Why SHA-224 and SHA-256 use different initial values?

Question

Wikipedia - SHA-2 says

SHA-224 is identical to SHA-256, except that:

• the initial variable values h0 through h7 are different, and
• the output is constructed by omitting h7.

RFC3874 - A 224-bit One-way Hash Function: SHA-224 says

The use of a different initial value ensures that a truncated SHA-256 message digest value cannot be mistaken for a SHA-224 message digest value computed on the same data.

My questions:

1. Is the above quoted reason the only reason why SHA-224 and SHA-256 use different initial values?

2. Why is it important to make sure that a SHA-256 message digest value cannot be mistaken for a SHA-224 message digest value?

3. If we use the same initial values for both hash functions, will the security of either hash function worsen? If yes, how?

Answer 1

1. This We called Domain Separation, when we use same algorithm for different output size.
2. Separation is necessary because if i found two messages which have hash value (SH256), differs only in last octet and then i can publish the hash value as first 7 octet showing i used SHA224. since i already have two messages colliding on SHA224 which i can use later for forgery attack. Using domain separation we can avoid such situation