commit 3601db73f0e355cb6e74720aadeedbd19d1acfee
parent b7c3a969c06037f3f56be3ff125723e89d8a5ece
Author: Jack Sangdahl <jack@pngu.org>
Date: Sat, 25 Oct 2025 21:42:23 -0600
challenge 25: break random access read/write aes ctr
Diffstat:
2 files changed, 126 insertions(+), 0 deletions(-)
diff --git a/data/25.txt b/data/25.txt
@@ -0,0 +1,64 @@
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diff --git a/src/25_break_rand_access_rw_aes_ctr.c b/src/25_break_rand_access_rw_aes_ctr.c
@@ -0,0 +1,62 @@
+// Break "random access read/write" AES CTR
+#include "aes.h"
+#include "base64.h"
+#include "file.h"
+#include "random.h"
+
+/*
+ * back to ctr. encrypt the recovered plaintext from [data/25.txt] (the ecb
+ * exercise) under ctr with a random key (for this exercise the key should be
+ * unknown to you, but hold onto it). now, write the code that allows you to
+ * seek into the ciphertext, decrypt, and re-encrypt with different plaintext.
+ * expose this as a function like 'edit(ciphertext, key, offset, newtext)'.
+ * imagine the "edit" function was exposed to attackers by means of an api call
+ * that didn't reveal the key or the original plaintext; the attacker has the
+ * ciphertext and controls the offset and "new text". recover the original
+ * plaintext. food for thought: a folkloric supposed benefit of ctr mode is the
+ * ability to easily "seek forward" into the ciphertext; to access byte n of the
+ * ciphertext, all you need to be able to do is generate byte 'n' of the
+ * keystream. imagine if you'd relied on that advice, to say, encrypt a disk.
+ */
+
+struct bytes edit(struct bytes ciphertext, const unsigned char *key, int offset,
+ char *newtext, size_t newtext_len) {
+ struct bytes plain =
+ aes_ctr(ciphertext.data, ciphertext.len, key, AES_BLOCK_SIZE, 0);
+ struct bytes prefix = {plain.data, offset};
+ struct bytes suffix = {plain.data + offset + newtext_len,
+ plain.len - offset - newtext_len};
+ char *data = (char *)malloc(ciphertext.len * sizeof(char));
+ memcpy(data, prefix.data, prefix.len);
+ memcpy(data + prefix.len, newtext, newtext_len);
+ memcpy(data + prefix.len + newtext_len, suffix.data, suffix.len);
+ struct bytes res = aes_ctr((const unsigned char *)data, ciphertext.len, key,
+ AES_BLOCK_SIZE, 0);
+ free(plain.data);
+ free(data);
+ return res;
+}
+
+int main(void) {
+ srand(time(NULL));
+ const unsigned char *rand_key = random_bytes(AES_BLOCK_SIZE);
+ const uint64_t nonce = 0;
+ char *input = NULL, *decoded = NULL;
+ int file_len = read_file("data/25.txt", &input);
+ int decoded_len = base64_decode(input, file_len, &decoded);
+ const char *key = "YELLOW SUBMARINE";
+ struct bytes res1 = aes_decrypt_ecb((unsigned char *)decoded, decoded_len,
+ (const unsigned char *)key, strlen(key));
+ struct bytes decrypted = aes_ctr(res1.data, res1.len, rand_key, 16, nonce);
+ char *zeros = (char *)calloc(decrypted.len, sizeof(char));
+ struct bytes keystream = edit(decrypted, rand_key, 0, zeros, decrypted.len);
+ unsigned char *recovered;
+ int res2 = xor(keystream.data, keystream.len, decrypted.data, decrypted.len,
+ &recovered);
+ printf("%s\n", recovered);
+ free(decoded);
+ free(res1.data);
+ free(decrypted.data);
+ free(keystream.data);
+ return 0;
+}
