RFC 1890 (rfc1890) - Page 3 of 18
RTP Profile for Audio and Video Conferences with Minimal Control
Alternative Format: Original Text Document
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