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TLS and SSL are defined as 'operating over some reliable transport layer', which places them as application layer protocols in the TCP/IP reference model and as presentation layer protocols in the OSI model.

Early research efforts towards transport layer security included the Secure Network Programming (SNP) application programming interface (API), which in 1993 explored the approach of having a secure transport layer API closely resembling Berkeley sockets, to facilitate retrofitting preexisting network applications with security measures.

While this can be more convenient than verifying the identities via a web of trust, the 2013 mass surveillance disclosures made it more widely known that certificate authorities are a weak point from a security standpoint, allowing man-in-the-middle attacks (MITM).

Before a client and server can begin to exchange information protected by TLS, they must securely exchange or agree upon an encryption key and a cipher to use when encrypting data (see Cipher).

It builds on the earlier SSL specifications (1994, 1995, 1996) developed by Netscape Communications Client-server applications use the TLS protocol to communicate across a network in a way designed to prevent eavesdropping and tampering.

Since applications can communicate either with or without TLS (or SSL), it is necessary for the client to indicate to the server the setup of a TLS connection.

As stated in the RFC, "the differences between this protocol and SSL 3.0 are not dramatic, but they are significant enough to preclude interoperability between TLS 1.0 and SSL 3.0".

TLS 1.0 does include a means by which a TLS implementation can downgrade the connection to SSL 3.0, thus weakening security.

Among the methods used for key exchange/agreement are: public and private keys generated with RSA (denoted TLS_RSA in the TLS handshake protocol), Diffie-Hellman (TLS_DH), ephemeral Diffie-Hellman (TLS_DHE), Elliptic Curve Diffie-Hellman (TLS_ECDH), ephemeral Elliptic Curve Diffie-Hellman (TLS_ECDHE), anonymous Diffie-Hellman (TLS_DH_anon), The TLS_DH_anon and TLS_ECDH_anon key agreement methods do not authenticate the server or the user and hence are rarely used because those are vulnerable to Man-in-the-middle attack.

Only TLS_DHE and TLS_ECDHE provide forward secrecy.

Once the client and server have agreed to use TLS, they negotiate a stateful connection by using a handshaking procedure.

The protocols use a handshake with an asymmetric cipher to establish cipher settings and a shared key for a session; the rest of the communication is encrypted using a symmetric cipher and the session key.

Developers of web browsers have also revised their products to defend against potential security weaknesses after these were discovered (see TLS/SSL support history of web browsers.) The TLS protocol comprises two layers: the TLS record protocol and the TLS handshake protocol.

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