Exception in generating the private key from RSA key in marshmallow device in android

Question

I am using this code to generate a private key from RSA key

public class ReadPKCS8Pem {
    private final static String PRIVATE_KEY = Keys.PRIVATE_KEY;

    public static PrivateKey getKey(){
        try{
            // Read in the key into a String
            StringBuilder pkcs8Lines = new StringBuilder();
            BufferedReader rdr = new BufferedReader(new StringReader(PRIVATE_KEY));
            String line;
            while ((line = rdr.readLine()) != null) {
                pkcs8Lines.append(line);
            }

            // Remove the "BEGIN" and "END" lines, as well as any whitespace

            String pkcs8Pem = pkcs8Lines.toString();
            pkcs8Pem = pkcs8Pem.replace("-----BEGIN RSA PRIVATE KEY-----", "");
            pkcs8Pem = pkcs8Pem.replace("-----END RSA PRIVATE KEY-----", "");
            pkcs8Pem = pkcs8Pem.replaceAll("\\s+","");

            // Base64 decode the result

            byte [] pkcs8EncodedBytes = Base64.decode(pkcs8Pem, Base64.DEFAULT);

            // extract the private key

            PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(pkcs8EncodedBytes);
            KeyFactory kf = KeyFactory.getInstance("RSA");
            PrivateKey privKey = kf.generatePrivate(keySpec);
            System.out.println(privKey);

            return privKey;
        }catch (Exception ex){
            ex.printStackTrace();
        }
        return null;
    }
}

In the line:

PrivateKey privKey = kf.generatePrivate(keySpec);

I am getting the exception:

java.lang.RuntimeException: error:0c0890ba:ASN.1 encoding routines:asn1_check_tlen:WRONG_TAG

• I get this exception in my marshmallow device but in my Oreo device, it works fine and i am able to generate the private key successfully
• How to resolve this

Answer 1

The header -----BEGIN RSA PRIVATE KEY----- in the PEM file means it contains a PKCS#1 key, but Java (and Android )does not support pkcs1, so you need to convert the key from pkcs1 to pcks8 using a tool like openssl

# openssl pkcs8 -topk8 -inform PEM -outform PEM -nocrypt -in pkcs1.key -out pkcs8.key

Alternatively you can load the key using Bouncycastle. See Read RSA private key of format PKCS1 in JAVA

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I do not know why it works in Oreo, it could be that the underliying provider will support the conversion even if it is not documented https://developer.android.com/reference/java/security/spec/PKCS8EncodedKeySpec