Re: An idea for refining penet-style anonymous servers

[frantz@netcom.com (Bill Frantz)]

At  8:15 AM 4/25/96 -0700, Alan Bostick wrote:
>The other night, while sick and feverish with the flu, a scheme popped
>into my head that would seem to make penet-style anonymous servers less
>vulnerable to compromise through seizure of the remailer equipment or of
>the address database...
>
>My scheme is the design of the address database.  It consists of two
>hash tables, one for sending messages (which maps anonymous IDs onto
>sender's addresses), and one for receiving them (mapping recipient's
>addresses onto anonymous IDs).  A cryptographically secure hash (say,
>MD5) is used for the index of both tables.
>
>The index of the sending message table is the MD5 hash of the sender's
>address.  The table entry the index points to is the sender's anonymous
>ID, encrypted by a symmetric algorithm (maybe IDEA).  The encryption key
>would be a different hash, by another algorithm (let's suppose it's
>SHA), of that same address.
>
>...
>
>The receiving message hash table is designed similarly, in reverse.  The
>index of the hash table is the MD5 hash of the anonymous ID; the entry
>in the table is the recipient's email address, encrypted with the SHA
>hash of the anonymous ID...

Assuming you have obtained the address database, it seems to me that this
scheme is subject to known address attacks:

(1) If you want to find out what newbie@slowresponse.com's anon ID is, you
just look it up.

(2) If you want to find out the real email addresses of all the users, you
test all the anon-ids with the reverse lookup table.  This attack could be
defeated by using sufficiently long random anon-ids.  If we assume 5 bits
of information/character, a 96 bit anon-id (sufficient to preclude
exhaustive search attacks) would require 19 character anon-ids.

Regards - Bill


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