Generating RSA keys in PKCS#1 format in Java

Question

When I generate an RSA key pair using the Java API, the public key is encoded in the X.509 format and the private key is encoded in the PKCS#8 format. I'm looking to encode both as PKCS#1. Is this possible? I've spent a considerable amount of time going through the Java docs but haven't found a solution. The result is the same when I use the Java and the Bouncy Castle providers.

Here is a snippet of the code:

KeyPairGenerator keygen = KeyPairGenerator.getInstance("RSA","BC"); keygen.initialize(1024); KeyPair pair = keygen.generateKeyPair(); PrivateKey priv = pair.getPrivate(); PublicKey pub = pair.getPublic(); byte[] privBytes = priv.getEncoded(); byte[] pubBytes = pub.getEncoded(); 

The two resulting byte arrays are formatted as X.509 (public) and PKCS#8 (private).

Any help would be much appreciated. There are some similar posts but none really answer my question.

Thank You

Answer 1

You will need BouncyCastle:

import org.bouncycastle.asn1.ASN1Encodable; import org.bouncycastle.asn1.ASN1Primitive; import org.bouncycastle.asn1.pkcs.PrivateKeyInfo; import org.bouncycastle.asn1.x509.SubjectPublicKeyInfo; import org.bouncycastle.util.io.pem.PemObject; import org.bouncycastle.util.io.pem.PemWriter; 

The code snippets below have been checked and found working with Bouncy Castle 1.52.

Private key

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 

Public key

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 

Thanks

Many thanks to the authors of the following posts:

• https://stackoverflow.com/a/8713518/1016580
• https://stackoverflow.com/a/14052651/1016580
• https://stackoverflow.com/a/14068057/1016580

Those posts contained useful, but incomplete and sometimes outdated info (i.e. for older versions of BouncyCastle), that helped me to construct this post.