The present invention relates to a cellular automata based authentication device for message communication systems. Message authentication schemes have their applications in electronic payment systems and other transactions in e-commerce. They can be used for validation of identity of system users and for verification of transactions in the distributed environment of large organizations or corporate houses. Many message authentication schemes are available these days. Some of the more popular and important schemes are described b e1ow. One of the most widely used message authentication scheme is MD—5 function developed by Ronald Rivest. It is based on MD-4 which is again an improvement of MD-2. All the three functions produce an 128-bit output signature for the input message padded to a fixed block length. Since MD-5 is by the most commonly used scheme a comparison of its performance has been done with that of the present invention. HAVAL is an extension of MD-5 proposed by Zheng, Pieprzyk and Seberry. It can produce variable length output from 92-bits to 256-bits. Haval can be run on different number of 'rounds' of internal algorithm, so that it can be made faster than MD-5, or alternately produces longer hash code output which is potentially more secure. SHA was prepared by NIST in collaboration with NSA. It is similar to MD-4 but rectified its cryptographic drawbacks and produces 160-bit output instead of 128-bit. This algorithm is designed to be securedc such that to find two different messages to produce same digest or to recover the message from the digest requires infeasible computational cost. In message authentication code (MAC) from the sender side, data Dsent in plaintext or ciphertext is created and another k-bit message authentication code MACsent is generated which is a function of Dsent and a secret key. The message transmitted (Msent) is Dsent with MACsent appended at the end. The receiver uses the same procedure to calculate message authentication code MACcal from the data received Dree; if MACcal is identical to the MACrec then the data received is authenticated. Checksums are calculated as simple linear or polynomial functions of the message input, and produces a small value (16 or 32 bits). They are easy to calculate and that is why very unreliable. A file can be easily altered so that its checksum value remains same before and after alteration, and hence prone to tampering. Ralph Merkle designed SNEFRU,which is essentially a one-way hash function for one-time signature as well as authentication of public files, also can run on various rounds of internal algorithm. But cryptographic analysis confirms that one can find arbitrary messages that hash to a given 128-bit string if 4 round SNEFRU is used. There were several attacks on MD-4 citing flaws in its efficacy; if round 1 (of total 3) is omitted from the compression function of MD—4, it was shown that collisions for different, set of inputs do take place; it was further confirmed that collision could take place even within milliseconds (if run even on a PC) for inputs differing only on 3-bits if round 3 of the compression function is omitted. These criticisms actually motivated Rivest to propose the MD-5 version. All these and other authentication functions that can be found in the literature in the past decade suffer from enormous time and resource requirements, as a result their hardware implementation became costly as well as complicated. The problem of message authentication can be described in simple t e rm s a s f o 1.1 ow s : Given : A received message (Mrec). Decide r Whether the received message should be accepted as identical to the message originated by the sender (Msent). Basically message authentication is a many-to-one function. The input to the function is the message block to be transmitted and the output is the digital signature which is appended at the end of transmission. The present invention relates to a cellular automata else if (n=3) then r rClD := 90; rC2D := 90; rC3D := 90; > else if (n=2k) then/* n even; split into two equal parts of length k * / r i for each part recursively call Construct-CA (k); Change the rules of cells k and k + 1 fromrule '90' to rule '150' or vice-versa; } else if (n=2k+l)then /* n odd; split into two parts of length k */ {' rCk + U:=90; for each part recursively call construct-CA(k); > endif endi f > end Any CA generated by the Construct-CA algorithm is TPSA. Some examples of 90-150 TPSA CA constructed by this algorithm have been noted in Table 1. It may be noted that in CA based data authentication scheme a cell configured with rule 90 depends only on its left and right neighbours, whereas, a cell controlled with rule 150 depends upon itself, left and right neighbours. The CA is initially loaded with the authenticationkey value agreed upon by both the parties. For n-bit key, length of the CA is n, and n-bit stream of message data is used to control the rule configuration of the individual CA cells. If the i-th bit of a stream is '0' , then i-th cell is configured to run under rule 90 else under rule 150. After applying an n-bit stream to configure the rule, the CA is run for one cycle Next,the CA is again configured for the rule using the next n-bit stream of the message, and is run for another cycle, and so on. To make the cracking complexity of the scheme high, the 90-150 TPSA CA and is complemented form between every two n-bit mesage streams have been used. That is, after running the CA with rule configured by a message stream, the CA is configured as a 90-150 TPSA CA, constructed as per Construct-CA algorithm and run for one cycle, and as a complemented one of the above and run for one clock cycle. Since the characteristic matrix of a TPSA CA is singular, its inverse does not exist.This iancreases the cracking complexity by a factor of two for every run. Complemented CA contributes to the cracking complexity in identical fashion, moreover, it ensures that signature is never all zero, and run of the CA stops only when all of message data array is exhausted. In the following a step—by-step enumeration of the authentication scheme is given. Step 1: Generate an n-bit TPSA CA, executing the algorithm Construet-CA. Step 2: Generate the corresponding complemented CA, usings Theorem 10 to get the complement vector F. Step 3: Let K be the key of length n, known to both the parties (we gave randomly generated key). Let M be the message length. Add pad (each a string of single one followed by all zeros) so that total length of key-code, message and pad is multiple of n. The first element of the message data array is now the key. Step 4: Load the CA with the seed - which is the first element of the data array, i.e. key. Step 5: i herein described and illustrated in the accompanying drawings. A cellular automate (CA) based authentication device for message communication system. The communication system comprises - - a programmable cellular automata PCA (1) rule vector of which can be modified dynamically; - a rule register (2) for holding individual message words and for forming rules for driving the PCA; - a two predecessors single attractor TPSA register (3) for holding the rule for the PCA; - a complement vector register (4) for holding the complement vector for running the PCA; - start and final address register (5,6) for providing authentication signature as PCA content; and - a controller (7) for generating timing and control signals for authentication.