Php – How to decrypt a string on OpenSSL that was previously encrypted via Crypto++

crypto++opensslpemPHPrsa

EDIT: I found out that the keys aren't the problem like I said in the comments. I can use them without any issues to encrypt and decrypt data on OpenSSL.

But I need to decrypt a string on OpenSSL that was previously encrypted via Crypto++
and that's not working.

I'll post additional details later.

Hi, I have encrypted a string using an RSA public key generated with Crypto++ and now I'm trying (still unsuccessful) to decrypt it via PHP and OpenSSL.

That is what I'm doing:

The private key which is NOT base64 or hex encoded is stored in a file called "rsa-private.key"
The encrypted message is stored in "message.txt" (hex encoded)

STEP 1: Load the private key via: $key = file_get_contents("rsa-private.key");

STEP 2: Convert the key into PEM format using the following function:

<?php
function pkcs8_to_pem($der) {

    static $BEGIN_MARKER = "-----BEGIN PRIVATE KEY-----";
    static $END_MARKER = "-----END PRIVATE KEY-----";

    $value = base64_encode($der);

    $pem = $BEGIN_MARKER . "\n";
    $pem .= chunk_split($value, 64, "\n");
    $pem .= $END_MARKER . "\n";

    return $pem;
    }

    $PEMprivatekey = pkcs8_to_pem($key); 
?>

( stackoverflow.com/questions/1357569/ )

STEP 3: Prepare the key for further use by OpenSSL: (without any problems)

<?php
$privateKey = openssl_get_privatekey($PEMprivatekey);
if (!$privateKey) {
    echo "Cannot get public key";
}
?>

STEP 4: Get the message and decode the message using the following function:

<?php
function hex_to_str($hex){

    for ($i=0; $i < strlen($hex)-1; $i+=2) {
    $string .= chr(hexdec($hex[$i].$hex[$i+1])); }
    return $string;
}

$message = file_get_contents("message.txt");` 
$encryptedstring = hex_to_str($message);
?>

STEP 5: Decrypt the string: (does not work)

<?php
openssl_private_decrypt($encryptedstring, $decrypteddata, $privateKey);
if (!$decrypteddata) {
    echo "........"; } else { echo $decrypteddata; }
?>

$decrypteddata is always empty.

I can't figure out why it's not working. Anyone noticed something I'm doing wrong?

Best Answer

phpseclib is fully interoperable with OpenSSL and is generally regarded as being much easier to use. The following URL provides several examples of how to inter-operate with OpenSSL:

RSA Encrypt in PHP to decrypt in .NET

Related Solutions

RSA Public Key format

You can't just change the delimiters from ---- BEGIN SSH2 PUBLIC KEY ---- to -----BEGIN RSA PUBLIC KEY----- and expect that it will be sufficient to convert from one format to another (which is what you've done in your example).

This article has a good explanation about both formats.

What you get in an RSA PUBLIC KEY is closer to the content of a PUBLIC KEY, but you need to offset the start of your ASN.1 structure to reflect the fact that PUBLIC KEY also has an indicator saying which type of key it is (see RFC 3447). You can see this using openssl asn1parse and -strparse 19, as described in this answer.

EDIT: Following your edit, your can get the details of your RSA PUBLIC KEY structure using grep -v -- ----- | tr -d '\n' | base64 -d | openssl asn1parse -inform DER:

    0:d=0  hl=4 l= 266 cons: SEQUENCE          
    4:d=1  hl=4 l= 257 prim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
  265:d=1  hl=2 l=   3 prim: INTEGER           :010001

To decode the SSH key format, you need to use the data format specification in RFC 4251 too, in conjunction with RFC 4253:

   The "ssh-rsa" key format has the following specific encoding:

      string    "ssh-rsa"
      mpint     e
      mpint     n

For example, at the beginning, you get 00 00 00 07 73 73 68 2d 72 73 61. The first four bytes (00 00 00 07) give you the length. The rest is the string itself: 73=s, 68=h, ... -> 73 73 68 2d 72 73 61=ssh-rsa, followed by the exponent of length 1 (00 00 00 01 25) and the modulus of length 256 (00 00 01 00 7f ...).

C# RSA encryption/decryption with transmission

well there are really enough examples for this, but anyway, here you go

using System;
using System.Security.Cryptography;

namespace RsaCryptoExample
{
  static class Program
  {
    static void Main()
    {
      //lets take a new CSP with a new 2048 bit rsa key pair
      var csp = new RSACryptoServiceProvider(2048);

      //how to get the private key
      var privKey = csp.ExportParameters(true);

      //and the public key ...
      var pubKey = csp.ExportParameters(false);

      //converting the public key into a string representation
      string pubKeyString;
      {
        //we need some buffer
        var sw = new System.IO.StringWriter();
        //we need a serializer
        var xs = new System.Xml.Serialization.XmlSerializer(typeof(RSAParameters));
        //serialize the key into the stream
        xs.Serialize(sw, pubKey);
        //get the string from the stream
        pubKeyString = sw.ToString();
      }

      //converting it back
      {
        //get a stream from the string
        var sr = new System.IO.StringReader(pubKeyString);
        //we need a deserializer
        var xs = new System.Xml.Serialization.XmlSerializer(typeof(RSAParameters));
        //get the object back from the stream
        pubKey = (RSAParameters)xs.Deserialize(sr);
      }

      //conversion for the private key is no black magic either ... omitted

      //we have a public key ... let's get a new csp and load that key
      csp = new RSACryptoServiceProvider();
      csp.ImportParameters(pubKey);

      //we need some data to encrypt
      var plainTextData = "foobar";

      //for encryption, always handle bytes...
      var bytesPlainTextData = System.Text.Encoding.Unicode.GetBytes(plainTextData);

      //apply pkcs#1.5 padding and encrypt our data 
      var bytesCypherText = csp.Encrypt(bytesPlainTextData, false);

      //we might want a string representation of our cypher text... base64 will do
      var cypherText = Convert.ToBase64String(bytesCypherText);


      /*
       * some transmission / storage / retrieval
       * 
       * and we want to decrypt our cypherText
       */

      //first, get our bytes back from the base64 string ...
      bytesCypherText = Convert.FromBase64String(cypherText);

      //we want to decrypt, therefore we need a csp and load our private key
      csp = new RSACryptoServiceProvider();
      csp.ImportParameters(privKey);

      //decrypt and strip pkcs#1.5 padding
      bytesPlainTextData = csp.Decrypt(bytesCypherText, false);

      //get our original plainText back...
      plainTextData = System.Text.Encoding.Unicode.GetString(bytesPlainTextData);
    }
  }
}

as a side note: the calls to Encrypt() and Decrypt() have a bool parameter that switches between OAEP and PKCS#1.5 padding ... you might want to choose OAEP if it's available in your situation

Best Answer

Related Solutions

RSA Public Key format

C# RSA encryption/decryption with transmission

Related Topic