I'm currently developing a C# application that communicates between a client and server using TCP/IP. After connecting I'm swapping session based RSA public keys to then pass a session based AES symmetrical key. This all works fine and means I have a secure channel to communicate on.
The trouble now is that I need to ensure that the server and client are both the ones I wish to communicate with. Before anyone suggests SSL/TLS: I'm not connected to the Internet to allow the use of a root CA.
Would it be valid, that after establishing a secure channel between the client and server that I provide a simple challenge-response method? For example, if the client sends an identifier (GUID?) the server compares this value to known clients and accepts or rejects it, and the same is repeated in the opposite direction. As the data is encrypted and the encryption is session based is this a valid method of verification?
I understand that the storage of these identifiers is the weak point.
Before anyone suggests SSL/TLS: I'm not connected to the Internet to allow the use of a root CA.
Before you are re-inventing the wheel: you don't need root CA with SSL/TLS but can simply use self-signed certificates with key pinning. Public root CAs are only usually used because it scales much better to provide only few common CAs instead of exchanging all self-signed certificates (or their fingerprint) to all peers of the communication.
Apart from that it looks like you proposal assumes a secure connection already to check credentials which you then will use to verify that the connection is really secure. Or in short: to provide a secure connection you need a secure connection first.
Related
I have a Client & Server application set, both written in C# but some client versions might be distributed in other languages in the future. I want to protect my applications.
I was looking for some kind of advice to stop just random people sending messages to a server and acting like a client, what kind of validation can I put in place?
My client applications I distribute will be obfuscated but is this enough? I'm just looking for some advice in this situation, is it wise for me to add some kind of encryption other than SSL, or am I just being over protective and over curious? Any input is welcomed & accepted.
It is impossible to determine if you are communicating remotely with "your client" or another piece of software that also knows how to communicate in the way that your client does.
What you can do is ensure that you are communicating with someone that is authorized to communicate with you by using client certificates for your SSL session.
The server proves who it is to the client and the client proves who it is to your server. The security then rests in whoever holds the private key to the client certificate (and the password for this key file).
The C# SslStream Class has support for this. Namely the AuthenticateAsClient method is relevant here.
In summary, if your software is only secure when communicating with a client you wrote, then your software isn't secure period. Instead, design your server in such a way that you can serve client requests securely. Using authentication is one of these ways.
You would want to do two things....one is look up certificate pinning. Your app will validate your SSL cert to thwart man in the middle attacks and it makes it hard to circumvent. The other is when making requests to the server have some type of user name / password block on the server side script before the server side does anything so the requests will simply be discarded by the server if they are from an unknown source.
I have a client server application written in C#.
Actually the tcp connection is crypted by rsa and rijandel (like poor man ssl). I've often read, that I should use something like SSL not a own implementation. I know the reasons for this.
But if I use ssl, I'll need two certificates, one for the client and one for the server, right? (Because also the server sends some data to the client and other way) How I do this with the client? Or should I use for this communication an other crypting channel?
SSL (or TLS) uses public key cryptography for two purposes. One is to encrypt the communication channel (really just to exchange a symmetric key) and one for server identification. The public key of a public private key pair is exchanged in a certificate that is digitally signed by an authority.
If you do not need your clients to authenticate themselves (which is different from your users logging in), you do not need a client certificate. The server certificate is enough to identify the server and to encrypt the communication channel.
Client certificates are typically used in scenarios where background services or daemons need to identify themselves with web services. In that case, there is no user who's identity can be used to authorize the request.
I am designing a system in a client server architecture - TCP based. There is a requirement that all the messages between the server & client should be encrypted. So I am thinking of using SslStream class in .NET Framework.
From SslStream MSDN, my understanding is that we need to use Client & server certificates to make a proper channel & communicate.
I don't want to use any certificate specific to particular machine(client) or server. I Just wanted to have a common key between the system.
Is there any possibility to use the SSL stream without certificates??
Whilst not required by the TLS spec, the use of the .NET SslStream implementation requires that the server has a certificate (and its associated private key). This allows any client to confirm that it is communicating with the server it expects to be. Optionally, clients can also be authenticated by having them provide a client certificate to the server. So, if you want to use SslStream, you're at a minimum going to have to create a certificate for the server, because that is how this particular implementation works.
If you don't wish to use certificates at all, then there are other options. E.g. if you are planning on using your application within a Windows domain, and both client/server are Windows based, you may be able to use NegotiateStream instead. This also supports encryption/authentication, but does not use certificates, and will additionally allow you to identify the user on the other side of the connection.
I am currently looking for the best way to establish a stateful and encrypted connection between a C# client and server application. First, I thought about using IPsec, but as it works on a low level (OSI: Internet Layer), I would be very hard to implement, if you want the functionality inside your program and don't want to rely on the OS.
What technologies would you recommend for this purpose? Is there some functionality already built into .NET (4.5)? It does not neccessarily have to be stateful, working with some kind of heartbeat would be a valid option, too.
You'll want to use a standard protocol such as SSL rather than trying to make your own. First the implementation will be much easier because the .NET framework will support it, and the transport protocol that runs underneath it is stateful (e.g. TCP). Second developing a cryptographic protocol that is secure is very difficult, and SSL has already been implemented so why reinvent the wheel?
SSL works by using PKI (Public Key Infrastructure) to generate a shared symmetric key. The handshake consists of a number of steps. First the client sends a request for a secure session, then the server responds with it's certificate, the client verifies the certificate by crawling up the ladder through the certificate authorities (e.g. Verisign, Thawte, GeoTrust etc...) or if it already trusts the server it can just accept the certificate that is self signed.... and once it finds the certificate is trustworthy it generates a symmetric key and picks an algorithm (e.g. AES, 3DES, RC4, IDEA etc...). The client then encrypts the key and algorithm being used with the public key, then the client sends that value to the server and a secure session can proceed using symmetric encryption which is much faster.
SSL itself is can be used in a stateful manner because it actually works over the transport layer in the OSI Model, HTTPS on the other hand is not a stateful protocol by design. HTTPS is HTTP over SSL so the two technically don't really have anything to do with each other, except that in HTTPS SSL is used to secure the application data that is being requested. With HTTPS as with HTTP once a request is made to the server it basically forgets about you (not exactly how it happens but for all intents and purposes you can think of it this way). I myself would prefer the use of HTTPS if you can get around having to have a stateful protocol. The main reason for doing so is so that I wouldn't have to write the code and possibly have a mistake in the implementation of SSL. All you have to do is build a WCF or REST based service that runs on IIS and get a certificate for your server.
That being said, if you still want to create your own SSL server that doesn't use HTTP on the application level you can use the TcpListener and TcpClient classes along with the SslStream class provided as part of .NET to create your own. MSDN has a good example of how to create an SSL server and client: http://msdn.microsoft.com/en-us/library/system.net.security.sslstream%28v=vs.110%29.aspx
Side Notes
Securing the transport of your data does not secure your app, do not make the mistake of thinking you get automatic security
If you choose to make your own server and client you can use either openssl to generate your certificate or you can use makecert which is part of .NET to make your certificate.
Just form a regular TCP connection between the applications, and write up a simple packet protocol (EG, 4 bytes indicate packet size, followed by packet data)
Except the data within this base-level packet is encrypted through System.Cryptography.AesManaged
If you have trouble encrypting the packets using AesManaged, try using The Encryptamajig - if that doesn't help, post further questions and we'll give you further specific help.
-- You can either have both sides know the password ahead of time (EG, tell the person at the other end the password in person), or quickly pass it unencrypted at the start of the connection (or, rather, encrypted with a default known password)
Not necessarily the best method but it should do the job.
Why not just the regular HTTPS? HTTP is just one level above TCP but it is far easier to work with and firewalls tend to be generally easy on HTTP/HTTPS ports namely 80 and 443. Of course, plain HTTP is not suitable for you but can you not use HTTPS instead of coming up with your own encrypted communication mechanism? In the client side (C#), all .NET classes such as HttpClient supports HTTPS very well. I quote Ayende in support of my suggestion to go with HTTP :)
I am about to start on a project that will be running as a windows service listening for incoming connections and doing some things locally then sending a reply message. I understand the basic concepts with sockets and communicating over the network, however the things the service are doing could very easily abused. I would like to authenticate the person connecting, preferably against the windows local users on the machine the service is running, to see if they have windows administrative/power user rights.
I know how to do it check the rights once I have their information but I know sending the user name and password to the application over the network in the clear is a no no. I was thinking of just encrypting the password with some secret key but I know "trying to be clever" is the worst possible thing you can do in cryptography so I wanted to know what is the "correct" way to handle this situation.
My second idea was just create a shared self signed certificate between the client and the server and just use TLS for the entire connection.
I may as well post what I was thinking of doing, if it is the right thing to do say so in the comments.
Both the client and server will have a PSK at run-time the server will send a random number to the client. the client will encrypt the credentials with the PSK and the random number as the IV. It will send back the encrypted blob plus whatever commands it needs done.
I am not concerned about replay or mitm attacks. I just want to authenticate the user and not have peoples passwords blasted all over the network.
Scott,
this may be a bit overkill and a bit off topic, but have you considered using a web service interface to serve your clients (instead of using raw sockets)?
ASP .Net web service interfaces are easy to implement, and in the end, you'll end up with a very well defined interface. They also have support for authentication and secure communication.
ASP .Net Web Service Tutorial
HTTP Security and ASP.NET Web Services