[Cryptography] Komitments

[Stephan Neuhaus]

If I understand your scheme, Bob can later claim to have committed to a 
different message.

For example, Bob has the message

s1 = The secret agent is Alice

He chooses some random r1 and computes and publishes w = H(s1 + r1)

When it later turns out that Dave is in fact the secret agent, he takes

s2 = The secret agent is David

and publishes s2 and r2 = s1 + r1 - s2. Since H(s2 + r2) = w = H(s1 + 
r1), Bob can thus "prove" that he knew that Dave had been the secret 
agent all along.

Did I misunderstand something?

You could perhaps save your scheme by using concatenation instead of 
addition. That looks as if it could work.

Cheers

Stephan