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Subject: Metadata for 1.1 Web Browser SSO Profile, Draft 06, 1 May 2003


In the case where the key distributed with the metadata is a public
signature-verification key, it is acceptable, desirable and conventional to
sign the metadata using the corresponding private key.  This is common
practice for X.509 certificates.  In addition, it allows the integrity of
the metadata to be confirmed using an out-of-band "digest".

As currently required, the integrity of the metadata has to be protected
with a separate key.  Presumably, it too has associated metadata that has to
be distributed, protected with another key, which (in-turn) has metadata.
Allowing the enclosed key to confirm the integrity of the metadata, breaks
this cycle.

A digest is considerably easier to distribute out-of-band than a public-key.

Here is a suggestion for a digest procedure:

The validation string is calculated from the binary form of the authority's
self-signed certificate by operating upon it with the SHA-1 hash algorithm.
The right-most 8 bytes of the resulting digest are discarded.  The left-most
3 bits of each of the remaining 12 bytes are discarded. The remaining twelve
5-bit values are represented as alphanumeric characters according to the
following table.

00000 > A
00001 > B
... omitting I
11000 > Z
11001 > 3
11010 > 4
11111 > 9

Finally, the alphanumeric string is divided into three sub-strings, each of
four characters, and the sub-strings are separated by hyphens.  For example:


This validation string is distributed out of band to the protocol.  It may
be distributed in software, printed on advertisements, letterhead and
business cards, read over a telephone link, or it may be included in a
register prepared by a third party, such as an industry association.

It is true that PKIX defined a digest procedure for this purpose.  But, it
resulted in a string (if I recall) of 32 or 40 characters.  The time taken
to recite a string of that length over a telephone call exceeds my attention

All the best.  Tim.

Tim Moses

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