使用公钥的InvalidKeySpecException

Question

I'm desperately trying to encrypt a message using asymmetric public / private key cryptography on an Android.

I'm on Windows and I've generated a public and private key using puttygen. I'm not sure what difference it makes but I've selected SSH-2 RSA. Here is the public key:

AAAAB3NzaC1yc2EAAAABJQAAAQEAh63orUzl0UTd7jj0KNYJg1+kNnty0QHyJu0r
Cajf5Kl7qWJaGXPfwsG8Qt3teafs5sv0JBSinab0s/5wfQmd1QPpXTMP93Wc4ucp
1VC/9B2o8XVi4fKoGTehB48yrSfI6KF2AIeASM1jUswydKxsuS4AS2mLGV/HuoKD
huMfCsRc8qK5zGQfVCoZTbQ66Z1yKdAzxMUuGmiTp7pVsle/P/UGbm6yFiee5r1/
dOR2CDyR6CP09Jaj7KSGfGuwPryCXPjEce1oCbN/FlLHVb7T1B5f6xhq+oY+Ij13
1IZPfShV8cs2kYKjsle2s23V5urSdWFv2tEcSJcpkUm2FlPdQw==

I've copied this to a text file in my main/assets folder. I read this in like so:

InputStream input = context.getAssets().open(filename);

This is then read in to a byte array through a fairly standard ByteArrayOutputStream method.

I then try and convert that to a public key as such:

public static PublicKey getPublicKey(byte[] keyBytes){
    PublicKey publicKey = null;

    if(keyBytes != null) {

        X509EncodedKeySpec spec = new X509EncodedKeySpec(keyBytes);
        KeyFactory kf = null;
        try {
            kf = KeyFactory.getInstance("RSA");
            publicKey = kf.generatePublic(spec);
        } catch (NoSuchAlgorithmException e) {
            Log.e(TAG, "NoSuchAlgorithmException");
            e.printStackTrace();
        } catch (InvalidKeySpecException e) {
            Log.e(TAG, "InvalidKeySpecException " + e.getMessage());
            e.printStackTrace();
        }
    }

    return publicKey;
}

Problem is I keep getting this error:

InvalidKeySpecException java.lang.RuntimeException: error:0D0680A8:asn1 encoding routines:ASN1_CHECK_TLEN:wrong tag

I've been attacking this for hours, and can't seem to get around it. Please please any suggestions welcome.

I've tried Base64 as such:

byte[] tempNewKey = Base64.decode(keyBytes, Base64.DEFAULT);

Which makes no difference and I've also tried using

RSAPublicKeySpec pubKeySpec = new RSAPublicKeySpec(new BigInteger(module), new BigInteger(exponent));

However putty doesn't tell me anything about an exponent? If I go ahead with this method I don't get the same error, but if I try and decrypt with my private key I just get gibberish.

Really hope you can help. Many Thanks

Answer 1

SSH keys are not X509 compatible keys. They are stored in a SSH proprietary format. You'll need a SSH capable libary to retrieve the key value.

If SSH functionality is not required then it is possible to generate keys in Java (using the keytool command line or KeyPairGenerator .

Alternatively it is also possible to use external applications or libraries such as the openssl command line. In the case of OpenSSL specify DER as output. Java expects a DER encoded SubjectPublicKeyInfo structure for X509EncodedKeySpec .