DES takes an 56-bit integer as the key. This could be represented as a decimal integer or hex string. But usually people want to use a password. The password needs to be turned into the key used for encryption. One way is to just use the string as the key. But this limits the size of the password. And makes the key easier to guess. The standard way is to hash the password with MD5 or another one-way hash function and use part of the result as the key.