I'm working with the Google DoubleClick ad exchange API. Their examples are in C++ and well, I'm pretty awful at C++. I'm trying to convert this to C# for something I'm working on and really, I think I just need some explanation of what is actually happening in certain blocks of this code sample. Honestly I kind of know what should happen over all but I'm not sure I am getting it 'right' and with encryption/decryption there isn't a 'sort of right'.
This is the full example from their API site:
bool DecryptByteArray(
const string& ciphertext, const string& encryption_key,
const string& integrity_key, string* cleartext) {
// Step 1. find the length of initialization vector and clear text.
const int cleartext_length =
ciphertext.size() - kInitializationVectorSize - kSignatureSize;
if (cleartext_length < 0) {
// The length can't be correct.
return false;
}
string iv(ciphertext, 0, kInitializationVectorSize);
// Step 2. recover clear text
cleartext->resize(cleartext_length, '\0');
const char* ciphertext_begin = string_as_array(ciphertext) + iv.size();
const char* const ciphertext_end = ciphertext_begin + cleartext->size();
string::iterator cleartext_begin = cleartext->begin();
bool add_iv_counter_byte = true;
while (ciphertext_begin < ciphertext_end) {
uint32 pad_size = kHashOutputSize;
uchar encryption_pad[kHashOutputSize];
if (!HMAC(EVP_sha1(), string_as_array(encryption_key),
encryption_key.length(), (uchar*)string_as_array(iv),
iv.size(), encryption_pad, &pad_size)) {
printf("Error: encryption HMAC failed.\n");
return false;
}
for (int i = 0;
i < kBlockSize && ciphertext_begin < ciphertext_end;
++i, ++cleartext_begin, ++ciphertext_begin) {
*cleartext_begin = *ciphertext_begin ^ encryption_pad[i];
}
if (!add_iv_counter_byte) {
char& last_byte = *iv.rbegin();
++last_byte;
if (last_byte == '\0') {
add_iv_counter_byte = true;
}
}
if (add_iv_counter_byte) {
add_iv_counter_byte = false;
iv.push_back('\0');
}
}
Step 1 is quite obvious. This block is what I am really not sure how to interpret:
if (!HMAC(EVP_sha1(), string_as_array(encryption_key),
encryption_key.length(), (uchar*)string_as_array(iv),
iv.size(), encryption_pad, &pad_size)) {
printf("Error: encryption HMAC failed.\n");
return false;
}
What exactly is happening in that if body? What would that look like in C#? There are a lot of parameters that do SOMETHING but it seems like an awful lot crammed in a small spot. Is there some stdlib HMAC class? If I knew more about that I might better understand what's happening.
The equivalent C# code for that block is:
using (var hmac = new HMACSHA1(encryption_key))
{
var encryption_pad = hmac.ComputeHash(iv);
}
It's computing the SHA1 HMAC of the initialization vector (IV), using the given encryption key.
The HMAC function is actually a macro from OpenSSL.
Just as a comment, I think it would be easier to implement this from their pseudocode description rather than from their C++ code.
While Decrypting Key I am getting error :
CryptographicException was unhandled by user code.
Length of the data to decrypt is invalid.
Now what I want to do is, I want to check whether the given string is able to decrypt or not. If it is able to decrypt than only I want to execute below code. So that I cannot get an error.
var byteBuff = Convert.FromBase64String(value);
var strDecrypted = Encoding.ASCII.GetString(
objDesCrypto.CreateDecryptor().TransformFinalBlock(byteBuff, 0, byteBuff.Length));
Is there any way to check it ?
I think a followed question could be: if it is not able to decrypt, what does your code want to do. Anyway with that considered, you can always use try...catch like:
try { /* your code */ }
catch (CryptographicException e) { /* whatever you need to if it is not able to */ }
I found Solution. Have to check for valid base64 string :
if ((value.Length % 4 == 0) && Regex.IsMatch(value, #"^[a-zA-Z0-9\+/]*={0,3}$", RegexOptions.None))
{
var byteBuff = Convert.FromBase64String(value);
decryptedString =
Encoding.ASCII.GetString(
objDesCrypto.CreateDecryptor().TransformFinalBlock(byteBuff, 0, byteBuff.Length));
}
I'm hoping you can help me with a weird error I've been getting while trying to implement a QueryString encryption module using RSA for my encryption type. My question is 2 fold:
Can you help me resolve the errors I'm getting?
Do you recommend something other than RSA to encrypt a QueryString?
Background/Important info:
I created an object called QueryString, which i store in the session (and which uses the SessionID to generate keys/salt). I instantiate it on Session Start, which generates the keys, and it naturally dies on Session.Abandon... I retrieve it in my BasePage and use it in my pages afterword much like i would a normal querystring (QueryString[key] for gets and stuff)... I store my Public and Private keys in the object itself, as internal properties to the object.
Another important thing is that my web site has a lot of grids, whithin which a record rows that contain links, and so they all have to be encrypted before they are set (href=...)... so the QueryString object i created can get taxed quite a bit and quite quickly (while looping with OnRowCreated or something to encrypt the hrefs).
The error(s):
i am currently getting intermittent errors, that cannot be reproduced (they happen at random... trust me... very random), of the following types when i try to either Encrypt or Decrypt:
Error type 1: CreateProvHandle
Error type 2: The specified file could not be found.
Error type 3: Attempted to perform an unauthorized operation.
For errors 1 and 2, i managed to deal with it so far by simply recursively calling the method (encrypt or decrypt) that caused it and they usually only recurse once (max i've had is 3 using my metrics) and the error magically disappears... so i blamed it on too many calls too fast to the object itself or something... but if someone has any clue as to why this would happen or how to solve this, i would love to take the recursing out of my methods and truly throw when a major exception occurs instead. On top of that i told my RSA params not to persist anything in the CSP store and so i thought the file thing didn't matter but apparently not...
For error 3, i simply cannot get my head around it! My RSA parameters say not to persist anything in the CSP so i don't know how, when or why it would even try to access files (yes, i am repeating myself!), let alone files that are restricted or that the user wouldn't have access to? Please help me!!
Here's some code for my RSA params... maybe you'll find something there that doesn't jive with what i'm trying to do (generate the keys once on object instantiation, store the object in the session, and use that from that point on/disconnect from anything remote/calls to server processes that are not part of the site or .NET)?
public static void AssignParameter()
{
const int PROVIDER_RSA_FULL = 1;
const string CONTAINER_NAME = "ICareContainer";
CspParameters cspParams;
cspParams = new CspParameters(PROVIDER_RSA_FULL);
cspParams.KeyContainerName = CONTAINER_NAME;
cspParams.Flags = CspProviderFlags.UseMachineKeyStore;
cspParams.ProviderName = "Microsoft Strong Cryptographic Provider";
CryptoKeyAccessRule rule = new CryptoKeyAccessRule("everyone", CryptoKeyRights.FullControl, AccessControlType.Allow);
cspParams.CryptoKeySecurity = new CryptoKeySecurity();
cspParams.CryptoKeySecurity.SetAccessRule(rule);
rsa = new RSACryptoServiceProvider(cspParams);
rsa.PersistKeyInCsp = false;
rsa.KeySize = 1024;
}
public static string[] GetKeys()
{
AssignParameter();
string[] keys = new string[2];
//privatekey
keys[0] = rsa.ToXmlString(true);
//publickey
keys[1] = rsa.ToXmlString(false);
return keys;
}
public static string EncryptData(string data2Encrypt, string key)
{
AssignParameter();
string publicOnlyKeyXML = key;
rsa.FromXmlString(publicOnlyKeyXML);
//read plaintext, encrypt it to ciphertext
byte[] plainbytes = System.Text.Encoding.Default.GetBytes(data2Encrypt);
byte[] cipherbytes = rsa.Encrypt(plainbytes, false);
return Convert.ToBase64String(cipherbytes);
}
public static string DecryptData(string data2Decrypt, string key)
{
AssignParameter();
byte[] getpassword = Convert.FromBase64String(data2Decrypt);
string publicPrivateKeyXML = key;
rsa.FromXmlString(publicPrivateKeyXML);
//read ciphertext, decrypt it to plaintext
byte[] plain = rsa.Decrypt(getpassword, false);
return System.Text.Encoding.Default.GetString(plain);
}
Ah yes... stupid mistake (aren't they always?):
Normal Class
Static Crypto Class
End Static
End Normal
Can you find the problem and why i was getting collision errors? I've changed the Static to be Normal and all is well in my neck of the woods.
Cheers!
I have devices with unique serial number (string incremetation) ex : AS1002 and AS1003.
I need to figure out an algorithm to produce a unique activation key for each serial number.
What would be the best approach for this ?
Thanks !
(This has to be done offline)
You have two things to consider here:
- Whatever key you generate must be able to be entered easily, so this eliminates some weird hash which may produce characters which will be cumbersome to type, although this can be overcome, it’s something you should consider.
- The operation as you stated must be done online
Firstly, there will be no way to say with absolute certainty that someone will not be able to decipher your key generation routine, no matter how much you attempt to obfuscate. Just do a search engine query for “Crack for Xyz software”.
This has been a long battle that will never end, hence the move to deliver software as services, i.e. online where the producer has more control over their content and can explicitly authorize and authenticate a user. In your case you want to do this offline. So in your scenario someone will attach your device to some system, and the accompanying software that you intend to write this routine on will make a check against the serial number of the device v/s user input.
Based on #sll’s answer, given the offline nature of your request. Your best, unfortunately would be to generate a set of random codes, and validate them when user’s call in.
Here is a method borrowed from another SO answer, I've added digits as well
private readonly Random _rng = new Random();
private const string _chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ123456789"; //Added 1-9
private string RandomString(int size)
{
char[] buffer = new char[size];
for (int i = 0; i < size; i++)
{
buffer[i] = _chars[_rng.Next(_chars.Length)];
}
return new string(buffer);
}
So, generating one for each of your devices and storing them somewhere might be your only option because of the offline considerations.
This routine will produce strings like this when set to create a 10 digit string, which is reasonably random.
3477KXFBDQ
ROT6GRA39O
40HTLJPFCL
5M2F44M5CH
CAVAO780NR
8XBQ44WNUA
IA02WEWOCM
EG11L4OGFO
LP2UOGKKLA
H0JB0BA4NJ
KT8AN18KFA
Activation Key
Here is a simple structure of the activation key:
Part
Description
Data
A part of the key encrypted with a password. Contains the key expiration date and application options.
Hash
Checksum of the key expiration date, password, options and environment parameters.
Tail
The initialization vector that used to decode the data (so-called "salt").
class ActivationKey
{
public byte[] Data { get; set; } // Encrypted part.
public byte[] Hash { get; set; } // Hashed part.
public byte[] Tail { get; set; } // Initialization vector.
}
The key could represent as text format: DATA-HASH-TAIL.
For example:
KCATBZ14Y-VGDM2ZQ-ATSVYMI.
The folowing tool will use cryptographic transformations to generate and verify the key.
Generating
The algorithm for obtaining a unique activation key for a data set consists of several steps:
data collection,
getting the hash and data encryption,
converting activation key to string.
Data collection
At this step, you need to get an array of data such as serial number, device ID, expiration date, etc. This purpose can be achieved using the following
method:
unsafe byte[] Serialize(params object[] objects)
{
using (MemoryStream memory = new MemoryStream())
using (BinaryWriter writer = new BinaryWriter(memory))
{
foreach (object obj in objects)
{
if (obj == null) continue;
switch (obj)
{
case string str:
if (str.Length > 0)
writer.Write(str.ToCharArray());
continue;
case DateTime date:
writer.Write(date.Ticks);
continue;
case bool #bool:
writer.Write(#bool);
continue;
case short #short:
writer.Write(#short);
continue;
case ushort #ushort:
writer.Write(#ushort);
continue;
case int #int:
writer.Write(#int);
continue;
case uint #uint:
writer.Write(#uint);
continue;
case long #long:
writer.Write(#long);
continue;
case ulong #ulong:
writer.Write(#ulong);
continue;
case float #float:
writer.Write(#float);
continue;
case double #double:
writer.Write(#double);
continue;
case decimal #decimal:
writer.Write(#decimal);
continue;
case byte[] buffer:
if (buffer.Length > 0)
writer.Write(buffer);
continue;
case Array array:
if (array.Length > 0)
foreach (var a in array) writer.Write(Serialize(a));
continue;
case IConvertible conv:
writer.Write(conv.ToString(CultureInfo.InvariantCulture));
continue;
case IFormattable frm:
writer.Write(frm.ToString(null, CultureInfo.InvariantCulture));
continue;
case Stream stream:
stream.CopyTo(stream);
continue;
default:
try
{
int rawsize = Marshal.SizeOf(obj);
byte[] rawdata = new byte[rawsize];
GCHandle handle = GCHandle.Alloc(rawdata, GCHandleType.Pinned);
Marshal.StructureToPtr(obj, handle.AddrOfPinnedObject(), false);
writer.Write(rawdata);
handle.Free();
}
catch(Exception e)
{
// Place debugging tools here.
}
continue;
}
}
writer.Flush();
byte[] bytes = memory.ToArray();
return bytes;
}
}
Getting the hash and data encryption
This step contains the following substeps:
create an encryption engine using a password and stores the initialization vector in the Tail property.
next step, expiration date and options are encrypted and the encrypted data is saved into the Data property.
finally, the hashing engine calculates a hash based on the expiration date, password, options and environment and puts it in the Hash property.
ActivationKey Create<TAlg, THash>(DateTime expirationDate,
object password,
object options = null,
params object[] environment)
where TAlg : SymmetricAlgorithm
where THash : HashAlgorithm
{
ActivationKey activationKey = new ActivationKey();
using (SymmetricAlgorithm cryptoAlg = Activator.CreateInstance<TAlg>())
{
if (password == null)
{
password = new byte[0];
}
activationKey.Tail = cryptoAlg.IV;
using (DeriveBytes deriveBytes =
new PasswordDeriveBytes(Serialize(password), activationKey.Tail))
{
cryptoAlg.Key = deriveBytes.GetBytes(cryptoAlg.KeySize / 8);
}
expirationDate = expirationDate.Date;
long expirationDateStamp = expirationDate.ToBinary();
using (ICryptoTransform transform = cryptoAlg.CreateEncryptor())
{
byte[] data = Serialize(expirationDateStamp, options);
activationKey.Data = transform.TransformFinalBlock(data, 0, data.Length);
}
using (HashAlgorithm hashAlg = Activator.CreateInstance<THash>())
{
byte[] data = Serialize(expirationDateStamp,
cryptoAlg.Key,
options,
environment,
activationKey.Tail);
activationKey.Hash = hashAlg.ComputeHash(data);
}
}
return activationKey;
}
Converting to string
Use the ToString method to get a string containing the key text, ready to be transfering to the end user.
N-based encoding (where N is the base of the number system) was often used to convert binary data into a human-readable text. The most commonly used in
activation key is base32. The advantage of this encoding is a large alphabet consisting of numbers and letters that case insensitive. The downside is that this encoding is not implemented in the .NET standard library and you should implement it yourself. You can also use the hex encoding and base64 built into mscorlib. In my example base32 is used, but I will not give its source code here. There are many examples of base32 implementation on this site.
string ToString(ActivationKey activationKey)
{
if (activationKey.Data == null
|| activationKey.Hash == null
|| activationKey.Tail == null)
{
return string.Empty;
}
using (Base32 base32 = new Base32())
{
return base32.Encode(activationKey.Data)
+ "-" + base32.Encode(activationKey.Hash)
+ "-" + base32.Encode(activationKey.Tail);
}
}
To restore use the folowing method:
ActivationKey Parse(string text)
{
ActivationKey activationKey;
string[] items = text.Split('-');
if (items.Length >= 3)
{
using (Base32 base32 = new Base32())
{
activationKey.Data = base32.Decode(items[0]);
activationKey.Hash = base32.Decode(items[1]);
activationKey.Tail = base32.Decode(items[2]);
}
}
return activationKey;
}
Checking
Key verification is carried out using methodes GetOptions an Verify.
GetOptions checks the key and restores embeded data as byte array or null if key is not valid.
Verify just checks the key.
byte[] GetOptions<TAlg, THash>(object password = null, params object[] environment)
where TAlg : SymmetricAlgorithm
where THash : HashAlgorithm
{
if (Data == null || Hash == null || Tail == null)
{
return null;
}
try
{
using (SymmetricAlgorithm cryptoAlg = Activator.CreateInstance<TAlg>())
{
cryptoAlg.IV = Tail;
using (DeriveBytes deriveBytes =
new PasswordDeriveBytes(Serialize(password), Tail))
{
cryptoAlg.Key = deriveBytes.GetBytes(cryptoAlg.KeySize / 8);
}
using (ICryptoTransform transform = cryptoAlg.CreateDecryptor())
{
byte[] data = transform.TransformFinalBlock(Data, 0, Data.Length);
int optionsLength = data.Length - 8;
if (optionsLength < 0)
{
return null;
}
byte[] options;
if (optionsLength > 0)
{
options = new byte[optionsLength];
Buffer.BlockCopy(data, 8, options, 0, optionsLength);
}
else
{
options = new byte[0];
}
long expirationDateStamp = BitConverter.ToInt64(data, 0);
DateTime expirationDate = DateTime.FromBinary(expirationDateStamp);
if (expirationDate < DateTime.Today)
{
return null;
}
using (HashAlgorithm hashAlg =
Activator.CreateInstance<THash>())
{
byte[] hash =
hashAlg.ComputeHash(
Serialize(expirationDateStamp,
cryptoAlg.Key,
options,
environment,
Tail));
return ByteArrayEquals(Hash, hash) ? options : null;
}
}
}
}
catch
{
return null;
}
}
bool Verify<TAlg, THash>(object password = null, params object[] environment)
where TAlg : SymmetricAlgorithm
where THash : HashAlgorithm
{
try
{
byte[] key = Serialize(password);
return Verify<TAlg, THash>(key, environment);
}
catch
{
return false;
}
}
Example
Here is a full example of generating the activation key using your own combination of any amount of data - text, strings, numbers, bytes, etc.
Example of usage:
string serialNumber = "0123456789"; // The serial number.
const string appName = "myAppName"; // The application name.
// Generating the key. All the parameters passed to the costructor can be omitted.
ActivationKey activationKey = new ActivationKey(
//expirationDate:
DateTime.Now.AddMonths(1), // Expiration date 1 month later.
// Pass DateTime.Max for unlimited use.
//password:
null, // Password protection;
// this parameter can be null.
//options:
null // Pass here numbers, flags, text or other
// that you want to restore
// or null if no necessary.
//environment:
appName, serialNumber // Application name and serial number.
);
// Thus, a simple check of the key for validity is carried out.
bool checkKey = activationKey.Verify((byte[])null, appName, serialNumber);
if (!checkKey)
{
MessageBox.Show("Your copy is not activated! Please get a valid activation key.");
Application.Exit();
}
By far the most secure way to do it is to have a centralized database of (serial number, activation key) pairs and have the user activate over the internet so you can check the key locally (on the server).
In this implementation, the activation key can be completely random since it doesn't need to depend on the serial number.
You want it to be easy to check, and hard to "go backwards". You'll see a lot of suggestions for using hashing functions, those functions are easy to go one way, but hard to go backwards. Previously, I phrased that as "it is easy to turn a cow into a hamburger, but hard to turn a hamburger into a cow". In this case, a device should know its own serial number and be able to "add" (or append) some secret (usually called "salt") to the serial and then hash or encrypt it.
If you are using reversible encryption, you want to add some sort of "check digit" to the serial numbers so that if someone does figure your encryption scheme out, there is another layer for them to figure out.
An example of a function that is easy enough to "go backwards" was one I solved with Excel while trying to avoid homework.
And you probably want to make things easier for your customers by making the encoding less likely to be messed up when the activation codes are handwritten (such as you write it down from the email then walk over to where the device is and punch the letters/digits in). In many fonts, I and 1, and 0 and O are similar enough that many encodings, such as your car's VIN do not use the letters i and o (and I remember older typewriters that lacked a key for the digit 1 because you were expected to use lowercase L). In such cases, Y, 4 and 7 can appear the same depending on some handwriting. So know your audience and what are their limits.
If your device has some secured memory which can not be read by connecting an programmator or an other device -you can store some key-code and then use any hashing algorithm like MD5 or SHA-1/2 to generate hash by:
HASH(PUBLIC_SERIALNUMBER + PRIVATE_KEYCODE)
And pairs of SERIALNUMBER + KEYCODE should be stored in local DB.
In this way: (offline)
Client calling you and asking for the Activation Code
You asking for a SERIALNUMBER of particular device
Then you search for a KEYCODE by a given SERIALNUMBER in your local DB and generate Activation Code (even using MD5 this will be sacure as long KEYCODE is privately stored in your DB)
Client enter Activation Code into the device, device able to generate hash
by own SERIALNUMBER and KEYCODE and then compare to Activation Code entered by user
This could be simplified by storing activation code itself if device has a secured memory onboard (like SmartCards has). In this way you can just keep own database of SerialCode - ActivationCode pairs.
How about: Invent a password that is not revealed to the user. Then concatenate this password with the serial number and hash the combination.
Anything you do can be broken by a dedicated enough hacker. The question is not, "Can I create absolutely unbreakable security?" but "Can I create security good enough to protect against unskilled hackers and to make it not worth the effort for the skilled hackers?" If you reasonably expect to sell 10 million copies of your product, you'll be a big target and there may be lots of hackers out there who will try to break it. If you expect to sell a few hundred or maybe a few thousand copies, not so much.
I'm trying to run a MathType in a C# app... using OLE in forms to signify the equations/images.
This is how I started with the code.
I got the CLSID object for math type equation. I create a new instance and run a verb to start Math Type. This works good until I try to set or get data of the IDataItem attribute I have.
Code:
string progID = "Equation.DSMT4";
comRetVal= CLSIDFromProgID(progID, out eqContainerGUID);
Type t = Type.GetTypeFromProgID(progID); //ok-> MT6 Equation
Object instance = Activator.CreateInstance(t);
IDataObject oleDataObject = instance as IDataObject;
MTSDKDN.MathTypeSDK.IOleObject oleObject = instance as IDataObject;
//run verb Run For Conversion - I'm not sure what this verb does
oleObject.DoVerb(2, (IntPtr)0, activeSite, 0, (IntPtr)this.Handle, new MathTypeSDK.COMRECT());
//up to here everything is find
//Now say I want to put a MathML string into the IDataObject
//set format
formatEtc.cfFormat = (Int16)dataFormatMathMLPres.Id; //<-this overflows. I verified that the only format that works is Presentation MAthML
formatEtc.dwAspect = System.Runtime.InteropServices.ComTypes.DVASPECT.DVASPECT_CONTENT;
formatEtc.lindex = -1;
formatEtc.ptd = (IntPtr)0;
formatEtc.tymed = TYMED.TYMED_HGLOBAL;
//set medium
ConnectSTGMEDIUM stgMedium = new ConnectSTGMEDIUM();
string mathEqn = "<math><mi>x</mi></math>";
stgMedium.unionmember = Marshal.StringToHGlobalAuto(mathEqn);
stgMedium.pUnkForRelease = 0;
//if now i write the equation to console from STGMEDIUM i see that after each char there is a null. Is this normal?
//now I try to set data in IDataObject and the OLE object
//I thought this set the data of the ole object to the MathML string I put in STGMEDIUM
oleDataObject.SetData(ref formatEtc, ref stgMedium, false);
The app now crashes with this exception:
System.Runtime.InteropServices.COMException was unhandled Message="Invalid FORMATETC structure (Exception from HRESULT: 0x80040064 (DV_E_FORMATETC))" Source="System" ErrorCode=-2147221404 StackTrace: at System.Runtime.InteropServices.ComTypes.IDataObject.GetData(FORMATETC& format, STGMEDIUM& medium)
I'm not sure what this means, but I think it might have to do with
formatEtc.cfFormat = (Int16)dataFormatMathMLPres.Id;
because that ID is 50000 and does not fit in a short (cfFormat is a short) so it overflows to something like -15000.
I have solved similar issue by converting it from unsigned to signed value. it means that if the value (dataFormatMathMLPres.Id) is grater than 32767. Use (dataFormatMathMLPres.Id - 65536) instead. It will fit signed short.