BouncyCastleUsage.java

import org.spongycastle.asn1.ASN1Encodable;
import org.spongycastle.asn1.ASN1Primitive;
import org.spongycastle.asn1.pkcs.PrivateKeyInfo;
import org.spongycastle.asn1.pkcs.RSAPrivateKey;
import org.spongycastle.crypto.params.AsymmetricKeyParameter;
import org.spongycastle.crypto.params.RSAKeyParameters;
import org.spongycastle.jcajce.provider.asymmetric.rsa.KeyFactorySpi;
import org.bouncycastle.util.io.pem.PemObject;
import org.bouncycastle.util.io.pem.PemWriter;

public class BouncyCastleUsage {
  
    public static void main(String[] args) {
      
        //Convert private key from PKCS8 to PKCS1: 
        PrivateKey priv = pair.getPrivate();
        byte[] privBytes = priv.getEncoded();

        PrivateKeyInfo pkInfo = PrivateKeyInfo.getInstance(privBytes);
        ASN1Encodable encodable = pkInfo.parsePrivateKey();
        ASN1Primitive primitive = encodable.toASN1Primitive();
        byte[] privateKeyPKCS1 = primitive.getEncoded();
      
        //Convert private key in PKCS1 to PEM:
        PemObject pemObject = new PemObject("RSA PRIVATE KEY", privateKeyPKCS1);
        StringWriter stringWriter = new StringWriter();
        PemWriter pemWriter = new PemWriter(stringWriter);
        pemWriter.writeObject(pemObject);
        pemWriter.close();
        String pemString = stringWriter.toString();
        // Check with command line OpenSSL that the key format is as expected:
        // openssl rsa -in rsa_private_key.pem -noout -text
      
        // Convert public key from X.509 SubjectPublicKeyInfo to PKCS1:
        PublicKey pub = pair.getPublic();
        byte[] pubBytes = pub.getEncoded();
        SubjectPublicKeyInfo spkInfo = SubjectPublicKeyInfo.getInstance(pubBytes);
        ASN1Primitive primitive = spkInfo.parsePublicKey();
        byte[] publicKeyPKCS1 = primitive.getEncoded();
      
        // Convert public key in PKCS1 to PEM:
        PemObject pemObject = new PemObject("RSA PUBLIC KEY", publicKeyPKCS1);
        StringWriter stringWriter = new StringWriter();
        PemWriter pemWriter = new PemWriter(stringWriter);
        pemWriter.writeObject(pemObject);
        pemWriter.close();
        String pemString = stringWriter.toString();
        // Check with command line OpenSSL that the key format is as expected:
        // openssl rsa -in rsa_public_key.pem -RSAPublicKey_in -noout -text
      
        // pkcs8Bytes contains PKCS#8 DER-encoded key as a byte[]
        PrivateKeyInfo pki = PrivateKeyInfo.getInstance(pkcs8Bytes);
        RSAPrivateKeyStructure pkcs1Key = RSAPrivateKeyStructure.getInstance(pki.getPrivateKey());
        byte[] pkcs1Bytes = pkcs1Key.getEncoded(); // etc.
    }
}