I need to use a hash function to generate a 128-bit key for a symmetric cipher. The specific cipher is from the eStream portofolio, called Rabbit.
I am using the SRP protocol for authentication (a type of EKE protocol). As a result of this, both parties will calculate the same large integer value and need to hash it into a key they can both use.
I would like to use SHA256 to generate the key, but its output is twice the length required.
Can I just truncate the hash to the lower 128 bits or something?
What is usually done in this case?