Add Merlin, squeeze_bits does not work

Cargo.toml

@@ -25,6 +25,7 @@ ark-serialize = { version = "^0.4.0", default-features = false, features = [ "de
 blake2 = { version = "0.10", default-features = false }
 sha2 = { version = "0.10", default-features = false }
 digest = { version = "0.10", default-features = false }
+merlin = { version = "3.0.0", default-features = false }
 
 ark-r1cs-std = { version = "^0.4.0", optional = true, default-features = false }
 ark-snark = { version = "^0.4.0", default-features = false }

src/sponge/merlin/mod.rs

@@ -0,0 +1,31 @@
+use crate::sponge::{Absorb, CryptographicSponge};
+use merlin::Transcript;
+
+impl CryptographicSponge for Transcript {
+    type Config = &'static [u8];
+
+    fn new(params: &Self::Config) -> Self {
+        Transcript::new(*params)
+    }
+
+    fn absorb(&mut self, input: &impl Absorb) {
+        self.append_message(b"", &input.to_sponge_bytes_as_vec());
+    }
+
+    fn squeeze_bytes(&mut self, num_bytes: usize) -> Vec<u8> {
+        let mut dest = Vec::with_capacity(num_bytes);
+        self.challenge_bytes(b"", &mut dest);
+        dest
+    }
+
+    fn squeeze_bits(&mut self, num_bits: usize) -> Vec<bool> {
+        let num_bytes = (num_bits + 7) / 8;
+        let mut tmp = Vec::with_capacity(num_bytes);
+        self.challenge_bytes(b"", &mut tmp);
+        let dest = tmp
+            .iter()
+            .flat_map(|byte| (0..8u32).rev().map(move |i| (byte >> i) & 1 == 1))
+            .collect::<Vec<_>>();
+        dest[..num_bits].to_vec()
+    }
+}

src/sponge/mod.rs

@@ -17,6 +17,11 @@ pub use absorb::*;
 /// [cos]: https://eprint.iacr.org/2019/1076
 pub mod poseidon;
 
+/// The sponge for Merlin
+///
+///
+pub mod merlin;
+
 #[cfg(test)]
 mod test;