Reference
The privacy glossary.
53 precise definitions — cryptography, regulation, measurement, and email infrastructure — written from implementations we actually run, not paraphrased marketing.
01 · Cryptography & authentication
AES-256-GCMAES-256-GCM is authenticated encryption: the AES block cipher with 256-bit keys in Galois/Counter Mode, providing confidentiality and integrity in one pass — tampered ciphertext fails authentication instead of decrypting to garbage.Argon2idArgon2id is the recommended variant of Argon2, winner of the Password Hashing Competition: a memory-hard key-derivation function that makes password guessing expensive on GPUs and custom hardware, while resisting side-channel attacks.Blind indexA blind index is a keyed, deterministic hash of a field (such as an email address) stored in place of the plaintext, so a database can look records up by exact match without holding — or leaking — the value itself.End-to-end encryption (E2EE)End-to-end encryption means data is encrypted on the sender’s device and decrypted only on the recipient’s — every intermediary, including the service operator, handles ciphertext it cannot read.HKDFHKDF (HMAC-based key derivation function, RFC 5869) expands one strong secret into any number of independent, purpose-bound keys via extract-then-expand — the standard way to turn a shared secret into separate encryption, signing, and session keys.JWT (JSON Web Token)A JWT is a compact, signed token carrying claims (user ID, expiry, scopes) that services can verify statelessly — any holder of the verification key can check authenticity without a database lookup.Key stretchingKey stretching turns a low-entropy secret like a password into a stronger key by making each derivation deliberately expensive — thousands to millions of iterations, or memory-hard computation — so brute-force guessing becomes uneconomical.OPAQUEOPAQUE is an asymmetric password-authenticated key exchange (aPAKE) protocol, standardized in RFC 9807, that lets a server verify a password without ever seeing it — the password never leaves the client, even during registration.OPRF (oblivious pseudorandom function)An OPRF is a two-party protocol where a server holding a secret key helps a client evaluate a pseudorandom function on the client’s input — without the server learning the input or the output, and without the client learning the key.PAKE (password-authenticated key exchange)A PAKE is a cryptographic protocol in which two parties use a shared password to establish a strong session key — without ever transmitting the password or exposing it to offline guessing from intercepted traffic.Passkeys (WebAuthn / FIDO2)Passkeys are public-key login credentials based on the FIDO2/WebAuthn standards: your device holds a private key and signs a per-site challenge, so there is no shared secret to phish, leak, or reuse across sites.PKCEPKCE (Proof Key for Code Exchange, RFC 7636) hardens the OAuth 2.0 authorization-code flow: the client commits to a one-time secret up front, so an intercepted authorization code is useless without it.SRP (Secure Remote Password)SRP is a 1990s password-authenticated key exchange that lets clients log in without sending the password.TOTP (time-based one-time password)TOTP (RFC 6238) generates six-digit codes from a shared secret and the current time, giving accounts a second factor that changes every 30 seconds and works fully offline in any authenticator app.Zero-knowledgeIn applied privacy engineering, zero-knowledge describes systems designed so the service operator cannot read user data or secrets — verification and storage work without the server ever holding the plaintext.Zero-knowledge authenticationAuthentication in which the server verifies you know your password without ever receiving it — during signup or login, in any form.
02 · Privacy & regulation
AnonymizationAnonymization is processing data such that the individual it once related to can no longer be identified by any means reasonably likely to be used — done correctly, the result falls entirely outside GDPR’s scope, unlike pseudonymized data.Consent bannerA consent banner is the on-page prompt asking a visitor to accept or decline non-essential cookies and tracking, required under the EU ePrivacy Directive whenever a site stores or reads anything on the visitor’s device that isn’t strictly necessary.Data controllerA data controller is the natural or legal person that determines the purposes and means of processing personal data — under GDPR, the controller bears primary responsibility for lawful processing and for honoring data subjects’ rights.Data processorA data processor is the natural or legal person that processes personal data on behalf of, and under the instructions of, a data controller — without independently deciding why or how that data is used.Data residencyData residency is the physical or jurisdictional location where an organization’s data is stored — a deliberate infrastructure choice, distinct from data sovereignty, which concerns whose laws govern that data regardless of where it sits.Data sovereigntyData sovereignty is the principle that data is subject to the laws of the jurisdiction that governs it — broader than data residency, since a legal claim can reach across borders regardless of where servers physically sit.DPA (data processing agreement)A data processing agreement (DPA) is the contract required by GDPR Article 28 between a data controller and a data processor, setting out the scope, purpose, and security obligations governing the processor’s handling of personal data on the controller’s behalf.ePrivacy DirectiveThe ePrivacy Directive (2002/58/EC, amended 2009) is the EU law governing confidentiality of electronic communications, including the cookie-consent rule: storing or reading anything on a user’s device requires prior consent unless it is strictly necessary.EU-US Data Privacy FrameworkThe EU-US Data Privacy Framework (DPF) is a 2023 adequacy decision permitting personal-data transfers to US companies that self-certify to its principles — successor to the invalidated Privacy Shield and Safe Harbor.FADP / nFADPThe FADP (Federal Act on Data Protection) is Switzerland’s national data protection law; its fully revised version (nFADP) took effect 1 September 2023, modernizing Swiss law to align closely — though not identically — with the GDPR.GDPRThe GDPR (General Data Protection Regulation, EU 2016/679) is the European Union’s data protection law, in force since 25 May 2018, governing how personal data of people in the EU/EEA is collected, processed, and transferred.Personal dataPersonal data, under GDPR, is any information relating to an identified or identifiable natural person — a deliberately broad definition covering not just names and IDs but anything that can be linked back to someone, directly or in combination with other data.Privacy by designPrivacy by design is the principle — codified as a legal obligation in GDPR Article 25 — that data protection must be built into a system’s architecture from the outset, not added afterward as a policy or a configuration toggle.PseudonymizationPseudonymization is processing personal data so it can no longer be attributed to a specific person without additional information, which is kept separately and secured — a GDPR-recognized risk-reduction measure, but the data remains personal data in scope.SCCs (standard contractual clauses)Standard contractual clauses (SCCs) are European Commission-approved contract templates that legalize personal-data transfers from the EU/EEA to countries without an adequacy decision by extending GDPR-equivalent protections.Sub-processorA sub-processor is a third party a data processor engages to help process personal data on a controller’s behalf — a cloud host, for example.Warrant canaryA warrant canary is a regularly published statement that an organization has not received a secret government order — its disappearance warns users implicitly, since a gag order would forbid saying so directly.
03 · Analytics & web
Bounce rateBounce rate is the share of visits in which a user views a single page and leaves without any further interaction or navigation — a rough signal of whether a landing page held attention.Browser fingerprintingBrowser fingerprinting identifies a device by combining device and browser attributes — screen size, installed fonts, GPU details, timezone, and more — into a composite signature, without storing anything on the device.Cookieless analyticsCookieless analytics measures website traffic without setting cookies or any other persistent client-side identifier — counts are derived from request-level signals, not a stored ID that follows a browser over time.Do Not Track (DNT)Do Not Track was an HTTP header and browser setting, standardized as DNT, that signaled a user’s preference not to be tracked across sites — largely abandoned because sites were never legally required to honor it.Global Privacy Control (GPC)Global Privacy Control is a browser-sent HTTP signal (Sec-GPC: 1) expressing a user’s opt-out of sale and sharing of personal data — legally binding in California and other US states, unlike Do Not Track.ReferrerThe referrer is the URL of the page a visitor was on immediately before arriving at the current one, sent by the browser in the HTTP Referer header (or omitted, per the page’s referrer policy).Session (analytics)In web analytics, a session is a group of interactions — pageviews, events — from one visitor treated as a single continuous visit, typically closed out after a period of inactivity (commonly 30 minutes).Third-party cookiesA third-party cookie is set by a domain other than the one the user is visiting — typically an ad or analytics vendor embedded via script — letting that vendor recognize the same browser across unrelated sites.Tracking pixelA tracking pixel is a tiny, invisible image embedded in a page or email whose load request signals to the hosting server that the content was opened, along with the viewer’s IP and client details.Unique visitorsUnique visitors counts distinct people (or devices) visiting a site over a period, as opposed to pageviews, which counts every page load — the same visitor loading ten pages is one unique visitor and ten pageviews.UTM parametersUTM parameters are standardized query-string tags (utm_source, utm_medium, utm_campaign, and related fields) appended to a URL so analytics tools can attribute traffic to a specific campaign, channel, or link.
04 · Email & infrastructure
Bot detectionBot detection distinguishes automated traffic from genuine human visitors before allowing an action — form submission, login, checkout — to prevent spam, credential stuffing, and scraping at scale.DKIMDKIM (DomainKeys Identified Mail, RFC 6376) lets a sending domain cryptographically sign outgoing email — a public key published in DNS verifies the signature, proving headers and body were not altered in transit.DMARCDMARC (RFC 7489) is a DNS policy record tying SPF and DKIM together: it requires one to align with the visible From address, tells receivers what to do on failure, and requests reporting on the results.Email open trackingEmail open tracking records when a recipient views a message, typically via an embedded tracking pixel that silently reports back to the sender the moment the email client loads remote images.HoneypotA honeypot, in bot detection, is a form field hidden from human view but present in the page’s HTML — automated submitters that fill in every field reveal themselves, while real visitors never see it to fill in.MTA-STSMTA-STS (RFC 8461) lets a domain require TLS for all inbound email delivery, published and fetched over HTTPS rather than trusted opportunistically — closing the downgrade attack that plain SMTP TLS leaves open.Proof-of-work challengeA proof-of-work challenge requires a client to perform a small, verifiable computation before a request is accepted — cheap for one user, expensive enough at scale to make bulk automated abuse uneconomical.SPFSPF (Sender Policy Framework, RFC 7208) is a DNS TXT record listing which mail servers may send email for a domain — receiving servers check the connecting IP against it to catch spoofed sender addresses.Transactional emailTransactional email is a message triggered by a specific user action or system event — password resets, receipts, verification codes, shipping notices — as opposed to marketing email sent to a list on a schedule.