impl Absorb for String (#138)

* impl Absorb for String

* add String from ark_std

* absorb length when converting String to sponge bytes

* add test to ensure that absorbing two strings is different from concat

crypto-primitives/src/sponge/absorb.rs

@@ -7,6 +7,7 @@ use ark_ec::{
 use ark_ff::models::{Fp, FpConfig};
 use ark_ff::{BigInteger, Field, PrimeField, ToConstraintField};
 use ark_serialize::CanonicalSerialize;
+use ark_std::string::String;
 use ark_std::vec::Vec;
 
 pub use ark_crypto_primitives_macros::*;
@@ -229,6 +230,17 @@ impl Absorb for isize {
     }
 }
 
+impl Absorb for String {
+    fn to_sponge_bytes(&self, dest: &mut Vec<u8>) {
+        self.len().to_sponge_bytes(dest);
+        dest.extend_from_slice(self.as_bytes())
+    }
+
+    fn to_sponge_field_elements<F: PrimeField>(&self, dest: &mut Vec<F>) {
+        self.as_bytes().to_sponge_field_elements(dest)
+    }
+}
+
 impl<P: TEModelParameters> Absorb for TEAffine<P>
 where
     P::BaseField: ToConstraintField<<P::BaseField as Field>::BasePrimeField>,
@@ -456,4 +468,31 @@ mod tests {
         let out_manual = sponge.squeeze_bytes(32);
         assert_eq!(out_derived, out_manual);
     }
+
+    #[test]
+    fn test_string_absort() {
+        // absorbing two strings should not be the same as absorbing the concatenated string
+        let s1 = "hello".to_string();
+        let s2 = "world".to_string();
+
+        let mut dest1 = Vec::new();
+        s1.to_sponge_bytes(&mut dest1);
+        s2.to_sponge_bytes(&mut dest1);
+
+        let mut dest2 = Vec::new();
+        let s3 = "helloworld".to_string();
+        s3.to_sponge_bytes(&mut dest2);
+
+        assert_ne!(dest1, dest2);
+
+        // Same for converting to sponge field elements
+        let mut dest1: Vec<Fr> = Vec::new();
+        s1.to_sponge_field_elements(&mut dest1);
+        s2.to_sponge_field_elements(&mut dest1);
+
+        let mut dest2 = Vec::new();
+        s3.to_sponge_field_elements(&mut dest2);
+
+        assert_ne!(dest1, dest2);
+    }
 }