CRYPTANALYSIS OF HASH FUNCTIONS USING ADVANCED MULTIPROCESSING

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1 CRYPTANALYSIS OF HASH FUNCTIONS USING ADVANCED MULTIPROCESSING Gómez J., Montoya F.G., Benedicto R., Jimenez A., Gil C. and Alcayde A. University of Almeria, Spain {jgomez, pagilm, rbenedicto, ajimenez, cgilm, Abstract. Every time it is more often to audit the communications in companies to verify their right operation and to check that there is no illegal activity. The main problem is that the tools of audit are inefficient when communications are encrypted. There are hacking and cryptanalysis techniques that allow intercepting and auditing encrypted communications with a computational cost so high that it is not a viable application in real time. Moreover, the recent use of Graphics Processing Unit (GPU) in high-performance servers is changing this trend. This article presents obtained results from implementations of brute force attacks and rainbow table generation, sequentially, using threads, MPI and CUDA. As a result of this work, we designed a tool (myechelon) that allows auditing encrypted communications based on the use of hash functions. Keywords: CUDA, MPI, hash, audit tools, rainbow tables, brute force. 1 Introduction From its origins computing has revolutionized the way in which companies communicate, coming to play an important role in their success. The constant emergence of new technologies and the ability to interconnect through networks gives significant improvements in productivity and business market. This has great benefits but also new challenges. The most important challenge is the computer security [1]. Given the importance of computer systems, laws and standards have been provided to regulate the use and transmission of information. For example, in Spain we can find the Data Protection Act [2], while at international level we can

2 2 find the ISO 1799/BS [3] and ISO / IEC [4]. All these legal advances, together with the increased complexity of computer systems have caused an increment of computer security audits. An audit of computer security [5] allows to check the security level of a computer system using all kinds of tools and techniques for finding the problems and weaknesses. To make a security audit is necessary to use a large set of tools to easily audit unencrypted communications. Therefore, the main challenge and objective of this work is to create an auditing tool that allows encrypted communications audit based on the use of hash functions. For that, in section two, we analyze the attacks on hash functions. Section three explains how to use the hash functions attacks on MyEchelon to audit encrypted communications. In sections four and five we analized the results of implementing the brute force attacks and generation of rainbows table using different technologies multiprocessing to provide greater power to MyEchelon. 2 Criptoanalysis of hash functions One of the great allies of computer security is cryptography. A clear example is the use of hash functions, which we can find in communications, to check the passwords, to verify the integrity of messages, etc. A hash function allows to calculate the trace (the summary) that uniquely identifies a particular set of data. Table 1 shows the characteristics of the most important hash functions: MD5 [6], SHA-1 [7] and NTLM/MD4 [8]. MD5 SHA1 NTLM / MD4 Summary size 128 bits 160 bits 128 bits Block size 512 bits 512 bits 512 bits Number of steps Message size Strength preimage Table 1 Comparative Hash functions The scientific community began to question about the security provided by the hash functions when Xiaoyun Wang [9] published the first results on the breaking of the MD5 hash function and Antoine Joux [] demonstrated vulnerabilities in the SHA-0 hash function. The classical way to break the hash functions is to use brute force attacks [11] that consists on generating all possibles solutions until you find the right one. This implies a high computational cost for large and complex passwords. For that, it becomes necessary to find an efficient solution to this problem. This solution was found in the use of tables Rainbow [12] that is an elegant solution from the known hash tables [13] created by Martin Hellman, avoiding as far as possible the large

3 3 number of collisions inherited from his predecessor. The disadvantage of both techniques is that they have a very high computational cost, making unfeasible its use in conventional equipments. 3 myechelon MyEchelon is created with the aim to automate the audit process of a system using a large set of tools. When myechelon runs, it analyzes and takes control of the network where you are. Once the network scans, it carries out an attack Man In The Middle to the networks devices to audit communications (see Figure 1). Logically, unencrypted communications can be audited but the problem is the encrypted communications. hash password Audited comunication Fig. 1. Architecture of a network auditing myechelon To allow auditing the encrypted communications, myechelon uses a high performance server which will be responsible for making the crypto-analysis (see Figure 1). As we will see, the brute force attacks can audit https secure communications and the use of rainbow tables allows obtaining the passwords that are transmitted over the network. To improve the performance of cryptanalysis it is going to be compared brute force attacks and generating rainbow tables sequentially, using threads [14], MPI [15] and using Graphics Processors Unit (GPU) using CUDA [16]. All results have been obtained on the server MX DUAL AZServer Xenon and NVIDIA TESLA 70. The server has two processors Dual / Quad Core Intel Xenon 2.66 GHz, 8 2Gb SDRAM and two SATA hard drives of 1TB in RAID 1. Moreover, the TESLA team has four cards tesla 70 T, which makes a total of 960 cores of 1.44 GHz each.

4 4 4 Brute force using advanced multiprocessing Basically, this type of cryptanalysis is based on the birthday attack [17]. This attack is that given a hash m, it has to find a text M whose value Hash (M ) is equal to the original hash (m). For example, in the case of SHA1 (160 bits), this attack requires to generate 280 tests to obtain the solution. One application of this cryptanalysis is the forgery of certificates https secure communications. An https security certificate is composed by a series of data that identifies the server (e.g. name, domain) that are signed by a certification authority. To ensure the integrity of the digital certificate is used a digital signature algorithm, which in the case of https certificate is the MD5 hash function. To audit the https encrypted communications is necessary to generate a certificate in real time so that the signature of the certificate is the same than the original. In this sense, Arjen Lenstra [18] demonstrated the creation of two X.509 certificates with different public keys and the same MD5 hash value. Thus, it is possible to modify an original certificate and find a collision to allow the new certificate had the same hash value. The process is computationally expensive because it must test a great set of solutions to obtain a collision. We implemented the brute force attack using different multiprocessing technologies for different hash functions. As an example, Figure 2 shows the comparative on the performance of different technologies to generate the brute force attack for the MD5 hash function. If we analyze the results we can see that CUDA has the best performance with more than 135 millions hash/sec. However, as happens to MPI, it needs a time to start the system which penalizes the generation of a small number of hashes Fig. 2. Generation MD5 Hash If we compare the results between the perfomance of different implementations with respect to the sequential implementation (see Figure 3 and Table 2), we see

5 5 that the use of threads and MPI represents a better performance of 780% and CUDA provides the best results with a performance of 44% over its sequential implementation. 6000% 5000% Avg Perfomance 4000% 3000% 2000% 00% Threads MPI CUDA 0% MD5 SHA1 NTLM Hash Function Fig. 3. Comparison of performance on the brute force attack MD5 SHA1 NTLM Threads 806% 769% 764% MPI 792% 766% 781% CUDA 5222% 4046% 3962% Table 2. Perfomance on the brute force attack 5 Rainbow Tables using advanced multiprocessing The use of Rainbow Tables is the fastest way to make brute force attacks. For the use of Rainbow Tables it has been used Rainbow-Crack project [19]. The process of breaking a hash consists of three phases: generation of tables (rigen), to put order in the tables (rtshort) and use of ordered tables to obtain the value of a given hash (rtcrack). The first two steps should be performed once, while the third step is repeated for each of the hash that we want to break. The main problem is that the required size and the time for generating tables are related exponentially to the size of the password that you want to analyze. For example, to generate a rainbow table that enables to break an alphanumeric password of seven character is necessary to generate a table of 2Gb for what it would be necessary about three days. To decrypt passwords of nine characters we need 500 tables of 2Gb and it would take about three years.

6 6 Basically, a rainbow table is composed of a set of vector data. The generation of each vector can be done completely independently and for this, as it is shown in Figure 4, it is necessary to use mainly three functions: 1) IndexToPlain is responsible for converting a numerical value to a string belonging to the set of possible values 2) PlainToHash is the most important function since it is responsible for calculating the hash of the string above, 3) HashTolndex converts the hash value to a numeric value. While (chain_num) While (chain_len) Generate Index Save Index IndexToPlain PlainToHash HashToIndex Save Index Fig. 4. Rainbow Tables Generation sequentially If we analyze the processing time we can observe that PlainToHash function represents a 44.58% of the time, IndexToPlain a 15.75%, HashTolndex a 8.27% and the 31.6% left is used in various operations reading and writing. The parallelization of the generation of Rainbow Tables has been made taking into account the characteristics of each technology. In the case of threads and MPI parallelization is easy and can be calculated separately each of vector in the table. But in CUDA case parallelization is more complicated because CUDA does not allow operations W/R and therefore it generates a CPU thread for each vector. To complete the calculations on the GPU functions have been implemented PlainToHash, IndexToPlain and HashTolndex in CUDA. Thus, the CPU thread is responsible for carrying out the operations W/R and GPUs performs the calculations. Figure 5 shows the results obtained by different technologies of multiprocessing. In this case, the use of threads and MPI represents a perfomance of 783% close to the ideal one while CUDA is much lower than the sequential implementation. The CUDA implementation for the generation of tables Rainbows presents the worst results. It is produced as it makes a continuous movement of data between the CPU and the GPU system.

7 7 900% 800% 700% Perfomance 600% 500% 400% 300% 200% Threads MPI CUDA 0% 0% MD5 SHA1 NTLM Hash Function Fig. 5. Performance Comparison in Rainbow Tables generation MD5 SHA1 NTLM Threads 765% 780% 770% MPI 800% 797% 787% CUDA 78% 36% 33% Table 3. Comparasion in Rainbow Tables generation 6 Conclusions This article presents the results of the implementation of brute force attacks and Rainbow Tables generation, so sequential, using threads, MPI, CUDA. Finally, we can say that CUDA has the best technology in brute force attacks while MPI presents the best results in the generation of tables Rainbow. These results have been applied to myechelon allowing intercepting and auditing encrypted communications using in each case the best multiprocessing technology. 7 Acknowledgements This work has been financed by the Excellence Project of Junta de Andalucia (P07-TIC02988), in part financed by the European Regional Development Fund (ERDF).

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