utils: add permutation with a PRG and Fisher-Yates

utils/Cargo.toml

@@ -11,3 +11,4 @@ blake3 = "1.3.3"
 ff = "0.13.0"
 num = "0.4.0"
 rand = "0.8.5"
+rand_chacha = "0.3.1"

utils/src/lib.rs

@@ -1,4 +1,5 @@
 pub mod bit_decompose;
 pub mod fixed_key_aes;
+pub mod permutation;
 pub mod prf;
 pub mod pseudorandom_conversion;

utils/src/permutation.rs

@@ -0,0 +1,65 @@
+use rand::{thread_rng, Rng, SeedableRng};
+use rand_chacha::ChaCha20Rng;
+
+pub trait Permutation {
+    type Key: Copy;
+
+    fn sample(log_domain_size: u32) -> Self::Key;
+    fn from_key(key: Self::Key) -> Self;
+    fn get_key(&self) -> Self::Key;
+    fn get_log_domain_size(&self) -> u32;
+    fn permute(&self, x: usize) -> usize;
+    // fn inverse(&self, x: usize) -> usize;
+    // fn permuted_vector() -> Vec<usize>;
+}
+
+#[derive(Clone, Copy, Debug)]
+pub struct FisherYatesPermutationKey {
+    log_domain_size: u32,
+    prg_seed: [u8; 32],
+}
+
+#[derive(Clone, Debug)]
+pub struct FisherYatesPermutation {
+    key: FisherYatesPermutationKey,
+    permuted_vector: Vec<usize>,
+}
+
+impl Permutation for FisherYatesPermutation {
+    type Key = FisherYatesPermutationKey;
+
+    fn sample(log_domain_size: u32) -> Self::Key {
+        Self::Key {
+            log_domain_size,
+            prg_seed: thread_rng().gen(),
+        }
+    }
+
+    fn from_key(key: Self::Key) -> Self {
+        let mut rng = ChaCha20Rng::from_seed(key.prg_seed);
+        let mut permuted_vector: Vec<usize> = (0..(1 << key.log_domain_size)).collect();
+        // To shuffle an array a of n elements (indices 0..n-1):
+        let n = 1 << key.log_domain_size;
+        for i in (1..n).rev() {
+            let j: usize = rng.gen_range(0..=i);
+            permuted_vector.swap(j, i);
+        }
+        Self {
+            key,
+            permuted_vector,
+        }
+    }
+
+    fn get_key(&self) -> Self::Key {
+        self.key
+    }
+
+    fn get_log_domain_size(&self) -> u32 {
+        self.key.log_domain_size
+    }
+
+    fn permute(&self, x: usize) -> usize {
+        assert!(x < self.permuted_vector.len());
+        self.permuted_vector[x]
+    }
+}