Reading cryptographic key information from snk/assembly

[nickdu]

After doing some searching on the web I found some code which shows how to
load an RSACryptoServiceProvider from an snk file or an assembly. That all
works. However I do have a question with regards to the code. There is a
snippet which indicates it has to do something a bit different when loading
an snk file in order for the ImportParameters() method to work. Here is the
snippet I'm referring to:

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

RSAParameters parameters = GetRSAParameters(bytes);

// 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);
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
rsa.ImportParameters(parameters);
return rsa;
}

Note that I commented out the lines and instead tried just using the
constructor with no parameters and everything still works. The code which
loads the key from an assembly was just calling the parameterless constructor
and then calling ImportParameters() so I was hoping I could do the same thing
with the snk file. It appears I can. Just want to confirm this.

By the way, the entire utility is below. I'll be able to combine some code
below if I can treat the byte[] from the snk file the same as the byte[] from
the assembly.

public sealed class SnkUtil
{
private const int MagicPrivateIndex = 8;
private const int MagicPublicIndex = 20;
private const int MagicSize = 4;

private static byte[] GetFileBytes(string path)
{
Stream stm = File.OpenRead(path);
using(stm)
{
byte[] buffer = new byte[stm.Length];
stm.Read(buffer, 0, buffer.Length);
return buffer;
}
}

private static byte[] Copy(byte[] src, int index, int size)
{
if ((src == null) || (src.Length < (index + size)))
return null;

byte[] dest = new byte[size];
Array.Copy(src, index, dest, 0, size);
return dest;
}

private static bool Check(byte[] bytes, byte[] check, int index)
{
if (bytes == null)
throw(new ArgumentNullException("bytes"));
if (check == null)
throw(new ArgumentNullException("check"));
if (bytes.Length < index + check.Length)
throw(new ArgumentException("index plus check length outside bounds of
bytes array."));

for (int i = 0; i < check.Length; i++)
{
if (bytes[i + index] != check)
return false;
}
return true;
}

/// <summary>
/// Check that RSA1 is in header (public key only).
/// </summary>
/// <param name="keypair"></param>
/// <returns></returns>
private static bool CheckRSA1(byte[] bytes)
{
// Check that RSA1 is in header.
// R S A 1
byte[] check = new byte[] {0x52, 0x53, 0x41, 0x31};
return Check(bytes, check, MagicPublicIndex);
}

/// <summary>
/// Check that RSA2 is in header (public and private key).
/// </summary>
/// <param name="keypair"></param>
/// <returns></returns>
private static bool CheckRSA2(byte[] bytes)
{
// Check that RSA2 is in header.
// R S A 2
byte[] check = new byte[] {0x52, 0x53, 0x41, 0x32};
return Check(bytes, check, MagicPrivateIndex);
}

/// <summary>
/// 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);
/// </summary>
/// <param name="keypair"></param>
/// <returns></returns>
private static RSAParameters GetRSAParameters(byte[] bytes)
{
if ((bytes == null) || (bytes.Length == 0))
throw new ArgumentNullException("bytes");

bool pubonly = (bytes.Length == 160);

if (pubonly && !CheckRSA1(bytes))
return new RSAParameters();

if (!pubonly && !CheckRSA2(bytes))
return new RSAParameters();

RSAParameters parameters = new RSAParameters();

int index = pubonly ? MagicPublicIndex : MagicPrivateIndex;
index += MagicSize + 4;
int size = 4;
parameters.Exponent = Copy(bytes, index, size);
Array.Reverse(parameters.Exponent);

index += size;
size = 128;
parameters.Modulus = Copy(bytes, index, size);
Array.Reverse(parameters.Modulus);

if (pubonly)
return parameters;

// Figure private params
// Must reverse order (little vs. big endian issue)

index += size;
size = 64;
parameters.P = Copy(bytes, index, size);
Array.Reverse(parameters.P);

index += size;
size = 64;
parameters.Q = Copy(bytes, index, size);
Array.Reverse(parameters.Q);

index += size;
size = 64;
parameters.DP = Copy(bytes, index, size);
Array.Reverse(parameters.DP);

index += size;
size = 64;
parameters.DQ = Copy(bytes, index, size);
Array.Reverse(parameters.DQ);

index += size;
size = 64;
parameters.InverseQ = Copy(bytes, index, size);
Array.Reverse(parameters.InverseQ);

index += size;
size = 128;
parameters.D = Copy(bytes, index, size);
Array.Reverse(parameters.D);

return parameters;
}

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

RSAParameters parameters = GetRSAParameters(bytes);

// 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);
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
rsa.ImportParameters(parameters);
return rsa;
}

/// <summary>
/// Returns RSA object from *.snk key file.
/// </summary>
/// <param name="path">Path to snk file.</param>
/// <returns>RSACryptoServiceProvider</returns>
public static RSACryptoServiceProvider GetRsaProvider(string path)
{
if (path == null)
throw new ArgumentNullException("path");

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

RSACryptoServiceProvider rsa = GetRsaProvider(bytes);
return rsa;
}

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

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

RSAParameters parameters = GetRSAParameters(bytes);
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
rsa.ImportParameters(parameters);
return rsa;
}
}

--
Thanks,
Nick

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