converting from string to unsigned char * and vice versa - c++

Question

I have two functions: 1. encryption (string text, string key) and 2. decryption (string text, string key). these functions encrypt/decrypt text under key . I use openssl library and functions:EVP_EncryptUpdate and EVP_DecryptUpdate the input arguments of these two functions are (EVP_CIPHER_CTX *ctx, unsigned char *out, int *outl, const unsigned char *in, int inl). my functions are below and the output is :
"Hello world\x5\x5\x5\x5\x5"
which should be "Hello world"
i suppose there is something wrong with castings.
std::string encrypt(std::string text, std::string k){
const unsigned char *plaintext=(const unsigned char*) text.c_str();
const unsigned char *key=(const unsigned char*) k.c_str();
EVP_CIPHER_CTX *ctx;
int len;
int plaintext_len=strlen((const char *)plaintext);
int ciphertext_len=plaintext_len+256;
unsigned char * ciphertext=new unsigned char[ciphertext_len];
/* Create and initialise the context */
if(!(ctx = EVP_CIPHER_CTX_new()))
handleErrors();
/*
* Initialise the encryption operation.
*/
if(1 != EVP_EncryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, NULL))
handleErrors();
/*
* Provide the message to be encrypted, and obtain the encrypted output.
* EVP_EncryptUpdate can be called multiple times if necessary
*/
if(1 != EVP_EncryptUpdate(ctx, ciphertext, &len, plaintext, plaintext_len))
handleErrors();
ciphertext_len = len;
/*
* Finalise the encryption. Further ciphertext bytes may be written at
* this stage.
*/
if(1 != EVP_EncryptFinal_ex(ctx, ciphertext + len, &len))
handleErrors();
ciphertext_len += len;
/* Clean up */
EVP_CIPHER_CTX_free(ctx);
std::string cipherteststring=(char const* )ciphertext ;
delete []ciphertext;
return cipherteststring;
}
std::string decrypt(std::string text, std::string k)
{
const unsigned char *ciphertext=(const unsigned char*)text.c_str();
const unsigned char *key=(const unsigned char*) k.c_str();
int ciphertext_len=strlen((const char *)ciphertext);
EVP_CIPHER_CTX *ctx;
int len;
int plaintext_len=ciphertext_len;
unsigned char *decryptedtext=new unsigned char[plaintext_len];
/* Create and initialise the context */
if(!(ctx = EVP_CIPHER_CTX_new()))
handleErrors();
/*
* Initialise the decryption operation.
*/
if(1 != EVP_DecryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, NULL))
handleErrors();
/*
* Provide the message to be decrypted, and obtain the plaintext output.
* EVP_DecryptUpdate can be called multiple times if necessary.
*/
if(1 != EVP_DecryptUpdate(ctx, decryptedtext, &len, ciphertext, ciphertext_len))
handleErrors();
/* Clean up */
EVP_CIPHER_CTX_free(ctx);
//std::string plaintextstring( reinterpret_cast< char const* >(decryptedtext) ) ;
std::string plaintextstring=(char const*)decryptedtext;
decryptedtext[plaintext_len] = '\0';
delete []decryptedtext;
return plaintextstring;
}
int main (){
std::string key ="01234567890123456789012345678901";
std::string plaintext ="Hello world";
std::string encrypted;
std::string decrypted;
std::string decryptedhexstr;
encrypted=encrypt (plaintext,key);
decrypted = decrypt(encrypted, key);
cout<<decrypted;
return 0;
}``