The terms you'll run into across every other guide on this site, in one place.
A TLS 1.3 feature allowing a client to send encrypted application data on the very first message of a resumed session, before the handshake fully completes — faster, but with a known replay-attack trade-off that's why it's not used for anything non-idempotent.
The protocol Let's Encrypt introduced and the IETF later standardized for fully automated certificate validation, issuance, and renewal — the basis for tools like Certbot and acme.sh.
A class of cipher modes, like AES-GCM and ChaCha20-Poly1305, that provide encryption and integrity checking together — the only cipher type TLS 1.3 permits.
An HTTP header letting a server advertise that it also supports a newer protocol, like HTTP/3, on an alternate connection — used for protocol upgrade discovery.
A TLS extension letting client and server agree on which application protocol (HTTP/1.1, HTTP/2) to use, during the handshake itself rather than after.
Encryption using a mathematically linked key pair — a public key that can be shared freely and a private key that must stay secret — the foundation of how TLS certificates work.
An organization trusted by browsers and operating systems to verify identity and issue certificates — e.g. Let's Encrypt, DigiCert, Sectigo.
An email standard that displays a verified brand logo next to a message in supporting email clients, requiring a Verified Mark Certificate (VMC) to prove trademark ownership.
The industry body of certificate authorities and browser vendors that sets the baseline requirements every publicly trusted certificate must meet.
A DNS record type letting a domain owner specify which certificate authorities are allowed to issue certificates for that domain, rejecting requests from any other CA.
The CA/Browser Forum's core rulebook governing how every publicly trusted certificate authority must validate domains and issue certificates — the actual document behind terms like "DV," "OV," and "EV."
The most widely used free ACME client, maintained by the Electronic Frontier Foundation, for automating Let's Encrypt certificate issuance and renewal.
The combined file containing a certificate along with the intermediate certificate(s) needed to complete its chain of trust back to a root.
Hardcoding a specific certificate or public key a client will trust for a given domain, rejecting any other certificate even if it's otherwise validly issued.
A system of public, append-only logs that record every publicly trusted certificate issued, letting anyone audit issuance and detect unauthorized certificates.
A certificate authority's own published documents describing exactly how it validates and issues certificates — when these fall out of sync with what a CA's systems actually do, the result is a compliance incident and mass revocation, regardless of intent.
The path from your certificate up through one or more intermediate certificates to a root certificate that's built into browsers and operating systems.
The specific combination of encryption, authentication, and key-exchange algorithms negotiated between browser and server for a session.
A legacy certificate field once used to state the primary domain a certificate covers — modern browsers ignore it entirely in favor of the SAN field.
A certificate issued to a client (a user, a device, a service) rather than a server, used to authenticate that client during mutual TLS.
A CA-published, signed list of every certificate serial number that's been revoked before its natural expiry.
An encoded request, generated on your server, containing your public key and domain details, submitted to a CA to obtain a certificate.
DNS-Based Authentication of Named Entities — a system using DNSSEC-secured DNS records to specify which certificate a service should present, an alternative trust model to the standard CA system.
The binary encoding format certificates and keys are fundamentally stored in — PEM format is simply DER data base64-encoded for text-safe handling.
A key exchange method letting two parties establish a shared secret over an insecure channel without ever transmitting the secret itself — the basis for forward secrecy in TLS.
A cryptographic proof that a specific piece of data was signed by the holder of a specific private key and hasn't been altered since.
An ACME validation method proving domain control by adding a specific DNS TXT record — the only validation method that supports wildcard certificates.
A set of extensions adding cryptographic signatures to DNS responses, protecting against DNS spoofing — a separate, complementary security layer to TLS.
The lightest certificate validation tier, confirming only that the requester controls the domain — fully automatable and the basis of free certificates.
See Domain validation — a certificate issued at the DV tier.
A cryptographic approach offering equivalent security to RSA at much smaller key sizes, increasingly the default choice for modern certificates.
Elliptic Curve Diffie-Hellman Ephemeral — the forward-secret key exchange method most modern TLS connections use, combining ECC's efficiency with per-session key generation.
A TLS 1.3 extension that encrypts the SNI hostname during the handshake, closing one of the last pieces of connection metadata still visible in plaintext.
The certificate actually issued for a specific domain or service, as opposed to the intermediate or root certificates that sign it — also called a leaf certificate.
The most rigorous certificate validation tier, verifying the legal, physical, and operational existence of an organization — no longer given distinct browser UI treatment.
See Extended validation — a certificate issued at the EV tier.
A property ensuring that a future key compromise can't be used to decrypt past recorded sessions, since each session uses its own independently generated, temporary key.
The negotiation process at the start of a TLS connection where the browser and server agree on protocol version, cipher suite, and exchange keys.
A one-way algorithm that converts data of any size into a fixed-length output — used throughout TLS for integrity checks and digital signatures.
A deprecated, now-removed browser feature that let a site specify pinned keys via an HTTP header — removed after misconfiguration risked permanently locking out visitors.
HTTP Strict Transport Security — a header that tells browsers to only ever connect to your site over HTTPS, even if a user types http://.
A hardcoded list, compiled into browser source code, of domains that enforce HTTPS-only connections from a visitor's very first request, before any header could even be received.
A major HTTP revision adding multiplexing and other performance improvements — in practice, requires HTTPS in every major browser.
The current HTTP version, built on the QUIC transport protocol instead of TCP, with encryption built in as a mandatory, non-optional part of the protocol itself.
A certificate that sits between your certificate and the root CA in the chain of trust, signed by the root and used to sign end-entity certificates.
The part of the TLS handshake where client and server agree on a shared secret key, using methods like ECDHE, without transmitting that key directly.
A file or store holding a private key alongside its certificate, commonly in Java environments (a JKS or PKCS#12 file) — the counterpart to a truststore, which holds trusted CA certificates instead.
The matched public and private key generated together for a certificate — the public half goes into the certificate; the private half stays secret on the server.
A free, automated, nonprofit certificate authority launched in 2016 that made HTTPS accessible to the entire web at no cost.
An emerging certificate architecture built for a post-quantum web, using the same Merkle-tree structure behind Certificate Transparency logs — Let's Encrypt has committed to this as its path toward quantum-resistant authentication.
An attack where a third party intercepts and potentially alters communication between two parties who believe they're communicating directly with each other.
A page loaded over HTTPS that also loads at least one resource — script, image, stylesheet — over plain HTTP.
A certificate extension that makes OCSP stapling mandatory rather than optional — a client will hard-fail the connection if the server doesn't include a current stapled response, rather than falling back to a weaker check.
A configuration where both the client and the server present and verify certificates, authenticating each other rather than just the server authenticating to the client.
A mechanism letting a mail domain publish a policy requiring TLS for incoming mail, defending against downgrade attacks on email delivery.
A way for browsers to check in real time whether a certificate has been revoked, by querying the issuing CA directly.
A server-side optimization where the server itself periodically fetches and caches its OCSP response, then includes it directly in the handshake rather than making every visitor query the CA separately.
A certificate validation tier that verifies domain control plus the legal existence of the requesting organization, sitting between DV and EV in rigor.
Cloudflare's own certificate authority for securing the connection between Cloudflare's edge and your origin server specifically — trusted by Cloudflare, but not intended for direct browser connections.
See Organization validation — a certificate issued at the OV tier.
The common text-based certificate and key format, base64-encoded and wrapped with BEGIN/END header lines — despite the name, it's used far beyond email today.
Related but distinct standards: PKCS#1 defines the original RSA private key format, PKCS#7 (also called P7B) defines a certificate chain bundle without a private key, and PKCS#8 defines a newer, algorithm-independent private key format.
A single binary file format bundling a certificate, its private key, and often the certificate chain together, commonly used on Windows and Java-based systems.
Cryptographic algorithms designed to remain secure even against a sufficiently powerful future quantum computer, an active area of TLS protocol development.
The secret half of a certificate's key pair, kept only on your server, used to decrypt data and prove ownership of the certificate.
The shareable half of a certificate's key pair, embedded directly in the certificate itself and used by others to encrypt data or verify signatures.
A transport protocol built on UDP with encryption built in from the ground up, serving as the foundation for HTTP/3.
A server that sits in front of one or more backend servers, forwarding client requests to them — commonly used to centralize TLS termination.
The act of a CA invalidating a certificate before its natural expiry, typically due to a suspected private key compromise.
A self-signed certificate at the top of the chain of trust, pre-installed in browsers and operating systems as an inherently trusted anchor.
The curated list of root certificates a browser or operating system ships with and trusts by default.
One of the oldest and most widely used asymmetric encryption algorithms, still commonly used for certificate key pairs alongside the increasingly popular ECC.
A field listing every domain and subdomain a certificate is valid for — required by modern browsers even for a single-domain certificate.
A certificate signed by its own private key rather than by a separate, trusted CA — valid for encryption but not trusted by browsers without manual configuration.
A mechanism letting a returning client and server skip most of a full handshake by reusing previously negotiated session parameters, speeding up reconnection.
A cryptographic hash function from the SHA-2 family, the current standard used for certificate signatures after SHA-1 was deprecated for being cryptographically weak.
A TLS extension letting a server know which hostname the client wants, so multiple certificates can be served from a single IP address.
The original name for the protocol TLS eventually replaced — SSL 2.0 and 3.0 are both now formally prohibited, though the term 'SSL certificate' still persists informally.
An attack that intercepts a visitor's initial unencrypted HTTP request and keeps them on plain HTTP for an entire session, rather than letting them reach the intended HTTPS version.
A multi-domain certificate covering several unrelated domains or subdomains under one certificate — originally built for Microsoft Exchange and Office Communications Server, now used more broadly.
The current, correct name for the protocol securing HTTPS connections — the direct successor to SSL, standardized by the IETF starting in 1999.
A file or store holding trusted root and intermediate CA certificates, used to validate certificates presented by others — the counterpart to a keystore, which holds your own private key and certificate.
The point in an infrastructure chain where an encrypted TLS connection is decrypted — commonly a load balancer or CDN rather than the origin server itself.
Another term for a root certificate — the point a chain of trust ultimately rests on without needing further verification.
A certificate proving trademark ownership of a logo, required to display that logo via BIMI in supporting email clients — currently issued by only a small number of authorized CAs.
The window of time, defined by a certificate's Not Before and Not After dates, during which a certificate is considered current and usable.
A certificate covering a domain and all of its first-level subdomains using a single entry, e.g. *.yourdomain.com.
The standard that defines the structure of digital certificates used throughout TLS — every certificate issued today is an X.509 certificate.
Loading comments…