[StructLayout(LayoutKind.Sequential)]
public struct CRYPT_HASH_BLOB
{
public int cbData;
public IntPtr pbData;
}

CRYPT_HASH_BLOB hashb = new CRYPT_HASH_BLOB();
hashb.pbData = Marshal.AllocHGlobal(sha1hash.Length);
Marshal.Copy(sha1hash, 0, hashb.pbData, sha1hash.Length);
hashb.cbData = sha1hash.Length;