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40-bit encryption is a key size for symmetric encryption representing where the key is forty bits in length (five bytes); this represents a relatively low level of security. 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 using a moderate amount computing power in a brute force attack — that is 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 email worm), could also break it rapidly. One powerful brute-force machine was the Electronic Frontier Foundation's Deep Crack Data Encryption Standard (DES) cracker, built by a group of enthusiasts for US$200,000 in 1999, albeit with some volunteer programming effort. This machine could break a 56-bit key in days, and would be able to break 40-bit encryption in around four seconds (assuming no hangups in initializing its processors; doing dozens of 40-bit keys on subsets of the machine would perhaps be easier). This machine could have been disproportionately improved with a larger budget by using more efficient chip designs and fabrication techniques, not to mention Moore's law. 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.
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