Initial version (not threaded).

3 files changed, 247 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
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--- /dev/null
+++ b/.gitignore
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+.idea
+.code
+
+# copied from https://github.com/github/gitignore/blob/main/CMake.gitignore
+CMakeLists.txt.user
+CMakeCache.txt
+CMakeFiles
+CMakeScripts
+Testing
+Makefile
+cmake_install.cmake
+install_manifest.txt
+compile_commands.json
+CTestTestfile.cmake
+_deps
+
+# cmake builds
+cmake-build-*
\ No newline at end of file
diff --git a/CMakeLists.txt b/CMakeLists.txt
new file mode 100644
index 0000000..5bdfa5a
--- /dev/null
+++ b/CMakeLists.txt
@@ -0,0 +1,12 @@
+cmake_minimum_required(VERSION 3.0)
+project(mobileqq_fp_cracker C)
+
+set(CMAKE_C_STANDARD 99)
+
+set(common_compiler_args "-Wall -Werror -Wno-unused")
+set(CMAKE_CXX_FLAGS_DEBUG "${common_compiler_args} -g -ggdb -fsanitize=address -DDEBUG")
+set(CMAKE_C_FLAGS_RELEASE "${common_compiler_args} -O3 -DNDEBUG")
+
+add_executable(fp_cracker main.c)
+
+target_link_libraries(fp_cracker libtomcrypt.a)
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diff --git a/main.c b/main.c
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+++ b/main.c
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+#include <tomcrypt.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <assert.h>
+
+const char *hexchars = "0123456789ABCDEF";
+
+typedef struct s_key_search_ctx {
+    uint64_t ciphertext; // first 8 bytes of the ciphertext in encrypted QQ flash image
+    uint64_t yield; // if routine `yield_possible_key` returns true, the possible 64-bit DES key will be stored here
+    uint32_t next_possible_key; // 4 byte effective key space
+    bool finished;
+} key_search_ctx;
+
+/* constructor of type `key_search_ctx` */
+void new_key_search_ctx(key_search_ctx *ctx, uint64_t ciphertext) {
+    ctx->finished = false;
+    ctx->next_possible_key = 0u;
+    ctx->ciphertext = ciphertext;
+}
+
+/* returns false if no result yield from this call and searching is finished */
+bool yield_possible_key(key_search_ctx *ctx) {
+    if (ctx->finished) return false;
+
+//    const char[] hexchars = {0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
+//                             0x38, 0x39, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46};
+#define FILL_KEY(buf, key, i) \
+        ((buf)[7-(i)] = hexchars[((key) >> (4*(i))) & 0xFu]) \
+        // buf: char[8], key: uint64_t, i:uint = 0, 1, 2, ..., 7
+    uint32_t k = ctx->next_possible_key;
+    uint64_t plaintext;
+    char key[8];
+    symmetric_key skey;
+    do {
+        // convert key uint32 to char[]
+        FILL_KEY(key, k, 0);
+        FILL_KEY(key, k, 1);
+        FILL_KEY(key, k, 2);
+        FILL_KEY(key, k, 3);
+        FILL_KEY(key, k, 4);
+        FILL_KEY(key, k, 5);
+        FILL_KEY(key, k, 6);
+        FILL_KEY(key, k, 7);
+        // decrypt file header with this key
+        int err;
+        if ((err = des_setup((const unsigned char *) key, 8, 0, &skey)) != CRYPT_OK) {
+            fprintf(stderr, "Err: setup: %s", error_to_string(err));
+            return false;
+        }
+        if (des_ecb_decrypt(
+                (const unsigned char *) &(ctx->ciphertext),
+                (unsigned char *) &plaintext,
+                (const symmetric_key *) &skey
+        ) != CRYPT_OK)
+            continue; // failed to decrypt
+        // validate first 3 bytes of the cleartext
+        if ((plaintext & 0xFFFFFFu) == 0xFFD8FFu) {
+            ctx->yield = *(uint64_t *) key;
+            // if next_possible_key rewinds to 0, finished will be set to true
+            ctx->finished = !++ctx->next_possible_key;
+            return true;
+        }
+    } while ((k = ++ctx->next_possible_key));
+    ctx->finished = true;
+    return false;
+}
+
+/* given buf, returns length of the pad, or -1 if the data is not padded with valid pkcs7 */
+int pkcs7_check_pad(const char *buf, size_t n) {
+    if (!n) return -1;
+    --n;
+    unsigned char pad = buf[n--];
+    if (!pad) return -1;
+    if (n < pad) return -1; // buf is shorter than a valid pad
+    for (int i = pad; i > 1; --i) {
+        if (buf[n--] != pad) return -1;
+    }
+    return pad;
+}
+
+int main(int argc, char *argv[]) {
+//    uint64_t ciphertext = 8022485120222247589;
+//    unsigned char plaintext[8];
+//    const char *key = "A0979B6D";
+//    symmetric_key skey;
+    FILE *fp;
+
+    if (argc != 2 && argc != 3) {
+        printf("Usage: %s <fp_file> [<where_to_save_the_decrypted_file>]\n"
+               "The decrypted image won't be saved if save path is not specified.\n",
+               argv[0]);
+        return 0;
+    }
+
+    const char* plaintext_save_path = (argc == 3) ? (argv[2]) : NULL;
+    const char* ciphertext_file_path = argv[1];
+
+    // open file
+    if (!(fp = fopen(ciphertext_file_path, "rb"))) {
+        perror("fopen");
+        return 1;
+    }
+
+    // test file header
+    char header[8];
+    if (fread(header, 1, 8, fp) != 8) {
+        fprintf(stderr, "Cannot read first 8 bytes from file.\n");
+        return 1;
+    }
+    if (*(uint64_t *) header != *(uint64_t *) "ENCRYPT:") {
+        fprintf(stderr, "Bad file header.\n");
+        return 1;
+    }
+
+    // read ciphertext into memory
+    fseek(fp, 0, SEEK_END);
+    long file_length = ftell(fp);
+    if (file_length <= 8) {
+        fprintf(stderr, "Invalid file length (%ld)\n", file_length);
+        return 1;
+    }
+
+    const unsigned long ciphertext_length = file_length - 8;
+    if (ciphertext_length % 8 != 0) {
+        fprintf(stderr, "Invalid file length: %ld can not be divided by 8.\n", file_length);
+        return 1;
+    }
+    char *ciphertext = malloc(ciphertext_length);
+    char *plaintext = malloc(ciphertext_length); // for the future decryption use, padded plaintext (pkcs7)
+    if (ciphertext == NULL || plaintext == NULL) {
+        perror("malloc");
+        return 1;
+    }
+
+    fseek(fp, 8, SEEK_SET);
+    if (fread(ciphertext, 1, ciphertext_length, fp) != ciphertext_length) {
+        fprintf(stderr, "Cannot read the whole file.\n");
+        return 1;
+    }
+
+    // start searching
+    printf("Searching key...\n");
+    fflush(stdout);
+
+    key_search_ctx ctx;
+    new_key_search_ctx(&ctx, *(uint64_t *) ciphertext);
+    // FOR DEBUGGING ONLY
+//    assert(*(uint64_t *) ciphertext == 8022485120222247589ull);
+//    ctx.next_possible_key = 0xA0979B6Du;
+    while (yield_possible_key(&ctx)) {
+        // found a possible correct key
+        // validate it by calculating md5 hashsum of the plaintext
+        printf("Possible key: %zu\n", ctx.yield);
+        fflush(stdout);
+
+        // decrypt the whole ciphertext
+        int err;
+        symmetric_key skey;
+        if ((err = des_setup((const unsigned char *) (&ctx.yield), 8, 0, &skey)) != CRYPT_OK) {
+            fprintf(stderr, "Err: setup: %s", error_to_string(err));
+            continue; // skip this key
+        }
+
+        uint_fast32_t blk_cnt = ciphertext_length >> 3;
+        for (uint_fast32_t blk = 0; blk < blk_cnt; ++blk) {
+            des_ecb_decrypt(
+                    (const unsigned char *) ((uint64_t *) ciphertext + blk),
+                    (unsigned char *) ((uint64_t *) plaintext + blk),
+                    (const symmetric_key *) &skey
+            ); // error checking is unnecessary
+        }
+
+        int pad_length = pkcs7_check_pad(plaintext, ciphertext_length);
+        const unsigned int unpadded_length = ciphertext_length - pad_length;
+        assert(pad_length < ciphertext_length);
+        if (pad_length < 0) {
+            // invalid pad, this key is incorrect, skip it
+            fprintf(stderr, "Invalid pad.\n");
+            continue;
+        }
+
+        // calculate md5 checksum of the decrypted plaintext
+        char md5_out[16];
+        hash_state md;
+        md5_init(&md);
+        md5_process(&md, (const unsigned char *) plaintext, unpadded_length);
+        md5_done(&md, (unsigned char *) md5_out);
+
+        // compare md5_out[0~3] with 8-byte ASCII hex string ctx.yield
+        char md5_hex[8+1]; // hex of first 4-byte of md5_out, 1 more byte to hold the '\0' terminator
+        snprintf(md5_hex, 8+1, "%02X%02X%02X%02X",
+                 md5_out[0] & 0xFFu, md5_out[1] & 0xFFu,
+                 md5_out[2] & 0xFFu, md5_out[3] & 0xFFu);
+        if (!memcmp(md5_hex, (const char *) (&ctx.yield), 8)) {
+            printf("[+] FOUND KEY: %zu\n", ctx.yield);
+
+            if (plaintext_save_path) {
+                FILE *fout = fopen(plaintext_save_path, "wb");
+                if (!fout) {
+                    perror("Cannot fopen for saving");
+                    return 1;
+                }
+                fwrite(plaintext, 1, unpadded_length, fout);
+                fclose(fout);
+                printf("Flash photo has been saved in: %s\n", plaintext_save_path);
+            }
+
+            return 0;
+        }
+        // otherwise the key is incorrect, continue searching
+    }
+
+    return 0;
+}
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