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//! Traits for hashing to field elements.
//!
//! <https://datatracker.ietf.org/doc/draft-irtf-cfrg-hash-to-curve>
mod expand_msg;
pub use expand_msg::{xmd::*, xof::*, *};
use crate::Result;
use generic_array::{typenum::Unsigned, ArrayLength, GenericArray};
/// The trait for helping to convert to a field element.
pub trait FromOkm {
/// The number of bytes needed to convert to a field element.
type Length: ArrayLength<u8>;
/// Convert a byte sequence into a field element.
fn from_okm(data: &GenericArray<u8, Self::Length>) -> Self;
}
/// Convert an arbitrary byte sequence into a field element.
///
/// <https://tools.ietf.org/html/draft-irtf-cfrg-hash-to-curve-11#section-5.3>
///
/// # Errors
/// See implementors of [`ExpandMsg`] for errors:
/// - [`ExpandMsgXmd`]
/// - [`ExpandMsgXof`]
///
/// `len_in_bytes = T::Length * out.len()`
///
/// [`ExpandMsgXmd`]: crate::hash2field::ExpandMsgXmd
/// [`ExpandMsgXof`]: crate::hash2field::ExpandMsgXof
#[doc(hidden)]
pub fn hash_to_field<'a, E, T>(data: &[&[u8]], domain: &'a [u8], out: &mut [T]) -> Result<()>
where
E: ExpandMsg<'a>,
T: FromOkm + Default,
{
let len_in_bytes = T::Length::to_usize() * out.len();
let mut tmp = GenericArray::<u8, <T as FromOkm>::Length>::default();
let mut expander = E::expand_message(data, domain, len_in_bytes)?;
for o in out.iter_mut() {
expander.fill_bytes(&mut tmp);
*o = T::from_okm(&tmp);
}
Ok(())
}
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