In 1997, Golic presented an attack based on solving sets of linear equations which has a time complexity of 2 40.16 (the units are in terms of number of solutions of a system of linear equations which are required). In this way the clocking of all three registers is determined and the second half of R3 can be computed. Anderson's basic idea was to guess the complete content of the registers R1 and R2 and about half of the register R3. The first attack on the A5/1 was proposed by Ross Anderson in 1994. The British proposed a key length of 48 bits, while the West Germans wanted stronger encryption to protect against East German spying, so the compromise became a key length of 54 bits. Audestad, Peter van der Arend, and Thomas Haug says that the British insisted on weaker encryption, with Haug saying he was told by the British delegate that this was to allow the British secret service to eavesdrop more easily. It is now believed that 128 bits would in fact also still be secure until the advent of quantum computing. At that time, 128 bits was projected to be secure for at least 15 years. In 2006 Elad Barkan, Eli Biham and Nathan Keller demonstrated attacks against A5/1, A5/3, or even GPRS that allow attackers to tap GSM mobile phone conversations and decrypt them either in real-time, or at any later time.Īccording to professor Jan Arild Audestad, at the standardization process which started in 1982, A5/1 was originally proposed to have a key length of 128 bits. In 2003, more serious weaknesses were identified which can be exploited in the ciphertext-only scenario, or by an active attacker. Originally, the weaknesses were passive attacks using the known plaintext assumption. Some attacks require an expensive preprocessing stage after which the cipher can be broken in minutes or seconds. Security The message on the screen of a mobile phone with the warning about lack of cipheringĪ number of attacks on A5/1 have been published, and the American National Security Agency is able to routinely decrypt A5/1 messages according to released internal documents. After this is completed, the cipher is ready to produce two 114 bit sequences of output keystream, first 114 for downlink, last 114 for uplink. Then the entire system is clocked using the normal majority clocking mechanism for 100 cycles, with the output discarded. Similarly, the 22-bits of the frame number are added in 22 cycles. The three shift registers are specified as follows: When operating in GPRS / EDGE mode, higher bandwidth radio modulation allows for larger 348 bits frames, and A5/3 is then used in a stream cipher mode to maintain confidentiality.Ī5/1 is based around a combination of three linear-feedback shift registers (LFSRs) with irregular clocking. This weakness was rectified with the introduction of Comp128v3 which yields proper 64 bits keys. Older fielded GSM implementations using Comp128v1 for key generation, had 10 of the key bits fixed at zero, resulting in an effective key length of 54 bits. A5/1 is initialised using a 64-bit key together with a publicly known 22-bit frame number. A5/1 is used to produce for each burst a 114 bit sequence of keystream which is XORed with the 114 bits prior to modulation. In a typical channel and in one direction, one burst is sent every 4.615 milliseconds and contains 114 bits available for information. A register is clocked if its clocking bit (orange) agrees with the clocking bit of one or both of the other two registers.Ī GSM transmission is organised as sequences of bursts. The Germans said it should be, as they shared a long border with the Warsaw Pact but the other countries didn't feel this way, and the algorithm as now fielded is a French design." Description The A5/1 stream cipher uses three LFSRs. Security researcher Ross Anderson reported in 1994 that "there was a terrific row between the NATO signal intelligence agencies in the mid-1980s over whether GSM encryption should be strong or not. In 2000, around 130 million GSM customers relied on A5/1 to protect the confidentiality of their voice communications. Though both were initially kept secret, the general design was leaked in 1994 and the algorithms were entirely reverse engineered in 1999 by Marc Briceno from a GSM telephone. A5/1 was developed in 1987, when GSM was not yet considered for use outside Europe, and A5/2 was developed in 1989. A5/2 was a deliberate weakening of the algorithm for certain export regions. A number of serious weaknesses in the cipher have been identified.Ī5/1 is used in Europe and the United States. It was initially kept secret, but became public knowledge through leaks and reverse engineering. It is one of several implementations of the A5 security protocol. A5/1 is a stream cipher used to provide over-the-air communication privacy in the GSM cellular telephone standard.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |