commit c41dcb673c914e2d755cf53624926a9093477feb
parent 406aa4552a412082f67d3eb20a165c31fa4474d5
Author: Jack Sangdahl <jack@pngu.org>
Date: Mon, 6 Oct 2025 14:03:02 -0600
challenge 7: aes in ecb mode
Diffstat:
5 files changed, 184 insertions(+), 2 deletions(-)
diff --git a/Makefile b/Makefile
@@ -1,7 +1,8 @@
DEBUG ?= 0
-CFLAGS = -Wall -Wextra -Werror -Wno-unused -pedantic -Ilib
-LDFLAGS =
+CFLAGS = -Wall -Wextra -Werror -Wno-unused -pedantic -Ilib \
+ $(shell pkgconf --cflags openssl)
+LDFLAGS = $(shell pkgconf --libs openssl)
ifeq ($(DEBUG),1)
CFLAGS += -g -fsanitize=address
diff --git a/data/07.txt b/data/07.txt
@@ -0,0 +1,64 @@
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diff --git a/lib/aes.c b/lib/aes.c
@@ -0,0 +1,66 @@
+#include "aes.h"
+
+int next_block(int length, size_t block_size) {
+ return (length + block_size) & ~(block_size - 1);
+}
+
+struct bytes pkcs7(const char *data, const size_t data_len, size_t block_size) {
+ size_t res_len = next_block(data_len, block_size);
+ unsigned char *res = (unsigned char *)malloc(res_len * sizeof(unsigned char));
+ unsigned char padding = res_len - data_len;
+ memcpy(res, data, data_len);
+ for (size_t i = data_len; i < res_len; i++)
+ res[i] = padding;
+ return (struct bytes){res, res_len};
+}
+
+struct bytes pkcs7_strip(unsigned char *data, size_t data_len) {
+ size_t padding = data[data_len - 1], res_len = data_len - padding;
+ unsigned char *res = (unsigned char *)malloc(res_len * sizeof(unsigned char));
+ memcpy(res, data, res_len);
+ return (struct bytes){res, res_len};
+}
+
+bool pkcs7_validate(unsigned char *data, size_t data_len) {
+ size_t padding = data[data_len - 1];
+ if (!padding || padding > data_len)
+ return false;
+ for (size_t i = 1; i < padding; i++)
+ if (padding != data[data_len - 1 - i])
+ return false;
+ return true;
+}
+
+struct bytes pkcs7_validate_strip(unsigned char *data, size_t data_len) {
+ struct bytes res = {NULL, 0};
+ if (pkcs7_validate(data, data_len))
+ res = pkcs7_strip(data, data_len);
+ return res;
+}
+
+struct bytes aes_encrypt_ecb(char *data, const size_t data_len,
+ const unsigned char *key, const size_t key_len) {
+ AES_KEY aes_key;
+ AES_set_encrypt_key(key, key_len * 8, &aes_key);
+ struct bytes padded = pkcs7(data, data_len, AES_BLOCK_SIZE);
+ unsigned char *res =
+ (unsigned char *)calloc(padded.len, sizeof(unsigned char));
+ for (size_t i = 0; i < padded.len; i += AES_BLOCK_SIZE)
+ AES_ecb_encrypt(padded.data + i, res + i, &aes_key, AES_ENCRYPT);
+ free(padded.data);
+ return (struct bytes){res, padded.len};
+}
+
+struct bytes aes_decrypt_ecb(const unsigned char *data, size_t data_len,
+ const unsigned char *key, size_t key_len) {
+ struct bytes res = {NULL, 0};
+ unsigned char *decrypted = calloc(data_len + 1, sizeof(unsigned char));
+ AES_KEY aes_key;
+ AES_set_decrypt_key(key, key_len * 8, &aes_key);
+ for (size_t i = 0; i < data_len; i += AES_BLOCK_SIZE)
+ AES_ecb_encrypt(data + i, decrypted + i, &aes_key, AES_DECRYPT);
+ if (data_len)
+ res = pkcs7_strip(decrypted, data_len);
+ free(decrypted);
+ return res;
+}
diff --git a/lib/aes.h b/lib/aes.h
@@ -0,0 +1,21 @@
+#pragma once
+
+#include <openssl/aes.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+
+struct bytes {
+ unsigned char *data;
+ size_t len;
+};
+
+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);
diff --git a/src/07_aes_ecb_mode.c b/src/07_aes_ecb_mode.c
@@ -0,0 +1,30 @@
+// AES in ECB mode
+#include "aes.h"
+#include "base64.h"
+#include "file.h"
+
+/*
+ * the base64 encoded content in [data/07.txt] has been encrypted via aes-128 in
+ * ecb mode under the key "YELLOW SUBMARINE" (case sensitive, without quotes,
+ * exactly 16 characters; i like "YELLOW SUBMARINE" because it's exactly 16
+ * bytes long, and now you do too). decrypt it. you know the key afterall.
+ * easiest way: use openssl cipher and give it aes-128 ecb as the cipher. you
+ * can obviously decrypt this using the openssl command line tool, but we're
+ * having you get ecb working in code for a reason. you'll need it a lot later
+ * on, and not just for attacking ecb.
+ */
+
+int main(void) {
+ static char *key = "YELLOW SUBMARINE";
+ char *input = NULL, *decoded = NULL;
+ int file_len = read_file("data/07.txt", &input);
+ int decoded_len = base64_decode(input, file_len, &decoded);
+ struct bytes res = aes_decrypt_ecb((unsigned char *)decoded, decoded_len,
+ (const unsigned char *)key, strlen(key));
+ if (res.data) {
+ printf("%s", res.data);
+ free(res.data);
+ }
+ free(decoded);
+ return 0;
+}
