Isn't it possible to simply capture the key while it's being negotiated between machines?
Let's say I connect to my bank's website. Browser will send request to bank server. Bank server will send me copy of its SSL certificate. If it's valid, browser will send signed ack. Isn't it possible to simply capture private key at that time?
Your private key (and bank's too) is never sent over the network. The bank's public key is a part of the X509 certificate that it sends to your browser. If banks signs anything it does it using its private key, and your browser verifies the signature using the bank's public key (from the certificate). The whole SSL session is encrypted using symmetric cipher (for performance reasons). To be able to do that both parties need to exchange the session key (different for each connection and renegotiated after defined interval of time). This exchange is done using Diffie-Hellman algorithm. During this exchange the key is not sent over the network, it is computed independently by both parties.
Isn't it possible to simply capture the key while it's being
negotiated between machines?
No because it is never sent, it is computed independently. You would need the server's private key, which is private, to repeat the independent computations that lead to the session key. So it is secure.
Only public key is shared. The bank dosen't need to share the private one.
Private key is not shared and is not stored in the cert.
Related
I came across a mbedTLS example on an embedded device and i see a server certificate validation (github.com as example) before downloading a firmware but the only certificate/key coded is the one of github (the CA chain, rather than the github cert itself).
My questions are:
If i perform a server CA verification do i only need the CA certs right?
After the verification of the server, is the communication between the two encrypted at all, meaning is the firmware file exposed in clear or is it crypted? Is the encryptiong done using the private key of the server? (which i suppose it is on the github server). If not what's the use of the server private key? Integrity?
Do i also need a client private key to establish an encrypted connection? If not is the client private key supposed to do the same thing of the server private key but on client side?
I've a general idea of what the answers are but i need certainties.
Regards,
If i perform a server CA verification do i only need the CA certs right?
correct
After the verification of the server, is the communication between the two encrypted at all, meaning is the firmware file exposed in clear or is it crypted? Is the encryptiong done using the private key of the server? (which i suppose it is on the github server). If not what's the use of the server private key? Integrity?
The validation of the servers certificate is part of the TLS handshake but not the end of the handshake. Only after the handshake is completed data are encrypted but also no application data are transferred before the handshake is completed. Thus, a firmware transferred over TLS is encrypted with whatever cipher client and server agreed on - which may be a weak or a strong cipher.
The application data are not encrypted by any private key but by symmetric cryptography. For more details see How does SSL/TLS work.
Do i also need a client private key to establish an encrypted connection? If not is the client private key supposed to do the same thing of the server private key but on client side?
A client does not need a private key for encryption. Apart from that the servers private key should be kept secret (private) and therefore it should not be possible that the client can just use the servers private key. Again, see How does SSL/TLS work for details.
Please help me fill out the blanks here -
The server keeps its private key and the public key is shared to the users. So the client trusts the content thats coming from the server using the public key. How does the client encrypt the contents he is sending back to the server ?. Using the public key of the server ? or does the client send a autogenerated private key and encrypts it using the public key which is then decypted by the server along with the message and used for furthter communication by both parties.
A Public and a Private key is required to do ssl communication. This Key pair is generated using a self signed certificate ?. How can a single self signed certificate contain both public and private keys.
One more thing On message level security -- im looking at a current configuration and am pulling my hair out -- Using IBM Ikeyman to look at the producer and Consumer JKS files-- for Message level security(Digital Signing) there is a Personal certificate at the Consumer and a Signer certificate at the Producer ... Isnt this the other way around? Is this current configuration incorrect --- Both the keys are same by the way.
The server keeps its private key and the public key is shared to the users.
Correct.
So the client trusts the content thats coming from the server using the public key.
No. There is no 'so' about it. The client trusts the server certificate because it is signed by someone he trusts, and he knows it belongs to the server because the server provides a digital signature that the client can verify, which only the private key owner can do. Therefore he knows that the server owns that public key.
How does the client encrypt the contents he is sending back to the server ?.
The client and server negotiate a shared session key independently using techniques described in RFC 2246. For the most part they don't involve PKI at all.
Using the public key of the server? or does the client send a autogenerated private key and encrypts it using the public key which is then decypted by the server along with the message and used for furthter communication by both parties.
Neither, see above. This is quite a large subject.
A Public and a Private key is required to do ssl communication.
No. One of the parties must have a private key and a corresponding certificate that the other party trusts, otherwise the communication is insecure.
This Key pair is generated using a self signed certificate
No. The statement doesn't even make sense. Key pairs are generated first, nothing to do with certificates yet. The certificate is a wrapper for the public key.
How can a single self signed certificate contain both public and private keys.
It can't, and it doesn't. Self-signing doesn't have anything to do with it either.
Public-key encryption 101:
The public and private keys form a pair: each key in the pair can decrypt messages encrypted with the other, but cannot decrypt messages encrypted with itself. If the client can decrypt a message with the public key, it knows the message was encrypted by the owner of the public key. Conversely, a message encrypted with the public key can only be decrypted by the owner of the private key.
The basic idea is that the client generates a key for a symmetric-key cypher, encrypts it with the public key, and sends that to the server. Both sides then use that symmetric key and cypher for the majority of the communication.
In SSL communication,when the client wants to interact with some server, the server sends its public key. Always remember a certificate is nothing but a public key with a bunch of supporting information.
The problem here is any hacker can masquerade as a server and can block the communication between server and client. So the server certificate must be signed by some certificate authority. The client only believes the server certificate if it is signed by a certificate authority.In that case the hacker in between can not masquerade as a server because its certificate will not be authenticated by the certificate authority.
So client accepts the certificate and gets the public key of server. Now the client can send its public key encrypted by the public key of the server. Since this encrypted message can only be decrypted by the private key of the server so only server can decrypt it.
But the use of public key and private key over the ssl communication can slow down the connection very much because these keys lengths are 1024 or 2048 bits.
So practically what happens is instead of sending its own public key, the client sends the symmetric key encrypted by the public key of the server. Server decrypts it with its private key and it gets to know the symmetric key. Now the further communication happens with this symmetric key encryption and decryption.Since no third party gets to know the symmetric key, the communication will be secure.
Remember symmetric key lengths are generally 64-128 unlike public keys hence less the time for encryption and decryption.
I was looking into SSL and some of the steps that are involved to set up an encrypted connection between a server and a client computer.
I understand that a server key and certificate is sent to the browser, and that a secret code is being calculated, like they say in the following video:
http://www.youtube.com/watch?v=iQsKdtjwtYI
around 5:22, they talk about a master secret code that is being calculated to start talking in an encrypted way.
My question now is: before the connection is actually encrypted (the handshake phase), all communication between the server and the client can be sniffed by a packet sniffer. Isn't it then possible to sniff the encryption key or other data that is used to set up a secure connection?
From the Wikipedia summary, I think the key part you're interested in is:
The client responds with a ClientKeyExchange message, which may
contain a PreMasterSecret, public key, or nothing. (Again, this
depends on the selected cipher.) This PreMasterSecret is encrypted
using the public key of the server certificate.
This is why the public key is so important. If you use the wrong public key, you're vulnerable to man in middle attacks.
Witness the justified worry whenever bogus SSL certificate issues (e.g. DigiNotar).
The secret key itself isn't transmitted at all, so it can't be sniffed. It is computed independently at both ends. The materials it is computed from is called a PreMasterSecret which is encrypted with the public key of the server certificate. So unless the server's private key has been compromised the secret key can be neither sniffed nor calculated independently.
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.