OpenSSL AES Decryption fails randomly C++

main.cc

#include <iostream>
#include <fstream>
#include <string>
#include <openssl/aes.h>
#include <openssl/bio.h>
#include <openssl/buffer.h>
#include <openssl/err.h>
#include <openssl/evp.h>
#include <openssl/pem.h>
#include <openssl/rsa.h>

using namespace std;

bool aesEncrypt(const std::string input, const std::string& key, std::string& output) {
    int ret = 0, len1 = 0, len2 = 0;
    EVP_CIPHER_CTX *ctx = EVP_CIPHER_CTX_new();

    ret = EVP_EncryptInit_ex(ctx, EVP_aes_128_ecb(), NULL, (const unsigned char*)key.c_str(), NULL);
    if (ret != 1) {
        EVP_CIPHER_CTX_free(ctx);
        return false;
    }

    string u;
    u.resize(input.size() + EVP_CIPHER_CTX_block_size(ctx));
    ret = EVP_EncryptUpdate(ctx, (unsigned char *)u.data(), &len1, (const unsigned char*)input.c_str(), input.size());
    u.resize(len1);
    if (ret != 1) {
        EVP_CIPHER_CTX_free(ctx);
        return false;
    }

    string f;
    f.resize(EVP_CIPHER_CTX_block_size(ctx));
    ret = EVP_EncryptFinal_ex(ctx, (unsigned char *)f.data(), &len2);
    f.resize(len2);
    if (ret != 1) {
        EVP_CIPHER_CTX_free(ctx);
        return false;
    }

    output = u + f;
    EVP_CIPHER_CTX_free(ctx);
    return true;
}

// 128 ecb
bool aesDecrypt(const std::string input, const std::string& key, std::string& output) {
    int ret = 0, len1 = 0, len2 = 0;

    EVP_CIPHER_CTX *ctx = EVP_CIPHER_CTX_new();
    ret = EVP_DecryptInit_ex(ctx, EVP_aes_128_ecb(), NULL, (const unsigned char*)key.c_str(), NULL);
    if (ret != 1) {
        EVP_CIPHER_CTX_free(ctx);
        return false;
    }

    string u;
    u.resize(input.size() + EVP_CIPHER_CTX_block_size(ctx));
    ret = EVP_DecryptUpdate(ctx, (unsigned char *)u.data(), &len1, (const unsigned char*)input.c_str(), input.size());
    u.resize(len1);
    if (ret != 1) {
        EVP_CIPHER_CTX_free(ctx);
        return false;
    }

    string f;
    f.resize(EVP_CIPHER_CTX_block_size(ctx));
    ret = EVP_DecryptFinal_ex(ctx, (unsigned char *)f.data(), &len2);
    f.resize(len2);
    if (ret != 1) {
        EVP_CIPHER_CTX_free(ctx);
        return false;
    }

    output = u + f;
    EVP_CIPHER_CTX_free(ctx);
    return true;
} 

int main(int argc, char* argv[]) {
    std::ifstream file(argv[1]);
    std::string content((std::istreambuf_iterator<char>(file)),
                            std::istreambuf_iterator<char>());

    string encrypted_content;
    if (!aesEncrypt(content, "input", encrypted_content)) return -1;
    
    // Note: decrypt failed randomly if uncomment 
    // std::ofstream encrypted_file(argv[2]);
    // encrypted_file << encrypted_content;
    // encrypted_file.close();    

    string decrypted_content;
    if (!aesDecrypt(encrypted_content, "input", decrypted_content)) {
        cout << "decrypt failed." << endl;
        return -1;
    } 
    cout << decrypted_content << endl; 
}