Introduction
In today’s applications, the use of web services is constantly growing. In the web services panorama, there are different ways of managing the authentications. One of the common methods is mutual certificates exchange.
Imagine that your application uses a web service that needs a customer’s certificate in order to correctly authenticate with the endpoint. Now, you can explain to your customer how to create the certificate, by generating one, the RSA key, and then creating a pfx container, positioning pfxcontainer file in the suggested folder, then inserting in your application the correct path to the pfx file. Hmm, it is a lot of work and is quite complicated. And what if you need to deploy it on many PCs for the same customer and your service is accessed directly from the client? If you exclude tricks, there is a lot of extra work.
What if you store only the pem strings of your certificate and private key, together with the password in database? Nice, but creating a valid X509Certificate object that contains the private key on .NET is not a trivial task. Don’t worry, this guide can help you.
In case some of the terms used in this introduction are not clear or known to you, but you still want to follow this guide, you will find some answers that will help you at the links given below:
Getting Started
In order to start and try the solution proposed in this guide, you need Microsoft Visual Studio 2010 in any of his many versions. This code will work even on previous versions of Visual Studio editions as 2008 or even 2005, but you’ll need to set up the project on your own, based on the code I suggested.
In order to create your own certificate, you can use your favourite tools but I will suggest you OpenSSL. You can download it from the following address:
Before installing OpenSSL, assure that you have installed the proper version of Visual C++ 2008 Redistributables. Once you have installed these libraries, you can proceed by installing OpenSSL.
Creating Certificates
Before creating a certificate, we need to generate the private key. Achieving this with OpenSSL is really simple. Open your command prompt window and get yourself into the bin directory under the folder where you installed OpenSSL (usually C:\Program Files\OpenSSL).
Now execute the following command:
opensslgenrsa 1024 >private.key
Once executed, you should see something like this:
Check in that folder, there should be a new file called private.key. It is the RSA key on which the certificate will be based. Now let’s create the certificate. Execute the following command.
opensslreq -new -x509 -nodes -sha1 -days 1100 -key private.key> public.cer
As soon as you hit enter, you will be prompted for a couple of questions about the details with whom the certificates attributes will be populated. Once finished, you’ll see the following:
Congratulations! Your certificate is done!
Certificate is represented as Base64 encoded DER certificate, enclosed between "-----BEGIN CERTIFICATE-----" and "-----END CERTIFICATE-----". Same is for the RSA signature key. For more details, consult:
Now, we can use a PKCS format file container in order to store both, public certificate and private key. This is not the goal of our project, but for the completeness I’ll show you how to do that via OpenSSL.
With the following command, you will create the required:
openssl pkcs12 -export -in public.cer -inkeyprivate.key –out cert_key.p12
After executing this command, you will get prompted about the Export password, this password will be used to encrypt your private key, so make it complex and unique.
Now you can load it to the X509Certificate2 object:
string certificatePath = @"cert_key.p12";
string certificatePassword = "password";
X509Certificate2 clientCertificate =
new X509Certificate2(certificatePath, certificatePassword);
In the next steps, we will replicate this behaviour directly from code.
Loading the Certificate
If only a certificate is a problem, X509Certificate2 class will do the job. With the following code, you can simply load the certificate:
string certificatePath = @"public.cer";
X509Certificate2 clientCertificate = new X509Certificate2(certificatePath);
Or if you want to load it directly from the string:
string publicCertString = "-----BEGIN CERTIFICATE----- MCIISFSDFEESd etc. example";
X509Certificate2 clientCertificate =
new X509Certificate2(Encoding.UTF8.GetBytes(publicCertString));
Now, the framework offers already much. But this is not enough. If we want to load a certificate from a certificate file, we can do it. Otherwise, if we want to load the certificate and private key from the pfx or pkcs12 container, we can do it. But what if we want to load dynamically the certificate and private key, choose the encryption password and use our object? The X509Certificate2 doesn’t offer an overload of constructor like X509Certificate2(string certFileName, string privateKeyFile, string password) or the similar overload that will accept a byte array in order to correctly initialize this object. In this case, we need to do some dirty work. So let’s do it.
Decoding RSA Private Key
I will not get into details on how the RSA key is decoded, you have the source code plain of comments, so if interested, read it, it says more than a thousand words. In order to decode the private key, we will use DecodeRsaPrivateKey method which will return RSACryptoServiceProvider instance representing our private key.
Creating the X509Certificate2
As described in the previous chapters, I will use the default X509Certificate2 constructor in order to create the certificate. Afterwards, I will assign the decoded RSA private key as RSACryptoServiceProvider to the X509Certificate2 instance property PrivateKey. If everything went well, we will have the proper instance of X509Certificate2 certificate container, containing both, the certificate and the key, encoded with chosen password. Here is the code sample:
byte[] certBuffer = Helpers.GetBytesFromPEM(publicCert, PemStringType.Certificate);
byte[] keyBuffer = Helpers.GetBytesFromPEM(privateKey, PemStringType.RsaPrivateKey);
X509Certificate2 certificate = new X509Certificate2(certBuffer, password);
RSACryptoServiceProvider prov = Crypto.DecodeRsaPrivateKey(keyBuffer);
certificate.PrivateKey = prov;
I forgot to mention the helper method GetBytesFromPEM, which “cleans” the string from header and footer information.
Demo Application
In the demo application, you can see how to load the described components, create the X509 certificate, persist and reload everything. The interface is just an example, because that is rudimental, simple and incomplete. But hey, this is a demo, and for a demo is already too much! The important stuff is under the hood, that’s in what you should be interested.
Here is a screenshot:
Further Improvements
How to improve this code? Well test it, I have some doubts about creating Key Containers, and I will test it too. Any new discovery will be promptly noticed and the article will be updated.
Other things that are on my mind are creating the extension methods to the X509Certificate2 in order to make code look simpler and cleaner.
Perhaps including these methods in your applications' framework cryptography library? Any idea is welcome so if you have any, feel free to contact me or to post the comment.
License
The idea and realisation are made completely by the author. The method DecodeRsaPrivateKey and all the code used inside were kindly provided by Mr Dan Maser; Dan, thanks for providing the code and for all the support. The way that RSACryptoServiceProvider is initialized is code written by me in order to overcome the encountered problems with .NET 4.0, all the traits-based size stuff and byte alignment are written by Dan Maser, but, speaking with Dan, we came to the following. The core of the ASN.1 parsing code came from a stackoverflow.com article. After analysing the code and after doing further research, I came across what I believe to be its original source, which is http://www.jensign.com/opensslkey/opensslkey.cs. The top of that file does include a standard copyright line "Copyright (C) 2008 JavaScience Consulting". I didn't knowingly use any copyrighted information when I originally wrote the code, but now it's kind of fuzzy. There's no direct indication I can find about any limitations on using or modifying the code directly on that website. At the bottom of http://www.jensign.com/JavaScience/cryptoutils/index.html, it does say "NOTE: These utilities and sample code are made available as-is with no support or guarantee of performance. They are intended to demonstrate specific technical implementation details with minimal error checking. Use at your own risk." which does seem to directly suggest that one can use the published utilities, at albeit one's own risk.
However, I think it is ok, but I guess you never know with the lawyers. If you find out that this code breaks the law, please contact me promptly and all code, as well as the article, will be removed.
If this is not the case, the article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL).
Joyful programming!
