Related
The server is sending data to the client and the client is using the public key to de-crypt the data. Simple enough. However, if someone with the public key is spoofing the line or gets his hands on the encrypted message packets, cant he/she steal the data using the public key?
What is wrong with my thought process.
Thank you
When using SSL/TLS the data transferred between the client and server is not encrypted using the public/private keys. Each connection uses its own set of encryption keys which are generated during connection setup. These keys are generated based on a shared secret that is provided by the client and protected by encrypting it with the public key before sending it to the server. As long as proper precautions have been taken to protect the private key this means that only the server will be able to decrypt the shared secret.
For more detailed information you can read the Wikipedia entry on TLS Handshake
Let's say we have a Server with a private and public key, the latter available to all Clients and we have a Client who doesn't have any asymmetrical keys of his own. We want to establish secure communication between the Server and the Client and the Client has to ensure Server's authenticity. This is a pretty common situation.
Now, it is my understanding that private/public key pair is usually used in such situations only to ensure authenticity (our Client can use the Server's public key to verify the Server's authenticity). To ensure two-sided communication between the Server and the Client Diffie–Hellman key exchange is used and the communication is then based on a shared secret key.
I can't stop by wonder why Diffie-Hellman is used at all in such situations. From what I understand Public-Key Cryptography could be used to both ensure authenticity AND to share a secret key between the Client and the Server. Server can send to Client a message encoded with its private key and Client can decode it using Server's public key to confirm its identity. Furthermore Client could use the Server's public key to send a new random secret key to the Server. Only Server would know this secret key as only Server knows his private key. I know some people advice not to use a public key to both encode and decode but nobody says why.
Obviously if both Client and Server had their own public/private keys they wouldn't even need to share any secret keys, but that's not a typical situation.
So... to sum it up. Why is Diffie-Hellman used instead of Secret Key Distribution with a Public-Key Cryptography? Any advantages?
I can't stop by wonder why Diffie-Hellman is used at all in such
situations. From what I understand Public-Key Cryptography could be
used to both ensure authenticity AND to share a secret key between the
Client and the Server.
Both Diffie-Hellman and RSA key exchange (where RSA is used for encryption) can be used with SSL/TLS. This depends on the cipher suite.
Server can send to Client a message encoded with its private key and
Client can decode it using Server's public key to confirm its
identity.
This is indeed what happens with DH key exchange with RSA or DSS authentication: the server signs its DH parameters using its private key, and the client can verify the signature.
Furthermore Client could use the Server's public key to send a new
random secret key to the Server. Only Server would know this secret
key as only Server knows his private key.
This is more or less what happens with RSA key exchange: the client encrypts the pre-master secret, which only the server can decipher.
You can read about all this in the Authentication and Key Exchange section of the TLS specification (leaving aside the anonymous key exchange). The choice of cipher suites (See Appendix A.5 and C), which depends on how the client and the server have been configured, will determine the key exchange mechanism used.
Fixed DH exchange is rather rare as far as I know. Ephemeral DH (DHE cipher suites) is more common. It can provider "Perfect Forward Secrecy": even if attackers get hold of the private key, they can't decipher existing traffic, since they would also need to have the DH parameters, which are not the same for all connections. However, this has a cost in terms of performance. You can find more on this topic in this article.
. Why is Diffie-Hellman used instead of Secret Key Distribution with a Public-Key Cryptography? Any advantages?
Ans: let's assume a hacker has been tracking the all messages exchanged between client and server and saving messages , but he can not decrypt those messages because he does not know the shared symmetric key. If private key of server is revealed then in case of diffie-hellman hacker still can not decrypt the messages because diffie Hellman algorithm never send the secret key on wire instead client and server agrees on same key without sharing, it's kind of key agreement algorithm not sharing. To attack diffie Hellman man in the middle should be live and should know server private key which was used for authentication. But in our case hacker has past diffie Hellman messages, so he can not use man in the middle attack. Meanwhile server changes it's private key so there is no damage. But if we use RSA keys to share secret key then on the disclosure of server's private key hacker can get all session symmetric secret keys and can decrypt all the past messages.
I am looking for a way to encrypt messages between client and server using the WCF. WCF offers a lot of built in security mechanisms to enrcypt traffic between client and server, but there seems to be nothing fitting my requirements.
I don't want to use certificates since they are too complicated, so don't suggest me to to use certificates please. I don't need confidentiality, so I though I'll go best using plain RSA.
I want real security, no hardcoded key or something. I was thinking about having a public/private keypair generated every time the server starts. Both keys will only be stored in RAM.
Then wen a client connects it should do exactly like SSL. Just as described here.
1.exchange some form of a private/public key pair; the server generates a key pair and keeps the private key to itself and shares the public key with the client (e.g. over a WCF message, for instance)
2.using that private/public key pair, exchange a common shared secret, e.g. an "encryption key" that will symmetrically encrypt your messages (and since it's symmetrical, the server can use the same key to decrypt the messages)
3.setup infrastructure on your client (e.g. a WCF extension called a behavior) to inspect the message before it goes out and encrypt it with your shared secret
That would be secure, wouldn't it?
Is there any existing solution to archive what I described? If not I'll create it on my own. Where do I start best? Which kind of WCF custom behaviour is the best to implement?
EDIT:
As this is NOT secure, I'll take the following approach:
When Installing the server component a new X509 certificate will be generated and automatially added to the cert store (of the server). The public part of this generated certificate will be dynamically included into the client setup. When running the client setup on the client machine the certificate will be installed into the trustet windows certificate store of the client.
So there's no extra work when installing the product and everything should be secure, just as we want it.
You've said you don't want to use certificates. I won't push certificate use on you, but one thing you are missing is that certificates serve a purpose.
A certificate proves that key you are negotiating an SSL connection with belongs to the entity you think it belongs to. If you have some way of ensuring this is the case without using certificates, by all means, use raw keys.
The problem is, in step 1:
1.exchange some form of a private/public key pair; the server generates a key pair and keeps the private key to itself and shares the public key with the client (e.g. over a WCF message, for instance)
How does the client know that the public key it received from the server wasn't intercepted by a man-in-the-middle and replaced with the MITM's key?
This is why certificates exist. If you don't want to use them, you have to come up with another way of solving this problem.
Do you have a small, well-known set of clients? Is it possible to preconfigure the server's public key on the client?
Alexandru Lungu has created an article on codeproject:
WCF Client Server Application with Custom Authentication, Authorization, Encryption and Compression
No, it would not be secure!
since there's no confidentiality, an attacker could do a men in the middle attack, and all the security is gone.
The only real secure way of encrypting messages between server and client IS to actually use digital certificates.
I'm sorry, the only two methods of providing secure communications are:
Use a public key infrastructure that includes a chain of trust relationships, a.k.a. certificates
or
Use a shared secret, a.k.a. a hardcoded key.
Nothing else addresses all of the known common attack vectors such as man-in-the-middle, replay attack, etc. That's the hard truth.
On the other hand I can offer you an alternative that may alleviate your problem somewhat: Use both.
Write a very, very simple web service whose only job is to generate symmetric keys. Publish this service via SSL. Require end user credential authentication in order to obtain a symmetric key.
Write the rest of your services without SSL but using the symmetric keys published via the first service.
That way your main app doesn't have to deal with the certificates.
This question already has answers here:
How does browser generate symmetric key during SSL handshake
(2 answers)
Closed 2 years ago.
I have been reading on HTTPS, trying to figure out how exactly it works. To me it doesn't seem to make sense, for example, I was reading this
https://ssl.trustwave.com/support/support-how-ssl-works.php
And notice it says this in the page
Step 4: xyz.com will next create a
unique hash and encrypt it using both
the customer's public key and
xyz.com's private key, and send this
back to the client.
Step 5: Customer's browser will decrypt the hash. This process shows
that the xyz.com sent the hash and
only the customer is able to read it.
What I don't understand is, couldn't a hacker just intercept the public key it sends back to the "customer's browser", and be able to decrypt anything the customer can?
Thanks for any response
Why is HTTPS required?
To verify whether the website is authenticated/certified or not (uncertified websites can do evil things). An authenticated website has a unique personal certificate purchased from one of the CA’s.
Who are CA’s (Certificate Authorities)?
CA’s are globally trusted companies like GoDaddy, GeoTrust, VeriSign etc who provide digital certificates to the websites.
What are public keys and private keys?
Keys are nothing but long random numbers used to encrypt/decrypt data.
Public keys are keys which can be shared with others. Private keys are meant to be kept private.
Suppose Jerry generates a private key and public key. He makes many copies of that public key and shares with others.
Now, others can only encrypt the data using the public key and that data can only be decrypted by the private key of Jerry.
Another approach is to use public keys to only decrypt the data and private keys to only encrypt the data.
How does a company get a certificate?
Website owner first generates a public key and private key, keeping the private key secret.
He gives a Certificate Signing Request file (CSR) and his public key to the CA.
CA then creates a personal certificate based on CSR including domain name, owner name, expiry date, serial no. etc and also adds an encrypted text (= digital signature) to the certificate and finally encrypts the whole certificate with the public key of the server and sends it back to the website owner.
This certificate is then decrypted with the private key of the website owner and finally, he installs it on the website.
Note: That encrypted text is the digital signature of the CA. That
text is encrypted by the private key of the CA and can only be
decrypted by a public key of CA.
When you install your operating
system or Browser, root-certificates from many trusted CA's like
GeoTrust, VeriSign, GoDaddy etc. come with it. These root-certificates
contain the public key of that CA provider which helps decrypt the
signature.
HTTPS security can be split into 2 parts (Handshakes):
1. To validate the certificate of a website:
1) When you enter the URL www.Google.com, Google’s server gives its public key and certificate (which was signed by GeoTrust) to the Browser.
2) Now browser has to verify the authenticity of the certificate i.e. it’s actually signed from GeoTrust or not.
As browsers come with a pre-installed list of public keys from all the major CA’s, it picks the public key of the GeoTrust and tries to decrypt the digital signature of the certificate which was encrypted by the private key of GeoTrust.
3) If it’s able to decrypt the signature (which means it’s a trustworthy website) then it proceeds to the next step else it stops and shows a red cross before the URL.
2. To create a secure connection (encrypts outgoing and incoming data) so that no one else can read it:
1) As I mentioned, Google sends its public key when you enter www.Google.com . Any data encrypted with this public key can only be decrypted by Google’s private key which Google doesn’t share with anyone.
2) After validating the certificate, browser creates a new key let’s call it Session Key and make 2 copies of it. These keys can encrypt as well as decrypt the data.
3) The browser then encrypts (1 copy of session key + other request data) with the Google's public key . Then it sends it back to the Google server.
4) Google’s server decrypts the encrypted data using its private key and gets the session key , and other request data.
Now, see, server and browser both have got the same copies of session
key of the browser. No one else has this key, therefore, only server
and browser can encrypt and decrypt the data. This key will now be
used for both to decrypt and to encrypt the data.
5) When Google sends the data like requested HTML document and other HTTP data to the browser it first encrypts the data with this session key and browser decrypts the data with the other copy of the session key.
6) Similarly, when browser sends the data to the Google server it encrypts it with the session key which server decrypts on the other side.
Note: This session key is only used for that session only. If the user
closes the website and opens again, a new session key would be
created.
can't get enough of web? behind the scenes when u type www.google.com in browser
What I don't understand is, couldn't a hacker just intercept the public key it sends back to the "customer's browser", and be able to decrypt anything the customer can.
First, understand that there are generally two steps of HTTPs communication.
1) Generate a shared symmetric key which can only be known between client and server, no one else knows it
2) With this shared symmetric key, client and server is able to safely communicate with each other without worrying about information being intercepted and decrypted by others.
So the question becomes, how can the client and server generate a secret shared key without being known by others in this open internet? This is the asymmetric algorithm coming to play, a demo flow is like below:
-- Client receives public key from server.
-- Client generates a key string "DummySharedKey" which will be later used as shared key, and encrypt it into "7$&^^%####LDD!!#" with server's public key, Man-in-the-middle may have the public key and might be able to intercept the encrypted data, but the data is useless to him as the data can only be decrypted by sever's private key.
-- Server receives the encrypted key string "7$&^^%####LDD!!#", and decrypt it into "DummySharedKey" with its private key
Above key exchange steps makes sure that only Client and Server can know the shared key is "DummySharedKey", no one else knows it.
So it's critical to understand that it is Client's responsibility to generate the shared key, NOT SERVER! (i think this is what confused you)
I also recommend you to take a look at this video which explains HTTPs very well.
https://www.youtube.com/watch?v=JCvPnwpWVUQ&list=FLt15cp4Q09BpKVUryBOg2hQ&index=3
What I don't understand is, couldn't a hacker just intercept the public key it sends back to the "customer's browser", and be able to decrypt anything the customer can.
Public/private key encryption is based on modulo arithmetics using prime numbers.
Such asymmetric encryption was only discovered in the mid-1970s. It is credited to Diffie and Hellman, and to Rivest, Shamir and Adleman. (Though, both actually rediscovered things already known by the British secret services.)
The wikipedia page on Diffie-Hellman has a detailed example of a secret key exchange through a public channel. While it does not describe SSL itself, it should be handy to make sense of why knowing a public key doesn't reveal the contents of a message.
You might also find this simple RSA example interesting.
I'm studying related topics and read several blogs like how-https-works and how-does-https-work-rsa-encryption-explained/.
I have summarized a sequence diagram based on my study and hope it can be helpful to someone who comes to this thread.
For more details, you can check the blog mentioned.
I have written a small blog post around SSL Handshake between the server/client. Please feel free to take a look.
SSL Handshake
A small snippet from the same is as follows:
"Client makes a request to the server over HTTPS. Server sends a copy of its SSL certificate + public key. After verifying the identity of the server with its local trusted CA store, client generates a secret session key, encrypts it using the server's public key and sends it. Server decrypts the secret session key using its private key and sends an acknowledgment to the client. Secure channel established."
It also describes the symmetric/asymmetric encryption which is used for SSL certificates and data transfer once secure transport is established.
For detailed explanation of SSL, see https://www.ietf.org/rfc/rfc2246.txt
Here are the brief ideas of SSL to answer your question:
1) Using certificates to authenticate. Server certificate is a must and client certificate is optional (only when the server requests it). A certificate is like something to prove who you are and it also contains a public key for asymmetric encryption.
2) Using asymmetric encryption (with public key in the server certificate) to establish a shared symmetric key which is used to transfer data between client and server securely by symmetric encryption (for performance reason because symmetric encryption is faster than asymmetric encryption).
The shared symmetric key is established by exchanging a premaster secret from client side (encrypted with server public key) and is derived from the pre-master secret together with client random and server random (thanks #EJP for pointing this out in the comment):
master_secret = PRF(pre_master_secret, "master secret",
ClientHello.random + ServerHello.random)
We need the server random and client random to prevent replay attacks that an attacker can capture the previous session and replay it for the new session.
What I don't understand is, couldn't a hacker just intercept the
public key it sends back to the "customer's browser", and be able to
decrypt anything the customer can.
The hacker cannot decrypt the message since he does not know the server private key. Be aware that public key cannot be used to decrypt the message.
it seams that, if using HTTPS protocol, packages transferred between browser and web server are all encrypted. And all the browser has to support its communication session with web server is so called certificates.
So I think it's clear that we can use the same certificates to decrypt packages that captured by Wireshark during the login of HTTPS sites.
I googled, and find some tutorials said the private key should be supplied, which i do think impossible and unpractical.
BTW. the following shell command get certificate from host , are there tools to yield the public key?
echo QUIT | openssl s_client -connect <site>:443 | sed -ne '/BEGIN CERT/,/END CERT/p'
Thanks in advance.
No, you cannot decrypt HTTPS/SSL sessions from a network capture, even if you do have the certificates. That is the whole point of Public Key Cryptography on which SSL is based.
What you have in your browser key-store is the certificates that will verify the validity of the public keys of the server. Said public keys are accessible, but they cannot be used to decrypt the session packets because the encryption algorithm is not symmetric.
You would need the private keys of the server in order to descrypt an SSL session and in normal situations it is quite hard to acquire those.
See also the Wikipedia article on TLS for more information.
EDIT:
I will add some more information in layman terms, mostly ripped off from the two links above, to make things a bit more clear.
Public Key Cryptography is based on asymmetric encryption algorithms. Those involve two keys, rather than one, a public key and a private key. Those two keys consist a key pair. Assuming a sufficiently strong algorithm, whatever is encrypted with the public key can only be encrypted with the private one and vice versa. The public key is widely accessible to "everyone" while the private key is stored securely and only used by its owner. How does this help?
The server has a key pair that consists of a world-readable public key and a securely stored (and inaccessible to anyone but the server itself) private key.
The client uses the server's public key to encrypt a rather long random key for a symmetric algorithm at the start of each encrypted session. That key is sent to the server.
Since the symmetric key is encrypted like this, it can only be decrypted using the server's private key. That means that it cannot be extracted from a captured network stream, and since it is used for the rest of the session, neither can any of the information that is transmitted later on.
The server uses its private key to retrieve the session symmetric key and all is well.
Is it? There is still one issue in the above scheme! How will the client get the server's public key? More specifically how does it know that it speaks with the server proper, and not some crook who managed to insert themselves in the middle (See Man In The Middle attack)?
The client cannot possibly know every single public key - there are millions of them. It will have to retrieve them from the servers and then somehow make sure they belong to the server they are interested in.
This is where another function of Public Key Cryptography comes in. Previously we used the public key to encrypt data so that only its intended destination could descrypt it. Now we
do the opposite: We use the private key to encrypt data so that it can only be decrypted with the public key. Essentially we sign the data so that if decrypts with the public key people will know the data came from the server. How does this help?
When the browser is installed it has a relatively small number of public keys (commonly known as certificates) pre-installed. Those belong to Certification Authorities (CA) that are entities (generally companies like Verisign, governments or even some Institutions) that are deemed trustworthy. The Certification Authorities use the corresponding private keys to sign other public keys available in the Internet and so on.
So when the browser contacts a server and receives its public key, that key should be signed with the private key from a certificate pair. That certificate pair would be likewise be signed by another and so on, until the browser comes up to a public key that was signed with the private key that corresponds to one of the Root Certificates that were preinstalled with it. You might want to read a previous answer of mine for more on Root Certificates, or just read the Wikipedia articles.
Since each public key was successively signed until we reached a trusted key, the client can now be certain that the server on the other end of the connection is indeed e.g. MyBank.com and not, say, some criminal's laptop in the next cafe table that is messing with the wireless connection.
While there have been weak SSL algorithms (hence the succession of SSL protocol versions), these days the attacks are generally targeted to the implementations:
Use specially crafted or malformed certificates to fool the browser (and its weak implementation) into thinking that a public key is signed by a trusted entity when it's really not.
Compromise the integrity of the client software (e.g. a virus) to capture the symmetric keys when they are generated. In most of those cases however, it is usually easier to just capture the interesting data as they are entered into the browser by e.g. capturing the keyboard strokes as the user types in their credit card number.
Compromise the server itself and steal the private key. While it's not as easy as targeting unsuspecting clients, it has happened and it has the potential to compromise a significantly larger number of encrypted sessions. The current Public Key Infrastructure (PKI) has a whole mechanism for the revocation and disabling of compromised or otherwise invalid keys.
The last attack can be defeated if the client sends a temporary public key of its own instead of a symmetric key and use an asymmetric algorithm for the whole session. That would make it impossible for an attacker to compromise past sessions by stealing the server's private key. This technique is the basis for ensuring Perfect Forward Secrecy, but unfortunately not all servers implement it yet due to compatibility or performance reasons.
thkala is spot on. You can, assuming you have MiTM, under common circumstances forward a self signed certificate to the victim for which you have the private key, then forward the requests by the victim to the destination server using the servers' real certificate. This will require the vicitm to accept a certificate warning though.
Another way of defeating Ssl is by stripping the ssl session, lookup sslstrip.