Quoted from the aircrack-ng tutorial:
Next, we look at cracking WEP with a dictionary. In order to do this, we need dictionary files with ascii or hexadecimal keys to try. Remember, a single file can only have ascii or hexadecimal keys in it, not both.
Example 64 bit hexadecimal key: “12:34:56:78:90” (Note the ”:” between each two characters.)