How to implement a simple hashlock contract with initial commit of a sha256 hash of a password string

Question

Im trying to implement a simple hashlock contract where I initially commit a sha256 hash of a password string and an address of the target receiver.

The contract compiles and tests work (since for tests I am using the smartpy hashing procedures). But once I deploy, I can't seem to get the reveal to work since the password string that I am passing does not result in the same hash.

How should I generate my initial hash of the string so that once the actual string is passed, the contract computes same hash?

So far I am using just a generic sha256 online generator. But I feel like smartpy sp.sha256(sp.pack()) procedure results in something I am unable to replicate outside.

What am i doing wrong? Is the issue in encoding of the sha256? What encoding does smartpy use? Can I specify what it uses? Code itself is here: Link

For example, I am trying to replicate the sp.pack("password") When i convert a string "password" into hex I get 70617373776f7264 But when I pass 70617373776f7264 into sha256 generator, I don't get the same hash as sp.sha256(sp.pack("password"))

So could someone clarify the correct procedure to replicate the steps?

Answer 1

You have to do the pack before compute the sha256.

If you do

scenario.show(sp.pack("test"))
scenario.show(sp.sha256(sp.pack("test")))

It results in 0x05010000000474657374 and 0x5cedc49191ca9f6a0af917b23dd897c11b125970af57bd6c8f033edccf913e68.

If you use compute the sha256, of the first result, you obtain the second. Pay attention that sp.pack(...) is an hex value, not a string.

Answer 2

The sp.pack adds a prefix bytes sequence that essentially says "this is a string". You can see it by doing scenario.show(sp.pack("password")) (output in log.txt). Instead of sp.pack you want sp.utils.bytes_of_string. Putting everything together, scenario.show(sp.sha256(sp.utils.bytes_of_string("password"))) indeed prints the SHA256 of "password".