You and your friend who lives far away want to fairly and randomly select which of the two of you will travel to the other’s home for a visit. Your friend remembers having learned about using hashes as cryptographic commitments and proposes to use a coin toss to decide. He tells you to randomly choose ‘heads’ or ‘tails’ by throwing a coin, hash the result using sha256, and send him the hash. Once you have committed to the outcome, he’ll guess ‘heads’ or ‘tails’, you’ll reveal to him your choice and he can verify that the hash is correct. If his choice matches your coin toss, then your friend wins and you have to make the trip. What is the problem with this scheme, as described?

a. there’s no way to prove that your coin toss is random
b. there’s no way to prove that sha256 is collision resistant
c. your friend can hash all possible options and discover your secret
d. unless you sign the hash message your friend can’t be sure it was you who sent it