Comparative Analysis of 128-bit Key Loading Techniques for the A5/1 Stream Cipher: Statistical, Structural and Performance Evaluation
DOI:
https://doi.org/10.65405/er87s043Keywords:
A5/1; stream cipher; key loading; equivalent keys; NIST SP 800-22; lightweight cryptography; GSM.Abstract
A5/1 is an encryption scheme that continues to be employed in the old-fashioned GSM network. It has a compact hardware architecture that makes it ideal for inexpensive modifications, but the size of its 64-bit key is not sufficient in view of modern computing capabilities. In this work, the performance of four methods of loading a 128-bit key in the same registers of A5/1 is examined: direct extension (D), extended warm-up with key salt (WS), key scrambling (SC) and state mixing (SX). The randomness of these variations was tested with the NIST SP 800-22 test suite applied to 100 sequences of 1000000 bits. The algorithm A5/1 is still used as the cryptographic scheme in the traditional GSM communication system. It has a small hardware structure, which makes it easy to implement any inexpensive alterations; however, the size of the 64-bit key is inadequate in the terms of contemporary computing resources. This paper evaluates four techniques for loading a 128-bit key into the registers of A5/1 without changing their structure: direct extension (D), extended warm-up with key salt (WS), key scrambling (SC), and state mixing (SX). Randomness was assessed for each technique with the NIST SP 800-22 test suite over 100 sequences of 10⁶ bits, and every one of them passed. The techniques were then compared on two further measures: how fully they draw on the 128-bit key, and what initialization costs they incur. D and SC leave part of the key unused, since their effective key space does not exceed 2⁶⁴, whereas WS and SX exploit all 128 key bits. SX achieves this while retaining the original 186-cycle initialization and 93% of the original throughput; WS, by contrast, requires 10,086 cycles. SX is therefore the recommended technique. Because the internal state remains 64 bits, the proposal improves key loading at low cost, but it does not amount to a complete security solution.
Downloads
References
Ahmad, F., Khan, R., & Nkembi, A. A. (2025). Improvement in the interception vulnerability level of encryption mechanism in GSM. Inventions, 10(4), 56.
Babbage, S. (1995). Improved exhaustive search attacks on stream ciphers. In European Convention on Security and Detection (IEE Conference Publication No. 408, pp. 161–166). IEE.
Bassham, L. E., Rukhin, A. L., Soto, J., Nechvatal, J. R., Smid, M. E., Leigh, S. D., Levenson, M., Vangel, M., Heckert, N. A., & Banks, D. L. (2010). A statistical test suite for random and pseudorandom number generators for cryptographic applications (NIST Special Publication 800-22 Rev. 1a). National Institute of Standards and Technology.
Biham, E., & Dunkelman, O. (2000). Cryptanalysis of the A5/1 GSM stream cipher. In Progress in Cryptology – INDOCRYPT 2000 (LNCS 1977, pp. 43–51). Springer.
Biryukov, A., & Shamir, A. (2000). Cryptanalytic time/memory/data tradeoffs for stream ciphers. In Advances in Cryptology – ASIACRYPT 2000 (LNCS 1976, pp. 1–13). Springer.
Biryukov, A., Shamir, A., & Wagner, D. (2001). Real time cryptanalysis of A5/1 on a PC. In Fast Software Encryption – FSE 2000 (LNCS 1978, pp. 1–18). Springer.
Briceno, M., Goldberg, I., & Wagner, D. (1999). A pedagogical implementation of the GSM A5/1 and A5/2 voice privacy encryption algorithms.
Fauzi, S., Othman, M., Shuib, F., Seman, K., & Rahim, K. (2018). Randomness evaluation of modified A5/1 stream cipher for global system for mobile communication. Malaysian Journal of Science Health & Technology, 2.
Golić, J. D. (1997). Cryptanalysis of alleged A5 stream cipher. In Advances in Cryptology – EUROCRYPT ’97 (LNCS 1233, pp. 239–255). Springer.
Jawad, N. H., Katran, L. F., Albermany, S., & Bani, S. J. (2024). Enhancement A5/1 stream cipher algorithm as lightweight generator. AIP Conference Proceedings, 3092, 070013.
Joshi, A., & Mishra, A. (2021). Enhanced A5 algorithm for secure audio encryption. In Proceedings (pp. 163–174). Springer Singapore.
Jurecek, M., Bucek, J., & Lorencz, R. (2019). Side-channel attack on the A5/1 stream cipher. In Proceedings of the 22nd Euromicro Conference on Digital System Design (DSD) (pp. 633–638). IEEE.
Koch, M., Nettersheim, C., Horstmann, T., & Rademacher, M. (2025). A5/1 is in the air: Passive detection of 2G (GSM) ciphering algorithms [Preprint]. arXiv.
Li, C., Chen, J., Wang, L., & Shu, Y. (2023). Design of ZUC-256 stream cipher coprocessor for 5G communication security. In Proceedings of the 3rd International Symposium on Computer Technology and Information Science (ISCTIS) (pp. 1122–1127). IEEE.
Mahalanobis, A., & Shah, J. (2014). An improved guess-and-determine attack on the A5/1 stream cipher. Computer and Information Science, 7(1).
Mammeri, Z. (2024). Cryptography: Algorithms, protocols, and standards for computer security. Wiley.
Morgan, D. (2025). Determining the A5 encryption algorithms used in 2G (GSM) networks (pp. 45–61). Springer Nature Switzerland.
Prajapat, R. P., Bhadada, R., & Choudhary, A. (2022). Information security enhancement by increasing randomness of stream ciphers in GSM. International Journal of Advanced Computer Science and Applications, 13(4).
Stallings, W. (2017). Cryptography and network security: Principles and practice (7th ed.). Pearson.
Sýs, M., Říha, Z., Matyáš, V., Márton, K., & Suciu, A. (2015). On the interpretation of results from the NIST statistical test suite. Romanian Journal of Information Science and Technology, 18(1), 18–32.
Turan, M. S., McKay, K., Chang, D., Kang, J., & Kelsey, J. (2025). Ascon-based lightweight cryptography standards for constrained devices (NIST Special Publication 800-232). National Institute of Standards and Technology.
