The SSL Certificate Practitioner Glossary: 200 Terms Defined the Way Buyers Actually Use Them

How this glossary differs from CA-published glossaries: CA glossaries are written to make certificates sound more complex and more necessary than they are. This glossary is written to tell you exactly what each term means in practice, when it matters, and when it does not. Where CA marketing language overstates what a term means, this glossary says so.

A

ACME (Automated Certificate Management Environment)  A protocol (RFC 8555) for automating the issuance and renewal of SSL/TLS certificates without human intervention. Let’s Encrypt uses ACME. When you run Certbot, it uses ACME to talk to Let’s Encrypt. Under the 47-day validity trajectory taking effect by 2029, ACME automation becomes operationally necessary, not optional.

AIA (Authority Information Access)  A field in an SSL certificate that tells browsers where to find the certificate issuer’s public key and where to check the certificate’s revocation status. Browsers use the AIA to fetch intermediate certificates if the server does not supply them and to reach the OCSP responder for revocation checking.

Algorithm  The mathematical method used to create and verify cryptographic signatures. Current standards: RSA (2048-bit minimum), ECDSA P-256 or P-384 (preferred for performance). SHA-256 for hashing. Post-quantum algorithms (ML-KEM, ML-DSA) are finalized by NIST as of August 2024 but not yet in browser-trusted certificates. [CA marketing note: ‘military-grade encryption’ typically means AES-256, which is universal across all certificate types :  it is not a premium feature]

Alternative Names  See Subject Alternative Names (SANs). The Common Name field in certificates is deprecated for browser trust purposes; all domains must be listed as SANs.

Anchor Certificate  See Root Certificate. The root CA certificate that is pre-installed in a browser’s or OS’s trust store and from which all certificate trust chains begin.

ASN.1 (Abstract Syntax Notation One)  The data encoding standard used to structure SSL certificate fields. You will encounter ASN.1 only if you are parsing raw certificate bytes. For practical purposes, ASN.1 is the reason certificate data looks like structured objects when you view a certificate in a browser.

B

Baseline Requirements  The CA/B Forum’s Baseline Requirements for the Issuance and Management of Publicly Trusted Certificates :  the core technical standards that every publicly trusted CA must follow. Defines DCV methods, certificate field requirements, validity periods, revocation timelines, and CA audit obligations. Updated by ballot. The current version is the authoritative reference for what CAs can and cannot do. [Why it matters to buyers: when a CA says ‘we follow industry standards,’ they mean the Baseline Requirements. They are mandatory, not voluntary]

Bearer Token  An authentication token used in API contexts, not specific to SSL. Not to be confused with certificate-based authentication. Mentioned here because it sometimes appears in SSL-adjacent security discussions.

Browser  For SSL purposes, the software application that maintains a root trust store and enforces TLS connection requirements. Chrome, Firefox, Safari, Edge, and Brave are the major browsers. Each maintains its own root store, which is why a CA must be accepted by multiple independent programs to be ‘universally trusted.’

Bundle (Certificate Bundle)  The set of certificate files required for installation on a web server: the end-entity certificate, any intermediate certificates, and sometimes the root certificate. Most hosting control panels accept a bundle as a single file. If your browser shows a ‘certificate chain incomplete’ error, the intermediate certificate is missing from the bundle.

C

CA (Certificate Authority)  An organization that issues SSL certificates. For a certificate to be trusted by browsers, the CA must be included in the browser’s root store. There are two types: public CAs (DigiCert, Sectigo, Let’s Encrypt :  trusted by browsers worldwide) and private CAs (used internally within organizations, trusted only on devices where the private root is manually installed).

CA/B Forum (CA/Browser Forum)  The industry governance body that writes the rules for how CAs issue publicly trusted certificates. Members include CAs (DigiCert, Sectigo, Let’s Encrypt) and browser vendors (Google, Mozilla, Apple, Microsoft). Decisions are made by ballot. The Baseline Requirements, EV Guidelines, and Code Signing Requirements are CA/B Forum documents. See: Ballot SC-081v3 for the 47-day validity schedule.

CAA Record (Certification Authority Authorization)  A DNS record that specifies which CAs are authorized to issue certificates for a domain. If your domain has a CAA record and the CA you purchased from is not listed, certificate issuance will fail. Check for CAA records before purchasing from a new CA. Example: issue ‘letsencrypt.org’ allows only Let’s Encrypt. [Common failure scenario: a domain with CAA record limiting to Let’s Encrypt will reject a paid Sectigo certificate]

Certificate  A digital document that binds a public key to an identity and is signed by a Certificate Authority. For websites, the certificate contains the domain name(s), the public key, the validity period, the CA’s identity, and the CA’s digital signature confirming it issued the certificate. Certificates are public documents; anyone can read them.

Certificate Chain  The sequence of certificates from the end-entity certificate (your website’s certificate) through one or more intermediate certificates to the root certificate. Browsers follow the chain to verify that the end-entity certificate was ultimately signed by a trusted root. A broken chain (usually a missing intermediate) causes browser trust errors even if the end-entity certificate is valid.

Certificate Lifecycle Management (CLM)  The organizational practice and software tooling for tracking, renewing, and replacing certificates across an infrastructure. At 47-day validity, CLM becomes operationally critical for organizations with more than a handful of certificates. Enterprise CLM tools include DigiCert ONE, Sectigo Certificate Manager, and Venafi. For small organizations, a spreadsheet plus calendar reminders is a basic CLM.

Certificate Pinning  A security mechanism in which an application (usually a mobile app) is programmed to accept only a specific certificate or public key, regardless of what the device’s trust store says. Prevents man-in-the-middle attacks even from trusted CAs. Major risk: if the pinned certificate expires and the application does not release an update, the application stops working. The Epic Games April 2021 outage extended due to certificate pinning cascade failures.

Certificate Policy (CP)  A formal document that a CA publishes describing the rules under which it issues certificates. The CP specifies what validation procedures are required for each certificate type. Paired with the Certificate Practices Statement (CPS). Publicly available from any legitimate CA’s website.

Certificate Practices Statement (CPS)  A detailed document from a CA describing exactly how it implements its Certificate Policy in practice. The CPS is the operational manual for the CA’s certificate issuance process. Publicly available. For regulated sectors, vendor security assessments may require review of the CA’s CPS.

Certificate Revocation List (CRL)  A list of certificates that a CA has revoked before their expiry date. Browsers can check CRLs to verify that a certificate they encounter has not been revoked. CRLs are large files and are checked less frequently than OCSP. Modern browsers typically use OCSP for revocation checking, not CRLs directly. [Practical note: revocation checking has known limitations :  most browsers fail soft (accept the certificate) if the CRL or OCSP check fails rather than reject the connection]

Certificate Signing Request (CSR)  The file you generate on your server and send to the CA when requesting a certificate. The CSR contains your public key and the domain name(s) you want the certificate to cover. It is signed with your private key, proving you have the key. The CA verifies domain control (and for OV/EV, business identity), then signs your public key to create the certificate. Generating the CSR on the server is important: the private key stays on the server and is never sent to the CA.

Certificate Transparency (CT)  A public logging system for SSL certificates. Every publicly trusted certificate must be logged in at least two public CT logs before browsers will trust it. The logs are public and append-only. Anyone can query CT logs to see all certificates issued for a domain. CT was introduced as a direct response to the DigiNotar breach of 2011 and became mandatory in 2018.

Cipher Suite  A named combination of encryption algorithms used for a TLS connection: key exchange algorithm, authentication algorithm, bulk encryption algorithm, and message authentication algorithm. Example: TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384. PCI DSS 4.0 prohibits weak cipher suites including those using RC4, 3DES, or NULL encryption. TLS 1.3 simplified cipher suites significantly.

Code Signing Certificate  A certificate used to sign software executables and scripts, not websites. Code signing certificates confirm the software publisher’s identity. Different from SSL/TLS certificates, though issued by the same CAs under different CA/B Forum guidelines.

Common Name (CN)  A field in a certificate that historically contained the domain name for website certificates. Now deprecated for browser trust purposes; the domain must appear in the Subject Alternative Names field. The CN still appears in certificate details but browsers validate against SANs, not CN. For OV certificates, the CN may contain the organization name rather than the domain name in some configurations.

Comodo  The former name of what is now Sectigo. Comodo CA was founded in 1998, became the world’s largest commercial CA by certificate volume, and was acquired by Francisco Partners private equity in 2018 and rebranded as Sectigo. ‘Comodo SSL’ and ‘Sectigo SSL’ refer to the same company’s products. A valid Comodo-branded certificate is trusted identically to a Sectigo-branded certificate. [Buyer note: resellers may still list products as ‘Comodo PositiveSSL’ and ‘Sectigo PositiveSSL’ :  they are the same certificate]

Crypto Agility  The ability to quickly update or replace cryptographic algorithms across an organization’s systems without service disruption. NIST CSWP 39 (2025) establishes crypto agility as a compliance objective for organizations preparing for the post-quantum transition. An organization with crypto agility can migrate from RSA to ML-DSA certificates without an extended emergency project.

CSR  See Certificate Signing Request.

D

DANE (DNS-based Authentication of Named Entities)  A protocol that allows SSL certificate information to be pinned in DNS records using DNSSEC. Allows a site operator to say ‘only trust this specific certificate for my domain’ at the DNS level. Not widely supported by browsers but used in email security (SMTP DANE). Mentioned here because it occasionally appears in SSL discussions as an alternative trust model.

DCV (Domain Control Validation)  The process by which a CA verifies that the person requesting a certificate controls the domain. Three methods: DNS CNAME (add a specific record), HTTP file (place a file at a specific path), and email (click a link in email to a domain address). DNS CNAME is fastest and most reliable. Email-based DCV was deprecated in July 2025 by the CA/B Forum.

DER (Distinguished Encoding Rules)  A binary encoding format for certificate files. DER files typically use the .cer or .der extension. You may encounter DER format when installing certificates on Java applications or non-Apache/Nginx servers. PEM format (Base64-encoded DER with header/footer lines) is more common for web servers.

DigiCert  One of the major commercial CAs, known for enterprise PKI, high-assurance OV and EV certificates, and managed certificate services. Acquired Symantec’s CA operations in 2017. Acquired by Clearlake Capital and TA Associates for $6.9 billion in 2024. Holds approximately 6.6% of new certificate issuance as of Q1 2026 (TechnologyChecker.io, May 2026). Considered a premium CA; higher prices reflect enterprise service infrastructure, not certificate quality. [Common misconception: DigiCert certificates are not more ‘secure’ than Sectigo certificates :  both are in all major root stores and use identical encryption standards]

Digital Signature  A cryptographic operation that uses a private key to create a verifiable proof that a specific entity signed a specific piece of data. The CA’s digital signature on a certificate is what makes browsers trust it: the CA has signed the certificate’s public key and domain information, and the browser can verify this signature using the CA’s public key (which is in the root certificate in the trust store).

Distinguished Name (DN)  The structured identifier in a certificate that represents the subject (for the website certificate) or the issuer (for the CA). Contains fields such as Common Name (CN), Organization (O), Organizational Unit (OU), Locality (L), State (ST), and Country (C). OV certificates contain a verified Organization field. DV certificates typically contain only the domain name in CN and no Organization field.

DV (Domain Validation) Certificate  The most common certificate type. The CA verifies only that the requester controls the domain. Takes minutes to issue. Free from Let’s Encrypt or $4-$15/year from authorized resellers. The Organization field in the certificate Subject is empty. Phishing sites use DV certificates :  Zscaler ThreatLabz 2024 found 90.5% of phishing sites use DV. The padlock that appears in browsers for DV and OV/EV sites is visually identical. [Marketing note: ‘industry-standard encryption’ on a DV certificate is accurate but misleading :  the encryption is identical to OV and EV; only the identity verification differs]

E

ECDSA (Elliptic Curve Digital Signature Algorithm)  A signature algorithm used as an alternative to RSA for certificates. ECDSA P-256 certificates are smaller, faster to process, and consume less CPU than RSA-2048 certificates. Preferred for modern TLS deployments. Gcore December 2025: ECDSA reduces TLS handshake CPU by 15-20% compared to RSA. Post-quantum migration will eventually replace ECDSA with ML-DSA, but for current deployments ECDSA P-256 or P-384 is the recommended certificate key type.

eIDAS 2.0  Regulation (EU) 2024/1183, entered force May 20, 2024. Establishes the EU Digital Identity Wallet framework and expands the legal recognition of Qualified Website Authentication Certificates (QWACs) issued by EU trust service providers. A draft provision (Article 45) proposed requiring browsers to trust EU-designated CAs regardless of their compliance track record; more than 500 security researchers opposed this provision. The final text was modified to preserve more of the CA/B Forum framework. Member State wallets due December 2026.

Encryption  The transformation of readable data into an unreadable form using a cryptographic key. TLS encryption for websites uses symmetric encryption (AES-256 or ChaCha20) for the data transmission, with the symmetric key itself exchanged using asymmetric cryptography (ECDHE or RSA key exchange). The SSL certificate establishes the trust chain that validates the server’s public key; the encryption of actual data uses a separate symmetric key negotiated during the TLS handshake. [Common confusion: the certificate itself does not ‘do’ the encryption :  it validates the server’s identity and enables the key exchange that produces the encryption keys]

End-Entity Certificate  The certificate at the end of a certificate chain :  the one installed on the web server and presented to browsers. As distinct from intermediate certificates (which sign end-entity certificates) and root certificates (which sign intermediate certificates). Also called a leaf certificate.

EV (Extended Validation) Certificate  The highest validation level certificate. Requires CA verification of legal entity, operational existence, physical address, phone number, and authorized representative authority. Takes 1-5 business days. EV new issuance has declined sharply: TechnologyChecker.io Q1 2026 data shows only ~50,000 EV certificates issued in April 2026, declining approximately 20% per month. The green address bar that distinguished EV in browsers was removed by Chrome in 2019 and all other major browsers subsequently. EV remains relevant in regulated sectors where it is specified in compliance frameworks. [Marketing note: ‘highest trust’ is accurate for validation level but misleading as a general purchase recommendation :  the visible browser difference between OV and EV is now minimal for most use cases]

Extended Key Usage (EKU)  A field in a certificate that specifies what the certificate is authorized to be used for: TLS server authentication, email protection, code signing, etc. A certificate with EKU set to Server Authentication can be used for HTTPS. A certificate with EKU set to Code Signing cannot be used for HTTPS. Browsers reject certificates used outside their authorized EKU.

F

FIPS (Federal Information Processing Standards)  US government cryptographic standards published by NIST. FIPS 140-2 and FIPS 140-3 are the cryptographic module standards relevant to SSL. FIPS 203, 204, and 205 (finalized August 2024) are the post-quantum cryptography standards. US federal agencies and contractors are required to use FIPS-validated cryptographic modules.

Forward Secrecy (Perfect Forward Secrecy / PFS)  A TLS property where each session uses a unique ephemeral key, so that compromising the server’s private key does not allow decryption of past recorded sessions. ECDHE key exchange provides forward secrecy. RSA key exchange does not. PCI DSS 4.0 requires forward secrecy. Without forward secrecy, an adversary who records encrypted traffic today can decrypt it if they later obtain the server’s private key :  the harvest-now-decrypt-later threat.

Fingerprint (Certificate Fingerprint)  A hash of the certificate data, used as a short identifier. SHA-256 fingerprints are the standard. Certificate fingerprints appear in browser certificate detail views and are used to verify certificate identity without comparing the entire certificate. When you see a fingerprint in Qualys SSL Labs output, it uniquely identifies that specific certificate.

Free Certificate  Commonly refers to Let’s Encrypt DV certificates, which are free to issue, automate, and renew. ‘Free’ refers to the certificate cost. Free certificates require ACME automation infrastructure to manage reliably. Free certificates do not include warranties, CA site seals, or organizational identity verification. Free certificates are trusted identically to paid DV certificates in browsers. [Marketing note: paid certificate features (warranty, site seal, OV identity) are real differentiators :  the price difference between free DV and $5/year paid DV at a reseller buys you only a warranty and site seal]

G

GoDaddy  A domain registrar and web hosting company that resells SSL certificates. GoDaddy is not a Certificate Authority; it purchases certificates from Sectigo, DigiCert, and other CAs and resells them with a significant markup. GoDaddy’s SSL renewal pricing: $94.99/year for a standard DV certificate whose equivalent from an authorized reseller costs $4.99-$7.66/year. The certificate issued is technically identical. GoDaddy’s value proposition for SSL is one-click integration with its hosting platform, not the certificate quality.

Grace Period  The time between the first renewal attempt and the certificate expiry, during which a site can continue operating on its current certificate while renewal is retried. Most ACME clients begin renewal 30 days before expiry, giving a 30-day grace period. At 47-day validity, the grace period shortens proportionally unless organizations configure earlier renewal initiation.

Green Padlock  The green padlock icon that Chrome and other browsers displayed for HTTPS connections until 2018-2023. Chrome removed the green color for all HTTPS connections in 2018 (moving to a grey padlock) and removed the padlock icon entirely in September 2023 (Chrome 117), replacing it with a neutral tune icon. Firefox and Safari retain modified padlock icons. The green padlock is no longer meaningful as a trust signal. [Historical note: the green padlock for EV certificates was a different shade and included the organization name :  this was removed by all major browsers between 2018 and 2020]

H

Harvest-Now-Decrypt-Later (HNDL)  A threat model in which an adversary records encrypted TLS traffic today with the intention of decrypting it when quantum computers become available. TLS without forward secrecy (RSA key exchange) is vulnerable to HNDL. TLS with ECDHE forward secrecy is partially resistant (past sessions are protected even if the private key is later compromised). Full protection requires PQC hybrid key exchange (X25519Kyber768 or ML-KEM-768). Banks and organizations with long-lived sensitive data are the primary HNDL risk category.

HPKP (HTTP Public Key Pinning)  A deprecated HTTP header that allowed websites to pin specific certificates or public keys, causing browsers to reject connections if the certificate did not match. Removed from Chrome in 2018 and deprecated in all major browsers. HPKP caused permanent inaccessibility when certificates were replaced without updating the pins. Not recommended for any new deployment.

HSTS (HTTP Strict Transport Security)  An HTTP response header that tells browsers to only connect to a site over HTTPS for a specified period (max-age). Once a browser has received an HSTS header from a site, it will not make HTTP connections to that site for the max-age duration. HSTS preloading adds sites to a browser-built-in list so HTTPS-only behavior applies even on first visit. PCI DSS 4.2.1.1 effectively requires HSTS for payment pages. [HSTS and certificate expiry: if a certificate expires on an HSTS-preloaded domain, the site becomes completely inaccessible (not just insecure) :  the US government shutdown incident of 2019 demonstrated this at scale]

HTTP-01 Challenge  The most common domain control validation method used by Let’s Encrypt and cPanel AutoSSL. The CA provides a token that must be placed at a specific URL path (http://domain.com/.well-known/acme-challenge/[token]). The CA fetches the URL and verifies the token. Fails when Cloudflare’s ‘Always Use HTTPS’ redirect is active, when DNS points to the wrong server, or when firewalls block the CA’s validation requests.

HTTPS  Hypertext Transfer Protocol Secure :  HTTP transmitted over a TLS-encrypted connection. The padlock in a browser address bar indicates HTTPS. HTTPS provides three things: encryption (data in transit cannot be read by network observers), authentication (the certificate validates the server’s identity), and integrity (data cannot be tampered with in transit). An HTTPS connection with a DV certificate provides encryption and integrity but minimal authentication (only domain control verified). An HTTPS connection with an OV certificate adds verified organizational identity.

I

Intermediate Certificate  A certificate in the chain between the root CA certificate and the end-entity certificate. CAs use intermediate certificates rather than signing end-entity certificates directly with the root, so that if the intermediate is compromised, the root remains secure. When you install an SSL certificate, you typically install both the end-entity certificate and one or more intermediate certificates. A missing intermediate is the most common cause of ‘certificate chain incomplete’ browser errors.

Issuance  The process of a CA signing and delivering a certificate. For DV certificates, issuance is automated and takes minutes. For OV certificates, issuance requires human organizational validation (1-3 business days). For EV certificates, issuance requires extended validation (1-5 business days). Issuance time is distinct from validity period :  a 199-day validity certificate issues once and is then valid for 199 days.

Issuer  The Certificate Authority that signed a certificate. The Issuer field in a certificate’s details shows the CA’s distinguished name. A certificate purchased through any authorized reseller shows the CA (e.g., Sectigo) as the Issuer, not the reseller. This is verifiable in certificate details and in Certificate Transparency logs.

K

Key Exchange  The part of the TLS handshake where the client and server agree on a shared secret that will be used to derive the session’s encryption keys. The two main key exchange algorithms: RSA key exchange (the server encrypts the session key with its public key; no forward secrecy) and ECDHE (both parties contribute to the session key using ephemeral keys; provides forward secrecy). TLS 1.3 mandates ECDHE and removes RSA key exchange. PQC hybrid key exchange (X25519Kyber768) adds an ML-KEM component alongside ECDHE.

Key Size  The length of the cryptographic key in bits. RSA minimum: 2048 bits (2048-bit RSA is adequate for current security; 4096-bit is available but adds overhead with no browser trust benefit). ECDSA: P-256 or P-384 (256-bit ECDSA provides equivalent security to 3072-bit RSA at lower computational cost). CA/B Forum requires RSA 2048-bit minimum; RSA 1024-bit is no longer permitted. [Marketing note: ‘2048-bit or 4096-bit’ is sometimes presented as a premium feature. 2048-bit RSA is adequate for current security requirements; the choice between them does not affect browser trust]

Key Usage  A field in a certificate that specifies what cryptographic operations the certificate’s key may be used for: digital signatures, key encipherment, key agreement, etc. TLS server certificates require Key Encipherment or Key Agreement (depending on the cipher suite). Certificates used for signing require Digital Signature.

L

Let’s Encrypt  A free, automated, and open Certificate Authority operated by the Internet Security Research Group (ISRG). Launched in 2015. As of mid-2025, held approximately 54-63% of all active SSL certificate issuance globally (the dominant CA by volume). Issues DV certificates only. 90-day validity (moving to 47 days by 2029). Uses ACME protocol for automation. Discontinued OCSP responders in May 2025 and expiry notification emails in June 2025. Certificates are trusted identically to paid commercial certificates in all major browsers. [Common question: ‘Is Let’s Encrypt less secure than paid certificates?’ No. Browser trust is identical. The difference is no warranty, no site seal, no OV/EV identity verification, and dependency on ACME automation working correctly]

Lifetime  See Validity Period.

M

Man-in-the-Middle Attack (MITM)  An attack where an adversary intercepts communications between two parties, reading or modifying the data. TLS with a valid certificate from a trusted CA prevents MITM because the browser verifies the server’s identity through the certificate chain. MITM attacks against TLS require either a compromised CA (as in DigiNotar 2011) or a certificate for a CA in the browser’s trust store that the attacker controls (as in the Russia NCA concern).

Maximum Merge Delay (MMD)  The maximum time between a certificate being submitted to a CT log and it being included in the log’s append-only structure. CT logs must have an MMD of 24 hours or less. An MMD exceeding 24 hours disqualifies a CT log from use for certificate issuance.

Mis-issuance  The issuance of a certificate that violates CA/B Forum requirements: issuing a certificate for a domain the requester does not control, issuing a certificate with incorrect validation, backdating certificates to avoid policy requirements. Documented mis-issuance is the primary trigger for CA distrust events (Symantec 30,000+ mis-issued certificates; WoSign backdated SHA-1 certificates).

Mixed Content  A web page that is loaded over HTTPS but which includes resources (images, scripts, iframes) loaded over HTTP. Browsers block mixed content that could affect page security. Mixed content is a common technical issue when migrating a site from HTTP to HTTPS :  all resource references must be updated to HTTPS. SSL certificates do not prevent mixed content; it requires application-level changes.

Multi-Domain Certificate (SAN Certificate / UCC Certificate)  A single certificate that covers multiple domain names, each listed as a Subject Alternative Name. Up to 100 domains on a single certificate with most CAs. More cost-effective than purchasing a separate certificate for each domain. Changing the domain list requires certificate reissuance.

Multi-Year Subscription  A certificate purchase agreement covering 1, 2, or 3 years under which all certificate reissuances during the term are free. At current 199-day validity, a 3-year subscription produces approximately 6 certificates (the initial issuance plus 5 reissuances). All are free within the subscription. Multi-year subscriptions lock in the current price against future price increases and reduce purchase transaction frequency. [Not to be confused with multi-year certificate validity :  CA/B Forum capped certificate validity at 398 days in 2020; certificates cannot be issued for 2 or 3 years, only subscriptions span multiple years]

N

Name Constraints  A certificate extension that limits which domains a CA or intermediate CA may issue certificates for. A constrained intermediate CA can issue certificates only for domains within the specified constraint. Used by large organizations that operate their own intermediate CA for internal use: the name constraint prevents the intermediate from issuing trusted public certificates for external domains.

Netcraft  A UK-based internet security company that tracks SSL certificate and web technology adoption globally. Netcraft’s research is widely cited as a primary source for SSL market share data (94.3% DV, 5.5% OV, 0.1% EV as of 2025). Netcraft also publishes research on phishing site SSL adoption and certificate authority market concentration.

NIST (National Institute of Standards and Technology)  The US federal standards body that publishes cryptographic standards. FIPS 203, 204, and 205 (ML-KEM, ML-DSA, SLH-DSA) :  the first post-quantum cryptography standards :  were finalized by NIST in August 2024. US federal agencies and contractors are required to use NIST-approved cryptographic algorithms. NIST’s standards strongly influence the global CA/B Forum trajectory.

Not Before / Not After  The validity window fields in a certificate. ‘Not Before’ is the earliest date the certificate is valid; ‘Not After’ is the expiry date. Browsers refuse connections from certificates where the current date falls outside this window. The ‘Not After’ date is publicly visible in Certificate Transparency logs from the moment of issuance :  expiry events are therefore always predictable, never surprise events.

O

OCSP (Online Certificate Status Protocol)  A real-time revocation checking protocol that allows browsers to verify whether a specific certificate has been revoked. The browser sends the certificate’s serial number to the CA’s OCSP responder and receives a signed response: good, revoked, or unknown. Let’s Encrypt discontinued its OCSP responders in May 2025. Commercial CAs (DigiCert, Sectigo) continue operating OCSP. OCSP responses add 50-200ms to connection time when checked in real time. [Privacy note: real-time OCSP checking tells the CA which websites you are visiting. OCSP Stapling eliminates this privacy concern]

OCSP Stapling  A TLS extension where the web server periodically retrieves a signed OCSP response from the CA and includes it in the TLS handshake. The browser can verify the response without contacting the CA directly, eliminating the latency and privacy concern of real-time OCSP. Let’s Encrypt discontinued OCSP in May 2025; OCSP stapling is therefore only relevant for certificates from commercial CAs.

OV (Organization Validation) Certificate  A certificate type where the CA verifies the legal entity operating the website: the organization’s legal name, registered address, and phone number are verified against independent business registries. The certificate Subject contains a verified Organization (O) field. Takes 1-3 business days to issue. OV certificates are the only type phishing sites cannot obtain (Zscaler 2024: 0% of phishing sites use OV). OV certificates cost $30-$80/year from authorized resellers. [The key distinction from DV: a visitor who inspects an OV certificate’s details sees a verified legal business name. A visitor who inspects a DV certificate’s details sees no organizational identity]

P

Padlock  The lock icon displayed in browsers for HTTPS connections. Chrome removed the padlock from the address bar in September 2023 (Chrome 117) and replaced it with a neutral tune icon because 90%+ of phishing sites had adopted HTTPS and the padlock no longer discriminated between safe and unsafe sites. Firefox and Safari retain modified padlock/lock icons. The padlock indicates connection encryption, not site identity or trustworthiness. See: The Padlock Lie article. [Historical note: Chrome also formerly showed a green padlock and ‘Secure’ label for all HTTPS, and a different green treatment for EV certificates :  both removed by 2021]

PCI DSS (Payment Card Industry Data Security Standard)  The security standard for organizations that process, store, or transmit credit card data. PCI DSS v4.0.1 became mandatory on March 31, 2025. SSL-relevant requirements: Requirement 4.2.1 (TLS 1.2 minimum, TLS 1.0/1.1 disabled, strong cipher suites, no RC4); Requirement 4.2.1.1 (documented certificate inventory for all cardholder data environment systems). DV certificates satisfy PCI DSS TLS requirements; OV or EV are not required by PCI DSS but support compliance documentation.

PEM (Privacy Enhanced Mail)  The most common certificate file format for web servers. A PEM file is a Base64-encoded DER certificate enclosed in —–BEGIN CERTIFICATE—– and —–END CERTIFICATE—– header/footer lines. Apache, Nginx, and most web servers accept PEM format. A PEM file may contain the end-entity certificate, intermediate certificates, or both (called a full chain or bundle).

PKI (Public Key Infrastructure)  The system of policies, procedures, software, and hardware used to manage digital certificates and public-private key pairs. A PKI includes the CAs that issue certificates, the directories where certificates are published, the revocation infrastructure, and the key management practices. The internet’s public PKI is the collection of globally trusted CAs and the root store programs that govern them.

Post-Quantum Cryptography (PQC)  Cryptographic algorithms designed to be resistant to attacks by quantum computers. NIST finalized three PQC standards in August 2024: ML-KEM (FIPS 203, for key exchange), ML-DSA (FIPS 204, for digital signatures), and SLH-DSA (FIPS 205, a backup signature algorithm). For TLS, PQC hybrid key exchange (pairing ECDHE with ML-KEM) is deployable now and implemented by Google Chrome and Cloudflare. Migrating certificate signature algorithms to ML-DSA requires CA infrastructure changes and is a later, more complex migration.

Private Key  The secret half of a public-private key pair. The private key never leaves the server where the certificate is installed. The private key is what proves to connecting clients that the server is the legitimate holder of the certificate. If a private key is compromised, the certificate must be revoked and replaced immediately. Generating the CSR on the server (rather than having the CA generate the key pair) ensures the private key stays under the buyer’s control.

Public Key  The publicly shareable half of a public-private key pair. The public key is embedded in the SSL certificate and visible to anyone who views the certificate. The CA’s signature on the certificate confirms that this public key belongs to the stated domain and organization. Clients use the public key to establish the encrypted connection and to verify the CA’s signature.

Q

QWAC (Qualified Website Authentication Certificate)  A certificate type defined under the EU’s eIDAS regulation, issued by EU-regulated trust service providers (TSPs). QWACs have legal recognition in the EU for electronic identity purposes. They are not in major browser trust stores (browsers maintain trust independently of EU regulatory designation). Under eIDAS 2.0, the EU proposed requiring browsers to trust QWACs :  a provision that faced significant security researcher opposition and was modified in the final regulation.

R

Reissuance  The process of generating a new certificate within an existing subscription, without purchasing a new subscription. At 199-day validity, a 3-year subscription requires approximately 5 reissuances after the initial issuance. Reissuances are free within active subscriptions. For DV certificates, reissuance takes minutes via the reseller’s portal. For OV certificates, reissuance may require re-verification of organizational details if the 398-day organization revalidation period has expired.

Renewal  Purchasing a new certificate subscription at or after the expiration of the current subscription. Distinct from Reissuance (generating a new certificate within an active subscription). Renewal involves a new purchase transaction and initiates a new subscription term. Renewal failures cause certificate expiry if not managed proactively.

Reseller  An authorized partner of a CA that sells that CA’s certificates, typically at prices significantly below the CA’s own retail price. The certificate issued through a reseller is identical to one purchased directly from the CA :  it passes through the CA’s own issuance infrastructure and shows the CA as the Issuer in certificate details. Sectigo PositiveSSL DV: $110/year at sectigo.com vs $4.99/year at authorized resellers.

Revocation  The process of invalidating a certificate before its natural expiry date. CAs revoke certificates when private keys are compromised, domain ownership changes, or certificates were mis-issued. The CA/B Forum requires revocation within 24 hours for high-priority incidents. Revocation is communicated through CRL and OCSP. Browsers ‘fail soft’ on revocation checks :  if the CRL or OCSP check cannot be completed, most browsers accept the certificate rather than blocking the connection.

Root CA (Root Certificate Authority)  The top-level Certificate Authority in a certificate chain. Root CA certificates are self-signed and pre-installed in operating systems and browsers. The four major root store programs (Chrome, Apple, Mozilla, Microsoft) independently evaluate which root CAs to trust. Root CA private keys are extremely sensitive and are typically kept offline in air-gapped hardware security modules.

Root Store  The collection of trusted root CA certificates maintained by an operating system or browser vendor. The four major root stores (Chrome Root Store, Apple Root Certificate Program, Mozilla CA Certificate Program, Microsoft Trusted Root Certificate Program) collectively determine which CAs are trusted by browsers globally. A certificate is ‘publicly trusted’ if the CA that issued it is in all four major root stores.

RSA  The most widely used public key algorithm, named after its inventors Rivest, Shamir, and Adleman. RSA-2048 is the current minimum key size for publicly trusted certificates. RSA key exchange (not the same as RSA signature) lacks forward secrecy and is not used in TLS 1.3. Post-quantum migration will require replacing RSA certificates with ML-DSA certificates, which is a medium-term migration project currently in the infrastructure preparation phase.

S

S/MIME  Secure/Multipurpose Internet Mail Extensions :  a standard for encrypting and digitally signing email. S/MIME certificates are different from SSL/TLS website certificates, though issued by the same CAs. An S/MIME certificate attached to an outgoing email proves to the recipient that the email was sent by the holder of the certificate and has not been tampered with in transit.

SANs (Subject Alternative Names)  The certificate extension that lists all domain names the certificate covers. Browsers validate domains against SANs, not against the Common Name. Every domain a certificate must cover must be listed in the SANs field. A wildcard SAN (*.example.com) covers all single-level subdomains. SANs are specified in the CSR at certificate request time; changing the SAN list requires certificate reissuance.

Sectigo  The world’s largest commercial CA by active certificate count. Formerly known as Comodo CA. Rebranded as Sectigo in 2018 after acquisition by Francisco Partners. Acquired Entrust’s public certificate business in January 2025. Issues DV, OV, EV, and code signing certificates under multiple product names (PositiveSSL, Essential SSL, Comodo SSL, ZeroSSL). Sectigo certificates are trusted identically to DigiCert certificates in all major browsers. [Market position note: Sectigo holds 11.7% of new certificate issuance as of Q1 2026, up 41.2% from Q4 2025 :  the surge reflects Entrust customer migration following the 2024 Entrust distrust]

Self-Signed Certificate  A certificate signed by the same entity that issued it, rather than by a CA. Not trusted by browsers or operating systems. Used for development environments, internal systems, and testing. Browsers display security errors for self-signed certificates on public sites. A self-signed certificate provides encryption but no verified identity. [Common misconception: ‘I just need it for encryption, so self-signed is fine’ :  browsers reject self-signed certificates on public sites with an interstitial warning that most users will not dismiss]

Serial Number  A unique identifier assigned by the CA to each certificate it issues. The serial number appears in the certificate details and is used in CRL and OCSP revocation records. CT log entries can be found by serial number. If you report a compromised certificate to a CA, the serial number is how they identify which certificate to revoke.

SHA (Secure Hash Algorithm)  The cryptographic hash algorithm used to create certificate fingerprints and signatures. SHA-1 was deprecated for certificate signatures in 2016 (the WoSign/StartCom distrust was partly caused by backdating SHA-1 certificates after the deadline). SHA-256 is current standard. SHA-384 and SHA-512 are also valid. NIST post-quantum standards use SHA-3 variants.

Site Seal  A clickable badge provided by a CA to indicate that a website’s certificate has been issued by that CA. A dynamic site seal links to a CA-hosted page showing the certificate details and validation date. Static site seals (image files) are not verifiable and have less trust value. OV and EV certificates from most CAs include site seal access. Let’s Encrypt and free DV certificates do not include site seals. [Marketing note: site seal recognition varies by CA brand :  the Norton Secured seal from DigiCert carries consumer recognition from the antivirus product; Sectigo and GlobalSign seals are less widely recognized by consumers]

SNI (Server Name Indication)  A TLS extension that allows a web server to present different certificates for different domain names on the same IP address. Before SNI, a unique IP address was required for each SSL certificate. SNI is supported by all modern browsers and is enabled by default on all modern web servers. Relevant when hosting multiple SSL-certified sites on shared server infrastructure.

SSL (Secure Sockets Layer)  The predecessor protocol to TLS. SSL was deprecated in 2015 (SSL 3.0) after the POODLE vulnerability. All modern ‘SSL certificates’ are technically TLS certificates. The term ‘SSL certificate’ persists as industry convention despite SSL itself being deprecated. When people say ‘SSL,’ they mean TLS in current usage. [Usage note: ‘SSL certificate’ is the correct colloquial term. ‘TLS certificate’ is the technically accurate term. Both are universally understood in context]

SSL Labs (Qualys SSL Labs)  A free web tool at ssllabs.com/ssltest that analyzes a website’s TLS configuration and produces a grade (A+ through F). Tests TLS version support, cipher suites, certificate chain, forward secrecy, HSTS, and more. Used for pre-sale due diligence, post-installation verification, PCI DSS compliance checking, and cyber insurance posture documentation. Running a scan produces a dated PDF report that serves as timestamped evidence of TLS configuration.

Subordinate CA (Sub-CA / Intermediate CA)  A CA that is authorized by a root CA to issue end-entity certificates. Sub-CA certificates are signed by the root and must follow the root CA’s certificate policy. Technically, any organization can operate a sub-CA if a root CA issues them an unconstrained intermediate certificate :  this is the mechanism that made CNNIC’s 2015 incident possible when CNNIC delegated authority to MCS Holdings without adequate controls.

T

TLS (Transport Layer Security)  The current encryption protocol for HTTPS. TLS 1.3 (RFC 8446, 2018) is the current version: faster handshake, better forward secrecy, no legacy cipher suites. TLS 1.2 is still widely deployed and acceptable per PCI DSS. TLS 1.0 and TLS 1.1 are deprecated and prohibited by PCI DSS 4.0. SSL 3.0 and earlier are insecure and should not be enabled.

TLS Handshake  The process by which a client and server establish a TLS connection. Sequence: client sends hello (supported TLS versions, cipher suites); server sends hello (chosen version and cipher suite), certificate, and key exchange parameters; client verifies the certificate chain against its trust store; both sides derive session keys from the key exchange; encrypted communication begins. TLS 1.3 reduced the handshake from 2 round trips to 1.

TLS 1.3  The current recommended TLS version. Key improvements over TLS 1.2: 1-RTT handshake (faster); 0-RTT session resumption option; forward secrecy required (RSA key exchange removed); only secure cipher suites permitted (no RC4, 3DES, NULL). Required for PQC hybrid key exchange. Qualys SSL Labs gives a bonus for TLS 1.3 support. Most modern servers support TLS 1.3 alongside TLS 1.2 for compatibility with older clients.

TLS Inspection (SSL Inspection)  A network security technique where enterprise security appliances decrypt TLS traffic, inspect it for malicious content, and re-encrypt it before delivery. Requires a private CA root certificate installed on all enterprise devices. Creates a man-in-the-middle position by design. Relevant for organizations operating enterprise security proxies; users on such networks see the proxy’s certificate rather than the server’s certificate.

Trust Anchor  See Root Certificate. The certificate at the top of the trust chain that a system trusts unconditionally because it is pre-installed.

Trust Chain  See Certificate Chain. The sequence of certificates from end-entity to root that allows browsers to verify a certificate’s legitimacy.

Trust Store  See Root Store. The collection of trusted root certificates maintained by an OS or browser.

V

Validity Period  The time a certificate is valid, from ‘Not Before’ to ‘Not After.’ Maximum validity has been progressively reduced: from multi-year to 2 years (2018), to 398 days (2020), to 199 days (DigiCert February 2026, Sectigo March 2026). CA/B Forum Ballot SC-081v3 (April 2025) schedules further reductions to 100 days (~March 2027) and 47 days (March 2029). The validity period affects how frequently certificates must be renewed :  at 47 days, approximately 7.7 renewals per year per certificate. [Not to be confused with subscription term: a 3-year subscription produces multiple certificates each valid for the maximum validity period, not a single certificate valid for 3 years]

Validation Level  The extent to which a CA verifies the identity of the certificate requester. Three levels: DV (domain control only), OV (legal entity verification), EV (extended legal and operational verification). Browser encryption is identical across all three levels. Validation level determines what identity information appears in the certificate and what assurance a verifier has about who operates the site. [CA marketing note: validation level is often presented as a quality spectrum from basic to premium. Technically, it is an identity verification spectrum :  all levels provide identical encryption]

W

Warranty  An indemnification commitment from the CA that pays out if a certificate is mis-issued and a relying party suffers a financial loss as a direct result. Typical warranty amounts: Sectigo PositiveSSL DV $50,000; DigiCert Basic DV $10,000; DigiCert EV up to $1,500,000. The warranty protects the relying party (a consumer who suffers a financial loss from a mis-issued certificate), not the certificate buyer. [Practical note: CA warranties are rarely paid out in practice because the conditions (documented financial loss directly caused by a mis-issued certificate, not just an expired one) are difficult to satisfy. The warranty should not be the primary reason to choose a CA or certificate type]

WebTrust  The primary audit standard for CAs seeking to maintain browser trust. A WebTrust audit by an accredited auditor verifies that a CA’s policies and practices comply with CA/B Forum requirements. Annual WebTrust audits are required for root store inclusion. DigiNotar’s 2011 breach occurred despite DigiNotar holding WebTrust certification, which prompted CA/B Forum reforms to expand audit requirements for intrusion detection and operational security controls.

Wildcard Certificate  A certificate that covers all single-level subdomains of a domain, specified as *.example.com. A wildcard for *.example.com covers www.example.com, mail.example.com, and app.example.com, but not sub.sub.example.com or example.com (the base domain). Wildcard certificates are typically OV-validated (DV wildcards are available but rarer). A wildcard and its matching base domain can be on the same certificate by listing both *.example.com and example.com as SANs.

X

X.509  The international standard that defines the format of public key certificates. SSL/TLS certificates are X.509 v3 certificates. The X.509 standard defines the fields that certificates contain: subject, issuer, validity period, public key, extensions, and CA signature. When you view a certificate’s details in a browser, you are seeing the contents of an X.509 certificate.

Z

ZeroSSL  A commercial SSL certificate provider that is now a Sectigo brand. ZeroSSL offers free DV certificates (alternative to Let’s Encrypt) and paid OV/EV certificates. ZeroSSL’s free certificates use the ACME protocol. ZeroSSL is owned by the same parent company as Sectigo following Sectigo’s acquisition history. ZeroSSL certificates show Sectigo as the Issuer CA.

Zero-Day  A vulnerability that is unknown to the software vendor and therefore has no patch available. Zero-day vulnerabilities in TLS libraries (such as the OpenSSL Heartbleed vulnerability of 2014) can affect certificate security by exposing private keys. When a zero-day vulnerability affects TLS infrastructure, all certificates whose private keys may have been exposed should be revoked and reissued.