Glossary

Glossary

Cryptographic Algorithms

Term	Definition
ML-DSA	Module-Lattice Digital Signature Algorithm (FIPS 204). Post-quantum signature scheme based on lattices. Replaces RSA/ECDSA for signatures. Security levels: ML-DSA-44 (Level 1), ML-DSA-65 (Level 3), ML-DSA-87 (Level 5).
ML-KEM	Module-Lattice Key Encapsulation Mechanism (FIPS 203). Post-quantum key exchange. Replaces ECDH/RSA for key establishment. Security levels: ML-KEM-512 (Level 1), ML-KEM-768 (Level 3), ML-KEM-1024 (Level 5).
SLH-DSA	Stateless Hash-based Digital Signature Algorithm (FIPS 205). Post-quantum signature based on hash functions. Conservative alternative to ML-DSA with larger signatures but different security assumptions.
ECDSA	Elliptic Curve Digital Signature Algorithm. Classical signature scheme using elliptic curves (P-256, P-384, P-521). Vulnerable to quantum attacks.
Ed25519	Edwards-curve Digital Signature Algorithm. Fast classical signatures using Curve25519. Vulnerable to quantum attacks.
RSA	Rivest-Shamir-Adleman. Classical asymmetric algorithm for signatures and encryption. Vulnerable to quantum attacks (Shor’s algorithm).

PKI Concepts

Term	Definition
CA	Certificate Authority. Entity that issues and signs digital certificates.
Root CA	Top-level CA in a hierarchy. Self-signed certificate, trust anchor for the PKI.
Issuing CA	Intermediate/subordinate CA that issues end-entity certificates. Signed by Root CA or another Issuing CA.
End-Entity	The subject of a certificate (server, client, user). Not a CA.
CSR	Certificate Signing Request. A message sent to a CA to request a signed certificate. Contains the public key and identity information.
CRL	Certificate Revocation List. Signed list of revoked certificate serial numbers published by the CA.
OCSP	Online Certificate Status Protocol (RFC 6960). Real-time certificate validity check as alternative to CRL.
TSA	Timestamp Authority (RFC 3161). Trusted service that provides cryptographic proof of when data existed.
SAN	Subject Alternative Name. X.509 extension for multiple identities (DNS names, IP addresses, email).
SKI/AKI	Subject/Authority Key Identifier. Extensions linking certificates in a chain.
Path Length	Constraint on how many CAs can exist below an issuing CA in the chain.

Post-Quantum Concepts

Term	Definition
PQC	Post-Quantum Cryptography. Algorithms designed to be secure against quantum computer attacks.
SNDL	Store Now, Decrypt Later. Threat where adversaries capture encrypted data today to decrypt with future quantum computers. Affects confidentiality.
HNDL	Harvest Now, Decrypt Later. Alternative term for SNDL, emphasizing data harvesting.
TNFL	Trust Now, Forge Later. Threat where classical signatures can be forged retroactively once quantum computers exist. Affects long-term signature validity.
Hybrid	Combining classical + post-quantum algorithms for defense in depth. If either algorithm is secure, the hybrid is secure.
Catalyst	ITU-T X.509 9.8 hybrid certificate format. Dual signatures stored in X.509 extensions, allowing graceful fallback to classical.
Composite	IETF hybrid format combining keys/signatures into single cryptographic objects. Both algorithms must be verified together.
Crypto-Agility	Ability to switch cryptographic algorithms without major infrastructure changes. Essential for PQC migration.
LTV	Long-Term Validation. Signatures that remain verifiable for decades, even after algorithm deprecation.
NIST Levels	Security strength categories (1, 3, 5) corresponding to AES-128, AES-192, AES-256 equivalent security.

SSH Certificate Concepts

Term	Definition
SSH Certificate	OpenSSH certificate format (PROTOCOL.certkeys). Signed by a CA, contains principals, validity period, and extensions. Alternative to authorized_keys.
Principal	An identity a certificate authorizes. For user certificates: usernames (alice, deploy). For host certificates: hostnames or IPs.
Critical Option	SSH certificate restriction enforced by sshd. If the server does not recognize a critical option, it rejects the certificate. Examples: force-command, source-address.
Extension	SSH certificate permission (user certificates only). Controls allowed operations: permit-pty, permit-port-forwarding, permit-agent-forwarding, etc.
KRL	Key Revocation List. OpenSSH binary format for efficiently revoking SSH certificates by serial number or key. Checked via ssh-keygen -Q.
TrustedUserCAKeys	sshd configuration directive specifying the CA public key file. All certificates signed by this CA are accepted for user authentication.

QPKI Concepts

Term	Definition
Profile	YAML template defining certificate policies (algorithm, validity, extensions, constraints). One profile = one certificate type.
Credential	Managed bundle of private key(s) + certificate(s) with coupled lifecycle management (enrollment, renewal, revocation).
Attestation	RFC 9883 mechanism where a signing certificate attests for a KEM key that cannot sign its own CSR. Required for ML-KEM certificates.
Related Certificate	X.509 extension linking separate classical and PQC certificates for the same subject.

Certificate Types

Term	Definition
TLS Server	Certificate for HTTPS server identity. Contains DNS names in SAN.
TLS Client	Certificate for client authentication in mTLS. May contain email or user identifier.
Code Signing	Certificate for signing software releases. Proves software authenticity.
Timestamping	Certificate for TSA service. Contains id-kp-timeStamping extended key usage.
OCSP Responder	Certificate for OCSP service. Contains id-kp-OCSPSigning extended key usage.
KEM Certificate	Certificate containing ML-KEM public key for key encapsulation (encryption).
Signature Certificate	Certificate containing signing key (ECDSA, ML-DSA, etc.) for digital signatures.

Standards

Term	Definition
FIPS 203	NIST standard for ML-KEM (key encapsulation). August 2024.
FIPS 204	NIST standard for ML-DSA (digital signatures). August 2024.
FIPS 205	NIST standard for SLH-DSA (hash-based signatures). August 2024.
X.509	ITU-T standard for public key certificates. Foundation of PKI.
RFC 5280	Internet X.509 PKI Certificate and CRL Profile.
RFC 6960	Online Certificate Status Protocol (OCSP).
RFC 3161	Time-Stamp Protocol (TSP). Standard for trusted timestamping services.
RFC 5652	Cryptographic Message Syntax (CMS). Format for signed/encrypted data.
RFC 9883	Use of Post-Quantum KEM in CMS. Defines CSR attestation for KEM keys.
ITU-T X.509 9.8	Alternative public-key and signature algorithms extension. Basis for Catalyst certificates.