ecash & remailers

It occurs to me that there is a problem with remailers using Chaum's
ecash as offered by MT bank and others.

The attacker could coerce the sender of an anonymous message into
revealing his blinding value, and use this to obtain the identity of
each remailer hop by colluding with the bank.  (The bank keeps a
database of the blinded coins minted against who they were given to;
unblinding reveals the coin which can then be compared against those
deposited by remailers, tallying sender with remailer, all the way to
the exit remailer.)

To stop this, the sender should discard the blinding values, thereby
removing his ability to be later coerced, or to later trace the
recipient of his cash.

Is there a way to purge blinding values from the ecash directory?

Adam
-- 
Have *you* exported RSA today? --> http://www.dcs.ex.ac.uk/~aba/rsa/

print pack"C*",split/\D+/,`echo "16iII*o\U@{$/=$z;[(pop,pop,unpack"H*",<>
)]}\EsMsKsN0[lN*1lK[d2%Sa2/d0<X+d*lMLa^*lN%0]dsXx++lMlN/dsM0<J]dsJxp"|dc`

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At 06:03 PM 5/22/97 +0100, Adam Back wrote:
>
>To stop this, the sender should discard the blinding values, thereby
>removing his ability to be later coerced, or to later trace the
>recipient of his cash.
>
>Is there a way to purge blinding values from the ecash directory?
>

Coins are removed from the cash db when they are paid out. They are then
stored in the payments db along with the payment headers, etc. The blinding
factors are not stored once the coins have been paid. Regardless, removing
the payments db will not cause any ill effect (that I have noticed, except for
the obvious loss of payment records), so you can do that anyway if you like.

(Note, this is based on a hex dump of the payments db, so it's possible I'm 
wrong. Anyone from Digicash who knows otherwise, please correct me.)

Jeremey.

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--
Jeremey Barrett                                  VeriWeb Internet Corp.
Crypto, Ecash, Commerce Systems                 http://www.veriweb.com/
PGP key fingerprint =  3B 42 1E D4 4B 17 0D 80  DC 59 6F 59 04 C3 83 64