RSA algorithm on swift and java

Question

I need to generate public/private key for RSA algorithm on IOS device and send public key to server with encrypted text. Server must read public key and decrypt user message.

I have code on swift:

 func generateKeys(){
    var publicKey: SecKey?
    var privateKey: SecKey?

    let publicKeyAttr: [NSObject: NSObject] = [kSecAttrIsPermanent:true as NSObject, kSecAttrApplicationTag:"publicTag".data(using: String.Encoding.utf8)! as NSObject]
    let privateKeyAttr: [NSObject: NSObject] = [kSecAttrIsPermanent:true as NSObject, kSecAttrApplicationTag:"privateTag".data(using: String.Encoding.utf8)! as NSObject]

    var keyPairAttr = [NSObject: NSObject]()
    keyPairAttr[kSecAttrKeyType] = kSecAttrKeyTypeRSA
    keyPairAttr[kSecAttrKeySizeInBits] = 4096 as NSObject
    keyPairAttr[kSecPublicKeyAttrs] = publicKeyAttr as NSObject
    keyPairAttr[kSecPrivateKeyAttrs] = privateKeyAttr as NSObject

    _ = SecKeyGeneratePair(keyPairAttr as CFDictionary, &publicKey, &privateKey)

    var error:Unmanaged?
    if #available(iOS 10.0, *) {
        if let cfdata = SecKeyCopyExternalRepresentation(publicKey!, &error) {
            let data:Data = cfdata as Data
            let b64Key = data.base64EncodedString(options: .lineLength64Characters)
            print("public base 64 : \n\(b64Key)")
        }
        if let cfdata = SecKeyCopyExternalRepresentation(privateKey!, &error) {
            let data:Data = cfdata as Data
            let b64Key = data.base64EncodedString(options: .lineLength64Characters)
            print("private base 64 : \n\(b64Key)")
        }
    }
    let encrypted = encryptBase64(text: "test", key: publicKey!)
    let decrypted = decpryptBase64(encrpted: encrypted, key: privateKey!)

    print("decrypted \(String(describing: decrypted))")

    self.dismiss(animated: true, completion: nil);
}


func encryptBase64(text: String, key: SecKey) -> String {
    let plainBuffer = [UInt8](text.utf8)
    var cipherBufferSize : Int = Int(SecKeyGetBlockSize(key))
    var cipherBuffer = [UInt8](repeating:0, count:Int(cipherBufferSize))

    // Encrypto  should less than key length
    let status = SecKeyEncrypt(key, SecPadding.PKCS1, plainBuffer, plainBuffer.count, &cipherBuffer, &cipherBufferSize)
    if (status != errSecSuccess) {
        print("Failed Encryption")
    }

    let mudata = NSData(bytes: &cipherBuffer, length: cipherBufferSize)
    return mudata.base64EncodedString()
}

func decpryptBase64(encrpted: String, key: SecKey) -> String? {
    let data : NSData = NSData(base64Encoded: encrpted, options: .ignoreUnknownCharacters)!
    let count = data.length / MemoryLayout.size
    var array = [UInt8](repeating: 0, count: count)
    data.getBytes(&array, length:count * MemoryLayout.size)

    var plaintextBufferSize = Int(SecKeyGetBlockSize(key))
    var plaintextBuffer = [UInt8](repeating:0, count:Int(plaintextBufferSize))

    let status = SecKeyDecrypt(key, SecPadding.PKCS1, array, plaintextBufferSize, &plaintextBuffer, &plaintextBufferSize)

    if (status != errSecSuccess) {
        print("Failed Decrypt")
        return nil
    }
    return NSString(bytes: &plaintextBuffer, length: plaintextBufferSize, encoding: String.Encoding.utf8.rawValue)! as String
}

This code returns public key in PKCS1. I found the library: SwCrypt This code helps me to convert PKCS1 into PKCS8 and read public key with java

SwKeyConvert.PublicKey.pemToPKCS1DER(publicKeyPEM)

But I can't decrypt user message. Can you help me with message decryption? I wrote small unit test.

import org.junit.Test;

import javax.crypto.Cipher;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.X509EncodedKeySpec;
import java.util.Base64;

import static org.junit.Assert.assertNotNull;

public class TestExample {
    String publicKeyContent = "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";
    String encryptedMessage = "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";

    @Test
    public void encryptTest() throws Exception {
        PublicKey publicKey = convertPublicKey(publicKeyContent);
        assertNotNull(publicKey);

        String s = decryptString(publicKey, encryptedMessage);
        assertNotNull(s);
    }

    private PublicKey convertPublicKey(String publicKey) throws RSAAlgorithmException {
        try {
            KeyFactory keyFactory = KeyFactory.getInstance("RSA");
            //generate public key
            byte[] publicBytes = Base64.getDecoder().decode(publicKey);
            X509EncodedKeySpec keySpec = new X509EncodedKeySpec(publicBytes);
            return keyFactory.generatePublic(keySpec);
        } catch (NoSuchAlgorithmException | InvalidKeySpecException e) {
            throw new RSAAlgorithmException("Unable to generate public key from string " + publicKey + " . " + e.getMessage());
        }
    }

    private String decryptString(PublicKey publicKey, String value) throws Exception {
        byte[] decodedBytes;
        try {
            Cipher c = Cipher.getInstance("RSA/ECB/PKCS1Padding");
            c.init(Cipher.DECRYPT_MODE, publicKey);
            decodedBytes = c.doFinal(value.getBytes());
        } catch (Exception e) {
            System.out.println("Error = " + e);
            throw new Exception(e);
        }
        return new String(decodedBytes);
    }
}

I have next error:

java.lang.Exception: javax.crypto.IllegalBlockSizeException: Data must not be longer than 512 bytes

RSA algorithm on swift and java

Answers (1)

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