Get RSA Public key from Assembly or SNK File

I use this class to return RSA public key from Signed Assembly (i.e. SN assembly.)  Works with SNK file too (I think Whidbey will include this too).

using System;
using System.IO;
using System.Text;
using System.Xml;
using System.Reflection;
using System.Security.Cryptography;
using System.Collections;

namespace WSESimpleTCPDLL
{
 ///

 /// One static method to get an RSACryptoServiceProvider from a *.snk file.
 /// NOTE:  These methods assume 1024 bit keys, the same as exported from sn.exe.
 ///

 public sealed class SnkUtil
 {
  #region Fields
  private const int magic_priv_idx = 0x08;
  private const int magic_pub_idx = 0x14;
  private const int magic_size = 4;
  #endregion

  #region Constructors
  private SnkUtil()
  {
  }
  #endregion

  #region Public Methods
  ///

  /// Returns RSA object from *.snk key file.
  ///

  /// Path to snk file.
  /// RSACryptoServiceProvider
  public static RSACryptoServiceProvider GetRSAFromSnkFile(string path)
  {
   if ( path == null )
    throw new ArgumentNullException("path");

   byte[] snkBytes = GetFileBytes(path);
   if ( snkBytes == null )
    throw new Exception("Invalid SNK file.");

   RSACryptoServiceProvider rsa = GetRSAFromSnkBytes(snkBytes);
   return rsa;
  }

  public static RSACryptoServiceProvider GetPublicKeyFromAssembly(Assembly assembly)
  {
   if ( assembly == null )
    throw new ArgumentNullException("assembly");

   byte[] pubkey = assembly.GetName().GetPublicKey();
   if ( pubkey.Length == 0 )
    throw new Exception("No public key in assembly.");

   RSAParameters p = SnkUtil.GetRSAParameters(pubkey);
   RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
   rsa.ImportParameters(p);
   return rsa;
  }

  ///

  /// Returns RSAParameters from byte[].
  /// Example to get rsa public key from assembly:
  /// byte[] pubkey = System.Reflection.Assembly.GetExecutingAssembly().GetName().GetPublicKey();
  /// RSAParameters p = SnkUtil.GetRSAParameters(pubkey);
  ///

  ///
  ///
  public static RSAParameters GetRSAParameters(byte[] keyBytes)
  {
   RSAParameters ret = new RSAParameters();

   if ((keyBytes == null) || (keyBytes.Length < 1))
    throw new ArgumentNullException("keyBytes");     
   
   bool pubonly = SnkBufIsPubLength(keyBytes);

   if ((pubonly) && (!CheckRSA1(keyBytes)))
    return ret;
      
   if ((!pubonly) && (!CheckRSA2(keyBytes)))
    return ret;
     
   int magic_idx = pubonly magic_pub_idx : magic_priv_idx;
   
   // Bitlen is stored here, but note this
   // class is only set up for 1024 bit length keys
   int bitlen_idx = magic_idx + magic_size;
   int bitlen_size = 4;  // DWORD

   // Exponent
   // In read file, will usually be { 1, 0, 1, 0 } or 65537
   int exp_idx = bitlen_idx + bitlen_size;
   int exp_size = 4;

   //BYTE modulus[rsapubkey.bitlen/8]; == MOD; Size 128
   int mod_idx = exp_idx + exp_size;
   int mod_size = 128;  

   //BYTE prime1[rsapubkey.bitlen/16]; == P; Size 64
   int p_idx = mod_idx + mod_size;
   int p_size = 64;

   //BYTE prime2[rsapubkey.bitlen/16]; == Q; Size 64   
   int q_idx = p_idx + p_size;
   int q_size = 64;

   //BYTE exponent1[rsapubkey.bitlen/16]; == DP; Size 64
   int dp_idx = q_idx + q_size;
   int dp_size = 64;

   //BYTE exponent2[rsapubkey.bitlen/16]; == DQ; Size 64   
   int dq_idx = dp_idx + dp_size;
   int dq_size = 64;

   //BYTE coefficient[rsapubkey.bitlen/16]; == InverseQ; Size 64
   int invq_idx = dq_idx + dq_size;
   int invq_size = 64;

   //BYTE privateExponent[rsapubkey.bitlen/8]; == D; Size 128
   int d_idx = invq_idx + invq_size;
   int d_size = 128;
       

   // Figure public params
   // Must reverse order (little vs. big endian issue)
   ret.Exponent = BlockCopy(keyBytes, exp_idx, exp_size);
   Array.Reverse(ret.Exponent);
   ret.Modulus = BlockCopy(keyBytes, mod_idx, mod_size);
   Array.Reverse(ret.Modulus);
     
   if (pubonly) return ret;

   // Figure private params    
   // Must reverse order (little vs. big endian issue)
   ret.P = BlockCopy(keyBytes, p_idx, p_size);
   Array.Reverse(ret.P);

   ret.Q = BlockCopy(keyBytes, q_idx, q_size);
   Array.Reverse(ret.Q);

   ret.DP = BlockCopy(keyBytes, dp_idx, dp_size);
   Array.Reverse(ret.DP);

   ret.DQ = BlockCopy(keyBytes, dq_idx, dq_size);
   Array.Reverse(ret.DQ);

   ret.InverseQ = BlockCopy(keyBytes, invq_idx, invq_size);
   Array.Reverse(ret.InverseQ);

   ret.D = BlockCopy(keyBytes, d_idx, d_size);
   Array.Reverse(ret.D);
   
   return ret;
  }
  #endregion

  #region Private Methods
  private static byte[] GetFileBytes(string path)
  {
   using ( FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read) )
   using ( BinaryReader br = new BinaryReader(fs) )
   {
    byte[] bytes = br.ReadBytes((int)fs.Length);
    return bytes;
   }
  }

  private static RSACryptoServiceProvider GetRSAFromSnkBytes(byte[] snkBytes)
  {
   if ( snkBytes == null )
    throw new ArgumentNullException("snkBytes");

   RSAParameters param = GetRSAParameters(snkBytes);

   // Must set KeyNumber to AT_SIGNATURE for strong
   // name keypair to be correctly imported.     
   CspParameters cp = new CspParameters();
   cp.KeyNumber = 2; // AT_SIGNATURE

   RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(1024, cp);     
   rsa.ImportParameters(param);
   return rsa;
  }
 
  private static byte[] BlockCopy(byte[] source, int idx, int size)
  {
   if ( (source == null) || (source.Length < (idx + size)) )
    return null;

   byte[] ret = new byte[size];
   Buffer.BlockCopy(source, idx, ret, 0, size);
   return ret;
  }

  ///

  /// Returns true if buffer length is public key size.
  ///

  ///
  ///
  private static bool SnkBufIsPubLength(byte[] keypair)
  {
   if ( keypair == null )
    return false;
   return (keypair.Length == 160);
  }

  ///

  /// Check that RSA1 is in header (public key only).
  ///

  ///
  ///
  private static bool CheckRSA1(byte[] pubkey)
  {
   // Check that RSA1 is in header.
   //                             R     S     A     1
   byte[] check = new byte[] { 0x52, 0x53, 0x41, 0x31 };
   return CheckMagic(pubkey, check, magic_pub_idx);
  }

  ///

  /// Check that RSA2 is in header (public and private key).
  ///

  ///
  ///
  private static bool CheckRSA2(byte[] pubkey)
  {
   // Check that RSA2 is in header.
   //                             R     S     A     2
   byte[] check = new byte[] { 0x52, 0x53, 0x41, 0x32 };
   return CheckMagic(pubkey, check, magic_priv_idx);
  }

  private static bool CheckMagic(byte[] keypair, byte[] check, int idx)
  {   
   byte[] magic = BlockCopy(keypair, idx, magic_size);
   if ( magic == null )
    return false;

   for (int i = 0; i < magic_size; i++)
   {
    if ( check[i] != magic[i] )
     return false;      
   }

   return true;
  }
  #endregion
 }
}

--William