A method of generating a cryptographic algorithm includes generating at least one key table on the basis of a seed value; generating, by using a round tweak bit string and an input bit string that is input to one of input branches of a Feistel structure, a first transformation function converting the input bit string into a first random bit string having a length that extends beyond a length of the input bit string, generating a second transformation function converting a second random bit string generated by referencing the one or more key tables into a third random bit string having a length that is the same as the length of the input bit string, and generating a block cryptographic algorithm of a Feistel structure which includes a round function to which the one or more key tables, the first transformation function, and the second transformation function are applied.