From 4a89edc74289d7b9e296148d896c8074d1a2027b Mon Sep 17 00:00:00 2001
From: Rodolfo P A <6721075+rodoufu@users.noreply.github.com>
Date: Wed, 13 Dec 2023 08:10:32 -0300
Subject: [PATCH 1/2] Adding mask_bits function

---
 starknet-core/src/crypto.rs | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/starknet-core/src/crypto.rs b/starknet-core/src/crypto.rs
index e703fc24..fe4790a7 100644
--- a/starknet-core/src/crypto.rs
+++ b/starknet-core/src/crypto.rs
@@ -98,6 +98,23 @@ pub fn ecdsa_verify(
     }
 }
 
+pub fn mask_bits(mask: usize, word_size: usize, slice: &mut [u8]) {
+    let mut excess = slice.len() * word_size - mask;
+
+    for v in slice {
+        let by = v;
+        if excess > 0 {
+            if excess > word_size {
+                excess -= word_size;
+                continue;
+            }
+            *by <<= excess;
+            *by >>= excess;
+            excess = 0;
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -226,4 +243,23 @@ mod tests {
 
         assert!(!ecdsa_verify(&public_key, &message_hash, &Signature { r, s }).unwrap());
     }
+
+    #[test]
+    fn test_mask_bits() {
+        // Given
+        let mut to_mask: [u8; 32] = [
+            72, 232, 75, 188, 182, 142, 15, 124, 115, 169, 5, 139, 168, 43, 109, 169, 193, 255,
+            220, 80, 46, 252, 240, 52, 231, 139, 12, 0, 60, 34, 236, 201,
+        ];
+
+        // When
+        mask_bits(250, 8, &mut to_mask[..]);
+        let expected: [u8; 32] = [
+            0, 232, 75, 188, 182, 142, 15, 124, 115, 169, 5, 139, 168, 43, 109, 169, 193, 255, 220,
+            80, 46, 252, 240, 52, 231, 139, 12, 0, 60, 34, 236, 201,
+        ];
+
+        // Then
+        assert_eq!(to_mask, expected);
+    }
 }

From a82c618d0414bcc750f359c4abea4189ea7d7ab0 Mon Sep 17 00:00:00 2001
From: Rodolfo P A <6721075+rodoufu@users.noreply.github.com>
Date: Wed, 13 Dec 2023 08:19:16 -0300
Subject: [PATCH 2/2] Adding comments

---
 starknet-core/src/crypto.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/starknet-core/src/crypto.rs b/starknet-core/src/crypto.rs
index fe4790a7..3b891ca9 100644
--- a/starknet-core/src/crypto.rs
+++ b/starknet-core/src/crypto.rs
@@ -98,6 +98,14 @@ pub fn ecdsa_verify(
     }
 }
 
+/// MaskBits masks the specified (excess) bits in a byte slice.
+///
+/// Parameters:
+/// - mask: is an integer representing the number of bits to mask
+/// - word_size: is an integer representing the number of bits in each element of the slice
+/// - slice: is a byte slice on which the masking operation is performed
+/// Makes the operation in place on slice, gets a new byte slice that contains the masked bits
+/// Porting the MaskBits from https://github.com/NethermindEth/starknet.go/blob/main/curve/utils.go#L128-L143
 pub fn mask_bits(mask: usize, word_size: usize, slice: &mut [u8]) {
     let mut excess = slice.len() * word_size - mask;