primefield: use modular sqrt from crypto-bigint (#1676)

Switches to the implementation added in RustCrypto/crypto-bigint#1139

Author: tarcieri

Cargo.lock

@@ -26,3 +26,5 @@ ed448-goldilocks = { path = "ed448-goldilocks" }
 hash2curve = { path = "hash2curve" }
 primefield = { path = "primefield" }
 primeorder = { path = "primeorder" }
+
+crypto-bigint = { git = "https://github.com/RustCrypto/crypto-bigint" }

Cargo.toml

@@ -1,8 +1,6 @@
 //! Field elements which use an internal Montgomery form representation, implemented using
 //! `crypto-bigint`'s [`MontyForm`].
 
-mod sqrt;
-
 use crate::ByteOrder;
 use bigint::{
     ArrayEncoding, ByteArray, Invert, Limb, Reduce, Uint, Word, ctutils,
@@ -444,7 +442,7 @@ where
     }
 
     fn sqrt(&self) -> CtOption<Self> {
-        self.sqrt()
+        self.inner.sqrt().map(|inner| Self { inner }).into()
     }
 
     fn sqrt_ratio(num: &Self, div: &Self) -> (Choice, Self) {