cryptopals

commit 8bdc32650426b580fc9972834fa2e3dd067fbe1e
parent 16c34670eeb091fb40430b3a87b4a0c0b1ffdb99
Author: Jack Sangdahl <jack@pngu.org>
Date:   Tue,  7 Oct 2025 22:13:38 -0600

challenge 9: implement pkcs#7 padding

Diffstat:
A
src/09_implement_pkcs7_padding.c
 | 
23
+++++++++++++++++++++++

1 file changed, 23 insertions(+), 0 deletions(-)
diff --git a/src/09_implement_pkcs7_padding.c b/src/09_implement_pkcs7_padding.c
@@ -0,0 +1,23 @@
+// Implement PKCS#7 padding
+#include "aes.h"
+
+/*
+ * a block cipher transforms a fixed-size block (usually 8 or 16) bytes of
+ * plaintext into ciphertext. but almost never want to transform a single block;
+ * we encrypt irregularly sized messages. one way we account for irregularly
+ * sized messages is by padding, creating a plaintext that is an even multiple
+ * of the block size. the most popular padding scheme is pkcs#7. so: pad any
+ * block to a specific length by appending the number of bytes of padding to the
+ * end of the block. for instance, "YELLOW SUBMARINE" ...padded to 20 bytes
+ * would be "YELLOW SUBMARINE\x04\x04\x04\x04"
+ */
+
+int main(void) {
+  static char *input_output = "YELLOW SUBMARINE";
+  struct bytes res = pkcs7(input_output, strlen(input_output), 16);
+  printf("Input: %s\n", input_output);
+  printf("Output: %s\n", res.data);
+  printf("Expected: %s\n", input_output);
+  free(res.data);
+  return 0;
+}