Multiple messages + entropy

The Internet PEM (Privacy Enhanced Mail) standard uses the
concept which Dean Tribble mentioned of multiple encryption
(using each recipient's public key) of a single session key
which encrypts the message.  PGP's data structures do not
currently provide for this but could be extended pretty easily
to allow it.

On the entropy measure - I thought entropy was how many bits
of information you get per character.  Encrypted binary text
would be pretty close to 8 bits per character.  The RFC1113
Ascii encoding used by PGP reduces this to 6 bits per character
(e.g. a character set with 64 printable characters) neglecting
line separators and message beginnings and endings.  So there
should be a little less than 6 bits per character for encrypted,
Ascii-encoded messages.

Hal
74076.1041@compuserve.com

Hal:
>Ascii encoding used by PGP reduces this to 6 bits per character
>(e.g. a character set with 64 printable characters) neglecting
>line separators and message beginnings and endings.  So there
>should be a little less than 6 bits per character for encrypted,
>Ascii-encoded messages.

Hal is, of course, right.  I was getting myself confused between
entropy lost in the encoding and the entropy of the encoding.  The
channel uses up two bits of entropy per character in the encoding.
What's left is six bits.

As punishment for getting this so egregiously wrong, I'm going to post
some C code for measuring entropy so that you all can play with it.

Eric