How do I hash a file of arbitrary size?

Noir
1

I need to prove that a person knew a file with a publicly known blake3 hash. The way I’d like to do it is to provide the file as a private parameter, the blake3 hash as a public parameter, and a signed message as another public parameter (so we know the person signed the message and provided it to the zero-knowledge system along with the file).

How do I deal with hashing a variable-length file? I tried to feel the blake3 function a slice, but it didn’t work.

fn main() {
    let mut slice: [u8] = &[0; 10];
    let hash = std::hash::blake3(slice);
}

The error is:

ori@CryptoDocLaptop:~/noir/hello_world$ nargo execute
warning: unused variable hash
  ┌─ src/main.nr:7:9
  │
7 │     let hash = std::hash::blake3(slice);
  │         ---- unused variable
  │

error: Expected type [u8; _], found type [u8]
  ┌─ src/main.nr:7:34
  │
7 │     let hash = std::hash::blake3(slice);
  │                                  -----
  │

Aborting due to 1 previous error
ori@CryptoDocLaptop:~/noir/hello_world$

All the tricks I tried to get it to work also failed.

Is there a way to do it, or does the application need to compile a new circuit for every file size?

2 Likes
2

To fix the error, the code should use a Vec<u8> for variable-length input data and pass that data into the hashing function. This way, the application can handle files of any size without compiling new circuits for each different file.

3

You actually can’t provide a variable length input to a circuit. It’s a limitation of working with circuits. You would need to generate a new circuit for different sizes. A workaround that people use to reduce the number of circuits needed is to have several circuits of various lengths, and with a given input, use the circuit with the smallest input possible. Then pad the input so that the input size will match what is expected.