What is SSL?
Also known as: Secure Sockets Layer
SSL (Secure Sockets Layer) was a cryptographic protocol for securing Internet communications, superseded by TLS in 1999. The term persists colloquially, but every modern secure connection uses TLS.
SSL (Secure Sockets Layer) is a deprecated cryptographic protocol that originally provided encrypted, authenticated communication between a client and a server over a network. Netscape developed SSL 1.0, which never saw public release, followed by SSL 2.0 in 1995 (RFC 6101 documents its specification) and SSL 3.0 in 1996. The protocol operates between the transport layer (TCP) and the application layer, meaning an application like HTTPS (HTTP over SSL) sends regular HTTP data through the SSL layer, which encrypts the payload before handing it to TCP.
SSL uses a handshake procedure to negotiate cipher suites, exchange certificates, and derive session keys. The handshake begins with the client sending a ClientHello message listing supported TLS/SSL versions and cipher suites. The server responds with its chosen version and cipher suite, its certificate, and optionally a request for the client's certificate. After key exchange (typically RSA or Diffie-Hellman in SSL), both sides compute shared encryption keys and finish the handshake. The record layer then encrypts application data using symmetric ciphers such as RC4, 3DES, or AES, each with different security characteristics.
By the late 1990s, multiple vulnerabilities had been found in SSL 3.0, including the POODLE attack (CVE-2014-3566). The IETF published TLS 1.0 (RFC 2246) in 1999 as a minor upgrade to SSL 3.0, effectively deprecating SSL. TLS 1.1, 1.2, and 1.3 followed, adding stronger cipher suites, removing weak algorithms entirely, and improving handshake security. Today, SSL 2.0 and 3.0 are prohibited by PCI DSS and rejected by all major browsers and servers. The term "SSL" survives in product documentation, configuration file names (nginx.conf uses ssl_certificate), and everyday speech, but the protocol actually negotiated on the wire will be TLS 1.2 or TLS 1.3 for any modern connection.
Key facts
- Netscape developed SSL 1.0 (unreleased), SSL 2.0 in 1995, and SSL 3.0 in 1996.
- SSL 3.0 was fataly vulnerable to the POODLE attack (CVE-2014-3566).
- TLS 1.0 (RFC 2246) replaced SSL 3.0 in 1999; no version of SSL is considered secure today.
- PCI DSS has prohibited use of any SSL version since June 2018.
- All browser vendors and server software now disable SSL 2.0 and 3.0 by default.
Related terms
References
More in TLS & PKI
ACME Protocol
ACME (Automated Certificate Management Environment) is a protocol that automates the issuance, renewal, and revocation of TLS certificates, defined in RFC 8555.
Certificate Authority
A Certificate Authority (CA) is a trusted entity that issues digital certificates after verifying that the requester controls the domain or identity named in the certificate.
Certificate Chain
A certificate chain is an ordered list of certificates, starting with the server certificate and ending with a root CA, that a client validates to establish trust in the server's identity.
Certificate Pinning
A security technique where an application trusts only a specific, pre-selected certificate or public key for a given server, bypassing the standard chain of trust.
Cipher Suite
A cipher suite is a named set of cryptographic algorithms negotiated during a TLS handshake, specifying key exchange, authentication, encryption, and integrity protection for secure communications.
Domain Validation
Domain Validation (DV) is the lowest level of certificate validation used in TLS/SSL, where the certificate authority verifies only that the applicant controls the domain name, typically via an HTTP or DNS challenge.
ECH
Encrypted Client Hello (ECH) is a TLS extension that encrypts the Client Hello message, including the Server Name Indication (SNI), to prevent on-path observers from learning the target hostname during the handshake.
Extended Validation
Extended Validation (EV) is the highest level of TLS certificate assurance, requiring the certificate authority to perform rigorous, human-verified checks on the legal identity and operational existence of the requesting organization before issuance.
HSTS
HTTP Strict Transport Security (HSTS) is a web security policy mechanism that forces browsers to interact with a website only over HTTPS, preventing downgrade attacks and cookie hijacking.
Intermediate Certificate
An intermediate certificate is a subordinate CA certificate signed by a root CA, used to sign end-entity certificates and enable path validation while the root remains offline.