SSL/TLS Glossary

A

ACME

Automatic Certificate Management Environment. The protocol that lets a server prove it controls a domain and obtain a certificate without human involvement. The standard underpinning Let's Encrypt and now used by many commercial CAs.

AEAD Cipher

Authenticated Encryption with Associated Data. A class of cipher (AES-GCM, ChaCha20-Poly1305) that combines encryption and integrity in a single primitive. All TLS 1.3 cipher suites are AEAD; older modes like CBC are not.

AIA

Authority Information Access. A certificate extension that tells a client where to fetch the issuer's certificate and OCSP status. Lets browsers retrieve missing intermediate certificates automatically — though many clients ignore it.

ALPN

Application-Layer Protocol Negotiation. A TLS extension that lets the client and server agree on an application protocol (HTTP/1.1, HTTP/2, HTTP/3) during the TLS handshake instead of after.

Asymmetric Cryptography

Encryption that uses a key pair — a public key anyone can know, and a private key the owner keeps secret. The basis of how SSL/TLS authenticates servers without sharing secrets in advance.

AutoSSL

cPanel's automated certificate service. Issues and renews free DV certificates for cPanel-hosted sites, typically backed by Let's Encrypt or Sectigo depending on the host's configuration.

B

Browser Trust Store

The list of root certificate authorities a browser (or its underlying OS) considers trustworthy by default. If your certificate's chain ends at a root not in the trust store, the browser shows a security warning.

C

CA (Certificate Authority)

An organisation that issues digital certificates. Browsers ship with a built-in list of trusted CAs (Let's Encrypt, DigiCert, Sectigo, GlobalSign, etc.). A certificate is only trusted by a browser if it's signed by one of these.

CA Bundle

A file containing the intermediate certificates needed to chain a leaf certificate up to a trusted root. Must be installed on the server alongside the leaf cert — without it, mobile browsers and many API clients will reject the certificate.

Certificate Chain

The sequence of certificates that proves your leaf certificate is trustworthy: leaf → intermediate(s) → root. Each certificate in the chain is signed by the next. Browsers walk the chain until they hit a certificate they trust.

Certificate Pinning

A practice where a client (typically a mobile app) hardcodes the expected certificate or public key, refusing to connect if the server presents a different one. Highly secure but operationally fragile — a renewed cert will break a pinned app unless pin rotation was planned.

Certificate Transparency (CT)

A public, append-only log of every TLS certificate issued by participating CAs. Lets domain owners detect misissuance, and lets browsers refuse certs that aren't logged. Required by Chrome since 2018.

Certbot

The most widely used ACME client, maintained by the EFF. Installed on a server, it handles certificate issuance and renewal from Let's Encrypt automatically.

Cipher Suite

The combination of algorithms a TLS connection uses for key exchange, authentication, encryption, and integrity (e.g. TLS_AES_256_GCM_SHA384). The client and server negotiate one during the handshake.

CN (Common Name)

The hostname in a certificate's Subject field. Historically used to match a certificate to a domain, now superseded by Subject Alternative Names — modern browsers ignore the CN entirely.

CSR (Certificate Signing Request)

A file containing your public key and identifying information, sent to a CA to request a certificate. Generated locally so the matching private key never leaves your server.

D

DANE

DNS-based Authentication of Named Entities. A way to publish certificate fingerprints in DNS (using TLSA records) so clients can verify a certificate without trusting browser CAs. Used in mail (SMTP) more than HTTPS.

Diffie-Hellman

A key exchange algorithm that lets two parties agree on a shared secret over an insecure channel. Modern TLS uses Elliptic Curve Diffie-Hellman (ECDHE) for forward secrecy.

DV (Domain Validation)

The lowest validation level. The CA only confirms the requester controls the domain (typically via an HTTP or DNS challenge). Free with Let's Encrypt; sufficient for most websites.

E

ECDHE

Elliptic Curve Diffie-Hellman Ephemeral. A key exchange method that produces a fresh shared secret per session, providing forward secrecy. Required for modern cipher suites.

EV (Extended Validation)

A high-assurance validation level requiring the CA to verify the legal identity of the requesting organisation. Once shown as a green bar in browsers; modern browsers no longer display EV differently from DV.

F

Forward Secrecy

A property of TLS connections where compromising the server's long-term private key in the future doesn't allow decrypting past recorded traffic. Achieved using ephemeral key exchange (ECDHE). Required for TLS 1.3.

Fullchain

A file containing the leaf certificate followed by all intermediate certificates needed to build a complete chain. Most servers accept either a fullchain plus the leaf, or a single fullchain file with the leaf first.

H

Handshake

The series of messages exchanged at the start of a TLS connection where the client and server authenticate, agree on a cipher suite, and establish a session key. TLS 1.3 reduced this from two round-trips to one.

HSTS (HTTP Strict Transport Security)

An HTTP response header that tells browsers to always connect to the domain over HTTPS, ignoring any HTTP links. Prevents downgrade attacks. See our HSTS guide.

HSTS Preload

A list of domains hardcoded into browsers as HTTPS-only. The strongest form of HSTS — even a fresh browser install will refuse HTTP for preloaded domains. Effectively irreversible.

HTTPS

HTTP layered on top of TLS. The name "HTTPS" technically refers to the URL scheme; the underlying security comes from TLS.

I

Intermediate Certificate

A certificate that sits between your leaf certificate and the trusted root CA in the chain. Used so root keys can be kept offline — intermediates do the day-to-day signing. Missing intermediates are the most common SSL misconfiguration.

K

Key Exchange

The handshake stage where the client and server establish a shared session key. Modern TLS uses ECDHE; older RSA key exchange is deprecated because it doesn't provide forward secrecy.

KeyUsage

A certificate extension that limits what a key can be used for (signing, encryption, key agreement). Helps enforce separation of duties between certificates with different roles.

L

Leaf Certificate

The certificate at the bottom of the chain — the one issued specifically for your hostname. Sometimes called the "end-entity" certificate.

Let's Encrypt

A free, automated, non-profit certificate authority operated by the Internet Security Research Group. Issues only DV certificates; the largest CA on the internet by certificate volume.

M

Mixed Content

A page loaded over HTTPS that contains resources (images, scripts, stylesheets) loaded over plain HTTP. Modern browsers block scripts and stylesheets and warn on images. Fix by switching all resources to HTTPS.

mTLS (Mutual TLS)

A TLS connection where both the client and the server present and verify certificates. Used for service-to-service authentication in microservices and zero-trust networks; rarely seen on public websites.

O

OCSP (Online Certificate Status Protocol)

A way for clients to check whether a certificate has been revoked, by querying the CA's OCSP responder in real time. Largely replaced for browsers by short-lived certs and CRLite.

OCSP Stapling

An optimisation where the server fetches its own OCSP response periodically and includes ("staples") it in the TLS handshake. Eliminates the privacy and performance cost of clients querying OCSP separately.

OV (Organisation Validation)

A validation level that includes business identity verification — the CA confirms the requesting organisation legally exists. Higher trust signal for compliance audits; not visually distinguished from DV in browsers.

P

PEM

A common file format for certificates and keys — base64-encoded data wrapped between -----BEGIN CERTIFICATE----- and -----END CERTIFICATE----- markers. Most Linux servers expect PEM.

PFX / PKCS#12

A binary file format that bundles a certificate and its private key in one password-protected file. Used by Windows/IIS.

Private Key

The secret half of an asymmetric key pair, held only by the certificate owner. Anyone with the private key can impersonate the certificate's identity. Treat it like a password — never share, never commit to git.

Public Key

The non-secret half of a key pair, embedded in the certificate. Used by clients to encrypt data only the private key can decrypt, and to verify signatures the private key produced.

R

Renewal

Issuing a new certificate before the current one expires. With short-lived certs (90 days), renewal must be automated; manual renewal is the leading cause of unexpected expiry.

Revocation

Marking a certificate as no longer valid before its expiry date, typically because the private key has been compromised. Browsers check revocation via OCSP, CRLs, or browser-vendor lists like CRLite.

Root Certificate

The certificate at the top of a chain. Self-signed and stored in browser/OS trust stores. Compromising a root would let an attacker impersonate any site signed by that CA — so root keys are kept offline and rarely used directly.

RSA

An asymmetric cryptography algorithm. Older TLS certificates use RSA keys (typically 2048 or 4096 bits). Modern certificates increasingly use ECDSA, which produces smaller, faster keys at equivalent security.

S

SAN (Subject Alternative Name)

A certificate extension listing every hostname the certificate covers. Modern browsers only check SANs (the Common Name is ignored). A "multi-domain" cert is one with many SANs. See our cert types guide.

Self-Signed Certificate

A certificate signed by its own private key rather than by a CA. Browsers don't trust them by default. Useful for local development or internal testing; not appropriate for public sites.

SHA-256

A cryptographic hash function used in modern certificate signatures. SHA-1, its predecessor, was deprecated for certificate use in 2017.

SNI (Server Name Indication)

A TLS extension where the client tells the server which hostname it's trying to connect to during the handshake. Lets one server with one IP host many sites with different certificates.

SSL

Secure Sockets Layer. The original protocol for encrypted web connections, deprecated since 2015 and replaced by TLS. The name "SSL" persists in popular usage but no modern site actually uses SSL — they use TLS.

Subject

The entity a certificate identifies — typically a domain or organisation. Listed in the certificate's Subject field; the hostnames are duplicated in the SAN extension for browser matching.

Symmetric Encryption

Encryption that uses the same key for both sides of the conversation. Faster than asymmetric encryption, so TLS uses asymmetric only to establish a shared symmetric key, then symmetric encryption (e.g., AES) for the actual data.

T

TLS

Transport Layer Security. The current protocol for encrypted connections, the successor to SSL. Versions 1.0 and 1.1 are deprecated; 1.2 and 1.3 are current. Despite the name, "SSL certificate" is still widely used to refer to TLS certificates.

TLS 1.2

The previous-generation TLS protocol, released in 2008. Still widely deployed and considered secure when configured correctly. Supports a wider range of cipher suites than 1.3, including some weak legacy options that should be disabled.

TLS 1.3

The current TLS protocol, released 2018. Removed legacy cryptography, reduced the handshake to one round-trip, and made forward secrecy mandatory. Should be enabled on every modern server. See our TLS 1.2 vs 1.3 guide.

U

UCC

Unified Communications Certificate. A SAN certificate marketed specifically for Microsoft Exchange and Lync/Skype for Business. Functionally equivalent to any multi-domain SAN cert.

W

Wildcard Certificate

A certificate covering all direct subdomains of a parent domain (*.example.com). Doesn't cover the apex domain itself or sub-subdomains. Convenient for SaaS with many tenant subdomains; carries a larger blast radius if the key is compromised.

X

X.509

The standard format for public-key certificates used in TLS, S/MIME, and many other protocols. Every "SSL certificate" you've ever installed is an X.509 v3 certificate.