Decryption in PHP

Question

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.

• ndkrempel: When you say an "8-byte number", do you mean that? Or do you mean it's an ASCII encoding of the number in decimal? Or something else?
• David Harkness: Do you know the unique event ID on your end?
• Aditya Singh: No, We don't now the event ID

Answer 1

$pad=hash_hmac("sha1",$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.

• Aditya Singh: you are right. Earlier the pad was 40 bytes, now its 20 bytes. Now strlen($uncipher) returns 8 but when i echo it on screen, it shows blank. Not sure why
• sarveshseri: try it now... I think we need to keep both of them in ASCII or as binary number.
• Aditya Singh: nop .. the strlen($pad) is now 3 . Unciphered is also 3 bytes.
• sarveshseri: try var_dump() to check on every variable.

Answer 2

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

Answer 3

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));