I’m trying to implement a message level encryption. Here is the current situation:
We have a mobile app client connects to server via oneway certificate https and we have username/password authentication and secure token for subsequent client to server invocations. So intention of message level encryption is not try to prevent 3rd part sniffing information or stealing client identity, instead to prevent the client user him/her self to something like,
1) Inspect and try to understand server - client protocol
2) Forgery request with other application than our app client
The initial idea is to use symmetric algorithm (DES, AES, or some simpler algorithm, as long as it could not be simply cracked by statistical or mathematical method without knowing the key). And the key is generated from a hash (SHA etc) from a string concatenated from a salt pre-agreed between client and server, and some information server tell client in non-encrypted content (for instance, the first call from client to server happens in non-encrypted context, and server returns a timestamp to client and also remember it for later key generating).
Does this do what I want to do for 1&2? And what is the major vulnerability if any?
There is no "secure" solution to what you try to achieve as long as you publish your software, because any key you use for the message level encryption will have to reach your software somehow. Either it is compiled into it, then a dedicated attacker can read it out of the binary, or it is transferred to the running software using the network, then the attacker can emulate the protocol your software is using to get the key.
The best you can hope for is to make it difficult for a reverse engineer to get to the key. That means you could assemble it in such a way that no complete piece of it can be found in the binary. But still if someone attaches a debugger at runtime she could still read it out of a variable trivially. In the end it remains an arms race between you and the reverse engineer.
Related
I'm looking for some guidance/opinion on best practice for both authenticating and signing (integrity checking) an HTTP request. I'm leaving this fairly open for language and technology used, but the basic parameters and requirements are as follows:
Need both authentication and integrity checking.
The request contains sensitive data and will be passed over HTTPS, but I cannot assume that the request cannot be sniffed by an attacker, either before it is HTTPS encrypted (at the client end by some illicit piece of installed software) or at the server end, e.g. by a sniffer placed between the HTTPS endpoint and the actual server. The HTTPS encryption/decryption handling has been passed off to an external load balancer so there is a small window for an attacker to insert a sniffer between the load balancer and the server.
I need to make sure that a transaction cannot be faked.
I need to make sure that a transaction cannot be replayed.
Requests can be GET or POST, and so any extra data needs to stay within the maximum size limit of a GET request.
Things that we have considered:
Username/password on each request. This guarantees authentication I guess, but if an attacker can sniff the username/password pair then it all fails.
Private key signing of each request. The server will have the public key, only the client will have the private key. This guarantees integrity because only the client could have generated the signature, but the server can check the signature. It generally guarantees authentication as well because the private key is not part of the request data and cannot be sniffed. However by itself it does not stop the transaction being replayed, because 2 transactions with the same data will have the same signature.
Using a cryptographic client nonce ( https://en.wikipedia.org/wiki/Cryptographic_nonce ) as part of the request data and including that in the data to be signed. We have had issues with these especially when they are generated on the client side because if they aren't sufficiently random then an attacker can find out how they are generated then the attacker can generate a sequence of nonces of their own, in advance of the client generating the same sequence, which can lead to a denial of service attack because the client will be trying to re-use a nonce that has already been used by the attacker. Generating nonces on the server side has been considered but it's an extra transaction and potentially a performance issue.
Including a date/time in the request data, however this can cause issues where the client's clock and the server's clock drift out of sync.
In case some admin decides to flag this as a duplicate, here are other Qs that I have considered that don't quite address the full scope of this issue:
What is the current standard for authenticating Http requests (REST, Xml over Http)?
Authenticity and Integrity of HTTP Requests
Assuming a 1-1 relationship between client and server, a HMAC with counter would solve this problem.
The client and server have a shared secret 128 bit key.
The client sends messages that have an HMAC with the secret key, and a counter. SHA-256 is recommended, as SHA-1 is on its way out (well SHA-1 HMAC is still considered secure, however that's another story).
e.g.
example.com?message=foo&counter=1&hmac=35ed8c76e7b931b06f00143b70307e90f0831682e7bdce2074ebb5c509d16cfb`
(This tool used for this post with secret bar.)
The HMAC is calculated over message=foo&counter=1.
Now, once the HMAC has been authenticated by the server and the counter checked, the server's counter is incremented to 2. Now the server will not accept any authenticated messages with a counter less than 2.
JSON Web Tokens could be used so that you are doing the above in a standard format. You could do the above with multiple clients, however you would need to keep track of the counter server-side for each client, and the client would have to identify itself in the message. Managing the shared secrets is the trickiest bit if you decide on a different key per client.
I'm working on a network program and I don't want anyone to know what kind of information is being passed when they sniff the network. Would using TLS achieve this? My main reason is that I want to keep the protocol I'm using to myself for now. If not please tell me if there is anything that can achieve my goal.
It depends on a lot of things, e.g. what your exact threat model is, and how much information leakage you can tolerate.
For TLS to provide adequate protection, these assumptions must be true:
Obviously, you should use a correct implementation, otherwise, if for instance, you are using SecureTransport from iOS 7.0.4, all bets are off.
You should enforce a minimum version requirement and only support secure ciphersuites. If you allow downgrade to SSLv2, you are setting yourself up for problems.
You check for validity of the server public key. You'd be surprised how many client apps skip this.
You use client certificates to authenticate the client, as well as the server, otherwise, it is possible to write a phony client that talks to your TLS server and reverse engineer your protocol. (You can also authenticate the client early in the protocol lifecycle using other means, but that part of your protocol would not be safe).
You keep the private keys secure.
(If you are using X509 certificates and trust chains:) Certificate authorities that you trust do what they are supposed to do, i.e. not sign certificates in your name for others.
You will still leak some packet length and timing information that you hope would not be complete enough for the reverse engineer.
The attacker does not control your client or server or have access to the binaries on any side. If, like an iPhone app, you are giving away the client binary, you have already lost.
Your higher level protocol cannot be tricked into say, redirecting to another server blindly, or lose its mind and do some other crazy thing when the client secure channel is interrupted. This can be hard to notice at times and depends on many other factors.
Something else I have probably missed here.
Would TLS prevent others reverse engineer my protocol?
Probably not. Pentesters do it all the time. They use something like Burp Suite to proxy the connection and watch all the web requests.
If not please tell me if there is anything that can achieve my goal.
Common practice is: if you don't want it stolen, copied, pilfered, abused, etc, then you don't put it on a client. So all sensitive code and data goes on a server you control. Since the client gets to see the request, you have to remove all sensitive information from it.
so I have this TCP connections between my server and client, and anyone can connect to my server. But I want to make sure that the client is really using MY client application and not just faking messages from a fake TCP client. What would be the ways to do that, check that the connection really is from my game client?
Thanks!
EDIT
If I'm gonna use TLS, can I solve that problem?
There will probably not be a complete solution to your problem, since whatever you do, the other party might always take your program, run it in a monitored environment, manipulate the runtime data and let it use its "secure" network protocol. Since the client application is in uncontrollable hands, you can never be sure that it is your own program.
Baby example: My application runs your application and plays back the data to your server, and forwards your response back to the application. How can you tell?
That said, it might be a very promising "99%" approach to use SSL and hardcode the client's private key into the application -- with some trickery you can try and make it hard to find (e.g. see how Skype does it). If you then also build integrity checks into your program that figure out whether anyone is manipulating the memory or debugging into your program, you can try and make it a bit harder for a potential adversary. (But note that you will always have to ship the private key with your application, so it isn't really safe from discovery.)
Others have suggested useful answers to your question, but I'm going to suggest another approach. Re-examine your requirements.
Ask yourself why you want to know the identity of the client program. Is it so that you can trust your client program more than you trust 3rd-party client programs?
If you need to trust the identity or integrity of software that you have already shipped to your customers, I claim your security model is broken. Once the software runs on a client's PC, you should assume it is evil, even if you originally wrote it.
Any status, any command, any data whatsoever that comes from the network must be checked before it is relied upon.
My default response is to use a challenge/response authentication.
After connection, send a random number from the server to the client
The client then computes, using a hash/key/.... a response message and returns that to the server
If the response matches the servers computation, your chances of authenticity are better. Note though that a reverse engineer of your client will leave this method open to fraud.
You could use a public/private key pair in order to verify that you are who you say you are.
http://en.wikipedia.org/wiki/RSA#Signing_messages
I have a client-server game, where the client connects to a server and stays connected during the game (approx 5-60 min).
I want new clients to be able to register securely, as well as allowing existing clients to authenticate with no worries that the login credentials are exposed.
The thing is that for performance reasons it would be best to stick with a simple and cheap encryption like RC4 for the game session, but a symmetric key does not make it easy to secure the registration procedure.
Since I will want to keep a separate login server anyway, my idea is like this:
Client sends a HTTPS request to the login server with credentials (or registration information)
The login server collects the user information, and generates a temporary RC4 session encryption key
The user information + RC4 session + timestamp + digest (I can rely on both servers to be synchronized timewise) with a secret symmetric key, shared between game server and login server.
The packaged data + RC4 session encryption key + ip address to the game server is sent as a reply to the HTTPS request to the client.
The client opens a connection to the game server, sends an initial unencrypted hello message with the encrypted user information as a payload.
The game server unpacks the data packaged in (3). It now knows the user and the RC4 encryption key it is supposed to use.
If the timestamp indicates that the login credentials has expired, an error is returned to the client (who is then to retrieve new information). If the decrypted user data cannot be verified with the digest a different error is returned.
If everything checks ok, the server sends an unencrypted LOGIN_OK, and the RC4 encrypted communication starts.
Possible security concerns:
The game server 100% trusts the user info it has decrypted. This makes the servers completely decoupled which is nice, but if the key is compromised, users could completely fake their user info. This could be alleviated somewhat by rotating these keys, so that every day or month has a new key. Both game and login servers could get this from a third server that manages their keys. It might be overkill since: a) in case of a break-in where source code is exposed on the servers, they can be restarted with a new key b) a good enough key + encryption should make brute force attacks hard (suggestions on algorithm?)
RC4 isn't the most secure algorithm, but I make sure to throw away the first 512 bytes or so and each key is only valid for a limited time, e.g. 24h.
Doesn't seem susceptible to man-in-the middle from what I can see: SSL secures the RC4 session key, in (5) the RC4 session key sent to the game server is encrypted as well. All that is possible is DoS and to cause the user request a key again. If the data in (2) is cached until it expires, this should not create a new packet.
The encryption in (3) could be improved by adding random bits to the key. Those random bits are sent together with the encrypted packet, and presented to the game server in (5). In (6) the game server adds those random bits to his key and uses the result to decrypt the data. This way and attacker cannot see when the packed data changes.
Are there any vulnerabilities I'm overlooking here?
A summary of payloads created:
Client login-credentials (protected by SSL), sent to login server
User info + timestamp + temporary game server session key + digest encrypted by login server using a secret key shared with game server, given to the client that - without modifying it - passes it to the game server. Ought to be temper resistant because: a) client does not know the secret key b) has timestamp to avoid resending same data c) digest to verify content was encrypted correctly
temporary game server session key sent by the login server to the client together with the encrypted payload. Protected by SSL.
Client game server login packet, consists of encrypted packet received by login server.
A summary of encryption keys:
Temporary game server session key: randomly generated by login server for encrypted game server <-> client communication. Generated by login server, given to client and game server.
Secret user info encryption key. Shared between game server and login server, used pass user info to game server with client as messenger. Client does not possess this key.
First of all I wouldn't use RC4. There are both faster and more secure stream ciphers around so if you control both client and server then you can probably do better than RC4. Discarding only 512 bytes may not be enough for the Fluhrer, Mantin and Shamir attack, but even if you discard more bytes there's also the Klein's attack etc. I don't know if it's worth the trouble.
Second, make sure that the keys you generate are random. I know it seems obvious but for an example see: http://www.debian.org/security/2008/dsa-1571
But the real problem is this part: "The game server 100% trusts the user info it has decrypted. This makes the servers completely decoupled which is nice, but if the key is compromised, users could completely fake their user info."
You have to assume that the user knows the key. His game client has to know the key so it can communicate with the server. If the user can use his real data to log in via ssl, get a key for stream cipher and then send whatever info he wants to the game server then all the attacker has to do is just get an account and do whatever he wants.
It doesn't matter how often you change the key because every time you change it you have to still give it to the client so you might as well change it after every byte and it still wouldn't matter here.
This is more important than the cipher used or the key generation because no one will brute force the key if he just gets it.
You should never trust the client. You should store the client data on the server side and match it with the key or sign the data and verify it or use HMAC etc. because if the game server 100% trusts the user info then you will have problems sooner o later. There is pretty much no way around it.
It sounds like you're trying to reinvent SSL. Why not issue each client a certificate (signed by your own root authority), and have them connect to the game server over SSL, with mutual authentication?
I understand you cannot use SSL between the game server and the client as you don't want to go through the handshake again.
The protocol seems ok from a first glance. There is no replay attack also as you really need the symmetric session key to do anything meaningful. The best thing you can do is switch to AES is also very fast and very secure. I highly doubt you will see any performance hit by switching to AES.
Also the first security concern bullet point you mentioned is not really a concern. Well it is a concern for all clients on the desktop, for example your browser has the same problem talking over HTTPS. So you don't really have to solve it. Your game logic somehow has to look for bad behavior if you want to actively monitor manipulation attempts. You cannot solve it by re-keying.
I ended up also posting on sci.crypt and I'll try to summarize the suggested changes (as far as I understand them) below in case it might be of interest.
Step 1: Client sends a HTTPS request to the login server with credentials
Assuming that the credentials take the form of a login token, also add a self-assigned unique id.
Step 3: The user information + RC4 session + timestamp + digest
Use an encryption algorithm that ensures integrity, instead of using a digest explicitly. E.g. AES-GCM or AES-CCM. Add the extra id field in step 1. Add the ip to the game server as well.
Step 4: The packaged data + RC4 session encryption key + ip address to the game server is sent as a reply.
Giving the timestamp to the client will allow the client to know when the session has expired. This avoids unnecessary connects to the game server with expired credentials.
Step 5: The client opens a connection to the game server, sends an initial unencrypted hello message with the encrypted user information as a payload.
Add the self-assigned id in step 1 unencrypted to the payload.
Step 6: The game server unpacks the data packaged in (3). It now knows the user and the RC4 encryption key it is supposed to use.
The game server matches both its own ip with the encrypted ip, as well as the encrypted id with the id given by the client. The first prevents the user from going to a different server with the same credentials.
Step 8: If everything checks ok, the server sends an unencrypted LOGIN_OK, and the RC4 encrypted communication starts.
At this point the game server cannot be sure of the client's identity. Use the session key and encrypt nonce + strictly increasing session id + login success state using AES-GCM/CCM and send it to the client.
The client decrypts and checks the login success state. If this is true, then the client knows that the game server knows the session key (GCM/CCM verifies that the packet has not been tampered with). The client returns sid + nonce.
The server verifies sid + nonce is the same as the values sent.
Finally the client and server creates new session keys by hash-ing the session key with sid + nonce + salt to create the key for the consequent communication, to prevent a possible replay attack.
Regarding RC4
There are vulnerabilities in RC4, but it probably would suffice for this scheme because of the rather aggressive key rescheduling. However, there are modern ciphers which are more secure and faster, such as Snow 2.0 or Trivium.
Just use SSL to the game server. Modern cryptanalysis has resulted in a few very fast implementations of some of the better encryption algorithms. For example, well optimized AES implementations can easily encrypt at better than 150MB/s on any remotely modern machine. Also while AES is held with high regard, it does have two weaknesses that I know of, but when used correctly those weaknesses become insignificant.
I noticed that you failed to mention that you would be using an advanced key scheduling algorithm between the client and the game server. Failing to do so would make the weaknesses of the encryption algorithm much more severe. SSL/TLS should do the key scheduling for you.
I'm using RSA to encrypt communication between a server and a client.
Lets say we have 2 Asymetric keys, key 1 and key2.
The server has key1 (Private) from the start and the client has the key1(public)
So here is the scenario:
the client generates key2
client connects to the server
sending key2(public) encrypted with key1(public)
from now on the server will send all data encrypted with the key2(public)
the client sends some random data to the server
the server sends back the same data hashed
the client verifies that the data is right
As far as I can see this should prevent a man-in-the-middle attack, or am I missing something?
At point 7 the client should know if someone is trying to give the server the wrong key to encrypt with, as no one else but the server can decrypt key2(public).
If there is anything that can be done to improve the security please tell me.
The best thing you can do to improve the security is to use an existing design and not try to reinvent the wheel. I'm not saying that what you've done is necessarily wrong, but just that many people much smarter than you and me have spent a lot of time thinking about this problem. Use TLS instead.
As long as key1 (private) has not been intercepted somehow by a third-party, your scenario looks secure.
I think I saw this somewhere in a paper actually. In it, Alice gave Bob an unlocked box (key 1 public), then Bob put a bunch of his own boxes (key 2 public) in it, locks it and sends it back to Alice. Alice then opens the box(key 1 private), and now she can securely seal the boxes that Bob just gave her.
Despite the box analogy, that's essentially what you're doing, so I'd say its secure.
I agree, just use TLS.
Also, what value do steps 5 through 7 provide? A MITM wanting to do an attack that would work after steps 1-4 (e.g. DoS of some sort by passing n transactions through and then stopping, forcing a retry from the start) could do so just as well after 5-7. What do they add?
-- MarkusQ
No, this protocol is not safe.
A man-in-the-middle can intercept the data sent by the client and send whatever it wants to the server, since you haven't specified any mechanism for the server to authenticate the client or verify the integrity of messages it receives.
Sure, you could doctor up your protocol to fix these glaring problems, but there would be others. If you ever fix them all, you'd have something that maps to TLS or SSH, so why not just start there?
#Petoj—the problem I was focusing on was that of the server trusting the messages it receives; your proposal doesn't provide any security there. However, if you are worried about confidentiality, you still have a problem, because the MITM could pass messages back and forth unaltered until he sees what wants to find because you don't have any privacy on the client messages.
Your proposal seems to be aimed at ensuring the integrity of messages from the client. You've developed the protocol to the point where the client can't distinguish between an attack and a network failure. Rather than trying to help the client determine whether the server acted on a tampered message, allow the server to verify the integrity of the message before acting on it.
I will agree with Greg that you are reinventing the wheel. What you are essentially describing is some basic form of key exchange. Incidentally, in order to ensure that it is secure against man-in-the-middle attacks you must also be certain of the server's identity, i.e. ensure that the client can know with certainty that what it believes to be public(key1) really is the server's and not the man-in-the-middle's (e.g. using a CA or having the server's public(key1) in secure storage on the client side.)
Moreover, there are additional considerations you must be aware from a systems standpoint, such as:
asymmetric key encryption is slower than symmetric key encryption, which is one of the reasons why existing solutions such as TLS will use asymmetric key encryption only to negotiate a temporary symmetric key, which is then used for channel encryption.
if traffic analysis by a third-party succeeds in cracking a temporary symmetric key, you have not compromised you asymmetric key pair. You are encouraged to re-negotiate the temporary key relatively often for this reason. Arguably, generating a new key2 in your scenario would mitigate this aspect.