Problem decrypting a Java RSA/ECB/PKCS1Padding encrypted string in Swift

Question

My iOS app needs to decrypt a string that was encrypted on a Java backend. The backend encrypts the string using these parameters: Cipher.getInstance("RSA/ECB/PKCS1Padding").

As a simple proof of concept, using OpenSSL from the Terminal I extracted the private key from the pfx certificate that they sent me:

$ openssl pkcs12 -in certificate.pfx -nocerts -out key.pem -nodes
$ openssl rsa -in key.pem -out server.key

Then, I hardcoded the private key as a String in my code and tried to decrypt the encrypted message using the SwiftyRSA lib:

import UIKit
import SwiftyRSA

class ViewController: UIViewController {

    private let rsaPemPrivateKey = """
      -----BEGIN RSA PRIVATE KEY-----
      THE ACTUAL PRIVATE KEY...
      -----END RSA PRIVATE KEY-----
      """

    private let encryptedString = "ciSvTLKRAMqz8d8SEX8epvkyyrfAoiX28Sd1RhYbsz7oOg3/taXZxZkTeCbVeFXB8Mf8eWn2SGMcVIrmFGuxqyRoC5SluJDgrup0wcG/XRg7lrt7oD6esVBAGZT5nmS79mUoAC4CKW5cHclCh66JUysATSIvI9qrrXNUnBApGJc="

    override func viewDidLoad() {
        super.viewDidLoad()

        do {
            try decryptWithPrivateKey()
        } catch {
            print("Exception:")
            print(error)
        }
    }

    private func decryptWithPrivateKey() throws {
        let privateKey = try PrivateKey(pemEncoded: rsaPemPrivateKey)
        let encrypted = try EncryptedMessage(base64Encoded: encryptedString)
        let clear = try encrypted.decrypted(with: privateKey, padding: SecPadding.PKCS1)

        print("Decrypted string is: \(try clear.string(encoding: .utf8))")
    }
}

This results in the following error, when the method encrypted.decrypted is called:

Exception:
chunkDecryptFailed(index: 0)

I also tried with other libraries, but similar errors occurred.

Here's the result with the SwiftRSA lib:

Exception:
decryptionFailed(error: Optional(Error Domain=NSOSStatusErrorDomain Code=-50 "RSAdecrypt wrong input (err -1)" UserInfo={NSDescription=RSAdecrypt wrong input (err -1)}))

And here's the result with the SwCrypt lib:

Exception:
decodeError

I'm not sure what could be wrong here. At first I thought the problem might be with the private key, but none of the libraries throw an exception when I instantiate their corresponding PrivateKey objects with it. Perhaps the problem is with the encrypted string I'm trying to decrypt, but it seems that no base64 decoding is required, as the EncryptedMessage object expects a base 64 encoded string anyway.

Is there anything else I can try to decrypt this RSA/ECB/PKCS1Padding encrypted string in Swift? (although in Java they pass ECB as the mode of operation, it looks like it don't actually use ECB under the hood, so I don't think I need to implement any custom management of the block size of the encrypted string as they say here).

-- Edit --

Here's a few other details as requested by zaitsman:

Dummy certificate's private key:

Java code used to encrypt (the minimal reproducible example):

I asked the backend team for an example of the code they use to encript, and they sent me this Kotlin function:

fun String.encryptWithPublicKey(publicKey: String): String {
    try {
        val base64Decoder = Base64.getDecoder()
        val decoded = base64Decoder.decode(publicKey.toByteArray(Charsets.UTF_8))
        
        val key = RSAPublicKey.getInstance(decoded)
        val keySpec = RSAPublicKeySpec(key.modulus, key.publicExponent)
        val rsaKey = KeyFactory.getInstance("RSA").generatePublic(keySpec)

        val cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding")
        cipher.init(Cipher.ENCRYPT_MODE, rsaKey)

        val encrypted = cipher.doFinal(this.toByteArray(Charsets.UTF_8))

        return ByteString.of(encrypted, 0, encrypted.size).base64()
    } catch (e: Exception) {
        throw e
    }
}

Sample encrypted string, b64 encoded:

Here's the base 64 encoded, encrypted string I get from the backend response:

ciSvTLKRAMqz8d8SEX8epvkyyrfAoiX28Sd1RhYbsz7oOg3/taXZxZkTeCbVeFXB8Mf8eWn2SGMcVIrmFGuxqyRoC5SluJDgrup0wcG/XRg7lrt7oD6esVBAGZT5nmS79mUoAC4CKW5cHclCh66JUysATSIvI9qrrXNUnBApGJc=

Sample raw value:

I don't have the exact value right now, but it should be a numeric value similar to this:

1234 5678 9012 3456

Problem decrypting a Java RSA/ECB/PKCS1Padding encrypted string in Swift

0 Answers