WEP encryption (Wired Equivalent Privacy, or Wired Equivalent Privacy) is a cipher, implemented in the 802.11 wireless connection protocol, which encrypts the information we will pass between two points so that it can only be access to them and interpret them the points that have the same key.
In general, a wireless router or Access Point will only allow access to those terminals that have the same WEP encryption key.
This key can be of three types:64-bit WEP key .-, 5 characters or 10 hexadecimal digits (0 to 9″”””A to F preceded by the string”0x”).128-bit WEP key .-, 13 characters or 26 hexadecimal digits (0 to 9″”””A to F preceded by the string”0x”).256-bit WEP key .-, 29 characters or 58 hexadecimal digits (0 to 9″”””A to F preceded by the string”0x”).
The one most often used is 128 bit, offering a good level of protection without being overly long and complicated.
The 256-bit WEP encryption is not supported by many devices.

A WEP key can decipher (there are programs for it), but this requires an uninterrupted traffic data for a given time (in fact, enough data and enough time).
Obviously, the higher the level of encryption and key more complicated the more difficult it will be to decipher.
It does not take the same (given data volume and time) to crack the WEP encryption key of 64 bits of 128 bits, and there are also no relationship between arithmetic, ie it does not take twice as crack a WEP key of 128 bits a 64-bit.
Although it is possible to crack these encryption keys, do not think that is easy or quick. A good key 128-bit WEP encryption (if not a 256-bit) can be almost unreadable if we have ensured that is sufficiently complicated.
Most key-cracking programs are based on a series of more or less logical sequence with which it begins to attack our system to enter. Obviously, a key rate takes seconds 1234567890 be located, but no one thinks (or should it occur) to this key.
We should avoid keys that contain sequences related to us (dates, names, places), and phrases, as it is the first thing you try this type of program. This is not only valid for a WEP key, but for any kind of key you say. We should also avoid key easy as consecutive sequences of keys or numbers.
For safety it is strongly recommended wherever possible to enable MAC address filtering. An also read my post on Wifi Security
A MAC (Media Access Control address) is a hexadecimal identifier of 48 bits. This address is unique for each device, not being a user-modifiable parameter (each card or network interface has its own MAC address, set by the manufacturer.)