PubConvert.java is a simply utility Java class which reads an X.509 encoded SubjectPublicKeyInfo file, instantiates an RSAPublicKey instance using an RSA Keyfactory, extracts the RSA public key components and writes:
java PubConvert SubjectPublicKeyInfo_FileSince Java generally reads/writes multi-byte data in big-endian format, and ASN.1 octets are always in big-endian order, but the PUBLICKEYBLOB has members formatted in little-endian order, the parsing in Java must reformat the data and reverse byte arrays. However, XML RSA keys are encoded in big-endian format so byte reversal is not necessary.
The class contains two static method:
private static byte subjectpublickeyinfoToPublickeyblob(byte encodedPubkey, int keyspec)
private static String subjectpublickeyinfoToXMLRSAPubKey(byte encodedPubkey)
The PUBLICKEYBLOB file is constructed according to the PUBLICKEYBLOB specification. The modulus data extracted in Java might contain a most significant (first byte) of zero, which must be removed before writing the PUBLICKEYBLOB and b64 encoding for the XML representation.
Pempublic: X.509 SubjectPublicKeyInfo Decoder
CryptoAPI PUBLICKEYBLOB to Java PublicKey Converter