| 摘要 |
PROBLEM TO BE SOLVED: To provide a cipher integrity evaluating device obtaining an unknown coefficient of a polynominal describing a plain text making a cipher sentence a variable within a realistic time by respectively providing a specified decipher means, a plain text inverter means, an interpolation means and the number of stage extraction means making a specified value the output of the cipher integrity evaluating device, or the like. SOLUTION: A decipher device 200 deciphers all cipher sentences c (xq) with a key kj to obtain Dkj (c (xq)). A plain text inverter device 500 performs the operation of P-1 (Dkj (c (xq))) from the decipher output to output sets yjq for all of the key kj, an origin xq of k to an interpolation device 130. The interpolation device 130 calculates yjq=Σai (kj) xjj-1 (ai (kj)ΕK) from an input yjq by a Lagrange interpolation formula, etc., and outputs it to a number of stage and base extraction device 140. The number of stage and base extraction device 140 uses the input ai (kj), and extracts (n) by an expression by the Gaussian elimination, etc., and the (n) is made the output of the cipher integrity evaluating device 100.
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