Code to decrypt the encrypted SHA256 code into string in crypto++

Question

I am using crypto++ to encrypt and decrypt the string .The code is shown below. The code encrypts the username and password .But I don't know how to I decrypt it one again into string. What is the code to decrypt the encrypted SHA256 code into string. Can any one help me.

#include <cryptopp/hex.h>
#include <cryptopp/sha.h>
#include <cryptopp/base64.h>
#include <iostream>
#include <string>

int main()
{
  CryptoPP::SHA256 hash;
  byte digest[CryptoPP::SHA256::DIGESTSIZE];
  std::string username, password, salt, output;
  std::cout << "Enter username: ";
  std::getline(std::cin,username);
  std::cout << std::endl << "Enter password: ";
  std::getline(std::cin,password);
  salt = username + password;

  hash.CalculateDigest(digest,(const byte *)salt.c_str(),salt.size());

  CryptoPP::HexEncoder encoder;
  CryptoPP::StringSink *SS = new CryptoPP::StringSink(output);
  encoder.Attach(SS);
  encoder.Put(digest,sizeof(digest));
  encoder.MessageEnd();

  std::cout << "The username/password salted hash is => " << output << std::endl;
  return 0;
}

Please read en.wikipedia.org/wiki/Cryptographic_hash_function - what you're asking is infeasible by design. — Jon Skeet
– Jon Skeet, Commented Feb 20, 2013 at 17:06

thiton · Accepted Answer · 2013-02-20 17:27:19Z

5

This code is not performing encryption, as the commentors already pointed out, but hashing. The central difference is that hashing, by design, is not reversible cheaply. This is important in password applications because you do explicitly not want to store the user's passwords in any accessible form, but only check against them.

So, in short words: You cannot "decrypt" your hash.

When you want to check a supplied password for correctness, you hash it again like in your code and compare the hash against the hash of the original password.

answered Feb 20, 2013 at 17:27

thiton

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1 Comment

jww Over a year ago

"... and compare the hash against the hash of the original password" - be sure to use Crypto++'s VerifyBufsEqual to avoid timing attacks on the comparison.

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