RFC 1890 (rfc1890) - Page 3 of 18

RTP Profile for Audio and Video Conferences with Minimal Control

RFC 1890                       AV Profile                   January 1996


   String-to-key mapping:  A user-provided string ("pass phrase") is
        hashed with the MD5 algorithm to a 16-octet digest. An n-bit key
        is extracted from the digest by taking the first n bits from the
        digest. If several keys are needed with a total length of 128
        bits or less (as for triple DES), they are extracted in order
        from that digest. The octet ordering is specified in RFC 1423,
        Section 2.2. (Note that some DES implementations require that
        the 56-bit key be expanded into 8 octets by inserting an odd
        parity bit in the most significant bit of the octet to go with
        each 7 bits of the key.)

   It is suggested that pass phrases are restricted to ASCII letters,
   digits, the hyphen, and white space to reduce the the chance of
   transcription errors when conveying keys by phone, fax, telex or
   email.

   The pass phrase may be preceded by a specification of the encryption
   algorithm. Any characters up to the first slash (ASCII 0x2f) are
   taken as the name of the encryption algorithm. The encryption format
   specifiers should be drawn from RFC 1423 or any additional
   identifiers registered with IANA. If no slash is present, DES-CBC is
   assumed as default. The encryption algorithm specifier is case
   sensitive.

   The pass phrase typed by the user is transformed to a canonical form
   before applying the hash algorithm. For that purpose, we define
   return, tab, or vertical tab as well as all characters contained in
   the Unicode space characters table. The transformation consists of
   the following steps: (1) convert the input string to the ISO 10646
   character set, using the UTF-8 encoding as specified in Annex P to
   ISO/IEC 10646-1:1993 (ASCII characters require no mapping, but ISO
   8859-1 characters do); (2) remove leading and trailing white space
   characters; (3) replace one or more contiguous white space characters
   by a single space (ASCII or UTF-8 0x20); (4) convert all letters to
   lower case and replace sequences of characters and non-spacing
   accents with a single character, where possible. A minimum length of
   16 key characters (after applying the transformation) should be
   enforced by the application, while applications must allow up to 256
   characters of input.

   Underlying protocol: The profile specifies the use of RTP over
        unicast and multicast UDP. (This does not preclude the use of
        these definitions when RTP is carried by other lower-layer
        protocols.)

   Transport mapping: The standard mapping of RTP and RTCP to
        transport-level addresses is used.




Schulzrinne                 Standards Track