Secret vs. Non-secret Initialization Vector

Question

Today I was doing some leisurely reading and stumbled upon Section 5.8 (on page 45) of Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm Crypto

Answer 1

The starting value of the counter in CTR mode encryption can be thought of as an IV. If you make it secret, you end up with some amount of added security over the security granted by the key length of the cipher you're using. How much extra is hard to say, but not knowing it does increase the work required to figure out how to decrypt a given message.