what is the format of RSA public key modulus and exponent in .net?

Question

in a short question: How can I fill in RSAParameters if I have having the following input from the third party?:

Modulus: 123456

Exponet: 111

In a long story, I use the following code:

RSACryptoServiceProvider RSAalg = new RSACryptoServiceProvider(2048);

RSAParameters rsaPublic = RSAalg.ExportParameters(false);

then I can get modulus and exponent of the rsa public key in byte[]. In order to write these information in the asn.1 format

      RSAPublicKey ::= SEQUENCE {
           modulus           INTEGER,  -- n
           publicExponent    INTEGER   -- e
       }

I use a asn.1 library to convert byte[] to its bigInteger format, but this byte[] should be in the format like {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9'} assuming it is decimal.

But it seems like rsaPublic.modulus and rsaPublic.exponent are not in this format, there are many not-digits in the byte[] of the modulus and exponent. So what is the format of rsaPublic.modulus and rsaPublic.exponent, and how convert them into a byte[] with the format like {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9'}?

Thanks a lot

Answer 1

The format is specified in the ASN.1 reference. The format used is called the DER encoding, and for Integers. A reference is available to these ugly encodings. Are you sure there isn't an easier way to get these values as bigints?

EDIT

It looks like you can get the byte[] arrays by getting an RSAParameters object via RSACryptoServiceProvider.ExportParameters() and then extracting from this the Exponent and Modulus fields.