1. An encrypted data computation system comprising a client terminal, a computation device, and a computation assist device, the client terminal comprising a numerical value storing circuit configured to store numerical values; a key generating circuit configured to generate an encryption key to encrypt each of the numerical values, and a secondary computation key and a tertiary computation key that constitute a decryption key corresponding to the encryption key, and an encryption circuit configured to encrypt each of the numerical values on the basis of the encryption key and generating encrypted data, the computation device comprising a tertiary computation key storing circuit configured to store the tertiary computation key, a primary computation circuit configured to execute primary computation processing on the basis of each of the encrypted data and thereby generate a primary computation result corresponding to data in which a computation result of an expression that has added and subtracted each of the numerical values is encrypted, a tertiary computation circuit configured to execute tertiary computation processing on the basis of a secondary computation result and the tertiary computation key when the secondary computation result in which the primary computation result is randomized is generated by the computation assist device, and thereby generate a tertiary computation result corresponding to data in which the computation result of the expression that has performed the addition-subtraction is kept secret by random numbers alone, and a magnitude deciding circuit configured to decide a magnitude relation between a minuend and a subtrahend in the expression on the basis of the tertiary computation result, the computation assist device comprising a secondary computation key storing circuit configured to store the secondary computation key, a random number generating circuit configured to generate the random numbers, and a secondary computation circuit configured to execute secondary computation processing on the basis of the primary computation result, the secondary computation key, and the random numbers and thereby generate the secondary computation result.