3.1.3 Initialization
LastGoodSSRC must be initialized to zero when the RTP session is created.
For RTPME, the encryption key MUST be derived as follows:
During negotiation, the encryption key phrase MUST be in base64-encoded format in the SDP as specified in [MS-SDP].
The key phrase MUST be base64 decoded into a Unicode string.
This Unicode string MUST be converted into a UTF-8 code point string.
This UTF-8 code point string MUST be hashed using the message digest algorithm 5 (MD5) algorithm.
The DES CBC key is the first 56 bits from output of the hash.