1 files changed, 39 insertions, 0 deletions
diff --git a/src/_cffi_src/hazmat_src/padding.c b/src/_cffi_src/hazmat_src/padding.c
new file mode 100644
index 00000000..570bad9f
--- /dev/null
+++ b/src/_cffi_src/hazmat_src/padding.c
@@ -0,0 +1,39 @@
+// This file is dual licensed under the terms of the Apache License, Version
+// 2.0, and the BSD License. See the LICENSE file in the root of this
+// repository for complete details.
+
+/* Returns the value of the input with the most-significant-bit copied to all
+   of the bits. */
+static uint8_t Cryptography_DUPLICATE_MSB_TO_ALL(uint8_t a) {
+    return (1 - (a >> (sizeof(uint8_t) * 8 - 1))) - 1;
+}
+
+/* This returns 0xFF if a < b else 0x00, but does so in a constant time
+   fashion */
+static uint8_t Cryptography_constant_time_lt(uint8_t a, uint8_t b) {
+    a -= b;
+    return Cryptography_DUPLICATE_MSB_TO_ALL(a);
+}
+
+uint8_t Cryptography_check_pkcs7_padding(const uint8_t *data,
+                                         uint8_t block_len) {
+    uint8_t i;
+    uint8_t pad_size = data[block_len - 1];
+    uint8_t mismatch = 0;
+    for (i = 0; i < block_len; i++) {
+        unsigned int mask = Cryptography_constant_time_lt(i, pad_size);
+        uint8_t b = data[block_len - 1 - i];
+        mismatch |= (mask & (pad_size ^ 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;
+}