X509PublicKey - non-standard additions??

[James Vanns]

In my quest for getting Java to open my Crypto++ RSA 1024-bit public key
I've noted that RSAFunction inherits from X509PublicKey.

X509PublicKey defines its method DEREncode as:

void DEREncode(BufferedTransformation &bt) const {
DERSequenceEncoder subjectPublicKeyInfo(bt);
DERSequenceEncoder algorithm(subjectPublicKeyInfo);
GetAlgorithmID().DEREncode(algorithm);
DEREncodeAlgorithmParameters(algorithm);
algorithm.MessageEnd();

DERGeneralEncoder subjectPublicKey(
subjectPublicKeyInfo, BIT_STRING);

subjectPublicKey.Put(0); // unused bits
DEREncodeKey(subjectPublicKey);
subjectPublicKey.MessageEnd();
subjectPublicKeyInfo.MessageEnd();
}

Now, according to ASN1 syntax X509 public keys should be represented as:

SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING }

So my question is: in the code above why are the algorithm parameters included?
And whats the extra 'unused bits' for? Now, admittedly even though I have a
modicum of understanding of Cryptography - my knowledge of PK standards is practically
non existent so I apologise in advance for any misunderstandings on my part.

I'm just trying to get Java to import my friggin key!! Java throws a BER Decode error, is this normal?
To decode a DER encoded key with a BER decoding algorithm? I'm guessing it is.

Any comments on my rantings above?

Cheers (again),

Jim

--
James Vanns BSc (Hons) MCP
Linux Systems Administrator
Software Engineer (Linux / C & C++)
Canterbury Christ Church University College
Public Key: http://pgp.mit.edu:11371/pks/lookup?op=get&search=0x24045370

[José Manuel Romero Ximil]

Hi James,

PKCS1 v2 specifies that RSA must include parameters and the value is NULL.
The unused bits field is necessary for asn1 bit strings, in this case is 0.

[James Vanns]

Thanks to all for putting up with my rantings again. I've come to the
root of the problem and it had bugger all to do with Crypto++. For those
who are interested, opening an RSA public key (saved in X509, DER-
encoded format) in Java, that was generated and saved using Crypto++, is
achieved simply like this:

KeyFactory factory = KeyFactory.getInstance ("RSA");
X509EncodedKeySpec specification =
new X509EncodedKeySpec (X509);
RSAPublicKey key = (RSAPublicKey)
factory.generatePublic (specification);

Where X509 is a byte[] holding the X509, DER-encoded public key. The
problem I had was with Java's String class constructed by a byte[] in a
different method. For some reason when I accessed the String' internal
byte array with getBytes() - it returned more bytes than it had
originally been constructed with!

Anyway, all works now (got rid of the String object) - and thanks for
your help again.

Regards

Jim Vanns