Conversion from BigInteger to octet?

Question

I am trying to manually create the signature tag for a web service call. I accessed the certificate from the keystore and accessed the public key for the certificate. I now have the problem in converting the RSAKeyValue to ds:CryptoBinary type. Code returns the Biginteger value for mudulus and exponent and I am looking for a method or algorithm to convert them to octets and then converting to Bas64. Here is my code

RSAPublicKey rsaKey  = (RSAPublicKey)certificate.getPublicKey();
customSignature.Modulus = rsaKey.getModulus(); 
customSignature.Exponent = rsaKey.getPublicExponent();

Is there any solution available in Java for converting the integers to octet representation?

Answer 1

Try following code using apache commons codec framework:

BigInteger modulus = rsaKey.getModulus();
org.apache.commons.codec.binary.Base64.encodeBase64String(modulus.toByteArray());

Answer 2

Unfortunatly, modulus.toByteArray() does not map directly to the XML Digital Signature's ds:CryptoBinary type, which also requires stripping leading zero octets. You need to do something like the following, before you do the base64 encoding

byte[] modulusBytes = modulus.toByteArray();
int numLeadingZeroBytes = 0;
while( modulusBytes[numLeadingZeroBytes] == 0 )
    ++numLeadingZeroBytes;
if ( numLeadingZeroBytes > 0 ) {
    byte[] origModulusBytes = modulusBytes;
    modulusBytes = new byte[origModulusBytes.length - numLeadingZeroBytes];
    System.arraycopy(origModulusBytes,numLeadingZeroBytes,modulusBytes,0,modulusBytes.length);
}