Secure sockets layer (SSL) is a deprecated cryptographic protocol that was used to encrypt communication between clients and servers in FTPS or HTTPS file transfers. SSL protects user credentials and data packets from interception by wrapping communications in an encrypted tunnel, and it protects clients from spoofing attacks by requiring a signed server certificate before fully establishing a session.
During the SSL handshake, the server presents a digital certificate for authentication. The client validates the certificate and then negotiates a cipher suite and session keys. Once agreed upon, the parties shift to encrypted mode, where even passive transfers, which traverse firewalls on high ports, retain confidentiality and integrity.
SSL has been replaced by transport layer security (TLS), which uses the same handshake model but provides more updated security tooling and options.
Common SSL vulnerabilities
SSL was the first widespread protocol for protecting file transfers, but its design carried flaws that modern threat models and computing power exposed. These included:
- A vulnerability that exploited predictable padding in cipher block chaining (CBC) mode to recover plaintext, known as the POODLE attack
- Exploits of the CBC mode of SSL that allowed attackers to perform a man-in-the-middle attack (known as the BEAST vulnerability)
- Lack of forward secrecy that allowed key compromise to decrypt past sessions
- Weak 40‑bit or RC4 cipher versions that allowed attackers to brute force encryption
TLS: The successor to SSL
TLS superseded SSL and is now the current encryption foundation used to protect HTTPS and FTPS file transfers. The current TLS version, 1.3, includes a number of security improvements over SSL. These include:
- Authenticated encryption that combines integrity and privacy
- Ephemeral key exchange that enables forward secrecy
- Optional session resumption that cuts handshake latency
- Removal of weaker ciphers, such as MD5 and SHA-224, and some elliptic curves
- SNI that lets multiple domains share one address without risk
- Strict certificate validation that blocks spoofed hosts
SSL vs. TLS
SSL is no longer used after TLS superseded it in 1999. SSL debuted in the 1990s as the first widely used encryption layer for internet protocols, but its handshake design and cipher catalog fall short under current threat models. TLS closes those gaps by hardening negotiation, adding authenticated encryption and removing weak cipher options.
Other key distinctions between SSL and TLS include:
- Ciphers: SSL allows RC4 and 40‑bit keys, but TLS mandates AES or ChaCha20.
- Efficiency: TLS session tickets and 0‑RTT cut handshake delay.
- Negotiation: SSL suffers downgrade attacks, whereas TLS blocks them with SCSV.
- Record layer: SSL uses MAC‑then‑encrypt, while TLS 1.2 and later use AEAD.
SSL’s legacy and TLS use cases
While SSL has been obsolete for more than 20 years, administrators may encounter its usage in older intranets or archival systems. Otherwise, “SSL” today actually refers to TLS. TLS performs the same functions with stronger keys, record framing and certificate rules.
TLS is one of the most widely used internet protocols, with common use cases including:
- B2B managed file transfer (MFT) hubs that re‑encrypt inbound sessions to object storage with client certificates
- Manufacturing controllers that upload production logs through implicit FTPS
- Modern REST APIs wrapped in HTTPS that use TLS 1.3 for data plane encryption
- Third‑party portals that accept explicit FTPS over port 21 running TLS
Secure sockets layer (SSL) FAQs
SSL inspection intercepts encrypted FTPS and HTTPS sessions at a controlled gateway, decrypts them, inspects the payload and then re‑encrypts the traffic before passing it to the final endpoint. Organizations perform this step to uncover hidden malware, ransomware commands or policy breaches that intrusion tools would normally miss because they cannot view the cipher stream.
Organizations seeking to comply with data security requirements such as PCI DSS, HIPAA or NIST must also provide proof that their transfers use approved ciphers and carry only authorized data. SSL inspection logs the certificate chain, cipher suite, file hash and user identity for each session so auditors can trace events and spot weak encryption. By validating both transport security and content integrity, this control strengthens data security without altering client or server workflows.
SSL and its successor TLS do not neatly fall into the OSI layer model (which itself is more of an information classification guide). TLS runs on top of TCP after the three‑way handshake, then manages key exchange, encryption and record framing before any application data flows. These duties match the OSI session and presentation layers, so the protocols sit between layer four and layer seven rather than belonging exclusively to either one.
SSL was created to secure client‑server communication and authenticate a server’s identity to prevent data interception and spoofing. Prior to SSL, eavesdroppers were much more easily able to view credentials, commands or file contents and tamper with client connections.
As internet cryptography and codebreaking evolved, numerous vulnerabilities in SSL emerged. In particular, its block cipher model was found to be vulnerable to attacks that could break its encryption in 2014, and SSL was officially deprecated in 2015 in favor of TLS.