docs: document behavior of hash function inside and outside a circuit

[gbotrel]

See #99 and here.

In short, when using MiMC inside a circuit, each input is treated as a field element on size(element) bytes.

Example

To mimic this the following (inside a circuit):

hash.Write([]byte{0x01})
hash.Write([]byte{0x02})

outside a circuit, one needs to add padding to the byte slices. The easiest way is to use fr.Element objects.

var b1, b2 fr.Element
b1.SetBytes([]byte{0x01})
b2.SetBytes([]byte{0x02})
hash.Write(b1.Marshal()) // writes size(element) bytes, like in a circuit, NOT a single byte.
hash.Write(b1.Marshal()) // writes size(element) bytes, like in a circuit, NOT a single byte.

[xenowits]

Hi @gbotrel . I'm looking to contribute to this repo. Can I take up one of these issues??

[gbotrel]

hi @xenowits -- sure, any contribution is welcome / appreciated :-) . Are you looking for something more dev? more doc? more crypto? Few hours work, or few days? (we've several things in mind that may not appear in the github backlog)

[xenowits]

Something more crypto would work @gbotrel . I actually want to learn about zksnarks in general. I can take up the easier ones first, u can assign me those

[gbotrel]

3 ideas come to mind, of independent tasks (crypto);

• SSWU hash to curve implementation in gnark-crypto
• adding new components in gnark/std; for example, we have a MiMC hash function available to use in a circuit, it could be interesting to add Poseidon hash function. That would likely involve implementing Poseidon in gnark-crypto and adding a circuit component under gnark/std .
• a PLONK solidity verifier

more software oriented, would be:

• #70 (find elegant ways to generate "type safe" witness in non-Go code bases)
• or explore any of the blockers that may arise with #74 #76 #75 (using gnark in WASM, iOS or Android).

more exploratory;

• add a (Groth16) batch-verify algorithm, from this

[xenowits]

@gbotrel can we connect on discord??

[gbotrel]

mhhh prefer email; can you reach out at our team email?