How to store and retrieve an RSA public key in Android keystore which is generated from server side application?

Question

The server side application generate an RSA key pair,and as a part of key exchange process i get this public key from server and now i want to store this key inside android KeyStore. I have seen examples with generating rsa key pair using KeyGenerator and storing them in android KeyStore but i have no clue on how to store an already generated RSA public key. Any help would be highly appreciated.

Daughter

Answer 1

I'm not sure what parts are confusing you, or what exactly you are trying to achieve. KeyStore can only hold three types of entries: symmetric keys, private keys, and certificates. Therefore, if you want to store a public key it must be formatted into a certificate. Whether your application uses any of the other features of X509 certificates is up to you.

It's also not clear if you want to use any old keystore on Android, or the AndroidKeyStore. The latter offers enhanced features, but whether you need them or not is dependent on your threat model. In any event, here is an example that shows how to import a google public key into the Android Keystore, how to set some properties on it, and how to use it to encrypt an AES key.

import android.security.keystore.KeyProperties;
import android.security.keystore.KeyProtection;
import android.util.Base64;
import android.util.Log;

import java.io.ByteArrayInputStream;
import java.nio.charset.StandardCharsets;
import java.security.Key;
import java.security.KeyStore;
import java.security.cert.Certificate;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;

public class MainActivity extends AppCompatActivity {

    private static final String THE_CERT = "-----BEGIN CERTIFICATE-----\n" +
            "MIIIPjCCByagAwIBAgIIWcyJ5Cnzp3UwDQYJKoZIhvcNAQELBQAwSTELMAkGA1UE\n" +
            "BhMCVVMxEzARBgNVBAoTCkdvb2dsZSBJbmMxJTAjBgNVBAMTHEdvb2dsZSBJbnRl\n" +
            "cm5ldCBBdXRob3JpdHkgRzIwHhcNMTgwMzEzMTgzMDQ1WhcNMTgwNjA1MTgxNjAw\n" +
            "WjBmMQswCQYDVQQGEwJVUzETMBEGA1UECAwKQ2FsaWZvcm5pYTEWMBQGA1UEBwwN\n" +
            "TW91bnRhaW4gVmlldzETMBEGA1UECgwKR29vZ2xlIEluYzEVMBMGA1UEAwwMKi5n\n" +
            "b29nbGUuY29tMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAtXqeeS6r\n" +
            "sLfE5dx5asD7dngw0Dev9rhgDYM9kAuV9VxbZJ2ehZM4Nk1zGGSlqidgRWsVSNrx\n" +
            "qb513IyrtxDSvTTGh8ihFGNTL/H61e+cYU565RCw4siOU0IevyhynPVh8D38pe5U\n" +
            "bkGDmkiP7tOVozQE+3Q7l6xaIvlq9hIAb0aTWdJ6AOm3r/iMRdiUv/kxIienQ4v/\n" +
            "RY/h3K/llz1E+S+TAyM2+As8o2nRMGrp9/hg8zIs3CLLv2km9VS/fgTQrM5pcfDf\n" +
            "iX6Tgzb+6RSGHnT7GgNA3R1LXo96gnwf3zlX3SqpvV8pQf2Y3TxhLRB7J28yZFef\n" +
            "P6d9t2EqlHZv+wIDAQABo4IFCzCCBQcwEwYDVR0lBAwwCgYIKwYBBQUHAwEwggPh\n" +
            "BgNVHREEggPYMIID1IIMKi5nb29nbGUuY29tgg0qLmFuZHJvaWQuY29tghYqLmFw\n" +
            "cGVuZ2luZS5nb29nbGUuY29tghIqLmNsb3VkLmdvb2dsZS5jb22CFCouZGI4MzM5\n" +
            "NTMuZ29vZ2xlLmNuggYqLmcuY2+CDiouZ2NwLmd2dDIuY29tghYqLmdvb2dsZS1h\n" +
            "bmFseXRpY3MuY29tggsqLmdvb2dsZS5jYYILKi5nb29nbGUuY2yCDiouZ29vZ2xl\n" +
            "LmNvLmlugg4qLmdvb2dsZS5jby5qcIIOKi5nb29nbGUuY28udWuCDyouZ29vZ2xl\n" +
            "LmNvbS5hcoIPKi5nb29nbGUuY29tLmF1gg8qLmdvb2dsZS5jb20uYnKCDyouZ29v\n" +
            "Z2xlLmNvbS5jb4IPKi5nb29nbGUuY29tLm14gg8qLmdvb2dsZS5jb20udHKCDyou\n" +
            "Z29vZ2xlLmNvbS52boILKi5nb29nbGUuZGWCCyouZ29vZ2xlLmVzggsqLmdvb2ds\n" +
            "ZS5mcoILKi5nb29nbGUuaHWCCyouZ29vZ2xlLml0ggsqLmdvb2dsZS5ubIILKi5n\n" +
            "b29nbGUucGyCCyouZ29vZ2xlLnB0ghIqLmdvb2dsZWFkYXBpcy5jb22CDyouZ29v\n" +
            "Z2xlYXBpcy5jboIUKi5nb29nbGVjb21tZXJjZS5jb22CESouZ29vZ2xldmlkZW8u\n" +
            "Y29tggwqLmdzdGF0aWMuY26CDSouZ3N0YXRpYy5jb22CCiouZ3Z0MS5jb22CCiou\n" +
            "Z3Z0Mi5jb22CFCoubWV0cmljLmdzdGF0aWMuY29tggwqLnVyY2hpbi5jb22CECou\n" +
            "dXJsLmdvb2dsZS5jb22CFioueW91dHViZS1ub2Nvb2tpZS5jb22CDSoueW91dHVi\n" +
            "ZS5jb22CFioueW91dHViZWVkdWNhdGlvbi5jb22CByoueXQuYmWCCyoueXRpbWcu\n" +
            "Y29tghphbmRyb2lkLmNsaWVudHMuZ29vZ2xlLmNvbYILYW5kcm9pZC5jb22CG2Rl\n" +
            "dmVsb3Blci5hbmRyb2lkLmdvb2dsZS5jboIcZGV2ZWxvcGVycy5hbmRyb2lkLmdv\n" +
            "b2dsZS5jboIEZy5jb4IGZ29vLmdsghRnb29nbGUtYW5hbHl0aWNzLmNvbYIKZ29v\n" +
            "Z2xlLmNvbYISZ29vZ2xlY29tbWVyY2UuY29tghhzb3VyY2UuYW5kcm9pZC5nb29n\n" +
            "bGUuY26CCnVyY2hpbi5jb22CCnd3dy5nb28uZ2yCCHlvdXR1LmJlggt5b3V0dWJl\n" +
            "LmNvbYIUeW91dHViZWVkdWNhdGlvbi5jb22CBXl0LmJlMGgGCCsGAQUFBwEBBFww\n" +
            "WjArBggrBgEFBQcwAoYfaHR0cDovL3BraS5nb29nbGUuY29tL0dJQUcyLmNydDAr\n" +
            "BggrBgEFBQcwAYYfaHR0cDovL2NsaWVudHMxLmdvb2dsZS5jb20vb2NzcDAdBgNV\n" +
            "HQ4EFgQU2Xh9D7F5dJYCBqsWcKChI16NReswDAYDVR0TAQH/BAIwADAfBgNVHSME\n" +
            "GDAWgBRK3QYWG7z2aLV29YG2u2IaulqBLzAhBgNVHSAEGjAYMAwGCisGAQQB1nkC\n" +
            "BQEwCAYGZ4EMAQICMDAGA1UdHwQpMCcwJaAjoCGGH2h0dHA6Ly9wa2kuZ29vZ2xl\n" +
            "LmNvbS9HSUFHMi5jcmwwDQYJKoZIhvcNAQELBQADggEBAFxBsH2U6j4KzZbNcyN1\n" +
            "UGiJnMn64DIXH8wsWrFEGAq3ONRhPgKd3AnbaBUdNdrRgOhfA3RtLvvnxsKn0rX6\n" +
            "Oz8+p5DZxJooUgWlet9NounLDe5um6m5NqLIGefdI49Ukn6IwBtCO5DD7rZTygTa\n" +
            "B499H9N0ixI9wGBdlZ37tOpCxayNb08eizU1uQEhb1/oxnXf0e6trPfC8krDL0Ks\n" +
            "Pyf3JgB5oBTiNAfix2zme1FrpXcKehOj2urnLQRr5EpminCJ+0uHI1sqiJbcSHrU\n" +
            "6TPQcOzZ7/haw1yY2bpy+sB4oXUMaNJxh6e2AiCeVf4MtX9EsYEnhsfc2XS50J32\n" +
            "nKo=\n" +
            "-----END CERTIFICATE-----";

    private void doExample() {
        try {
            X509Certificate googleCert = (X509Certificate) CertificateFactory.getInstance("X509").generateCertificate(new ByteArrayInputStream(THE_CERT.getBytes(StandardCharsets.UTF_8)));
            KeyStore keyStore = KeyStore.getInstance("AndroidKeyStore");
            KeyProtection keyProtection = new KeyProtection.Builder(KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_VERIFY)
                    .setDigests(KeyProperties.DIGEST_SHA1, KeyProperties.DIGEST_SHA256)
                    .setRandomizedEncryptionRequired(true)
                    .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_RSA_OAEP)
                    .setSignaturePaddings(KeyProperties.SIGNATURE_PADDING_RSA_PSS)
                    .setUserAuthenticationRequired(false)
                    .build();
            keyStore.load(null);
            keyStore.setEntry("googlecert", new KeyStore.TrustedCertificateEntry(googleCert), keyProtection);
            // Now use the entry
            KeyGenerator aesKeygen = KeyGenerator.getInstance("AES");
            aesKeygen.init(128);
            Key aesKey = aesKeygen.generateKey();
            // Wrap key for transport
            KeyStore keyStore2 = KeyStore.getInstance("AndroidKeyStore");
            keyStore2.load(null);
            Certificate googleCert2 = keyStore2.getCertificate("googlecert");
            Cipher rsaCipher = Cipher.getInstance("RSA/ECB/OAEPwithSHA-256andMGF1Padding");
            rsaCipher.init(Cipher.WRAP_MODE, googleCert2);
            byte[] rsaEncrypted = rsaCipher.wrap(aesKey);
            Log.d("crypt", Base64.encodeToString(rsaEncrypted, Base64.DEFAULT));
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}

Amateur

I was trying to store public key in AndroidKeyStore as if we store a symmetric key. Now i understood that it should be formatted into a certificate in order to store in AndroidKeyStore. – Daughter

One question, since there is nothing a hacker can do with a public key, is it okay to store it in SharedPreference or embed it in a string constant? – Daughter

Yes, you can store it in almost any way. It's doesn't need to be secret, but it shouldn't be changeable by a hacker. – Amateur