CODEWITHSUNDEEP
phpencryptionsha1hmac
Aditya Singh
Aditya Singh

Reputation: 852

Decryption in PHP

I am trying to decrypt data being communicated to our server. Its a certain 8 digit number which is encrypted using a scheme. I have the encryption and integrity keys with me. I have a documentation on how to decrypt it which says -

The value is encrypted using a custom encryption scheme. The encryption scheme uses a keyed HMAC-SHA1 algorithm to generate a secret pad based on the unique event ID. The encrypted value has a fixed length of 28 bytes. It is comprised of a 16-byte initialization vector, 8 bytes of ciphertext, and a 4-byte integrity signature. The encrypted value is web-safe base-64-encoded, according to RFC 3548, with padding characters omitted. Thus, the 28-byte encrypted value is encoded as a 38 character web-safe base-64 string. The value is encrypted as:

value xor HMAC-SHA1(encryption_key, initialization_vector)>

so decryption calculates:

HMAC-SHA1(encryption_key, initialization_vector)

and xor's with the encrypted value to reverse the encryption. The integrity stage takes 4 bytes of

HMAC-SHA1(integrity_key, value||initialization_vector)>

where || is concatenation.

So i wrote the following PHP Code.

$value= "[VALUE]"; //38 character base64
$ekey=hextostr("[ENCRYPTIONKEY]"); //64 byte hex encoded key . 32 byte key
$ikey=hextostr("[INTEGRITYKEY]"); //64 byte hex encoded key . 32 byte key

$value=str_replace("-","+",$value);
$value=str_replace("_","/",$value);
$value=$value."==";
$dvalue=base64_decode($value); //Gets a 28 byte encrypted string.

$initvec=substr($dvalue,0,16);
$ciphertext=substr($dvalue,16,8);
$integritysig=substr($dvalue,24,4);

$pad=hash_hmac("sha1",$initvec,$ekey);    //Generates 40 byte pad

$uncipher=$ciphertext^$pad;

print($uncipher); //This is 8 byte binary. Dumps some binary on screen. Result should be a 8 byte number

Unable to get around this problem. Please advice.

Upvotes: 2

Views: 6134

Answers (3)

sm9sh
sm9sh

Reputation: 11

Try this

function decrypt_google_winning_price($value, $ekey, $ikey, &$reason = '') {
if (strlen($value) != 38)
{
    $reason = "Wrong encrypted value length";
    return false;
}

$ekey = base64_decode($ekey);
$ikey = base64_decode($ikey);
$value = strtr($value, '-_,', '+/=') . "==";
$enc_value = base64_decode($value); //Gets a 28 byte encrypted string.
if (strlen($enc_value) != 28)
{
    $reason = "Wrong encrypted value length after base64_decode()";
    return false;
}

$iv = substr($enc_value, 0, 16);// initialization vector (16 bytes - unique to the impression)
$p = substr($enc_value, 16, 8); // encryption key (32 bytes - provided at account set up)
$sig = substr($enc_value, 24, 4);// integrity signature (4 bytes)
$price_pad = hash_hmac("sha1", $iv, $ekey, true);
$price = $p ^ $price_pad;// XOR

$conf_sig = substr(hex2bin(hash_hmac("sha1", $price . $iv, $ikey)), 0, 4);

if ($sig !== $conf_sig)
{
    $reason = "Signature is not valid";
    return false;
}

return  hexdec(bin2hex($price)); //This is 8 byte binary. Dumps some binary on screen. Result should be a 8 byte number
}


$value = "[VALUE]"; //38 character base64
$ekey = "[ENCRYPTIONKEY]"; //64 byte hex encoded key . 32 byte key
$ikey "[INTEGRITYKEY]"; //64 byte hex encoded key . 32 byte key

var_dump(decrypt_google_winning_price($value, $ekey, $ikey));

Upvotes: 1

n3vrax
n3vrax

Reputation: 51

Your posted code should look like this

$value= "[VALUE]"; //38 character base64
$ekey=hextostr("[ENCRYPTIONKEY]"); //64 byte hex encoded key . 32 byte key
$ikey=hextostr("[INTEGRITYKEY]"); //64 byte hex encoded key . 32 byte key

$value=str_replace("-","+",$value);
$value=str_replace("_","/",$value);
$value=$value."==";
$dvalue=base64_decode($value); //Gets a 28 byte encrypted string.

$initvec=substr($dvalue,0,16);
$ciphertext=substr($dvalue,16,8);
$integritysig=substr($dvalue,24,4);

//here is the change
$pad=hash_hmac("sha1",$initvec,$ekey, true);

$uncipher=$ciphertext^$pad;

print(hexdec(strToHex($uncipher))); //This is 8 byte binary. Dumps some binary on screen. Result should be a 8 byte number

Upvotes: 1

sarveshseri
sarveshseri

Reputation: 13985

$pad=hash_hmac("sha1",$initvec,$ekey); // returns a hexstring, but XOR interprets
                                       // as ASCII string and converts to binary
                                       // accordingly

$ciphertext=substr($dvalue,16,8); // this is ASCII, converted to binary by XOR

$uncipher=$ciphertext^$pad; // so the XOR operation is confused in interpretation.

try changing it to,

function bin2asc($in)#syntax - bin2asc("binary to convert");
{
  $out = '';
  for ($i = 0, $len = strlen($in); $i < $len; $i += 8)
  {
    $out .= chr(bindec(substr($in,$i,8)));
  }
  return $out; 
}

$pad= hash_hmac("sha1",$initvec,$ekey, true); // now it will return in binary 
$pad = bin2asc($pad);

$uncipher=$ciphertext^$pad;

hope this solve your problem.

Upvotes: 1

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