Add no-std rand implementation for fields

Cargo.toml

@@ -12,6 +12,7 @@ license = "MIT OR Apache-2.0"
 # no-alloc, no-std
 cfg-if = "1.0"
 hex = { version ="=0.4.3", default-features = false }
+rand_core = "0.6"
 subtle = { version="2.5", default-features = false }
 tracing-subscriber = { version = "0.3", default-features = false }
 zeroize = { version = "1.7", default-features = false }

src/fields/fp.rs

@@ -1,7 +1,5 @@
-// Fiat-crypto generates some unused type aliases, but we don't want to edit the generated code at all.
-#![allow(dead_code)]
-
 use cfg_if::cfg_if;
+use rand_core::CryptoRngCore;
 
 use crate::EncodingError;
 
@@ -115,7 +113,6 @@ impl Fp {
             }) // let acc =
     }
 
-    ///
     /// Convert bytes into an Fp element, returning None if these bytes are not already reduced.
     ///
     /// This means that values that cannot be produced by encoding a field element will return
@@ -132,6 +129,17 @@ impl Fp {
     pub fn to_bytes(&self) -> [u8; N_8] {
         self.to_bytes_le()
     }
+
+    /// Sample a random field element uniformly.
+    pub fn rand<R: CryptoRngCore>(rng: &mut R) -> Self {
+        // Sample wide, reduce
+        let bytes = {
+            let mut out = [0u8; N_8 + 16];
+            rng.fill_bytes(&mut out);
+            out
+        };
+        Self::from_le_bytes_mod_order(&bytes)
+    }
 }
 
 #[cfg(test)]

src/fields/fq.rs

@@ -1,7 +1,5 @@
-// Fiat-crypto generates some unused type aliases, but we don't want to edit the generated code at all.
-#![allow(dead_code)]
-
 use cfg_if::cfg_if;
+use rand_core::CryptoRngCore;
 
 use crate::EncodingError;
 
@@ -116,6 +114,17 @@ impl Fq {
     pub fn to_bytes(&self) -> [u8; N_8] {
         self.to_bytes_le()
     }
+
+    /// Sample a random field element uniformly.
+    pub fn rand<R: CryptoRngCore>(rng: &mut R) -> Self {
+        // Sample wide, reduce
+        let bytes = {
+            let mut out = [0u8; N_8 + 16];
+            rng.fill_bytes(&mut out);
+            out
+        };
+        Self::from_le_bytes_mod_order(&bytes)
+    }
 }
 
 #[cfg(test)]

src/fields/fr.rs

@@ -1,7 +1,5 @@
-// Fiat-crypto generates some unused type aliases, but we don't want to edit the generated code at all.
-#![allow(dead_code)]
-
 use cfg_if::cfg_if;
+use rand_core::CryptoRngCore;
 
 use crate::EncodingError;
 
@@ -108,6 +106,17 @@ impl Fr {
     pub fn to_bytes(&self) -> [u8; N_8] {
         self.to_bytes_le()
     }
+
+    /// Sample a random field element uniformly.
+    pub fn rand<R: CryptoRngCore>(rng: &mut R) -> Self {
+        // Sample wide, reduce
+        let bytes = {
+            let mut out = [0u8; N_8 + 16];
+            rng.fill_bytes(&mut out);
+            out
+        };
+        Self::from_le_bytes_mod_order(&bytes)
+    }
 }
 
 #[cfg(test)]