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Encyclopedia > Transport Secure Layer

Secure Sockets Layer (SSL) and Transport Layer Security (TLS), its successor, are cryptographic protocols which provide secure communications on the Internet. There are only slight differences between SSL 3.0 and TLS 1.0, but they are not interchangeable. The term "SSL" as used here applies to both protocols unless clarified by context.



SSL provides endpoint authentication and communications privacy over the Internet using cryptography. In typical use, only the server is authenticated (i.e. its identity is ensured) while the client remains unauthenticated; mutual authentication requires PKI deployment to clients. The protocols allow client/server applications to communicate in a way designed to prevent eavesdropping, tampering, and message forgery.

SSL involves a number of basic phases:

  • Peer negotiation for algorithm support
  • Public-key encryption-based key exchange and certificate-based authentication
  • Symmetric cipher-based traffic encryption

During the first phase, the client and server negotiate which cryptographic algorithms will be used. Current implementations support the following choices:


SSL runs on layers beneath application protocols such as HTTP, SMTP and NNTP and above the TCP transport protocol, which forms part of the TCP/IP protocol suite. While it can add security to any protocol that uses TCP, it is most commonly used with HTTP to form HTTPS. HTTPS serves to secure World Wide Web pages for applications such as Electronic commerce. It uses public key certificates to verify the identity of endpoints.

While an increasing number of client and server products can support SSL natively, many still do not. In these cases, a user may wish to use standalone SSL products like Stunnel to provide encryption.

History and development

Developed by Netscape, SSL version 3.0 was released in 1996, which later served as a basis to develop Transport Layer Security (TLS), an IETF standard protocol. The first definition of TLS appeared in RFC 2246: "The TLS Protocol Version 1.0". Visa, MasterCard, American Express and many leading financial institutions have endorsed SSL for commerce over the internet.

SSL operates in modular fashion: its authors designed it for extendability, with support for forwards and backwards compatibility and negotiation between peers.

Early weak keys

Some early implementations of SSL could use a maximum of only 40-bit symmetric keys because of US government restrictions on the export of cryptographic technology. The US government explicitly imposed a 40-bit keyspace small enough to be broken by brute-force search by law enforcement agencies wishing to read the encrypted traffic, while still presenting obstacles to less-well-funded attackers. A similar limitation applied to Lotus Software's 'Notes' product in export versions. After several years of public controversy, a series of lawsuits, and eventual US government recognition of changes in the market availability of 'better' cryptographic products (within and without the US), the authorities relaxed some aspects of the export restrictions. The 40-bit key size limitation has mostly gone away. Modern implementations use 128-bit (or longer) keys for symmetric key ciphers.


The first definition of TLS appeared in RFC 2246: "The TLS Protocol Version 1.0".

Other RFCs subsequently extended TLS, including:

  • RFC 2712: "Addition of Kerberos Cipher Suites to Transport Layer Security (TLS)". The 40-bit ciphersuites defined in this memo appear only for the purpose of documenting the fact that those ciphersuite codes have already been assigned.
  • RFC 2817: "Upgrading to TLS Within HTTP/1.1", explains how to use the Upgrade mechanism in HTTP/1.1 to initiate Transport Layer Security (TLS) over an existing TCP connection. This allows unsecured and secured HTTP traffic to share the same well known port (in this case, http: at 80 rather than https: at 443).
  • RFC 2818: "HTTP Over TLS", distinguishes secured traffic from insecure traffic by the use of a different 'server port'.

See also

External links

  • The IETF TLS Workgroup (http://www.ietf.org/html.charters/tls-charter.html)
  • RFC 2246 - The TLS Protocol, Version 1.0
  • SSL 3.0 specification (http://wp.netscape.com/eng/ssl3/)
  • Webmaster.org: Web Hosts offering SSL (http://www.webmaster.org/webhosting/plans/ssl.htm)
  • Using OpenPGP keys for TLS authentication (http://www.ietf.org/internet-drafts/draft-ietf-tls-openpgp-keys-05.txt)
  • SSL Security Forum (http://www.ssl-forum.com)
  • The GNU Transport Layer Security Library (http://www.gnu.org/software/gnutls/manual/index.html) implements both SSL and TLS
  • Mozilla's Network Security Services (NSS) (http://www.mozilla.org/projects/security/pki/nss/) is dual-licensed under the MPL and the GPL
  • Quick introduction to SSL and Certificates (http://www.pseudonym.org/ssl/ssl_intro.html)


This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

  Results from FactBites:
Transport Layer Security - Wikipedia, la enciclopedia libre (1566 words)
Secure Sockets Layer (SSL) y Transport Layer Security (TLS) -Seguridad de la Capa de Transporte-, su sucesor, son protocolos criptográficos que proporcionan comunicaciones seguras en Internet.
RFC 3546: "Transport Layer Security (TLS) Extensions", añade un mecanismo para negociar extensiones de protocolos durante la inicialización de sesión y define algunas extensiones.
RFC 4279: "Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)", añade tres conjuntos de nuevas familias de cifrados para que el protocolo TLS permita la autenticación basada en claves previamente compartidas.
RFC 2246 (rfc2246) - The TLS Protocol Version 1.0 (16076 words)
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Note that higher layers should not be overly reliant on TLS always negotiating the strongest possible connection between two peers: there are a number of ways a man in the middle attacker can attempt to make two entities drop down to the least secure method they support.
Security analysis The TLS protocol is designed to establish a secure connection between a client and a server communicating over an insecure channel.
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