request: Fr::from_bytes() and Fr::to_bytes()

[dan-da]

Even though Fr is really just an [u64; 4] internally, I don't presently see an easy way to go to/from bytes.

Can we add this please?

[iancoleman]

There's the convert functions, if that helps?

blsttc/src/convert.rs

Lines 10 to 37 in 94a71e6

	/// Convert big endian bytes to bls12_381::Fr
	pub fn fr_from_be_bytes(bytes: [u8; SK_SIZE]) -> FromBytesResult<Fr> {
	let mut le_bytes = bytes;
	le_bytes.reverse();
	let mut fr_u64s = [0u64; 4];
	for i in 0..4 {
	let mut next_u64_bytes = [0u8; 8];
	for j in 0..8 {
	next_u64_bytes[j] = le_bytes[i * 8 + j];
	}
	fr_u64s[i] = u64::from_le_bytes(next_u64_bytes);
	}
	Ok(Fr::from_repr(FrRepr(fr_u64s))?)
	}
	/// Convert bls12_381::Fr to big endian bytes
	pub fn fr_to_be_bytes(fr: Fr) -> [u8; SK_SIZE] {
	let mut bytes = [0u8; SK_SIZE];
	// iterating 4 u64s which are in order suiting little endian bytes
	// and must be reversed to get big endian bytes
	fr.into_repr().0.iter().enumerate().for_each(|(le_i, n)| {
	let be_i = 4 - le_i - 1;
	n.to_be_bytes().iter().enumerate().for_each(|(j, byte)| {
	bytes[8 * be_i + j] = *byte;
	});
	});
	bytes
	}

[dan-da]

thx, I missed / forgot that. definitely helps me right now.

Still, the reason I didn't see it is because I was looking all over Fr and FrRepr in the docs and didn't see anything about bytes. Since other types have ::from_bytes() and ::to_bytes() methods I thought Fr would/should also.