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?

Tellurize

Answer 1

Use Bouncy Castle's SubjectPublicKeyInfo, like this:

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

Keeter

This doesn't serve the pourpose. here we have just casted the public key into a RSAPublicKey object, but when i say RSAPublicKey.getEncoded(), i still get the key in x.509 format and not RSA format. – Tellurize

Thanks a bunch! Your approach worked. I am posting the exact snippet i used. – Tellurize

The above comment from @AshishKumarShah seems incorrect; I do get a DER encoded key of course, but it does seem to be a PKCS#1 formatted key (ps Hi Martijn, hope everything is OK). – Peroneus

Hey Maarten. @AshishKumarShah's comment was probably about my answer before I edited (back in 2012 :) ). – Keeter

Answer 2

Without BouncyCastle:

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

Everara

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();

Tellurize

The getPublicKey() is a deprecated function now. – Deragon

new SubjectPublicKeyInfo(..) is deprecated as well: Use SubjectPublicKeyInfo.getInstance(...). And instead of getPublicKey() use parsePublicKey(). – Melaniemelanin