How to get human readable std::string from RSA_private_encrypt - c++

Question

I have RSA encrypted my string but it's now a unsigned char *. How do I create a human readable std::string that I can output for the user? I want to use it in an amazon signed url. Here are the meat and potatoes of the code from GitHub
unsigned char* RSA_SHA1_Sign(std::string policy, RSA *privateKey) throw(std::runtime_error)
{
//sha1 digest the data
unsigned char hash[SHA_DIGEST_LENGTH] = {'0'};
SHA1((const unsigned char *)policy.c_str(), policy.length(), hash);
// Sign the data
int rsaSize = RSA_size(privateKey);
// std::unique_ptr<unsigned char[]> signedData(new unsigned char[size]);//if c++11 available
unsigned char *signedData = (unsigned char *)malloc(sizeof(unsigned char) * rsaSize);
unsigned int signedSize = 0;
//use RSA_sign instead of RSA_private_encrypt
if(!RSA_sign(NID_sha1, hash, SHA_DIGEST_LENGTH, signedData, &signedSize, privateKey)){
throw std::runtime_error("Failed to sign");
}
return signedData;
}
std::string base64Encode(unsigned char *signedData)
{
//prepare
BIO *b64 = BIO_new(BIO_f_base64());
BIO *bmem = BIO_new(BIO_s_mem());
BIO_set_flags(b64, BIO_FLAGS_BASE64_NO_NL);
b64 = BIO_push(b64, bmem);
//write
BIO_write(b64, signedData, 256);
BIO_flush(b64);
//create string
BUF_MEM *bptr;
BIO_get_mem_ptr(b64, &bptr);
std::string base64String(bptr->data);
BIO_free_all(b64);
return base64String;
}
int main(int argc, const char * argv[]) {
RSA *privateKey = createRSAFromPrivateKeyFile("/path/to/privatekey");
std::string sourceString = "testing";
std::string signature = RSA_SHA1_Sign(sourceString, privateKey);
std::string encodedSignature = base64Encode(signature);
std::cout << "RESULT: " << encodedSignature << std::endl;
return 0;
}
UPDATE: I was using the wrong sign function. Once updated, using base64 encode gave me the correct string.
RSA_PKCS1_PADDING
PKCS #1 v1.5 padding. This function does not handle the algorithmIdentifier specified in PKCS #1.
When generating or verifying PKCS #1 signatures, RSA_sign(3) and RSA_verify(3) should be used.

Answer 1

To save all the data, use this std::string constructor: std::string( char *data, int size ). The size will be useful as the output MIGHT contain a null character.
To send it to amazon over an url, consider using the base64 encoding, again, as the encrypted data might contain NULLs and other shenanigans.

Answer 2

Firstly, to get it into an std::string object, which will probably be helpful in general:
std::string s{private_key, size};
However, to then make that compatible with Amazon's scheme you'll need to pick out (or write your own) Base64 library and URL encoder to escape special URL chars. A cursory search of Google or StackOverflow will provide you with what you need in this respect and it's beyond the scope of this question to write out how to do Base64 encoding and URL escaping in C++.
Also, since you're using C++, consider std::unique_ptr<unsigned char[]> rather than straight-up malloc();
std::unique_ptr<unsigned char[]> signedData{new unsigned char[size]};