40-bit encryption is a key size for symmetric encryption representing a low-level of security where the key is forty bits in length (five bytes). Forty bits can represent a total of 240 possible keys. Although this is a large number (about a trillion, and nearly two hundred times the world's human population), it is possible to break this degree of encryption quite rapidly by using computing power in a brute force attack — trying out each possible key in turn.
With dedicated (and rather expensive) hardware, a 40_bit key might be broken in seconds. On a typical home computer, it might take a little longer — on the order of weeks. A large corporate network, or a set of home PCs subverted by a Trojan horse (say, by employing an emailworm), could also break it rapidly. One powerful brute-force machine was the Electronic Frontier Foundation's Deep CrackData Encryption Standard (DES) cracker, built by a group of enthusiasts for US$250,000 in 1999. This machine could break a 56-bit key in days, and would be able to break 40-bit encryption in around four seconds. In the light of this, it is difficult to recommend 40-bit encryption for any serious cryptographic use given the ability of today's computers to defeat it.
40-bit encryption was common in software when algorithms with larger key lengths could not legally be exported from the United States. Examples include web browsers (for secure e-commerce) as well as dedicated software packages. 40-bit encryption is now considered badly outdated. Some web servers will not communicate with a client which does not implement 128-bit encryption.
However, note that use of 128-bit keys does not in itself provide security: it is merely a requirement that, to avoid vulnerability to brute force key space search, the key size must be large enough to make brute force search too expensive, or too time consuming, to be feasible. High quality algorithms are required as well, for there are easily breakable ones which employ large keys. It should also be noted that asymmetric encryption requires keys far longer than 128 bits; see that article, and key size, for more details.
The bit rate of the radio channel is 270.833 kbit/sec, which corresponds to a timeslot duration of 156.25 bits.
Two bits from the 116-bit payload are used by the Fast Associated Control Channel (FACCH) to signal that a given burst has been borrowed, leaving a total of 114 bits of payload.
These 260 bits are divided into 182 class 1 and 78 class 2 bits based on a subjective evaluation of their sensitivity to bit errors, with the class 1 bits being the most sensitive.
Feeds |
Contact