To provide a measured billing service excluding illegal conducts com mitted by transaction parties in network commerce.
A user generates a secret integer S and prime numbers (p) and (q) to calculate N=p×q and T=S2, and discloses T and N (400). When the user receives a service, he/she generates secret random numbers R1 and R2, calculates X1=R12 and X2=R22, and transmits them to a seller (405). The seller generates a random number a = an element of {0,1}, and transmits it to the user (406). The user generates Y1=S×R1 and Y2=R2 in the case of a=0 or Y1=R1 and Y2=S×R2 in the case of a=1, and transmits the result to the seller (407). The seller verifies Y12=T×X1 and Y22=X2 in the case of a=0 or Y12=X1 and Y22=T×X2 in the case of a=1. If the verification is successful, the seller provides goods to the user by a unit amount (408). In the above, an expression of (mod N) in equations is omitted.
UENO MASAMI
AGAWA YUSUKE
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