Only re-create an SRTP session when needed; respond with correct crypto policy

Only re-create an SRTP session when needed; respond with correct crypto policy

In r356604, SRTP handling was fixed to accomodate multiple crypto keys in an
SDP offer and the ability to re-create an SRTP session when the crypto keys
changed.  In certain circumstances - most notably when a phone is put on
hold after having been bridged for a significant amount of time - the act
of re-creating the SRTP session causes problems for certain models of phones.
The patch committed in r356604 always re-created the SRTP session regardless
of whether or not the cryptographic keys changed.  Since this is technically
not necessary, this patch modifies the behavior to only re-create the SRTP
session if Asterisk detects that the remote key has changed.  This allows
models of phones that do not handle the SRTP session changing to continue
to work, while also providing the behavior needed for those phones that do
re-negotiate cryptographic keys.

In addition, in Asterisk 1.8 only, it was found that phones that offer
AES_CM_128_HMAC_SHA1_32 will end up with no audio if the phone is the
initiator of the call.  The phone will send an INVITE request specifying
that AES_CM_128_HMAC_SHA1_32 be used for the cryptographic policy; Asterisk
will set its policy to that value.  Unfortunately, when the call is Answered
and a 200 OK is sent back to the UA, the policy sent in the response's SDP
will be the hard coded value AES_CM_128_HMAC_SHA1_80.  This potentially
results in Asterisk using the INVITE request's policy of
AES_CM_128_HMAC_SHA1_32, while the phone uses Asterisk's response of
AES_CM_128_HMAC_SHA1_80.  Hilarity ensues as both endpoints think the other
is crazy.

This patch fixes that by caching the policy from the request and responding
with it.  Note that this is not a problem in Asterisk 10 and later, as the
ability to configure the policy was added in that version.

(issue ASTERISK-20194)
Reported by: Nicolo Mazzon
Tested by: Nicolo Mazzon

Review: https://reviewboard.asterisk.org/r/2099

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@372709 65c4cc65-6c06-0410-ace0-fbb531ad65f3