Remove BigInt dependency

src/symmetric_crypto/nonce.rs

@@ -8,7 +8,6 @@ use core::{
     convert::{TryFrom, TryInto},
     fmt::{Debug, Display},
 };
-use num_bigint::BigUint;
 use rand_core::{CryptoRng, RngCore};
 
 /// Trait defining a nonce for use in a symmetric encryption scheme.
@@ -26,7 +25,7 @@ pub trait NonceTrait: Send + Sync + Sized + Clone {
 
     /// Increment the nonce by the given value.
     #[must_use]
-    fn increment(&self, increment: usize) -> Self;
+    fn increment(&self, increment: u64) -> Self;
 
     /// Xor the nonce with the given value.
     #[must_use]
@@ -62,12 +61,26 @@ impl<const NONCE_LENGTH: usize> NonceTrait for Nonce<NONCE_LENGTH> {
     }
 
     #[inline]
-    fn increment(&self, increment: usize) -> Self {
-        let mut bi = BigUint::from_bytes_le(&self.0);
-        bi += BigUint::from(increment);
-        let mut bi_bytes = bi.to_bytes_le();
-        bi_bytes.resize(NONCE_LENGTH, 0);
-        Self(bi_bytes.try_into().expect("This should never happen"))
+    fn increment(&self, increment: u64) -> Self {
+        let increment = increment.to_le_bytes();
+        assert!(NONCE_LENGTH > 8, "Consider using a longer Nonce!");
+        // add the first bytes
+        let mut res = [0; NONCE_LENGTH];
+        let mut carry = 0;
+        for (i, (b1, b2)) in self.0.iter().zip(increment).enumerate() {
+            (res[i], carry) = adc(*b1, b2, carry);
+        }
+        // take into account the potentially remaining carry
+        res[increment.len()] = self.0[8] + carry;
+        // copy the rest of the input nonce
+        for (res, b) in res
+            .iter_mut()
+            .rev()
+            .zip(self.0.iter().rev().take(NONCE_LENGTH - 7))
+        {
+            *res = *b;
+        }
+        Self(res)
     }
 
     #[inline]
@@ -85,6 +98,12 @@ impl<const NONCE_LENGTH: usize> NonceTrait for Nonce<NONCE_LENGTH> {
     }
 }
 
+#[inline]
+const fn adc(a: u8, b: u8, carry: u8) -> (u8, u8) {
+    let ret = (a as u16) + (b as u16) + (carry as u16);
+    (ret as u8, (ret >> 8) as u8)
+}
+
 impl<'a, const NONCE_LENGTH: usize> TryFrom<&'a [u8]> for Nonce<NONCE_LENGTH> {
     type Error = CryptoCoreError;
 
@@ -100,7 +119,7 @@ impl<const NONCE_LENGTH: usize> From<[u8; NONCE_LENGTH]> for Nonce<NONCE_LENGTH>
 }
 
 impl<const NONCE_LENGTH: usize> Display for Nonce<NONCE_LENGTH> {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "{}", hex::encode(self.0))
     }
 }
@@ -128,7 +147,7 @@ mod tests {
     fn test_increment_nonce() {
         const NONCE_LENGTH: usize = 12;
         let mut nonce: Nonce<NONCE_LENGTH> = Nonce::from([0_u8; NONCE_LENGTH]);
-        let inc = 1_usize << 10;
+        let inc = 1 << 10;
         nonce = nonce.increment(inc);
         println!("{}", hex::encode(nonce.0));
         assert_eq!("000400000000000000000000", hex::encode(nonce.0));