The larger the key, the more security one gets. Since most attempts to decrypt rely on searching through the key space, adding another bit to the key doubles the size of the search space. One can see that large keys will generate large search spaces. 64 bit keys can probably be broken in under 245 hours, 128 bit keys need the development of faster computers to be breakable in a realistic time. However, large keys do slow the speed at which encryption can take place, but not very much. And if speed is essential, one can always buy a faster computer. More importantly, large keys are frowned upon by governments, since it then becomes difficult for government agencies to decrypt all the encrypted traffic flowing over the Internet which is generated by criminals - or at least that is the excuse that governments offer for refusing to allow good encryption.
Instead, one can use key escrow, where a part of your key is held ina secure registry, which only a trusted member of the security agencies can get at it. Thus, when you encrypt, anyone who isn't a member of the government has to decode using the full length of key, but government agencies get a head start by knowing a large portion of your key. In this way, only the government agencies can read your email and listen to you audio calls. Doesn't that make you sleep happier at night?
Next: Public Key Cryptography Up: Symmetric Cryptography Previous: Symmetric Cryptography Jon CROWCROFT