Implementation in Java of a Cryptosystem using a Dynamic Huffman Coding and Encryption Methods

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					                                                                                              (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                              Vol. 9, No. 3, March 2011

Implementation in Java of a Cryptosystem using a
Dynamic Huffman Coding and Encryption Methods
                                                                                                    Eug` ne C. Ezin
                                                                 Institut de Math´ matiques et de Sciences Physiques
                                                                 e                                                   e
                                                             Unit´ de Recherche en Informatique et Sciences Appliqu´ es
                                                                            e                    e              e
                                                                  Universit´ d’Abomey-Calavi, R´ publique du B´ nin

   Abstract—Data transmission through a secure channel is im-                                                       The paper is organized as follows: section II presents some
portant in our daily experience with the objective to ensure that                                                   basic concepts about secure systems and cryptography; section
the receiver is the only one authorized and able to discover the                                                    III reviews the dynamic Huffman coding we introduced for
message content. In this paper, and based upon the proposed
model in [1], we implement a cryptosystem for data transmission.                                                    data compression [1]. Section IV presents the DES and RSA
The plaintext message to transmit is first compressed using                                                          algorithms. The model description is given in section V which
the dynamic Huffman coding algorithm. The security level is                                                         consists of the alphabet construction for the plaintext message
reinforced by transmitting the resulting message through an                                                         in subsection V-A whereas subsections V-B and V-C present
encrypted subsystem using DES or/and RSA algorithms. The full                                                       respectively the encryption model and the decryption model.
description of the whole system is given and the results analyzed.
                                                                                                                    In section VI, we present the cryptosystem implementation in
   Keywords-component—Data compression, Huffman coding tech-                                                        detail using Java programming language meanwhile section
nique, DES and RSA algorithms.                                                                                      VII shows the testing results of the proposed cryptosystem.
                                                                                                                    We finally conlude this work and give some perspectives in
                                            I. I NTRODUCTION                                                        section VIII.
   Data transmission occurs when there is a channel between
two machines. To exchange a datum, an encoding must be                                                                                  II. S ECURE S YSTEMS
chosen for the transmission signals. This basically depends
                                                                                                                       The term secure systems is somehow misleading and it
upon the physical medium used to transfer the data, the
                                                                                                                    implies that systems are either secure and insecure. In truth,
guaranteed data integrity and the transmission speed. Data
                                                                                                                    there is no absolute security. Every system can be broken given
transmission can be simple if there are only two machines
                                                                                                                    enough time and money. According to Knudsen et al. [2],
communicating, or if only a single piece of data is sent.
                                                                                                                    there are more secure and less secure systems but no totally
Otherwise, it is necessary to install several transmission lines
                                                                                                                    secure systems. When people talk about secure systems, they
or to share the line among several different communication
                                                                                                                    mean systems where security is a concern or was considered
actors. Transmitting a message sometimes requires a safe
                                                                                                                    as part of the design. The security of any application is
channel to prevent an unauthorized person from discovering
                                                                                                                    determined by the security of the platform it runs on, as well
its content.
                                                                                                                    as the security features designed into the application itself.
   In our previous work [1], we proposed a model that can be
                                                                                                                    The most important tool used by applications for security is
summarized as in Fig. 1. This model is based upon a dynamic
                                                                                                                    cryptography, a branch of mathematics that deals with secret
          Message to tranmit                                                             Received message           writing.
                                                                                                                       Cryptography is the biggest tool in the application pro-
          Plaintext message     Dynamic Huffman   Coded message   DES / RSA encryption Delivered message            grammer’s arsenal even though a cryptographycally-enabled
                                  coding                                methods                                     program is not necessarily a secure one. When correctly used,
                                                                                                       Receiver     cryptography provides the standard security features that are
                                                                                                                    confidentiality, integrity, authentification and non-repudiation
             Fig. 1.           Block diagram for transmitting the original message.                                 [2], [3].
                                                                                                                       Confidentiality assures that data cannot be viewed by unau-
Huffman coding and encryption methods.                                                                              thorized people.
   In this paper we implement such a cryptosystem considering                                                          Integrity assures that data cannot be changed without certain
an alphabet with 256 symbols in the ASCII code based                                                                knowledge. The receiver of a message should be able to check
on dynamic Huffman coding and existing DES and RSA                                                                  whether the message has been modified during transmission,
algorithms. By doing so, security is reinforced at two levels.                                                      either accidentally or deliberately. No one should be able to

                                                                                                                                              ISSN 1947-5500
                                                       (IJCSIS) International Journal of Computer Science and Information Security,
                                                       Vol. 9, No. 3, March 2011

substitute a false message for the original message, or for parts     left child inserts a 0. The result is a unique bit pattern for
of it.                                                                each symbol that can be uniquely decoded. This is due to the
   Authentification assures that people dealing with the system        fact that no codeword can be a prefix of any larger-length
are not imposters. The receiver of a message should be able           codeword.
to verify its origin. No one except Alice, should be able to             The dynamic aspect of the Huffman coding we introduced is
send a message to Bob and pretend to be Alice (data origin            the following: instead of assigning frequency to each symbol
authentification). When initiating a communication, Alice and          of the alphabet according to predefinied values, we rather as-
Bob should be able to identify each other i.e. the entity             sign to each symbol of the alphabet a random value belonging
authentification.                                                      to the interval [0, 1]. We then normalized each frequency. By
   Non-repudiation assures that one can prove to a third party        doing so, it is impossible to decode the compressed data by
that he/she received a message from a specific and known               using frequency analysis. A full description about the dynamic
sender. The sender should not be able to later deny that he/she       Huffman algorithm can be found in [1].
sent a message.
                                                                                        IV. E NCRYPTION M ETHODS
              III. DYNAMIC H UFFMAN C ODING                              Encryption is a tool one can use to protect secrets. A cipher
   Applications of Huffman coding are pervasive in computer           encrypts or decrypts data. Ciphers come in three flavors:
science. This coding scheme is not limited to encoding mes-              • Symmetric, or private-key ciphers use a single secret key
sages. Indeed, Huffman coding can be used to compress parts                 to encrypt and decrypt data. Symmetric keys can be useful
of both digital photographs and other files such as digital sound            in applications like hard-disk file encryption, when the
files (MP3) and ZIP files. In [4], Massey showed that the                     same person encrypts and decrypts data. Data Encryption
techniques of source coding readily applied in cryptography.                System (or DES) is a symmetric cipher. The term symmet-
Data reduction is accomplished by Patra and Sanka in [5]                    ric encryption is used since both communication partners
in using Huffman coding and encryption by the insertion of                  use the same key k for encryption and decryption.
shuffled cyclic redondancy code. Klein et al. [6] analyzed                • Asymmetric, or public key ciphers use a pair of keys. One
the cryptographic aspects of Huffman codes used to encode                   key is public and may be freely distributed while the other
a large natural language on CD-ROM and concluded that                       key is private and should be kept secret. Data encrypted
this problem is N P −complete for several variants of the                   with either key can be decrypted using the other key. In
encoding process [7]. Rivest et al. [8] cryptanalysed a Huffman             practice, the public key which may be freely distributed is
encoded text assuming that the cryptanalyst does not know the               used for data encryption and the private key which must
codebook. According to them, cryptanalysis in this situation                be kept secret is used for data decryption.
is surprisingly difficult and even impossible in some cases due           • Hybrid systems use a combination of symmetric and
to the ambiguity of the resulting encoded data. Data compres-               asymmetric ciphers. Asymmetric ciphers are much slower
sion algorithms have been considered by cryptographers as a                 than their symmetric counterparts. In a hybrid system, an
ciphering scheme [9]. In summary, many researchers still pay                asymmetric cipher is used to exchange a private key (also
attention to Huffman coding. The following paragraphs briefly                called a secret key or a session key). The secret key is
present the Huffman coding technique.                                       used with a symmetric cipher for data encryption and
   The Huffman (or variable length coding) method is a                      decryption.
lossless data compression algorithm based on the fact that            Symmetric and asymmetric ciphers are described by algo-
some symbols have a higher frequency of occurence than                rithms. A hybrid system, is at a higher level, a protocol that
others. These symbols are encoded in fewer bits than the fixed         uses both public and private key algorithms [2].
length coding producing in average a higher compression. The             In subsections IV-A and IV-B we present the DES and RSA
idea behind Huffman coding is to use shorter bit patterns             algorithms.
for more frequent symbols. To achieve that goal, Huffman
algorithm needs the probability of occurence of each symbol.          A. Presentation of DES Algorithm
These symbols are stored in nodes and then sorted in ascending           In 1973, The National Institute of Standards and Technol-
order of their probability values.                                    ogy published a solicitation for cryptosystem in the Federal
   The algorithm selects two nodes (children) with the smaller        Register. This led to the adoption of the DES which stands
probabilities and constructs a new node (parent) with a proba-        for Data Encryption Standard. It’s a symmetric cipher, first
bility value equal to the sum of the probabilities of its children.   published in 1975 and based largely on research performed at
This process ends when the node with the probability equal to         IBM. The National Security Agency (NSA) also had a hand in
1 is constructed. The result of this algorithm is a binary tree       the algorithm, although its involvement and motives are still a
data structure with the root being the node with probability          subject of debate. A paper on the history of DES was written
equal to 1, and the last nodes (leaves) represent the original        by Smid and Brandstad [10].
symbols. The binary tree is then crossed to reach its leaves             DES processes a plaintext blocks of n = 64 bits producing
when a certain symbol is needed. Moving to a right child              64-bit ciphertext blocks with a secret key of 56 bits [11]. Its
inserts a 1 in the bit pattern of the symbol, while moving to         weakest part is that its 56-bit key size is stored in 8 bytes,

                                                                                                  ISSN 1947-5500
                                                                 (IJCSIS) International Journal of Computer Science and Information Security,
                                                                 Vol. 9, No. 3, March 2011

which makes it vulnerable to key search attacks1 . For short,                       Bob computes c = me (mod n).
The DES algorithm takes 56-bit keys and 64-bit plaintext                          Data decryption. Alice can decrypt the message by doing
messages as inputs and outputs a 64-bit cryptogram.                            the following:
               DES : {0, 1}56 × {0, 1}64 −→ {0, 1}64                     (1)      • Use the private key d.
                                                                                  • Compute m = e mod n.
  If the key k is chosen, we get                                                  RSA encryption/decryption is substantially slower than the
               DESk : {0, 1}64            −→  {0, 1}64                         commonly used symmetric-key encryption algorithms such as
                         x                −→ DES(k, x).                        DES [17]. In practice, RSA encryption is most commonly
                                                                               used for the transport of symmetric-key encryption algorithm
   An encryption with DESk consists of 16 major steps, called                  keys and for the encryption of small data items. More detailed
rounds. In each of the 16 rounds, a 48-bit round key ki is used.               information about the RSA encryption can be found in [19].
DES decryption is identical to encryption but the key scheduler
is reverse [12].                                                                                                 V. M ODEL D ESCRIPTION
   The well-known drawback of a symmetric-key system is                          The full description of the proposed model in concern is
that it requires a prior communication of the key between the                  done in [1]. Some extensions are done for its implementation
involved people before any ciphertext is transmitted. In prac-                 that we mentioned in the following subsections by presenting
tice, this may be very difficult to achieve. A deep explanation                 the used alphabet, data encryption system modeling, and data
about the DES can be found in [13].                                            decryption system modeling.
B. Presentation of RSA Algorithm                                               A. Presentation of the Alphabet
   The general idea about public-key cryptography was intro-                      An alphabet is important for the definition of the plaintext
duced in the open literature by Diffie and Hellman in 1976                      message. Indeed, a readable text is possible when both the
[14]. The RSA encryption falls into this category of cryptog-                  sender and receiver use the same alphabet. Instead of using
raphy. For a general survey article on public-key cryptography                 a restricted alphabet which consists of 36 symbols as we did
the reader could refer to [15].                                                for symplicity in [1], we now consider the 256 symbols in the
   The RSA cryptosystem, named after its inventors R. Rivest,                  ASCII code which is used by many mailers through the world
A. Shamir, and L. Adleman, is the most widely used public-                     [2]. Extension can be done easily using other codes such as
key cryptosystem [16]. It may be used to provide both se-                      Unicode [20], etc.
crecy and digital signatures and its security is based on the
intractability of the integer factorization problem.                           B. Modeling Data Encryption System
   Let us assume Bob and Alice want to communicate using                            The proposed model for data transmission is shown in Fig.
RSA encryption method. First, both of them create an RSA                       2.
public key and a corresponding private key. The algorithm has
three steps, namely: the key generation for RSA public-key                                                 Identification of                  Huffman
encryption, the RSA public-key encryption and the decryption.                                              different symbols                 codewords

Key generation for RSA public-key encryption. Alice should                                                                                                Compressed message
do the following [17], [18]:                                                                        Message             Random probability
                                                                                                   to transmit          value assignment                 with Huffman algorithm
  • Generate two distinct large random primes p and q, each
     roughly the same size.
  • Compute n = pq and ϕ = (p − 1)(q − 1).
  • Select a random integer e, 1 < e < ϕ such that
                                                                                                                          Transmitted                            Encryption
     gcd(e, ϕ) = 1.                                                                                                                                               method
  • Use the extended Euclidean algorithm to compute this
     unique integer d, 1 < d < ϕ such that ed = 1 (mod ϕ).                              Receiver

  • Alice’s public key is (n, e); Alice’s private key is d.                              Fig. 2.       Block diagram for transmitting the original message.

  Key generation for RSA public-key encryption. Bob en-
crypts the plaintext message m for Alice by doing the follow-                     The sender uses the alphabet described in subsection V-A
ing:                                                                           to compose the plaintext message to transmit.
                                                                                  Once the original message is formed, the set of different
  • Bob obtains Alice authentic public-key (n, e).
                                                                               symbols in such a message is considered with assignment
  • Bob represents the message as an integer m in the interval
                                                                               of random value probability to each symbol [1]. Huffman
     [0, n − 1].
                                                                               algorithm is then applied to compress the data to transmit.
  1 The   56 bits of the key are packed with 8 bits of parity.                    Next, an encryption method is used to encrypt the message
  2 gcd   stands for great common divisor.                                     before delivering it to the receiver.

                                                                                                                               ISSN 1947-5500
                                                               (IJCSIS) International Journal of Computer Science and Information Security,
                                                               Vol. 9, No. 3, March 2011

  In this paper, we consider the DES algorithm in the group                     In the second part, according to the encryption algorithm
of symmetric cipher methods and the RSA algorithm as                         used, one can have one or two keys. In the case of DES
belonging to asymmetric cypher methods.                                      encryption method (or more generally a symmetric encrpytion
                                                                             algorithm), only a private key is in concern. In the case of
C. Modeling Data Decryption System                                           RSA encryption method (or more generally an asymmetric
   On the receiver’s side, the user needs to know the key of                 encryption algorithm), a private key (to keep secret) and a
the encryption method and the codebook constructed based                     public key (can be freely distributed) are involved. Fig 4
on the dynamic Huffman coding. A full descrption about keys                  presents how the whole process of data encryption is involved
involved in the system is given in subsection VI-A. Fig. 3                   with the management of the keys.
shows the process for decrypting the received message.

       message                           codewords

                       Decryption                    Huffman decoding
                        method                       method


                       Alphabet symbols
                       and their codewords


        Fig. 3.   Block diagram for decoding the received message.                     Fig. 4.   Model describing the management of the keys.

                                                                                The major problem is about how to transfer the different
                                                                             keys in a secure mode to avoid their descovering by imposters
   There are a number of excellent cryptographic libraries in                or men in the middle.
C and C++ [21], but Java serves as the reference since it is                    We suggest the following solutions when the DES encryp-
very popular for new business and server application. Indeed,                tion is chosen:
memory management is automatically done eliminating entire                      • First, send the encrypted message.
classes of bugs. It provides a standard cryptographic API                       • Then, transmit the decoding key, followed by the encryp-
which, while not perfect or complete, is about as universal                        tion key.
as we can get [22].                                                             • Finally, the receiver should authentify himself or herself

A. Keys Description                                                                before decrypting the message using the two keys.
   Two subsystems can be clearly identified in the proposed                      We suggest the following solutions when the DES and RSA
model. The first one concerns the data compression which uses                 encryptions are chosen:
the dynamic Huffman coding and the second part concerns                         • First, send the encrypted message.

encryption methods. Depending upon the used encryption                          • Then, transmit the decoding key.

method, two or three keys are involved in the whole model.                      • Next, get the public key to encrypt the coded message.

   In the first part, the compressed data are obtained using the                 • Finally, the receiver should authentify himself or herself

dynamic Huffman coding [1]. The decoding process should                            before decrypting the message using the decoding key
involve the codewords registrered as a binary tree or the                          and the private key.
symbols with respect to their distrubution frequencies. To                   These solutions are not without weakness as it is the case with
prevent the previous information from being as clear as a                    data transmission problems. The question How do we exhange
plaintext for data decrpytion, one can encrypt it to obtain                  the keys in a safe mode ? is still a challenge in cryptography.
the key. We call this latter, the decoding key. Therefore, the                  For security reasons the sender can use different channels
decoding key can be one of the previous encrypted data namely                to transmit each of the keys to the receiver. By doing so, even
the codebook or the symbols with respect to their distribution               though a man in the middle intercepts one of the keys, it will
frequencies. Any encryption algorithm can be used at this                    be hard for him to recover the original message.
stage. In our case, we used the DES encryption method for
practical purpose. The use of the RSA algorithm at this step                 B. Interface of the Cryptosystem
should involve two keys. That is the first type of key involved                 The cryptosystem tool is built for a peer to peer architecture
in the model.                                                                but it can easily be extended to distributed environment.

                                                                                                            ISSN 1947-5500
                                                           (IJCSIS) International Journal of Computer Science and Information Security,
                                                           Vol. 9, No. 3, March 2011

  Figure 5 presents the interface for the encryption process.              The cryptosystem tool is built for a peer to peer architecture,
The transaction form has information about the sender and                but it can be easily extended to distributed environment.
the receiver. Once the message is written, it must first be
coded by a dynamic Huffman technique (DH coding). The
encrypted decoding key, thanks to DES algorithm, is generated
and stored into a file. The sender can then chose the DSA or
RSA algorithms to encrypt the previous coded message. The
generated two keys are stored into different files in case of
RSA algorithm and into one file in case of DSA algorithm.
The labeled button Send is used to send all data to the receiver
by electronic mail.

                                                                                         Fig. 6.   Interface for receiving a message.

                                                                         C. Overview of Classes used in the implemented Cryptosystem
                                                                            The Java Cryptography Encryption [2] includes many
                                                                         classes to handle various algorithms in cryptography. For
                                                                         instance, the class javax.crypto.KeyGenerator is suitable for
                                                                         randomly generating a single key. It uses a getInstance()
                                                                         factory method taking as parameter the name of the algorithm
                                                                         of interest. The general syntax is
              Fig. 5.   Interface for sending a message.
                                                                         KeyGenerator kg = KeyGenerator.getInstance(”AlgoName”);
  Once the receiver downloads all the files attached to the               The secret key of the DES algorithm is generated thanks to
message in his inbox related to the transaction, he can use              the class javax.crypto.SecretKey. SecretKey sk = KeyGenera-
the reception form in Fig. 6 for the decryption process. All             tor.getInstance(”DES”).generatesKey();
files are stored in a specific folder. The receiver selects the
                                                                         The private and public keys of the RSA algorithm are gener-
encrypted file and choses the appropriate decryption algorithm.
                                                                         ated thanks to the classes and
Then he clicks on the button Decryption. The receiver obtains
the original plaintext message by clicking on the button DH
Decoding for the dynamic Huffman decoding procedure.                     KeypairGenerator kp = KeyGenerator.getInstance(”RSA”);

                                                                                                        ISSN 1947-5500
                                                      (IJCSIS) International Journal of Computer Science and Information Security,
                                                      Vol. 9, No. 3, March 2011

kp.initialize(512);                                                                                R EFERENCES
KeyPair keys = kp.generateKeyPair();                                 [1] E. C. Ezin, “Modeling data transmission through a channel based on
                                                                         huffman coding and encryptions methods,” International Journal of
We develop a package which contains four classes.                        Computer Science and Information Security, vol. 8, no. 9, pp. 195–199,
 • The class DES has two methods cryptage() and de-                  [2] J. Knudsen, Java Cryptography. O’Reilly and Associates, Inc., 1998.
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                                                                     [8] R. L. Rivest et al., “On breaking a huffman code,” in Proc. IEEE
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                                                                         and future,” in Contemporary Cryptology, the Science of Information
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of the interface (see Fig. 5) to send the plaintext message         [12] D. R. Stinson, Cryptography: Theory and Practice, Third Edition
cryptography handles a part of security to Alice.                        (Discrete Mathematics ans its applications). Taylor abd Francis Group
                                                                         LLC, 1995.
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to Alice through an electronic message within five minutes                Notice of the American Mathematical Society, vol. 47, 2000, pp. 450–
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                                                                         pp. 120–126.
   In this paper, we implemented in Java a cryptosystem based       [17] P. v. O. A. Menezes and S. Vanstone, Handbook of Applied Cryptogra-
on dynamic Huffman coding and DES and RSA encryption                     phy. CRC Press, 1996.
methods. The advantage of such a tool can be observed at            [18] B. Schneier, Applied Cryptography. John Wiley & Sons Inc., 1996.
                                                                    [19] R. Boney, “Twenty years of attacks on the rsa cryptosystem,” in Notices
two levels.                                                              of the American Mathematical Society, vol. 46, 1999, pp. 203–216.
   The use of dynamic Huffman coding algorithm introduces           [20] J. K. Korpela, Unicode Explained. O’Reilly and Associates, Inc., 2006.
a security level for decoding the plaintext message without         [21] M. Welschenbach, Cryptography in C and C++, Second Edition.
                                                                         Apress, 2005.
any knowledge about the symbol frequency. The key plaintext         [22] N. Galbreath, Crptography for Internet and Database Applications.
information to decode the coded message is encrypted before              Wiley Publishing, Inc., 2002.
its delivering to the receiver.
   The second level of security is introduced by DES and RSA                                    AUTHOR PROFILE
algorithms. When using the DES algorithm, the private key                                     Eug` ne C. Ezin received his Ph.D
should be kept as secret as possible as it is for the symmetric                           degree with highest level of distinction
encryption methods. When using the RSA algorithm, the                                     in 2001 after research works carried out
private generated and public keys also should be kept in a                                on neural and fuzzy systems for speech
safe way.                                                                                 applications at the International Institute
   The drawback of the proposed system is the transmission                                for Advanced Scientific Studies in Italy.
of the different keys in a safe mode. This is a general problem                           Since 2007, he has been a senior lecturer
of symmetric and asymmetric encryptions. The idea to send                                 in computer science. He is a reviewer of
the crypted message and different keys within five minutes is        Mexican International Conference on Artificial Intelligence.
formulated in order to limit the risk of a man in the middle        His research interests include high performance computing,
trying to intercept the crypted message to discover its contents.   neural network and fuzzy systems, signal processing, cryptog-
   Future works will include the security aspects of the whole      raphy, modeling and simulation. He is currently in charge of
system for data transmission. The channel coding for the data       the master program in computer science and applied sciences
transmission will also be explorated.                                                      e
                                                                    at the Institut de Math´ matiques et de Sciences Physiques of
                                                                    the Abomey-Calavi University in the Republic of Benin.
  We thank anonymous reviewers for their review efforts.

                                                                                                    ISSN 1947-5500