Add (not particularly efficient) from_str to PrimeField.

src/bls12_381/fq.rs

@@ -1766,6 +1766,7 @@ fn fq_field_tests() {
     ::tests::field::random_field_tests::<Fq>();
     ::tests::field::random_sqrt_tests::<Fq>();
     ::tests::field::random_frobenius_tests::<Fq, _>(Fq::char(), 13);
+    ::tests::field::from_str_tests::<Fq>();
 }
 
 #[test]

src/bls12_381/fr.rs

@@ -1478,6 +1478,7 @@ fn fr_field_tests() {
     ::tests::field::random_field_tests::<Fr>();
     ::tests::field::random_sqrt_tests::<Fr>();
     ::tests::field::random_frobenius_tests::<Fr, _>(Fr::char(), 13);
+    ::tests::field::from_str_tests::<Fr>();
 }
 
 #[test]

src/lib.rs

@@ -477,6 +477,46 @@ pub trait PrimeField: Field
     /// representation.
     type Repr: PrimeFieldRepr + From<Self>;
 
+    /// Interpret a string of numbers as a (congruent) prime field element.
+    /// Does not accept unnecessary leading zeroes or a blank string.
+    fn from_str(s: &str) -> Option<Self> {
+        if s.len() == 0 {
+            return None;
+        }
+
+        if s == "0" {
+            return Some(Self::zero());
+        }
+
+        let mut res = Self::zero();
+
+        let ten = Self::from_repr(Self::Repr::from(10)).unwrap();
+
+        let mut first_digit = true;
+
+        for c in s.chars() {
+            match c.to_digit(10) {
+                Some(c) => {
+                    if first_digit {
+                        if c == 0 {
+                            return None;
+                        }
+
+                        first_digit = false;
+                    }
+
+                    res.mul_assign(&ten);
+                    res.add_assign(&Self::from_repr(Self::Repr::from(c as u64)).unwrap());
+                },
+                None => {
+                    return None;
+                }
+            }
+        }
+
+        Some(res)
+    }
+
     /// Convert this prime field element into a biginteger representation.
     fn from_repr(Self::Repr) -> Result<Self, PrimeFieldDecodingError>;
 

src/tests/field.rs

@@ -1,5 +1,5 @@
 use rand::{Rng, SeedableRng, XorShiftRng};
-use ::{SqrtField, Field};
+use ::{SqrtField, Field, PrimeField};
 
 pub fn random_frobenius_tests<F: Field, C: AsRef<[u64]>>(characteristic: C, maxpower: usize) {
     let mut rng = XorShiftRng::from_seed([0x5dbe6259, 0x8d313d76, 0x3237db17, 0xe5bc0654]);
@@ -87,6 +87,40 @@ pub fn random_field_tests<F: Field>() {
     }
 }
 
+pub fn from_str_tests<F: PrimeField>() {
+    {
+        let a = "84395729384759238745923745892374598234705297301958723458712394587103249587213984572934750213947582345792304758273458972349582734958273495872304598234";
+        let b = "38495729084572938457298347502349857029384609283450692834058293405982304598230458230495820394850293845098234059823049582309485203948502938452093482039";
+        let c = "3248875134290623212325429203829831876024364170316860259933542844758450336418538569901990710701240661702808867062612075657861768196242274635305077449545396068598317421057721935408562373834079015873933065667961469731886739181625866970316226171512545167081793907058686908697431878454091011239990119126";
+
+        let mut a = F::from_str(a).unwrap();
+        let b = F::from_str(b).unwrap();
+        let c = F::from_str(c).unwrap();
+
+        a.mul_assign(&b);
+
+        assert_eq!(a, c);
+    }
+
+    {
+        let mut rng = XorShiftRng::from_seed([0x5dbe6259, 0x8d313d76, 0x3237db17, 0xe5bc0654]);
+
+        for _ in 0..1000 {
+            let n: u64 = rng.gen();
+
+            let a = F::from_str(&format!("{}", n)).unwrap();
+            let b = F::from_repr(n.into()).unwrap();
+
+            assert_eq!(a, b);
+        }
+    }
+
+    assert!(F::from_str("").is_none());
+    assert!(F::from_str("0").unwrap().is_zero());
+    assert!(F::from_str("00").is_none());
+    assert!(F::from_str("00000000000").is_none());
+}
+
 fn random_multiplication_tests<F: Field, R: Rng>(rng: &mut R) {
     for _ in 0..10000 {
         let a = F::rand(rng);