1. An authenticated encryption device for applying a two-round Feistel structure using an encryption function assigned with an auxiliary variable for a round function per two blocks to an input plaintext or encrypted text, generating an encrypted text or decoded plaintext, and applying the encryption function to a checksum calculated by use of some bits in the input plaintext or the decoded plaintext, and generating an authentication tag, the authenticated encryption device comprising:a non-transitory memory storing instructions; and

a processor configured to execute the instructions to:

input a plaintext to be encrypted and an initial vector;

generate the auxiliary variable given to each of the encryption functions based on the initial vector and a size of the input plaintext;

apply the two-round Feistel structure to each plaintext chunk when dividing the plaintext into chunks per two blocks thereby to generate encrypted text chunks corresponding to the plaintext chunks;

calculate the checksum of the plaintext, and applying the encryption function assigned with the auxiliary variable to the resultant checksum thereby to generate the authentication tag,

assuming the initial vector N, a chunk index i, an i-th plaintext chunk MC[i]=(M[i_1], M[i_2]), a pair of auxiliary variables (N, Tw_i_1) and (N, Tw_i_2) corresponding to two plaintext blocks included in the plaintext chunk MC[i], and the encryption function F_K(*, *), find an i-th encrypted text chunk CC[i]=(C[i_1], C[i_2]) as:

C[i_1]=F_K((N,Tw_i_1),M[i_1])xor M[i_2],

C[i_2]=F_K((N,Tw_i_2),C[i_1])xor M[i_1], and

calculate the checksum of the plaintext by use of a plaintext block M[i_2] included in each plaintext chunk, and assuming the resultant checksum SUM, an auxiliary variable (N, Tw_T_1) given to the encryption function used for generating the authentication tag, and the encryption function F_K(*, *), find the authentication tag T as: T=F_K((N, Tw_T_1), SUM);

when the input plaintext is divided into even blocks and a size of the last block is less than a predetermined block size, apply a predetermined two-round Feistel structure to the last plaintext chunk including the last plaintext block thereby to generate the last encrypted text chunk including the last encrypted text block; and

when the input plaintext is divided into even blocks and the size of the last block is less than the predetermined block size, calculate the checksum of the plaintext by use of the input plaintext and output from applying the predetermined two-round Feistel structure, and apply an encryption function assigned with an auxiliary variable to the resultant checksum thereby to generate the authentication tag,

assuming an index m of the last plaintext chunk, the last plaintext block M[m_2], the last plaintext chunk MC[m]=(M[m_1], M[m_2]), a pair of auxiliary variables (N, Tw_m_1) and (N, Tw_m_2) corresponding to two plaintext blocks included in the last plaintext chunk MC[m], the encryption function F_K(*, *), a size s of the last plaintext block, a block size n, a padding processing pad_n( ) from s bits to n bits, and a cutting processing cut_s( ) from n bits to s bits, find the last encrypted text chunk CC[m]=(C[m_1], C[m_2]) including the last encrypted text block C[m_2] with s bits as:

C[m_2]=cut_s(Z)xor M[m_2],

C[m_1]=F_K((N,Tw_m_2),pad_n(C[m_2]))xor M[m_1],

where Z=F_K((N, Tw_m_1), M[m_1]), and

calculate the checksum of the plaintext by use of the plaintext block M[i_2] included in each plaintext chunk except the last plaintext chunk, the Z, and C_n[m_2] obtained by padding the C[m_2] to n bits, and finds the authentication tag T as: T=F_K((N, Tw_T_2), SUM) assuming the resultant checksum SUM, an auxiliary variable (N, Tw_T_2) given to the encryption function used for generating the authentication tag, and the encryption function F_K(*, *).