PSK的目的

  我们都知道TLS需要依赖非对称算法（RSK，EC，DS，DH...）完成秘钥交换，身份认证的功能，但是非对称算法的耗时和耗计算资源的特性在对资源或者耗时敏感的场景下，你就想把他优化掉。本文我们就简绍一种TLS标准本身提供的优化方式:PSK.

PSK的江湖地位

  PSK的方式应该是最古老的一种秘钥交换和认证方式，但是它在TLS中的江湖地位是比较低的，从最早的非正式的优化方案到有了自己的RFC编号RFC4279(December 2005)对比TLS的历史

TLS中PSK的核心目的

  一下是RFC中的原文摘录

This document specifies three sets of new ciphersuites for the
Transport Layer Security (TLS) protocol to support authentication
based on pre-shared keys (PSKs).
These pre-shared keys are symmetric
keys, shared in advance among the communicating parties.

一，The first set of ciphersuites uses only symmetric key operations for  authentication.

TLS_PSK_WITH_RC4_128_SHA           PSK           RC4_128       SHA
TLS_PSK_WITH_3DES_EDE_CBC_SHA      PSK           3DES_EDE_CBC  SHA
TLS_PSK_WITH_AES_128_CBC_SHA       PSK           AES_128_CBC   SHA
TLS_PSK_WITH_AES_256_CBC_SHA       PSK           AES_256_CBC   SHA

二，The second set uses a Diffie-Hellman exchange authenticated with a pre-shared key, and

TLS_DHE_PSK_WITH_RC4_128_SHA       DHE_PSK       RC4_128       SHA
TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA  DHE_PSK       3DES_EDE_CBC  SHA
TLS_DHE_PSK_WITH_AES_128_CBC_SHA   DHE_PSK       AES_128_CBC   SHA
TLS_DHE_PSK_WITH_AES_256_CBC_SHA   DHE_PSK       AES_256_CBC   SHA

三，the third set combines  public key authentication of the server with pre-shared key authentication of the client.

TLS_RSA_PSK_WITH_RC4_128_SHA       RSA_PSK       RC4_128       SHA
TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA  RSA_PSK       3DES_EDE_CBC  SHA
TLS_RSA_PSK_WITH_AES_128_CBC_SHA   RSA_PSK       AES_128_CBC   SHA
TLS_RSA_PSK_WITH_AES_256_CBC_SHA   RSA_PSK       AES_256_CBC   SHA