cryptopals

commit 1b2b4397beceb8fab31dcaaae7f5340d77e95034
parent 2bc60b7d2f6c303799d76aedb948bb547f491c3e
Author: Jack Sangdahl <jack@pngu.org>
Date:   Tue, 14 Oct 2025 09:24:55 -0600

challenge 15: pkcs#7 padding validation

Diffstat:
A
src/15_pkcs7_padding_validation.c
 | 
34
++++++++++++++++++++++++++++++++++

1 file changed, 34 insertions(+), 0 deletions(-)
diff --git a/src/15_pkcs7_padding_validation.c b/src/15_pkcs7_padding_validation.c
@@ -0,0 +1,34 @@
+// PKCS#7 padding validation
+#include "aes.h"
+
+/*
+ * write a function that takes a plaintext, determines if it has valid pkcs#7
+ * padding, and strips the padding off. the sring "ICE ICE BABY\x04\x04\x04\x04"
+ * has valid padding, and produces the result "ICE ICE BABY". the string "ICE
+ * ICE BABY\x05\x05\x05\x05" does not have valid padding, nor does "ICE ICE
+ * BABY\x01\x02\x03\x04". if you are writing a language with exceptions, like
+ * python, make your function throw an exception on bad padding.
+ */
+
+int main(void) {
+  static char *valid = "ICE ICE BABY\x04\x04\x04\x04";
+  static char *invalid1 = "ICE ICE BABY\x05\x05\x05\x05";
+  static char *invalid2 = "ICE ICE BABY\x01\x02\x03\x04";
+  struct bytes stripped = pkcs7_validate_strip((unsigned char *)valid, 16);
+  if (stripped.data)
+    printf("%s\n", stripped.data);
+  else
+    printf("Invalid padding\n");
+  stripped = pkcs7_validate_strip((unsigned char *)invalid1, 16);
+  if (stripped.data)
+    printf("%s\n", stripped.data);
+  else
+    printf("Invalid padding\n");
+  stripped = pkcs7_validate_strip((unsigned char *)invalid2, 16);
+  if (stripped.data)
+    printf("%s\n", stripped.data);
+  else
+    printf("Invalid padding\n");
+  free(stripped.data);
+  return 0;
+}