cryptopals

commit 3912bee57a60f9c2cea400325f5e129dae621fab
parent bdd46ff96d02167e197596336bf8c7b061f4458e
Author: Jack Sangdahl <jack@pngu.org>
Date:   Sat,  4 Oct 2025 20:03:33 -0600

challenge 5: implement repeating key xor

Diffstat:
A
src/05_repeating_key_xor.c
 | 
45
+++++++++++++++++++++++++++++++++++++++++++++

1 file changed, 45 insertions(+), 0 deletions(-)
diff --git a/src/05_repeating_key_xor.c b/src/05_repeating_key_xor.c
@@ -0,0 +1,45 @@
+// Implement repeating-key XOR
+#include "hex.h"
+#include "xor.h"
+#include <assert.h>
+#include <stdio.h>
+
+/*
+ * here is the opening stanza of an import work of the english language:
+ * Burning 'em, if you ain't quick and nimble
+ * I go crazy when I hear a cymbal
+ * encrypt it, under the key "ICE", using repeating-key xor. in repeating
+ * key-xor, you'll sequentially apply each byte of the key; the first byte of
+ * plaintext will be xor'd against I, the next C, the next E, then again for the
+ * 4th byte and so on. It should come out to:
+ * 0b3637272a2b2e63622c2e69692a23693a2a3c6324202d623d63343c2a26226324272765272
+ * a282b2f20430a652e2c652a3124333a653e2b2027630c692b20283165286326302e27282f
+ * encrypt a bunch of stuff using your repeating-key xor function. encrypt your
+ * mail. encrypt your password file. your .sig file. get a feel for it. i
+ * promise, we aren't wating your time with this.
+ */
+
+int main(void) {
+  static char *input = "Burning 'em, if you ain't quick and nimble\nI go crazy "
+                       "when I hear a cymbal";
+  static char *key = "ICE";
+  static char *output =
+      "0b3637272a2b2e63622c2e69692a23693a2a3c6324202d623d63343"
+      "c2a26226324272765272a282b2f20430a652e2c652a3124333a653e"
+      "2b2027630c692b20283165286326302e27282f";
+  char *hex = NULL;
+  unsigned char *xord = NULL;
+  size_t input_len = strlen(input), key_len = strlen(key);
+  int res = xor((unsigned char *)input, input_len, (unsigned char *)key,
+                key_len, &xord);
+  if (res == -1)
+    return -1;
+  ascii_to_hex((char *)xord, &hex);
+  printf("Input: %s\n", input);
+  printf("Output: %s\n", hex);
+  printf("Expected: %s\n", output);
+  assert(!strcmp(hex, output));
+  free(xord);
+  free(hex);
+  return 0;
+}