# MDC-2 recommended IV values?

I'm currently reading Menezes' "Handbook of Applied Cryptography", and in it, when describing the MDC-2 cryptographic hash function, has the following line:

Choose the 64-bit non-secret constants $IV$, $\widehat{IV}$ (the same constants must be used for MDC verification) from a set of recommended prescribed values. A default set of prescribed values is (in hexadecimal): $IV = 0x5252525252525252, \widehat{IV} = 0x2525252525252525$.

My questions are firstly, is it OK to use these "default" values more than once (as implied by the word "default")? And secondly, is there a full list and reasoning for these prescribed values? I cannot find the MDC-2 specification and so was hoping someone here would know more about it.

## 1 Answer

You can use those values more than once and there isn't much of a reason to choose another pair – except longer values for a cipher with a larger block size.

The only real requirement for the values is that they differ. However, if someone found a fixed point or other cycle for MDC-2 with a given block cipher, they could choose that point as an IV and be able to generate collisions for any string. That's why using a pair of non-suspicious nothing-up-my-sleeve numbers is better.