New Practical Algebraic Public-Key Cryptosystem and Some Related Algebraic and Computational Aspects ()

S. K. Rososhek

Faculty of Mathematics and Mechanics, Tomsk State University, Tomsk, Russia.

**DOI: **10.4236/am.2013.47142
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Faculty of Mathematics and Mechanics, Tomsk State University, Tomsk, Russia.

The most popular present-day public-key
cryptosystems are RSA and ElGamal cryptosystems. Some practical algebraic
generalization of the ElGamal cryptosystem is considered-basic modular matrix
cryptosystem (BMMC) over the modular matrix ring *M*_{2}(Z_{n}).
An example of computation for an artificially small number *n* is presented. Some possible attacks on the cryptosystem and
mathematical problems, the solution of which are necessary for implementing
these attacks, are studied. For a small number *n*, computational time for compromising some present-day public-key
cryptosystems such as RSA, ElGamal, and Rabin, is compared with the
corresponding time for the ВММС. Finally, some open mathematical and computational problems are
formulated.

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Rososhek, S. (2013) New Practical Algebraic Public-Key Cryptosystem and Some Related Algebraic and Computational Aspects. *Applied Mathematics*, **4**, 1043-1049. doi: 10.4236/am.2013.47142.

1. Introduction

Security of some present-day public-key cryptosystems is based on computational complexity of some numbertheoretical problems. Two of these problems are used most often: the integer factorization problem and the discrete logarithm problem. These problems ensure the security of the RSA and ElGamal cryptosystems, as well as of the corresponding digital signature schemes [1].

However, the true level of the computational complexity of these problems is unknown. That is to say, they are widely believed to be intractable, although no proof of this fact is known.

In [2], randomized polynomial-time algorithms for computing discrete logarithms and integer factoring were presented for the quantum computer.

Nevertheless, some alternatives should be proposed. One of possible approaches is to replace number-theoretical cryptosystems by such algebraic cryptosystems that would be resistant to an attack on a quantum computer.

Let us now consider some scheme of cryptosystems, namely, cryptosystems of group rings.

In the author’s work [3,4], a scheme of group ring cryptosystems was proposed. The idea to apply group rings in cryptography is based on the fact that if we fix the cardinality of a finite ring R, the cardinality of the group ring RG for a finite group G is an exponent of the cardinality of the group G. Then, a legal user can perform cryptographic transformations separately in the ring R and in the group G using polynomial algorithms and the illegal user has to solve computationally difficult problems in the group ring RG.

Let us consider the standardization problem in the group ring and two its aspects. The direct standardization problem is to construct a standard automorphism g of the group ring RG from an automorphism a of the group G and automorphism b of the ring R in the following way: if an element х of the group ring RG is represented as a formal linear combination of elements of the group G with coefficients r_{i} from the ring R, then the image of the element х under the action of g is a formal linear combination of images of the elements g_{i} of the group G under the action of a with coefficients that are images of the coefficients r_{i} under the action of b.

The inverse standardization problem is formulated as follows. For a given automorphism g of a group ring RG, find an automorphism a of the group G and an automorphism b of the ring R such that g can be constructed from a and b by the way that was mentioned in the direct standardization problem or prove that such automorphisms a and b do not exist.

It is easy to see that, in the case of an efficient specification of the automorphism a in the group G and of the automorphism b in the ring R, one can efficiently compute the action of the automorphism g on any element of the group ring RG, i.e., efficiently specify the automorphism g of the ring RG.

As for the inverse standardization problem, there are some reasons to believe that this problem is computationally difficult. However, there is no proof for this statement.

In [5] some generalization of group ring cryptosystem is considered in the case of quasigroup ring.

The question “For which finite commutative rings R and finite groups G all automorphisms of the group ring RG are standard automorphisms?” was partially answered in [6-8]. It should be noted that an inner automorphism of an integral group ring of a finite group is not a standard automorphism as a rule. This is why, together with the standard automorphisms of the group ring, where G is a finite group, we use inner automorphisms. In [9] the group ring, where is the permutation group for three symbols, is represented in a matrix form as block diagonal matrices of the fourth degree with two one-dimensional blocks and one two-dimensional block. In [9,10] it is shown that the unit group of the group ring is a semi-direct product of trivial units and a free subgroup of rank 3. Since matrices of the fourth degree from this subgroup contain two identity one-dimensional blocks, we can restrict ourselves by a free group of matrices of the second degree with the free generators [9]:

If we fall outside the limits of the matrix representation of, we consider arbitrary matrices of the second degree from the ring and its unit group, which contains free rank 3 subgroups with the free generators

where and [11]. For example, if, we obtain a free rank 3 subgroup with the aforesaid free generators А, В, and С.

It should be also noted that all automorphisms of the group ring are inner [12].

New practical algebraic generalization of the ElGamal cryptosystem will be given in the Section 2, some attacks on this cryptosystem—in the Section 4, new hard computational problems—in the Section 5, comparison of the security level of classical RSA, ElGamal and Rabin cryptosystems with security level of this cryptosystem for the same small number—in the Section 7, some related open mathematical and computational problems— in the Section 8. It should be noted, that some other theoretical algebraic generalizations of the ElGamal cryptosystem are given in [13,14].

2. Basic Modular Matrix Cryptosystem (BMMC)

2.1. Key Generation

User А does the following:

1) picks large random positive integer n;

2) picks the random words and in the alphabet in a free rank 3 group with free generators А, В, and С;

3) computes the noncommuting matrices by replacing the symbols А, В, and С in the words and by the corresponding matrices

and performing matrix computations modulo n, i.e.,

If and commute, then return to 2);

4) let be the cardinality of the group over -residue ring modulo n, then user A picks the random integers

5) public key of user А is

and its private key is

Remark 1. Orders of matrices

in the group are equal to n;

Remark 2. The cardinality of the group in the case, p is a prime number, i is a positive integer, is equals to

[15].

As consequence in the case are primes, we have

2.2. Encryption

User В does the following:

1) writes the plaintext as a sequence of N numbers from, where N is a multiple of 4, , adding, if necessary, numbers from the first quadruple by a cyclic permutation at the end of the sequence;

2) writes each quadruple of numbers of the obtained sequence similarly as matrix:

3) picks session keys-random integers

for each of obtained matrices;

4) computes the ciphertext block for each matrix:

2.3. Decryption

Using the private key, user А computes for each ciphertext block:

After obtaining the sequence of matrices

the sequence of numbers and hence the plaintext can be reconstructed uniquely.

Theorem. Decryption in the ВММС is correct.

Proof. It is sufficiently to consider a case of one block of the ciphertext:

It should be noted that algorithms of the BMMC are implemented using the algorithm of matrix modular exponentiation similar to the usual modular exponentiation algorithm in which multiplication of integers is replaced by multiplication of matrices with reduction of their elements modulo n. In addition parallel computations may be used in matrix multiplications to increase the computational efficiency of the cryptosystem.

Let n be a large 256 bit integer, then the cardinality bit length of the group would be near 800 bits or more. For comparing in the case of the ElGamal cryptosystem the bit lengths of p and the cardinality of corresponding multiplicative group of residue field are equal. But one reduction modulo 1024 bit number in the ElGamal cryptosystem costs as some reductions modulo 256 bit number in the BMMC. Therefore, under corresponding choice of parameters the BMMC may be faster than the ElGamal cryptosystem with the same security level, because the gybrid problem and the transformation problem are harder than the discrete logarithm problem in the groups of the same cardinality.

3. Example

3.1. Key Generation

User А does the following:

1) picks two prime numbers and and computes;

2) picks the words in the free group:

3) computes matrices modulo n:

matrices and do not commute and, therefore, the user passes to the next step;

4) picks the integers

5) the public key is

the private key is.

3.2. Encryption

User В does the following:

1) writes the plaintext as a sequence of numbers from. The length of this sequence is multiple of 4. If necessary, some numbers are added. For example, let the plaintext be

here, a number should be added to the last block by shifting the first number cyclically, the user obtains two quadruples of numbers from:

2) writes the plaintext as two matrices from:

3) encrypts each block (matrix) separately choosing different session keys. For example, the first block is encrypted as follows;

4) picks the session key for the first block r_{1} = 2, t_{1} = 1;

5) computes the ciphertext of the first block modulo:

The ciphertext of the second block is computed similarly with the choice of another session key.

3.3. Decryption

User А, having obtained the ciphertext from user В, does the following: using its private key, for each ith block, computes

in particular, for the first block, he obtains

4. Some Attacks on ВММС

4.1. Find the Private Key by the Public Key

1) Let the cardinality of the group be

Since, the cryptanalyst can try to solve the equation with two unknowns Y and х:

where

2) Since

the cryptanalyst can try to solve the equation with two unknowns Z and х:

where, what leads to the private key by applying 1) to each solution (which we call the transforming matrix).

4.2. Find the Private Key by the Ciphertext

Since the private key is applied in the ciphertext not directly but only via the public key, the knowing of only the ciphertext does not yield additional possibilities to the attacks from 4.1 for the attack on the private key.

4.3. Find the Session Key by the Ciphertext

1) Since, the cryptanalyst can try to solve the equation with two unknowns Z and у:

where

2) For any solution of the equation from 1), the cryptanalyst can try to solve the equation with two unknowns Y and х:

where

4.4. Find the Corresponding Plaintext m or the Session Key by a Chosen Ciphertext

Cryptanalyst chooses the random and computes, then send it to user A for decryption. User A computes:

and send the result to cryptanalyst, which computes the plaintext:

Hence for protecting cryptosystem the modification of encryption algorithm is:

the modification of decryption algorithm is:

5. Computational Problems in Ensuring ВММС Security

From the consideration of attacks 4.1-4.4 one can formulate some problems, the solution of which is necessary to implement the corresponding attacks.

5.1. The Transformation Problem

Let a matrix be conjugated with an unknown integral power of a matrix for two given matrices. Find all solutions of the equation with two unknowns Z and у:

where.

Let us consider a particular case of Problem 5.1.

1) The conjugation problem.

For two given conjugated matrices and from the group, find a transforming matrix, i.e., matrix Т such that

5.2. The Hybrid Problem

Find all solutions of the equation with two unknowns Y and х

where in the group.

Let us also consider two particular cases of Problem 5.2.

1) The discrete logarithm problem in a cyclic subgroup of the group.

Let be a fixed cyclic subgroup of order j of the group with the generator, be an arbitrary element. Find the unique solution of the equation

where х is an integer such that.

2) The problem of extracting a root of the ith power in the group (the matrix RSA problem).

Let be an arbitrary element, be a fixed integer satisfying the condition and.

Find all solutions of the equation with a single unknown Y:

According to the Problem 2), in turn, one can also discern the following problem.

The problem of square-root extraction in.

Find all solutions of the equation with a single unknown Y:

where.

6. Computational Complexity of Problems 5.1, 5.2

If the order is a large number, then, the fact that the generators in a cyclic group are indistinguishable and random choice of k in the key generation show, on the one hand, that the identification of matrices in Problem 5.1 is a hard problem and, on the other hand, the impossibility to implement the exhausting search in practice for a large number j.

Considering Problem 5.1 1), it should be noted that this problem is solvable in the free subgroup of the group (see [16]). The possibility to extend this algorithm for a subgroup of the group depends on the solution of the following problem: for a given matrix, find the word and matrix whose reduction modulo n yields the matrix.

Nevertheless, even in the case of a solved problem of extension, the problem about the existence of an efficient algorithm for solving Problem 5.1 1) remains open.

Let us now consider Problem 5.2. As it is a problem with two unknowns, this problem is more complicated in the general case than its particular cases, the discrete logarithm problem and the problem of extracting a matrix root modulo n. It is worth to note that the square-root extracting problem is computationally difficult for large number, p and q are primes.

Let us now turn to the discussion of the cardinality of the set of secret keys for ВMМС. Note that, for classical cryptosystems, the uniqueness of the secret key can be reached by fitting of parameters. For BMMC, the situation is other. Indeed, if a matrix transforms the matrix into the matrix, i.e.,

then the matrix also transforms into for any matrix, where is a centralizer of in, because

Thus, if the secret key is considered as initial, the cryptanalyst can compromise the BMMC by any real key of the form, where. Then, for the cardinality of the set of real keys, we have

and when generating a key it is necessary to choose matrix so that

was negligibly small, e.g.,

This protects from random guessing of the private key.

7. Comparison of Computational Security of Classical RSA, ElGamal, and Rabin Cryptosystems with ВММС

For demonstrativeness, we compare the cryptosystems for a very small number.

7.1. RSA Cryptosystem

Let the public key be.

In this case, the cryptanalyst instantaneously compromises RSA by factorization, from which finds, and computation of the secret key either by the extended Euclid’s algorithm or by exhaustive search. Then the cryptanalyst finds the secret key:

7.2. Modified ElGamal Cryptosystem

In the unit group of the ring one has to choose an element of the maximal order. For this purpose, is factorized as, and the generators are chosen in the groups and, e.g.,

and.

Then the element of maximal order in is obtained from the solution of the following simultaneous congruences either by inspection or by the Chinese reminder theorem:

It follows that and its order is.

Let one of cyclic subgroups of order 12, for example, be chosen in the group. In the group G, another generator may be chosen, e.g.,.

Let the modified ElGamal cryptosystem be considered in a cyclic group G of order 12 with a generator and let the public key be

In this case, the cryptanalyst instantaneously compromises the modified ElGamal cryptosystem using exhaustive search in the cyclic group of order 12 finding the secret key а = 5 since.

Remark. In the case of choice n as, where p is a prime number, we compare BMMC with classical ElGamal cryptosystem.

7.3. Rabin Cryptosystem

Let the public key be, then the cryptanalyst instantaneously compromises the Rabin cryptosystem in this case by factorizing the number by prime multipliers.

One can see that, in all three cases, the cryptanalyst instantaneously compromises these classical cryptosystems for. Let us now the case of the BMMC cryptosystem for.

7.4. BMMC

Let the public key be

be the ciphertext of a certain matrix m.

Compromising BMMC in this case needs essentially more efforts than for the classical cryptosystems and exhausting search in the space of the search containing 775,760 matrices gives; secret key—

; session key—,.

8. Some Open Mathematical and Computational Problems

1) For which finite groups G and rings R the unit group of group ring RG is a semi-direct product of trivial units and a free subgroup of a finite rank?

2) For which groups G and rings R every automorphism of the group ring RG has a standard form?

3) For which subgroups of the group it takes place the property of small centralizers i.e. every element has a cyclic centralizer?

Remark. It is well-known [16] that in the free group of finite rank centralizer of any element is a cyclic subgroup.

4) Is there a polynomial-time algorithm for constructing cyclic centralizer of any element in a free group of finite rank?

5) Is there a polynomial-time algorithm for solving the membership problem for cyclic subgroup of the a) free group of finite rank, b) subgroup by modulo n in a group?

6) Is there a polynomial-time algorithm for solving the modular factorization problem, i.e. to represent every matrix from the subgroup by modulo n in a group as a word in an alphabet of by modulo n?

7) How to compute the number for arbitrary positive integers n? More exactly, is there a polynomialtime algorithm for computing?

8) Is there a polynomial-time algorithm for computing maximal order elements in a subgroup by modulo n in the group? What is a cardinality of this subgroup?

9. Conclusion

The practicality of the BMMC is provided by the absence of the necessity in the computer algebra systems used for computer realization of cryptosystem algorithms and efficient matrix computations by modulo number of essentially less bit length than that are usually used in classical cryptosystems under the same security level.

Conflicts of Interest

The authors declare no conflicts of interest.

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