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TLS-PSK (Pre-Shared Key) is a set of TLS/SSL cipher suites that use
symmetric (pre-shared) keys for mutual authentication of peers. These
cipher suites are in some scenarios more suitable than those based on
public key cryptography and X.509. See RFC 4279 (Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)) for more information.

OpenSSL currently supports only the following PSK cipher suites:
* TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256
* TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384
* TLS_PSK_WITH_3DES_EDE_CBC_SHA
* TLS_PSK_WITH_AES_128_CBC_SHA
* TLS_PSK_WITH_AES_256_CBC_SHA
* TLS_PSK_WITH_RC4_128_SHA

The last four cipher suites mutually authenticate the peers and
secure the connection using a pre-shared symmetric key. These cipher
suites do not provide Forward Secrecy -- once the pre-shared key is
compromised, all previous communications secured with that key can be
decrypted. The first two cipher suites combine the pre-shared
symmetric key with an ephemeral key obtained from an ECDH key
exchange performed during the TLS/SSL handshake, thus providing
Forward Secrecy.

Users of TLS-PSK are expected to provide an implementation of
PSKKeyManager to SSLContext.init and then enable at least one PSK
cipher suite in SSLSocket/SSLEngine.

Bug: 15073623
Change-Id: I8e59264455f980f23a5e66099c27b5b4d932b9bb