Here's my attempt at explanatory comments - the ScriptPubkey is a program (written in Bitcoin Script) which is attached to an output of a Bitcoin transaction -

Here's my attempt at explanatory comments - the ScriptPubkey is a program (written in Bitcoin Script) which is attached to an output of a Bitcoin transaction - parameters must be provided that make it return "true". # Compute the SHA256 of the value on the top of the stack (spender input), # then add the result to the top of the stack OP_SHA256 # Check whether this value is equal to Y, the hash of the key under which # the solution was encrypted. <Y> OP_EQUAL # If the correct key was provided, add the seller's public key to the stack OP_IF <Seller Pubkey> # Otherwise, if it has been 100 blocks or more, add the buyer's public key # to the stack so they may refund themselves if the seller does not complete # the protocol OP_ELSE <block_height+100> OP_CHECKLOCKTIMEVERIFY OP_DROP <Buyer Pubkey> OP_ENDIF # Verify the signature against the public key on the top of the stack OP_CHECKSIG