[SOLVED] Decrypting a string with private key (RSA 2048 Key size)

Decrypting a string with private key (RSA 2048 Key size)

I am working with an API that returns sensitive data. This data will need to be decrypted on the client side with the key pair that has been generated by the client.

Below is the JSON Payload that will be sent to the server. This payload is used to encrypt the Sensitive Data. Note: I do not control the of API server.

{
    "exponent": "010001"
    "keyId": 0,
    "identifier": "RSA Key Identifier",
    "appName": "App name",
    "modulus": "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",
}

I do not have any prior knowledge of cryptography, but i researched on how to create a RSA key pair with openssl

Below the Config used to create the RSA key pair

$config = array(
    "digest_alg" => "sha512",
    "private_key_bits" => 2048,
    "private_key_type" => OPENSSL_KEYTYPE_RSA,
);


//Continue to create the key PAIR//
$resource = openssl_pkey_new($config);
openssl_pkey_export($resource, $privateKey);

$pubKeyArr = openssl_pkey_get_details($resource);
$publicKey = $pubKeyArr["key"];

file_put_contents('publicKey',$publicKey);
file_put_contents('privateKey', $privateKey);

Below is a copy of my generated public & private keys respectively.

Here are the challenges I faced.

I do not know how to generate the Modulus
I do not know how to come by the Exponent for my keys
I do not know how to generate the identifier for the RSA keys i generated earlier.

Here is what i tried so far.

Response from the API server (Base64 Encoded)

$extendedDetails = '{"ExtendedDetails": {
    "CardNumber": "{base64encryptedString}",
    "ExpiryDate": "{base64encryptedString}",
    "CVV2": "{base64encryptedString}",
    "CardLimits": []
}}';


$cDetails = json_decode($extendedDetails, true);

$pan = base64_decode($cDetails['ExtendedDetails']['CardNumber']);
$exp = base64_decode($cDetails['ExtendedDetails']['ExpiryDate']);
$cv2 = base64_decode($cDetails['ExtendedDetails']['CVV2']);

$decryption_result = openssl_private_decrypt($pan, $decryptedPan, file_get_contents('privateKey'));

var_dump($decryption_result); // Returns bool(false)//
var_dump($decryptedPan); // Empty String//

openssl_private_decrypt returns false, my data is not decrypted.

Solution

As pointed out by Topaco, your modulus field does not contain just the modulus, it contains the entire public key including the exponent, in SubjectPublicKeyInfo (or SPKI) format. To get just the modulus, you need to do something like:

$pem_pub = "-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAuIwf4CRrGzTnYYThKjX8
aMRYIcOZSC7U6/HF9qx5+MEPMx1lsAC8K0G35RMD1KLVpoX68eYGnDCj3dJf69wC
+pdY98vjGm9GQpN1TEQeyKBUVD8V7Fr5SmTaVFBWgllj3hjvogSImDS9quq2XshJ
fuLzTCXHm5MnK67goPitTgE3lZsBh18B1xxjnhbOG4lEceGC8zIqUAKZtTZteYMz
6rXuI+RmLE/YCoLYPryXTFGWkCwzkAPEPKFry/nP34yi1QfpTqwJCb7zTBJ3IzM3
ImteSreX6jHtiWKDU9NiPsNK+viiZ8+Gusxt2gWd7TXXZM5rHrxfMoqdrPjuzrWa
lQIDAQAB
-----END PUBLIC KEY-----";

$rsa_pub = openssl_pkey_get_public($pem_pub);
$pub_key_details = openssl_pkey_get_details($rsa_pub);
$modulus = $pub_key_details["rsa"]["n"];
$modulus_hex = bin2hex($modulus);