SECURE COMPUTATION DEVICE, SECURE COMPUTATION METHOD, AND PROGRAM

1 . A secure computation device that performs computation while keeping input data secret, the input data including an encrypted value sequence including a missing value, the secure computation device comprising:

processing circuitry configured to

generate a flag sequence that is a sequence of flags indicating a value obtained by encrypting 0, when a corresponding value is a missing value, and a value obtained by encrypting 1, when the corresponding value is not a missing value; and

compute the number of true data items excluding a missing value by summing values of flags in the flag sequence.

2 . The secure computation device according to claim 1 , processing circuitry configured to

set products of the values and corresponding flags as correction values and generate a correction value sequence; and

compute a mean of the input data by dividing a sum of the correction values by the number of true data.

3 . The secure computation device according to claim 1 ,

processing circuitry configured to

overwrite the missing value with a value larger than a maximum value of values in the true data;

sort data including the value sequence and the flag sequence in ascending order based on the values;

generate an index flag sequence which is a sequence of index flags in which a value corresponding to a first row of the data is a value obtained by encrypting 0, and a value corresponding to another row is a value obtained by encrypting a value obtained by adding 1 to a value corresponding to a previous row;

compute, as a median index, a value obtained by rounding down after a decimal point of (m−1)/2, where m represents the number of true data;

generate an odd flag sequence which is a sequence of odd flags indicating a value obtained by encrypting a value of 0, when values of the index flag are not equal to values of the median index, and a value obtained by encrypting a value of 1, when the values of the index flag are equal to the values of the median index;

generate an even flag sequence that is a sequence of even flags in which a value corresponding to a case where values of the odd flag are 1 is 1 and which indicate a value obtained by encrypting a value of 0, when the values of the index flag are not equal to a value obtained by adding 1 to the median index, and a value obtained by encrypting a value of 1, when the values of the index flag are equal to a value obtained by adding 1 to the median index; and

compute, as a median, a sum of products of the values and the odd flag, when the number of true data is an odd number, and compute, as a median, a value obtained by dividing a sum of products of the values and the even flag by 2 when the number of true data is an even number.

4 . The secure computation device according to claim 1 ,

processing circuitry configured to

overwrite the missing value with a value larger than a maximum value of values in the true data;

sort data including the value sequence and the flag sequence in ascending order based on the values;

generate a boundary flag sequence which is a sequence of boundary flags in which a value corresponding to a predetermined value is a value obtained by encrypting 0, when a value of the predetermined value is equal to a value of a next value after the predetermined value, a value corresponding to the predetermined value is a value obtained by encrypting 1 when the value of the predetermined value is not equal to the value of the next value after the predetermined value, and a value corresponding to a value located in a lowermost row of the value sequence is a value obtained by encrypting 1;

generate value vectors having values corresponding to a value of 1 of the boundary flag as respective elements;

compute a frequency vector that is a sum of values of the flags in each group delimited based on the value of 1 of the boundary flag; and

sort data including a set of individual elements of the value vector and the frequency vector in descending order by using the frequency vector as a key and output an element of a first value vector as a mode.

5 . A secure computation method executed by a secure computation device that performs computation while keeping input data secret, the input data including an encrypted value sequence including a missing value, the secure computation method comprising:

a flag sequence generating step of generating a flag sequence that is a sequence of flags indicating a value obtained by encrypting 0, when a corresponding value is a missing value, and a value obtained by encrypting 1, when the corresponding value is not a missing value; and

a true data count computing step of computing the number of true data items excluding a missing value by summing values of flags in the flag sequence.

6 . A non-transitory computer readable medium storing a computer program for causing a computer to function as the secure computation device according to claim 1 .