In this paper, a session based symmetric key cryptographic technique has been proposed and it is termed as Matrix Based Block Encoding Technique (MBBET). MBBET consider the input plain text as a stream of finite number of binary bits. This binary bit stream is chopped into manageable-sized blocks. The bits of these blocks fit column-wise from top to bottom into a square matrix of suitable order n. Now bits are collected from the square matrix along diagonally upward starting from (n, n) cell in a right to left trajectory to form the encrypted binary string and from this string cipher text is formed.