly, a tunable number of times — simple and standard, but more cheaply parallelizable on GPUs than memory-hard alternatives like Argon2.
PCI DSS (Payment Card Industry Data Security Standard)
The PCI Security Standards Council's data-handling standard for anyone who stores, processes, or transmits cardholder data — including the explicit rule that a CVV/CVC can never be stored after authorization. Enforced through the card networks' own merchant and processor contracts rather than government law.
Perfect Forward Secrecy (PFS)
A stronger phrasing of forward secrecy, emphasizing that every session's key exchange uses fresh ephemeral keys, so no single key compromise — past or future — exposes more than one session.
PIN block
A PIN packed into a fixed-width 8-byte structure (see ISO 9564) before encryption, so every PIN produces identically shaped ciphertext regardless of its actual length, and XOR'd with a PAN-derived field so two customers sharing a PIN never produce the same encrypted block.
PKCE (Proof Key for Code Exchange)
An OAuth2 extension for clients that can't safely hold a secret (mobile apps, single-page apps): the client generates a random secret locally and proves possession of it at token exchange, so an intercepted authorization code alone can't be redeemed by an attacker.
PKCS#1 v1.5
The original, deterministic RSA padding scheme, vulnerable to the Bleichenbacher padding-oracle attack against encryption. Superseded by OAEP for encryption (PSS is the modern equivalent for signatures).
PKCS#11 (Cryptoki)
The vendor-neutral API for talking to hardware security tokens — HSMs, smart cards, USB keys — through a fixed set of functions (C_GenerateKeyPair, C_Sign, C_WrapKey) that every conforming token implements identically. Its CKA_SENSITIVE / CKA_EXTRACTABLE object attributes are what let a key exist provably in exactly one place.
Pohlig-Hellman algorithm
An algorithm that breaks the discrete logarithm problem — classical or elliptic-curve — whenever the group's order is smooth (factors into small primes), by solving the discrete log in each small subgroup and recombining the results with the Chinese Remainder Theorem.
Post-compromise security
The property that a session can recover security even after an attacker briefly compromises its state, because ongoing fresh Diffie-Hellman exchanges (as in the Signal Protocol's Double Ratchet) eventually heal it.
PQC (Post-Quantum Cryptography)
Cryptographic algorithms believed to resist attack by both classical and quantum computers, standardized by NIST to replace RSA, Diffie-Hellman, and ECC once Shor's algorithm becomes practical against them.
Preimage resistance
The property that, given only a hash digest, it should be computationally infeasible to find any input that produces it — costing roughly 2^n operations for an n-bit hash.
Private key
The half of an asymmetric key pair kept secret by its owner, used to decrypt messages encrypted to the matching public key, or to produce digital signatures.
Public key
The half of an asymmetric key pair that can be freely shared, used to encrypt messages to its owner or verify their digital signatures.
Q
Qubit
The basic unit of quantum information, capable of existing in a superposition of states. Breaking RSA-2048 with Shor's algorithm is estimated to require several thousand error-corrected (logical) qubits — far beyond today's hardware.
S
Salt
A unique random value appended to a password before hashing, ensuring identical passwords don't produce identical hashes — defeats precomputed rainbow-table attacks, but does nothing to slow down an attacker targeting one specific hash (that's the KDF's job).
SAML (Security Assertion Markup Language)
An older, XML-based authentication and single-sign-on standard, predating JWTs — an Identity Provider issues a signed XML assertion (via XML-DSig) that a browser carries to a Service Provider via redirect or form POST. Still common in enterprise SSO alongside the more modern OIDC.
Schnorr signature
A discrete-log signature scheme built on the same commit-challenge-response structure as ElGamal encryption — simpler and more efficient than DSA, and the basis of Bitcoin's Taproot upgrade.
Session key
A symmetric key derived for the duration of a single communication session (e.g. via TLS or SSH's Diffie-Hellman handshake), used for fast bulk encryption instead of slower public-key operations.
Shamir's Secret Sharing
A (k, n)-threshold scheme splitting a secret into n shares so that any k reconstruct it exactly and any k−1 reveal nothing, built on the fact that k points uniquely determine a degree-(k−1) polynomial. Offers information-theoretic, not just computational, security below the threshold.
Shor's algorithm
A quantum algorithm, published in 1994, that solves integer factorization and discrete logarithms in polynomial time — the specific mathematical result that makes RSA, classical Diffie-Hellman, and ECC breakable on a sufficiently large, low-error quantum computer.
Side-channel attack
An attack that recovers secret data not by breaking an algorithm's mathematics, but by observing something about its physical execution — timing, power draw, cache access patterns, or electromagnetic emissions.
Signal Protocol
The end-to-end encryption protocol combining X3DH (initial key agreement) and the Double Ratchet (per-message key derivation), used by Signal, WhatsApp, and others to keep even the server operator from reading messages.
SHA-2 / SHA-256
The current widely deployed hash function standard, using the Merkle-Damgård construction and producing a 256-bit digest (for SHA-256) — used throughout TLS, code signing, and blockchain.
SHA-3
A structurally different hash standard (sponge construction rather than Merkle-Damgård), standardized in 2015 as a hedge against future cryptanalysis of SHA-2 — naturally immune to length-extension attacks.
SLH-DSA
NIST's standardized hash-based post-quantum digital signature algorithm (based on SPHINCS+, standardized as FIPS 205) — its security rests only on hash function properties, offering a structurally different, conservative alternative to ML-DSA.
Symmetric encryption
Encryption where the same key is used to both encrypt and decrypt, requiring the key to be shared secretly in advance. AES is the standard symmetric cipher; it's typically combined with asymmetric key exchange (as in TLS) to establish that shared key.
T
Timing attack
A side-channel attack that recovers secret data by measuring how long an operation takes, exploiting code whose execution time depends on secret values — mitigated by constant-time implementations.
TLS (Transport Layer Security)
The protocol behind HTTPS, combining a key exchange (ECDHE), certificate-based authentication, and symmetric encryption (typically AES-GCM) to protect data between a client and a server.
Token Requestor ID (TRID)
The identifier a network token vault binds a token to — typically one per merchant or wallet provider. A token presented under a different Token Requestor ID than the one it was issued for is declined, which is the entire security benefit of domain-bound tokenization.
TPK / ZPK (Terminal PIN Key / Zone PIN Key)
The two ends of one PIN-translation hop: a PIN block is encrypted under the TPK at the terminal, then an HSM decrypts it and immediately re-encrypts it under the ZPK for the next hop toward the issuer — the PIN is never allowed to exist in the clear outside that HSM's own protected memory.
TRNG (True Random Number Generator)
A randomness source that samples a physical process believed to be genuinely non-deterministic — thermal noise, oscillator jitter, radioactive decay, or quantum vacuum fluctuations — rather than a CSPRNG's deterministic (if unpredictable-looking) algorithm. Raw TRNG output usually still needs statistical testing and conditioning (whitening) before it's trusted as a seed.
Trapdoor function
A one-way function that becomes easy to reverse if you possess a specific secret (the "trapdoor") — the structural idea behind RSA, where factoring the public modulus is the trapdoor known only to the private key holder.
Triple DES (3DES)
DES run three times (encrypt-decrypt-encrypt) with two or three keys, closing the meet-in-the-middle gap that makes naive double-DES barely stronger than single DES — itself since superseded by AES.
V
Vigenère cipher
A polyalphabetic substitution cipher that shifts each letter by a different amount, following a repeating keyword, specifically to defeat frequency analysis — believed unbreakable for roughly three centuries until Kasiski examination cracked it.
X
X.509
The standard format for public-key certificates, defining fields like subject, issuer, public key, validity period, and signature — the format underlying HTTPS certificates.
X3DH (Extended Triple Diffie-Hellman)
The Signal Protocol's initial key agreement, letting a sender compute a shared secret and send a first encrypted message even while the recipient is offline, by combining several Diffie-Hellman exchanges against a pre-published key bundle.
Z
Zero-knowledge proof
A proof that lets one party convince another that a statement is true without revealing any information beyond the statement's truth itself — not covered in depth in this catalog, but a growing area adjacent to it.