How to generate Private key from string using BouncyCastle

I have a String stored in a variable:

I generate public key as:

public static PublicKey getFromString(String keystr) throws Exception
{
  //String S1= asciiToHex(keystr);
   byte[] keyBytes = new sun.misc.BASE64Decoder().decodeBuffer(keystr);
   ASN1InputStream in = new ASN1InputStream(keyBytes);
   DERObject obj = in.readObject();
   RSAPublicKeyStructure pStruct = RSAPublicKeyStructure.getInstance(obj);
   RSAPublicKeySpec spec = new RSAPublicKeySpec(pStrcut.getModulus(), pStruct.getPublicExponent());
   KeyFactory kf = KeyFactory.getInstance("RSA");
   return kf.generatePublic(spec);
}

How to generate the PrivateKey using bouncy castle in android

{Edit}

Without using bouncy castle i am generating private key like this:

public static PrivateKey getKey(String mKey){
        try{
            // Read in the key into a String
            StringBuilder pkcs8Lines = new StringBuilder();
            BufferedReader rdr = new BufferedReader(new StringReader(mKey));
            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;
    }

I want to achieve the same using Bouncy Castle

Solution

I'm a little confused on why you insist on using bouncycastle, but if you really want to use bouncycastle then the CMS/PKIX library has a nice helper class called PEMParser that will shorten the code needed, e.g:

public static PrivateKey getPemPrivateKey(String mKey) throws Exception {
    PEMParser pemParser = new PEMParser(new StringReader(mKey));
    final PEMKeyPair pemKeyPair = (PEMKeyPair) pemParser.readObject();
    final byte[] encoded = pemKeyPair.getPrivateKeyInfo().getEncoded();
    KeyFactory kf = KeyFactory.getInstance("RSA");
    return kf.generatePrivate(new PKCS8EncodedKeySpec(encoded));

}