How to - SHA1 hash of plaintext using SHA1CryptoServiceProvider in C++/CLI? - hash

Question

I want to hash a string but looking at the example on MSDN I got stuck on the DATA_SIZE. What is this? and how do I know ahead of time what the size of the array is if the plaintext can vary in length?
Also I need to return the result as a vector (consuming method expects this)
Code from MSDN
array<Byte>^ data = gcnew array<Byte>( DATA_SIZE );
array<Byte>^ result;
SHA1^ sha = gcnew SHA1CryptoServiceProvider;
// This is one implementation of the abstract class SHA1.
result = sha->ComputeHash( data );
My method so far looks like
std::vector<byte> sha1(const std::string& plaintext)
{
//#define SHA1_BUFFER_SIZE ????
//array<System::Byte>^ data = gcnew array<System::Byte>(DATA_SIZE);
//convert plaintext string to byte array
array<System::Byte>^ result;
SHA1^ sha = gcnew SHA1CryptoServiceProvider;
result = sha->ComputeHash(data);
//return result as a vector<byte>
}

Answer 1

First, answering your questions:
On the referred MSDN page, it was stated before the example:
This example assumes that there is a predefined constant DATA_SIZE.
So, it was assumed that there was a #define or a enum constant predefined
Of course you don't know the size of the plaintext. But you'll see that in this approach you don't need the DATA_SIZE constant
If you want your function to receive as input a std:string and to output a std::vector, there are a lot of steps to be done. It would look something like that:
std::vector<unsigned char> sha1(const std::string& message)
{
// Steps #1 and #2: convert from std::string to managed string
// and convert from a string to bytes (UTF-8 as example)
array<Byte>^data = Encoding::UTF8->GetBytes(gcnew String(message.c_str() ));
// Steps #3 and #4: perform hash operation
// and store result in a byte array
SHA1^ sha = gcnew SHA1CryptoServiceProvider;
array<Byte>^ array_result = sha->ComputeHash(data);
// Step #5: convert from a managed array
// to unmanaged vector
std::vector<unsigned char> vector_return(array_result->Length);
Marshal::Copy(array_result, 0, IntPtr(&vector_return[0]), array_result->Length);
return vector_return;
}
don't forget to declare the namespaces you're using:
using namespace System;
using namespace System::Text;
using namespace System::Security::Cryptography;
using namespace System::Runtime::InteropServices;
int main(array<System::String ^> ^args)
{
std::string str_test("This is just a test.");
std::vector<unsigned char> vec_digest( sha1(str_test) );
return 0;
}
Final thoughts:
It's a bit misleading to name the string you want to hash as plaintext. More accurate terms would be message (for the input data) and digest (for the hash value)
Avoid mixing the use of std::string and managed String^ if you can.