I see how this sort of approach can tie a particular timestamp and
sender_id / email address together in a MAC which can be validated, but
I'm missing how the MAC gets coupled to a given message.
Was such a coupling intended?
If not, what's to prevent a spammer who gets his hands on one of these
(a valid one) from then using it to send a million messages of his own
(where of course he'll force all the other headers as necessary).
Confused,
Bob
-----Original Message-----
From: asrg-admin(_at_)ietf(_dot_)org [mailto:asrg-admin(_at_)ietf(_dot_)org] On
Behalf Of
Justin Mason
Sent: Wednesday, April 02, 2003 2:09 PM
To: asrg(_at_)ietf(_dot_)org
Hi all --
[...]
From: jm
There's 2 entirely separate components; the SMTP part, adding the
header;
and the CGI script, validating the header. The only data they need to
share is the secret passphrase, so they do not even need to be on the
same
network! Here's how that works:
- header contains:
sender_id (usually email addr?)
timestamp
opaque_md5_sum = md5(sender_id, timestamp, secretkey)
- CGI parses header to get:
sender_id
timestamp
opaque_md5_sum
- CGI already has:
secretkey
- it then computes md5(sender_id, timestamp, secretkey) and compares it
with opaque_md5_sum.
If it matches, ok, if not, it's an invalid signature.
[...]
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