Glossary

44 terms, in plain English, each linked to the guide that explains it properly.

Argon2: A memory-hard password hashing function, specified in RFC 9106. Its cost settings control time, memory and parallelism, which makes large-scale guessing on GPUs expensive. See password hashing explained. See: Password hashing explained: salts and cost factors · Password length limits: minimums, maximums, truncation
Autofill: A password manager or browser filling saved login details into a form. Because it offers a login only on the domain it was saved for, it quietly refuses to fill look-alike phishing pages. NIST SP 800-63B-4 says verifiers SHALL allow it. See: How password managers work · Phishing and passwords: why strength doesn't help
bcrypt: A long-established password hashing function with an adjustable cost factor. Most implementations only use the first 72 bytes of the password, which is why it matters for length limits. See: Password hashing explained: salts and cost factors · Password length limits: minimums, maximums, truncation
Bit: One binary digit, and the unit of password strength. Each extra bit doubles the number of guesses needed to cover all possibilities: 40 bits is twice as many guesses as 39. See: Password entropy, explained in bits · How long does it take to crack a password?
Blocklist: A list of passwords a service refuses because they are common, expected or known from breaches. NIST requires services to check new passwords against one and to tell the user why a password was rejected. See: What makes a password common? · Password rules that backfire
Brute-force attack: Guessing by trying every possible value in a space, such as all 10,000 four-digit PINs. Against a truly random password, brute force is the best an attacker can do, so the size of the space sets the strength. See: How long does it take to crack a password? · Password entropy, explained in bits
Composition rules: Requirements such as “at least one uppercase letter, one digit and one symbol”. NIST SP 800-63B-4 says verifiers SHALL NOT impose them, because people meet them in predictable ways such as “Password1!”. See: Password rules that backfire · Why length beats complexity
Credential stuffing: Taking username and password pairs leaked from one site and trying them automatically on many others. It works only where a password has been reused. See credential stuffing and password reuse. See: Credential stuffing and password reuse · How data breaches expose passwords
CSPRNG: Cryptographically secure pseudo-random number generator: a generator whose output cannot be predicted from earlier output. In browsers it is reached through crypto.getRandomValues; Math.random is not one. See: How password generators work · Modulo bias and rejection sampling, worked through
Diceware: A method of making a passphrase by rolling dice to pick words from a numbered list. With the EFF large wordlist, five dice select one of 7,776 words, worth 12.925 bits each. See diceware with real dice. See: Making a passphrase with real dice · How passphrases work
Dictionary attack: Guessing from a list of likely candidates, such as common passwords, words and leaked passwords, often with variations like capitals and digits added. It succeeds against human-chosen passwords far faster than brute force. See: What makes a password common? · Keyboard patterns and letter substitutions
Entropy: For a randomly generated secret, log2 of the number of equally likely possibilities, measured in bits. It describes the process that made the password, not the string itself. See entropy explained. See: Password entropy, explained in bits · How password generators work
FIDO2: The combination of the W3C WebAuthn browser API and the FIDO Alliance’s CTAP protocol for talking to authenticators. It underlies passkeys and security keys. See: Passkeys explained: public keys instead of passwords · Two-factor authentication explained
Hash function: A function that turns any input into a fixed-size output that cannot practically be reversed. General-purpose hashes such as SHA-256 are fast, which is why passwords need slower, purpose-built password hashing functions. See: Password hashing explained: salts and cost factors · How data breaches expose passwords
HOTP: HMAC-based one-time password, defined in RFC 4226. A code is computed from a shared secret and a counter that advances with each use. TOTP is HOTP with a time-based counter. See: How authenticator apps make 6-digit codes
k-anonymity: A privacy property where a query reveals only that the item is one of many. The Pwned Passwords range API uses it: the client sends just the first 5 hex characters of a password’s SHA-1 hash and checks the returned list itself. See: Checking breached passwords with k-anonymity
Keyboard walk: A password made by moving across neighboring keys, such as qwerty or 1qaz2wsx. It looks random but is one of the first things guessing tools try. See: Keyboard patterns and letter substitutions · What makes a password common?
Keyspace: The total number of possible values a secret could take. A 6-digit PIN has a keyspace of 10^6 = 1,000,000; 8 characters from 94 printable ASCII characters have 94^8, about 6.1 × 10^15. See: Password entropy, explained in bits · How long does it take to crack a password?
Leetspeak: Swapping letters for similar-looking digits or symbols, such as p@55w0rd for “password”. Guessing tools undo these swaps automatically, so they add very little strength. See: Keyboard patterns and letter substitutions · Password rules that backfire
Modulo bias: The unevenness that results from reducing a random number with % n when its range is not a multiple of n. One random byte mod 94 makes 68 characters 1.5 times as likely as the other 26. See: Modulo bias and rejection sampling, worked through · How password generators work
Multi-factor authentication (MFA, 2FA): Signing in with two or more different kinds of evidence: something you know, something you have, something you are. Two-factor authentication (2FA) is the two-factor case. See two-factor authentication explained. See: Two-factor authentication explained · Passkeys explained: public keys instead of passwords
NIST SP 800-63B: The US National Institute of Standards and Technology’s guideline on authentication. Revision 4, published in 2025, sets current rules such as a 15-character minimum for single-factor passwords and no composition rules. See: Password rules that backfire · Should you change passwords regularly? · Password length limits: minimums, maximums, truncation
Offline attack: Guessing against stolen password hashes on the attacker’s own hardware, with no login server to slow it down. Speed then depends on the hash function, from thousands to billions of guesses per second. See: How long does it take to crack a password? · Password hashing explained: salts and cost factors
One-time password (OTP): A code that is valid for a single login, such as a 6-digit code from an authenticator app or a text message. Because it is typed by hand, NIST does not consider it phishing-resistant. See: How authenticator apps make 6-digit codes · Two-factor authentication explained
Online attack: Guessing by trying passwords against a live login. Rate limiting and lockouts restrict it; NIST allows no more than 100 consecutive failures before an authenticator is disabled. See: How long does it take to crack a password? · PINs explained: why four digits can be enough
Passkey: A login credential made of a public-private key pair, created per site and bound to its domain under the WebAuthn standard. The site stores only the public key, and a look-alike site cannot use it. See passkeys explained. See: Passkeys explained: public keys instead of passwords · Two-factor authentication explained
Passphrase: A password made of several words. When the words are picked at random from a known list, its strength is exactly the number of words times log2 of the list size. See how passphrases work. See: How passphrases work · How many words should a passphrase have?
Password hashing function (KDF): A deliberately slow function that turns a password, a salt and a cost setting into a stored value; also called a key derivation function (KDF). Examples are Argon2, scrypt, bcrypt and PBKDF2. See: Password hashing explained: salts and cost factors · How password managers work
Password manager: Software that stores logins in an encrypted vault unlocked by one master password, generates random passwords, and fills them in on matching sites. See password managers explained. See: How password managers work · Choosing a master password
Password spraying: Trying a few very common passwords against many accounts, slowly enough to stay under per-account lockout limits. It relies on some accounts using the most predictable choices. See: Credential stuffing and password reuse · What makes a password common?
PBKDF2: Password-Based Key Derivation Function 2, defined in RFC 8018. It slows guessing by repeating an HMAC many times; it is not memory-hard, so its iteration count needs to be high. See: Password hashing explained: salts and cost factors
Pepper: A secret key, stored apart from the password database, that is mixed into password hashing. If only the database leaks, the attacker cannot test guesses without it. NIST says verifiers SHOULD use one. See: Password hashing explained: salts and cost factors · How data breaches expose passwords
Phishing: Tricking someone into entering credentials on a fake site or message that imitates a real one. Long, random passwords do not help against it; domain-bound methods do. See phishing and passwords. See: Phishing and passwords: why strength doesn't help · Passkeys explained: public keys instead of passwords
Phishing resistance: NIST’s term for an authentication method that cannot leak usable secrets to an impostor site, without relying on the user to notice. It requires cryptographic authentication bound to the site, such as WebAuthn; typed codes never qualify. See: Passkeys explained: public keys instead of passwords · Two-factor authentication explained
Rainbow table: A precomputed table that maps hashes back to passwords for a given unsalted hash function. A unique salt per password makes such tables useless. See: Password hashing explained: salts and cost factors · How data breaches expose passwords
Rate limiting: Restricting how many login attempts can be made, for example with delays, lockouts or bot challenges. It is what makes short secrets like 6-digit codes safe online. See: PINs explained: why four digits can be enough · Two-factor authentication explained
Recovery codes: One-time codes, usually issued when 2FA is set up, for signing in when your usual second factor is lost. NIST calls them look-up secrets, which must be at least six decimal digits; saved recovery codes need at least 64 random bits. See: Recovery codes and backups: how they work · Two-factor authentication explained
Rejection sampling: Discarding random values that fall in the uneven top end of a range and drawing again, so every outcome is exactly equally likely. It is the standard fix for modulo bias. See: Modulo bias and rejection sampling, worked through · How password generators work
Salt: A random value stored with each password hash and fed into the hashing, so identical passwords produce different hashes. It defeats precomputed tables; NIST requires at least 32 bits. See: Password hashing explained: salts and cost factors · How data breaches expose passwords
scrypt: A memory-hard password hashing and key derivation function, defined in RFC 7914. Requiring lots of memory per guess makes specialized guessing hardware less effective. See: Password hashing explained: salts and cost factors
Security key: A small hardware device, connected by USB, NFC or Bluetooth, that holds private keys and signs WebAuthn login challenges. Keys stored in it typically cannot be exported, which makes them device-bound. See: Passkeys explained: public keys instead of passwords · Two-factor authentication explained
TOTP: Time-based one-time password, defined in RFC 6238. An app and a site share a secret and compute HOTP over floor(Unix time / 30), giving a 6-digit code that changes every 30 seconds. See: How authenticator apps make 6-digit codes · Two-factor authentication explained
WebAuthn: The W3C Web Authentication API that lets websites create and use public-key credentials through the browser. Each credential is scoped to a site’s domain, which is what makes it phishing-resistant. See: Passkeys explained: public keys instead of passwords
Wordlist: A fixed list of words for building passphrases. The EFF large wordlist has 7,776 words, chosen to be easy to type and remember, so each randomly chosen word adds 12.925 bits. Browse it on the wordlist page. See: How passphrases work · Making a passphrase with real dice