how to get the preimage size for implementing sha256

[zhfnjust]

hello , I am implementing sha256.

before that i should know the preimage size to do the padding.

  export class Sha256 extends Circuit {
    @circuitMain
    static main(preimage: Field, @public_ hash: Field) {

      let size = preimage.toBits().length

    }
  }

but when calling toBits().length, it returns 255 all the tme.

so any idea to get the preimage size?

[mitschabaude]

Is there anything wrong with using 255? That's the (max) bit length of a Field. (Sorry I'm not familiar with the sha256 algorithm)

[zhfnjust]

sha256 algorithm need to append some padding bytes according the preimage length. but in this case I have no way of knowing the real size of the preimage。

[zhfnjust]

I also need to add a parameter preimageSize: Field to indicate.

[mitschabaude]

You could do that, yeah! But I still don't get what's wrong with always using 255 as the length.

EDIT: Is it a problem for sha256 if some significant bits of that 255 bits are zero? (this would surprise me, to be honest..)

[zhfnjust]

another question:

if i use slice() on array when writing a circuit, it crash right?

[mitschabaude]

if i use slice() on array when writing a circuit, it crash right?

no! the code inside a circuit is normal JavaScript. Normal JS doesn't crash when you use slice on an array, except when that array is undefined or something like that.

[mitschabaude]

Closing this issue, let's continue debugging in discord