I recently used a custom construction as a commitment scheme, which was taken from the standard picture you give people while explaining commitment schemes.
Basically commitment schemes can be described in terms of physical objects, as "put the information to commit to in a lockable box, give the other person the box and keep the key. As soon as you want to prove that you had the information at the previous time, give him the key to verify".
Now Schneier (in Applied Cryptography 2nd edition p. 86) describes a block cipher based commitment following the exact same principle (with block ciphers). Choose some random value (B does) and let A encrypt it using the key to which A wants to commit, basically exchanging "key" and "content".
Now for the custom scheme:
Suppose you are using an authenticated encryption scheme (e.g. AES-GCM) and you want to commit to a value $R$.
- Encrypt $R$ using a randomly chosen $k$ and choose a random IV and append the tag. ($=IV||C||tag$)
- Send $k$ to the verifier at the point of verification.
So the question:
Is authenticated encryption the computer equivalent to a lockable box (in this context)? or equivalently: Can the above commitment scheme be broken (and if yes, how?)
For me it looks like the above question is equivalent to:
Given a ciphertext with tag, there's only one key that will produce the correct tag with the given ciphertext, meaning the corresponding plaintext can't be altered.