Converting A public key in SubjectPublicKeyInfo format to RSAPublicKey format java

Question

The PublicKey.getEncoded(), returns a byte array containing the public key in SubjectPublicKeyInfo (x.509) format, how do i convert it to RSA public key encoding?

Answer 1

Use Bouncy Castle's SubjectPublicKeyInfo, like this:

byte[] encoded = publicKey.getEncoded();
SubjectPublicKeyInfo subjectPublicKeyInfo = new SubjectPublicKeyInfo(
    ASN1Sequence.getInstance(encoded));
byte[] otherEncoded = subjectPublicKeyInfo.parsePublicKey().getEncoded();

Answer 2

Without BouncyCastle:

PublicKey publicKey = KeyFactory.getInstance("RSA").generatePublic(new X509EncodedKeySpec(publicKeyBinary));                

Answer 3

The following snippet of code worked for me, had to use BouncyCastle though.

byte[] keyBytes = key.getEncoded(); // X.509 for public key
SubjectPublicKeyInfo subPkInfo = new SubjectPublicKeyInfo((ASN1Sequence)ASN1Object.fromByteArray(keyBytes));
byte[] rsaformat = subPkInfo.getPublicKey().getDEREncoded();