Add padding check for ANSI X.923

All padding bytes must be 0.

3 files changed, 28 insertions, 1 deletions

diff --git a/src/_cffi_src/hazmat_src/padding.c b/src/_cffi_src/hazmat_src/padding.c
index 570bad9f..1a0c869d 100644
--- a/src/_cffi_src/hazmat_src/padding.c
+++ b/src/_cffi_src/hazmat_src/padding.c
@@ -37,3 +37,27 @@ uint8_t Cryptography_check_pkcs7_padding(const uint8_t *data,
     /* Now check the low bit to see if it's set */
     return (mismatch & 1) == 0;
 }
+
+uint8_t Cryptography_check_ansix923_padding(const uint8_t *data,
+                                            uint8_t block_len) {
+    uint8_t i;
+    uint8_t pad_size = data[block_len - 1];
+    uint8_t mismatch = 0;
+    /* Skip the first one with the pad size */
+    for (i = 1; i < block_len; i++) {
+        unsigned int mask = Cryptography_constant_time_lt(i, pad_size);
+        uint8_t b = data[block_len - 1 - i];
+        mismatch |= (mask & b);
+    }
+
+    /* Check to make sure the pad_size was within the valid range. */
+    mismatch |= ~Cryptography_constant_time_lt(0, pad_size);
+    mismatch |= Cryptography_constant_time_lt(block_len, pad_size);
+
+    /* Make sure any bits set are copied to the lowest bit */
+    mismatch |= mismatch >> 4;
+    mismatch |= mismatch >> 2;
+    mismatch |= mismatch >> 1;
+    /* Now check the low bit to see if it's set */
+    return (mismatch & 1) == 0;
+}
diff --git a/src/_cffi_src/hazmat_src/padding.h b/src/_cffi_src/hazmat_src/padding.h
index 4d218b1a..fb023c17 100644
--- a/src/_cffi_src/hazmat_src/padding.h
+++ b/src/_cffi_src/hazmat_src/padding.h
@@ -3,3 +3,4 @@
 // repository for complete details.
 
 uint8_t Cryptography_check_pkcs7_padding(const uint8_t *, uint8_t);
+uint8_t Cryptography_check_ansix923_padding(const uint8_t *, uint8_t);
diff --git a/src/cryptography/hazmat/primitives/padding.py b/src/cryptography/hazmat/primitives/padding.py
index 08c6ca03..81883404 100644
--- a/src/cryptography/hazmat/primitives/padding.py
+++ b/src/cryptography/hazmat/primitives/padding.py
@@ -165,4 +165,6 @@ class _ANSIX923PaddingContext(_BytePaddingContext):
 class _ANSIX923UnpaddingContext(_ByteUnpaddingContext):
 
     def _check_padding(self):
-        return True
+        return lib.Cryptography_check_ansix923_padding(
+            self._buffer, self.block_size // 8
+        )