SHA-512 Length Extension Attack Question

Hi,

I'm using SHA-512 to have another party send me the SHA-512 now to be able to prove in the future that they actually had Computer File X with certain contents.

This is very similar to the "Alice Solves Hard Math Problem ..." example here:

http://en.wikipedia.org/wiki/Cryptographic_hash_function#Applications

Just a quick question:

A "length extension attack" only implies that given h(m) (but not m), an attacker may be able to calculate h(m||m') for an m' that the attacker chooses, right?

So, a "length extension attack" would seem to not apply to the "Alice solves hard math problem ... " example and to my application, right?

This attack wouldn't allow someone to find a different file with the same h(m), right?

Thanks, just being sure I understand it correctly.

Datesfat

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