<b>Symmetric Key Generation Algorithm in Linear Block Cipher Over LU Decomposition Method </b> In symmetric key algorithm in linear block cipher to encrypt and decrypt the messages using matrix and inverse matrix. In this proposed technique generate lower and upper triangular matrices from the square matrix using decomposition. In encryption process, the key is a lower triangular matrix and decryption process, the key is upper triangular matrix under modulation of the prime number. We illustrate the proposed technique with help of examples. liner block cipher, symmetric matrix key, encryption, and decryption, lower and upper triangular decomposition. Issue-4 Volume-1 P.Sundarayya | M.G.Vara Prasad