Using Base64 encoded Public Key to verify RSA signature

Question

In a nutshell, this is my problem:

private string publicKeyString = \"MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDVGUzbydMZS+fnkGTsUkDKEyFOGwghR234d5GjPnMIC0RFtXt         


        

Answer 1

First base64 is only an encoding of some binary data. There's more than one way to encode, in binary, an RSA public key. However if you got this from OpenSSL it's likely a DER-encoded RSAPublicKey structure.

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

In general you would need a ASN.1 decoder, Mono.Security.dll provides one, but for such a simple structure you might want to do it by hand since ASN.1 is basically a Tag, Length and a Value.

Answer 2

If your partner also uses .NET, he/she can derive from Mono's makecert https://github.com/mono/mono/blob/bf55818da11240bd108dc51b374fae3d3b5482ce/mcs/tools/security/makecert.cs to generate certificate files and send to you directly.

In that case, you can easily load certificates instead of raw bytes,

http://www.lextm.com/2012/02/simple-publicprivate-key-signing-sample-code/

Answer 3

Your string is the base64 encoding of a SubjectPublicKeyInfo. You can use Bouncycastle.net to decode it like this:

byte[] publicKeyBytes = Convert.FromBase64String(publicKeyString);
AsymmetricKeyParameter asymmetricKeyParameter = PublicKeyFactory.CreateKey(publicKeyBytes);
RsaKeyParameters rsaKeyParameters = (RsaKeyParameters) asymmetricKeyParameter;
RSAParameters rsaParameters = new RSAParameters();
rsaParameters.Modulus = rsaKeyParameters.Modulus.ToByteArrayUnsigned();
rsaParameters.Exponent = rsaKeyParameters.Exponent.ToByteArrayUnsigned();
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
rsa.ImportParameters(rsaParameters);