AES CBC 128 decryption returns a wrong result

[gwynn-1]

I use openssl to decrypt an encrypted epub file in Android environment. Firstly I will decompress the epub file, then let's say after the decompression, I have 4 html files that is encrypted:

• titlepage.xhtml
• index_split_000.html
• index_split_001.html
• toc.ncx
  My function will decrypt these 4 files in order. It worked perfectly with titlepage.xhtml, but the next one, index_split_000.html, it still returned success but the decrypted result is wierd and unexpected. And so is index_split_001.html. Then to the toc.ncx, it worked fine again. I have tried to decrypt 2 index files using another tool and also the openssl command to decrypt, nothing was wrong then, the decrypted data is right what i want. So I think that the EVP_DecryptFinal_ex() function is working correctly.

This is my decryption logic code :

char *Decryption::standalone_decryptAES_CBC_128(const unsigned char *cipherText, int cipherTextLength,
                                                const char *key) {
    if(key && !key[0]) {
        __android_log_print(ANDROID_LOG_INFO, "Decryption", "Key is empty");
        return (char *) cipherText;
    }
    const unsigned char iv[] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
    // Convert char* key to unsigned char*
    unsigned char* unsignedKey = reinterpret_cast<unsigned char*>(const_cast<char*>(key));

    EVP_CIPHER_CTX *ctxNew = EVP_CIPHER_CTX_new();
    if (!ctxNew) {
        handleOpenSSLError();
        __android_log_print(ANDROID_LOG_INFO, "Decryption", "Can't create new Cipher");
        return (char *) cipherText;
    }

    if (EVP_DecryptInit_ex(ctxNew, EVP_aes_128_cbc(), NULL, unsignedKey, reinterpret_cast<const unsigned char*>(iv)) != 1) {
        handleOpenSSLError();
        EVP_CIPHER_CTX_free(ctxNew);
        __android_log_print(ANDROID_LOG_INFO, "Decryption", "Can't init Cipher");
        return (char *) cipherText;
    }

    // Set PKCS7 padding
    EVP_CIPHER_CTX_set_padding(ctx, EVP_PADDING_PKCS7);

    int maxPlainTextLength = cipherTextLength + EVP_CIPHER_block_size(EVP_aes_128_cbc());
    unsigned char* decryptedText = new unsigned char[maxPlainTextLength];

    int outLen = 0;
    if (EVP_DecryptUpdate(ctxNew, decryptedText, &outLen, cipherText, cipherTextLength) != 1) {
        handleOpenSSLError();
        __android_log_print(ANDROID_LOG_INFO, "Decryption", "Can't Update decrypted text to Cipher");
        delete[] decryptedText;
        EVP_CIPHER_CTX_free(ctxNew);
        return (char *) cipherText;
    }

    int tmpLen = outLen;
    int result_dec = EVP_DecryptFinal_ex(ctxNew, decryptedText + outLen, &outLen);
    if (result_dec != 1) {
        handleOpenSSLError();
        if(result_dec == 0) {
//            __android_log_print(ANDROID_LOG_INFO, "Decryption", "padding is incorrect");
            EVP_CIPHER_CTX_free(ctxNew);
            return (char *) cipherText;
        }

        delete[] decryptedText;
        EVP_CIPHER_CTX_free(ctxNew);
        __android_log_print(ANDROID_LOG_INFO, "Decryption", "Can't decrypt cipher");
        return (char *) cipherText;
    }
    tmpLen += outLen;
    // Null-terminate the decrypted text
    decryptedText[tmpLen] = '\0';

    EVP_CIPHER_CTX_free(ctxNew);

    char* clonedDecryptedText = new char[tmpLen + 1];
    strcpy(clonedDecryptedText, reinterpret_cast<const char *>(decryptedText));

    // Move the pointer forward by 16 bytes in the cloned string
    char* modifiedDecryptedPtr = clonedDecryptedText + 16;
    delete[] decryptedText;
    return modifiedDecryptedPtr;
}

This is the correct decrypted content of titlepage.xhtml.

Then when the program continue to the index_split_000.html, the same function that decrypt the last file correctly is broken.

but the result it returned is still 1, which mean success.

So is the index_split_001.html.

Then the process continues to the toc.ncx and it worked again.

Do notice that these 4 files share the same key and I have checked the 2 index files. There's nothing wrong with it, I use the key that decrypt titlepage.xhtml and toc.ncx successfully to decrypt 2 index files by another tool (still AES CBC 128 bit of course), and it generated the expected decrypt data.

Openssl Version: 3.3.0
Environment: Android Studio
MacOS Sonoma 14.4.1, Chip Apple M3

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