commit 419e41293a37b19d53da3dd4fd7a37bdce5566cb
parent 8bdc32650426b580fc9972834fa2e3dd067fbe1e
Author: Jack Sangdahl <jack@pngu.org>
Date: Wed, 8 Oct 2025 09:08:51 -0600
challenge 10: implement cbc mode
Diffstat:
6 files changed, 223 insertions(+), 0 deletions(-)
diff --git a/data/10.txt b/data/10.txt
@@ -0,0 +1,64 @@
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diff --git a/lib/aes.c b/lib/aes.c
@@ -131,3 +131,99 @@ struct bytes aes_decrypt_ecb(const unsigned char *data, size_t data_len,
free(plaintext);
return unpadded;
}
+
+struct bytes aes_encrypt_cbc(char *data, const size_t data_len,
+ const unsigned char *key, const size_t key_len,
+ unsigned char *iv) {
+ if (key_len != 16 && key_len != 24 && key_len != 32)
+ return (struct bytes){NULL, 0};
+ EVP_CIPHER_CTX *ctx = EVP_CIPHER_CTX_new();
+ const EVP_CIPHER *cipher;
+ switch (key_len) {
+ case 16:
+ EVP_aes_128_cbc();
+ break;
+ case 24:
+ EVP_aes_192_cbc();
+ break;
+ case 32:
+ EVP_aes_256_cbc();
+ break;
+ }
+ if (EVP_EncryptInit_ex(ctx, cipher, NULL, key, iv) != 1) {
+ EVP_CIPHER_CTX_free(ctx);
+ return (struct bytes){NULL, 0};
+ }
+ struct bytes padded = pkcs7(data, data_len, AES_BLOCK_SIZE);
+ if (!padded.data) {
+ EVP_CIPHER_CTX_free(ctx);
+ return (struct bytes){NULL, 0};
+ }
+ EVP_CIPHER_CTX_set_padding(ctx, 0);
+ unsigned char *ciphertext =
+ (unsigned char *)malloc(padded.len * sizeof(unsigned char));
+ int len;
+ if (EVP_EncryptUpdate(ctx, ciphertext, &len, padded.data, padded.len) != 1) {
+ free(padded.data);
+ free(ciphertext);
+ EVP_CIPHER_CTX_free(ctx);
+ return (struct bytes){NULL, 0};
+ }
+ int ciphertext_len = len;
+ if (EVP_EncryptFinal_ex(ctx, ciphertext + len, &len) != 1) {
+ free(padded.data);
+ free(ciphertext);
+ EVP_CIPHER_CTX_free(ctx);
+ return (struct bytes){NULL, 0};
+ }
+ ciphertext_len += len;
+ free(padded.data);
+ EVP_CIPHER_CTX_free(ctx);
+ return (struct bytes){ciphertext, ciphertext_len};
+}
+
+struct bytes aes_decrypt_cbc(unsigned char *data, size_t data_len,
+ const unsigned char *key, size_t key_len,
+ unsigned char *iv) {
+ if (key_len != 16 && key_len != 24 && key_len != 32)
+ return (struct bytes){NULL, 0};
+ if (!data_len || data_len % AES_BLOCK_SIZE)
+ return (struct bytes){NULL, 0};
+ EVP_CIPHER_CTX *ctx = EVP_CIPHER_CTX_new();
+ const EVP_CIPHER *cipher;
+ switch (key_len) {
+ case 16:
+ cipher = EVP_aes_128_cbc();
+ break;
+ case 24:
+ cipher = EVP_aes_192_cbc();
+ break;
+ case 32:
+ cipher = EVP_aes_256_cbc();
+ break;
+ }
+ if (EVP_DecryptInit_ex(ctx, cipher, NULL, key, iv) != 1) {
+ EVP_CIPHER_CTX_free(ctx);
+ return (struct bytes){NULL, 0};
+ }
+ EVP_CIPHER_CTX_set_padding(ctx, 0);
+ unsigned char *plaintext =
+ (unsigned char *)malloc(data_len * sizeof(unsigned char));
+ int len;
+ if (EVP_DecryptUpdate(ctx, plaintext, &len, data, data_len) != 1) {
+ free(plaintext);
+ EVP_CIPHER_CTX_free(ctx);
+ return (struct bytes){NULL, 0};
+ }
+ int plaintext_len = len;
+ if (EVP_DecryptFinal_ex(ctx, plaintext + len, &len) != 1) {
+ free(plaintext);
+ EVP_CIPHER_CTX_free(ctx);
+ return (struct bytes){NULL, 0};
+ }
+ plaintext_len += len;
+ EVP_CIPHER_CTX_free(ctx);
+ struct bytes res = pkcs7_validate_strip(plaintext, plaintext_len);
+ free(plaintext);
+ return res;
+}
diff --git a/lib/aes.h b/lib/aes.h
@@ -1,5 +1,6 @@
#pragma once
+#include "xor.h"
#include <openssl/aes.h>
#include <openssl/evp.h>
#include <stdbool.h>
@@ -12,11 +13,19 @@ struct bytes {
};
int next_block(int length, size_t block_size);
+
struct bytes pkcs7(const char *data, const size_t data_len, size_t block_size);
struct bytes pkcs7_strip(unsigned char *data, size_t data_len);
bool pkcs7_validate(unsigned char *data, size_t data_len);
struct bytes pkcs7_validate_strip(unsigned char *data, size_t data_len);
+
struct bytes aes_encrypt_ecb(char *data, const size_t data_len,
const unsigned char *key, const size_t key_len);
struct bytes aes_decrypt_ecb(const unsigned char *data, size_t data_len,
const unsigned char *key, size_t key_len);
+struct bytes aes_encrypt_cbc(char *data, const size_t data_len,
+ const unsigned char *key, const size_t key_len,
+ unsigned char *iv);
+struct bytes aes_decrypt_cbc(unsigned char *data, size_t data_len,
+ const unsigned char *key, size_t key_len,
+ unsigned char *iv);
diff --git a/lib/misc.c b/lib/misc.c
@@ -0,0 +1,11 @@
+#include "misc.h"
+
+bool contains(const unsigned char *data, size_t data_len, const char *expr) {
+ size_t expr_len = strlen(expr);
+ if (expr_len > data_len)
+ return false;
+ for (size_t i = 0; i < data_len - expr_len; i++)
+ if (!memcmp(data + i, expr, expr_len))
+ return true;
+ return false;
+}
diff --git a/lib/misc.h b/lib/misc.h
@@ -0,0 +1,6 @@
+#pragma once
+
+#include <stdbool.h>
+#include <string.h>
+
+bool contains(const unsigned char *data, size_t data_len, const char *expr);
diff --git a/src/10_implement_cbc_mode.c b/src/10_implement_cbc_mode.c
@@ -0,0 +1,37 @@
+// Implement CBC mode
+#include "aes.h"
+#include "base64.h"
+#include "file.h"
+
+/*
+ * cbc mode is a block cipher mode that allows us to encrypt irregularly sized
+ * messages, despite the fact that a block cipher natively only transforms
+ * individual blocks. in cbc mode, each ciphertext block is added to the next
+ * plaintext block before the next call to the cipher core. the first plaintext
+ * block, which has no associated previous ciphertext block, is added to a "fake
+ * 0th ciphertext block" called the initialisation vector, or iv. implement cbc
+ * mode by hand by taking the ecb function you wrote earlier, making it encrypt
+ * instead of decrypt (verify this by deccrypting whatever you encrypt to test),
+ * and using your xor function from the previous exercise to combine them.
+ * [data/10.txt] is intelligible (somewhat) when cbc decrypted against "YELLOW
+ * SUBMARINE" with an iv of all ascii 0 (\x00\x00\x00 &c). do not use openssl's
+ * cbc code to do cbc mode, even to verify your results.
+ */
+
+int main(void) {
+ static char *key = "YELLOW SUBMARINE";
+ char *input = NULL, *decoded = NULL;
+ int file_len = read_file("data/10.txt", &input);
+ int decoded_len = base64_decode(input, file_len, &decoded);
+ unsigned char iv[16] = {0};
+ struct bytes res =
+ aes_decrypt_cbc((unsigned char *)decoded, decoded_len,
+ (const unsigned char *)key, strlen(key), iv);
+ if (res.data) {
+ printf("%s\n", res.data);
+ free(res.data);
+ }
+ if (decoded)
+ free(decoded);
+ return 0;
+}
