Aditya Singh Aditya Singh - 1 year ago 88
PHP Question

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

$dvalue=base64_decode($value); //Gets a 28 byte encrypted string.


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

Answer Source
$pad=hash_hmac("sha1",$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);


hope this solve your problem.

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