Authentication and Secure Communication

Secure Communication

Security protocol: A sequence of messages exchanged to achieve a security goal.
Key establishment: Getting a shared secret key to the parties that need it, and only them.
Pre-shared key: A secret distributed out of band before communication begins.
Trusted third party: A party that participants rely on to vouch for keys or identities.
Public key transport: One party generating a key and sending it encrypted under the other’s public key.
Diffie-Hellman key agreement: Two parties computing a shared secret from values exchanged in the open.
Long-term key: A key that identifies a party across many connections.
Session key: A key that protects one conversation and is then discarded.
Adversary in the middle (AiTM): An attacker relaying traffic between two parties who believe they are talking directly. Also called man in the middle (MITM).
Replay attack: Reusing a valid message outside the context in which it was sent.
Freshness: Evidence that a message belongs to the current exchange or a recent period.
Nonce: A value intended for one use.
Timestamp: The sending time, included in a protected message so the receiver can judge recency.
Sequence number: An increasing counter that lets a receiver reject duplicated or reordered messages.
Challenge-response authentication: Proving knowledge of a key by computing a response to a fresh challenge.
One-way authentication: One party authenticating the other, but not the reverse.
Mutual authentication: Each party authenticating the other.
Reflection attack: Sending a party’s own challenge back so that it answers itself.
Session hijacking: Taking over a session after authentication has finished.
Authenticated key exchange: A protocol that establishes shared keys and authenticates one or both participants.

Key Distribution with a Trusted Third Party

Key distribution center (KDC): A trusted server that shares a key with every participant and issues session keys.
Ticket: A credential created by a trusted third party that only the recipient can decrypt.
Needham-Schroeder protocol: A 1978 protocol for obtaining a session key from a trusted server.
Denning-Sacco attack: Replaying an old Needham-Schroeder ticket whose session key was compromised.
Otway-Rees protocol: A key distribution protocol that uses nonces from both parties instead of clocks.
Kerberos: A ticket-based authentication system built around a key distribution center.
Authentication server (AS): The Kerberos service that verifies a user at login.
Ticket-granting server (TGS): The Kerberos service that issues tickets for individual services.
Ticket-granting ticket (TGT): A Kerberos ticket used to request service tickets without the password.
Service ticket: A ticket for one service, encrypted under that service’s key.
Authenticator: The client’s name and current time, encrypted under a ticket’s session key.
Single sign-on: One login giving access to many services.

Public Key Authentication and TLS

Elliptic-curve Diffie-Hellman (ECDH): Diffie-Hellman key agreement using elliptic curves, with smaller public values.
Hybrid cryptosystem: Establishing a key with public-key cryptography and protecting data with symmetric encryption.
Hybrid key establishment: Combining a classical and a post-quantum method to derive one key.
Ephemeral key: A key-agreement value used for one handshake and then erased.
Forward secrecy: Keeping recorded sessions secret after a long-term key is compromised. Also called perfect forward secrecy.
Transport Layer Security (TLS): The protocol that authenticates servers and protects HTTPS connections.
TLS handshake: The messages that negotiate algorithms, authenticate the server, and establish keys.
Record: A chunk of the TLS data stream, encrypted and authenticated as a unit.
Transcript hash: A hash of the handshake messages exchanged so far.
Downgrade attack: Forcing parties to negotiate a weaker protocol version or algorithm.
Mutual TLS (mTLS): TLS with certificates authenticating both the client and the server.
0-RTT: Early TLS application data sent before the server replies, which can be replayed and lacks forward secrecy.

Passwords

Identification: Claiming an identity.
Authentication: Verifying a claimed identity.
Authorization: Determining what an authenticated party is allowed to do.
Authentication factor: A category of evidence: something you know, have, or are.
Multi-factor authentication (MFA): Requiring evidence from two or more different factor categories.
Password Authentication Protocol (PAP): Sending the password to the server, which compares it with its records.
Challenge-Handshake Authentication Protocol (CHAP): Proving password knowledge by hashing it with a challenge from the server.
Online guessing: Submitting password guesses to the login service.
Offline guessing: Testing password guesses against stolen or captured data.
Dictionary attack: Guessing likely passwords and their common variations first.
Rainbow table: A compact precomputed table for reversing unsalted hashes of likely passwords.
Salt: A random value stored with each password hash to make it unique.
Password hashing function: A deliberately slow hash with a tunable cost, used to store passwords.
bcrypt: A password hashing function with an adjustable cost parameter.
scrypt: A password hashing function that requires a large amount of memory per hash.
Argon2: A password hashing function with adjustable time and memory costs, recommended for new systems.
PBKDF2: A password hashing function that repeats HMAC many times.
Credential stuffing: Trying username and password pairs leaked from one site on other sites.
Password spraying: Trying a few common passwords against many accounts.
Password manager: Software that generates and stores a unique password for each site.

Beyond Passwords

One-time password (OTP): A code accepted for at most one successful authentication.
Hash chain: A sequence of values produced by repeatedly hashing a seed.
HMAC-based one-time password (HOTP): A one-time code computed from a shared key and a counter.
Time-based one-time password (TOTP): A one-time code computed from a shared key and the current time.
SIM swap: Moving a victim’s phone number to a SIM card the attacker controls.
Push fatigue: Sending repeated login prompts until the user approves one. Also called MFA fatigue.
Number matching: Requiring the user to enter a number from the login screen into the app.
Session cookie: A token the browser sends to show it has already authenticated.
Passkey: A public-key credential bound to one site, whose private key stays with the user.
Synced passkey: A passkey copied to a user’s approved devices through a sync provider.
Device-bound passkey: A passkey whose private key never leaves one authenticator.
Security key: A USB or NFC device that holds keys and answers login challenges.
WebAuthn: The browser interface for creating and using public-key credentials.
FIDO2: The standards for public-key authentication to websites, including WebAuthn.
Origin: A web page’s scheme, host, and port.

Biometric Authentication

Biometric authentication: Verifying identity from physical or behavioral characteristics.
Template: The stored biometric features created at enrollment.
Enrollment: Capturing samples and building a user’s biometric template.
Threshold: The similarity cutoff for accepting a biometric match.
False accept rate (FAR): The probability that the system accepts an impostor.
False reject rate (FRR): The probability that the system rejects the legitimate user.
Receiver operating characteristic (ROC) curve: A plot of correct acceptances against false acceptances across thresholds.
Operating point: The threshold a deployment chooses, and its resulting error rates.
Biometric verification: Comparing a sample with the template of one claimed identity.
Biometric identification: Searching every template to find whom a sample belongs to.
Minutiae: Points where fingerprint ridges end or split.
Presentation attack: Fooling a biometric sensor with a substitute, such as a photograph or mold.
Liveness detection: Checking that a sample comes from a live person at the sensor.