In the beginning of an SSL query, the client sends a CLIENT_HELLO message.
The server replies with a certificate message that gives the chain of verifications going back to a known trusted agent.
Suppose for efficiency I wanted to store certificates locally for a new protocol. The current design in TLS is always to require getting the certificate. What could happen to a certificate that would require me to know?
I am trying to understand possible attack scenarios. Consider doing online banking, and suppose a certificate has been compromised. In such as case, the bank is not criminal, but they have been hacked and have to issue a new certificate. Is this reasonable?
If you consider that the bank itself is corrupt, then it seems to me there is no point in worrying about the certificate since they have your money and can just steal it. If the entity you are dealing with goes criminal, does the certificate matter?
Under what circumstances can certfiicates be revoked? I am trying to understand why SSL sends the certificate each time -- it seems really wasteful, but there is probably a good reason.
Would it be possible instead to keep all certificates stored on the client, but check a timestamp with a trusted server? It seems like one could at least send less data across the network
TLS Certificate message after `ServerHello is mandatory in mostly cases, so caching won't have any useful effect. See RFC5246
7.4.2. Server Certificate
When this message will be sent:
The server MUST send a Certificate message whenever the agreed- upon key exchange method uses certificates for authentication (this includes all key exchange methods defined in this document except DH_anon). This message will always immediately follow the ServerHello message.
TLS has its own methods to improve performance. When client sends a valid session_id in ClientHello the session can be resumed and the parties must proceed directly to the Finished messages
Also RFC5077 specifies how to resume sessions without server-side state
EDITED - added comments to specific questions
Suppose for efficiency I wanted to store certificates locally for a new protocol. The current design in TLS is always to require getting the certificate. What could happen to a certificate that would require me to know?
"always" is not correct. TLS sends certificates during handshake. Once the shared key is negotiated, the session can be resumed later by client using sessionid (the usual behaviour). Then, the server does not send the certification chain.
The server sends the certification chain. The client must verify that the presented certificate is reliable:
checking a digital signature performed with the private key of the server certificat
the certificate is issued by a trusted CA. Is supposed that client has a trust store with the root certificates of the certification authorities it trust. The client builds the certification chain presented by server until it finds the root certificate in local truststore
You can perfectly skip the sending of certificates from the server in the second step step if client has a copy of the server certificate in a local truststore
I am trying to understand possible attack scenarios. Consider doing online banking, and suppose a certificate has been compromised. In such as case, the bank is not criminal, but they have been hacked and have to issue a new certificate. Is this reasonable?
In this scenario the attacker could make a MITM attack. The certificate must be revoked by CA and client should check revocation. This is out of scope of TLS
If you consider that the bank itself is corrupt, then it seems to me there is no point in worrying about the certificate since they have your money and can just steal it. If the entity you are dealing with goes criminal, does the certificate matter?
Seems in this case the certificate is the least of the problems...
Under what circumstances can certfiicates be revoked?
Each CA stablish its own procedure. There is no a standard but there are "good practices": When certificate data changes (e.g email) or becomes invalid (Representative of a company), after a renewal revoke the older one, when key is compromised or certificate is lost
Would it be possible instead to keep all certificates stored on the client, but check a timestamp with a trusted server?
Yes it is possible as commented above: Verify a digital signature, verify revocation and stablish a refreshing mechanism
But if you're looking for performance comparing with TLS, the session resumption will probably have better results
Related
Scenario 1: If you are using a self-signed certificate on an internal website, you are still using encryption. The biggest security issue (that I am aware of) is the fact that your browser won't recognize the cert as trusted, and when you tell your browser to trust it, most people won't verify that the certificate you are trusting is actually the certificate on the web server, and not a man-in-the-middle system that replaced your certificate with their own. So the security issue here is clear.
Scenario 2: With a valid X509 certificate from an actual, globally-trusted CA, when the certificate expires, if you choose to bypass your browser's warning and use the website to login, what is the security issue? You're still using encryption. The private key is still secure on the web server. If a man-in-the-middle system tries to replace the certificate, you'll presumably get a browser warning about the certificate being invalid, rather than a warning about it being expired.
PS. There's an entire article about the Dangers of SSL Certificate Expiration, but all it does is mention the business downsides (not technical downsides) that only apply to public websites (not internal websites), and mention a generic statement like "Personal information at risk from man-in-the-middle attacks" with zero explanation for why they think that is the case. I'm not sure they even know. I feel like most websites on the internet do this for a complex topic like this - they say a generic statement that they assume is true, but don't know why.
A certificate has a life time to simplify certificate revocation. Once the certificate is expired it is considered invalid, which means no revocation information for this certificate need to be kept by the CA and provided on request by the client (i.e. revocation checks with CRL, OCSP, ...).
Thus, if an expired certificate gets compromised (private key known to the attacker) the certificate owner can not revoke it, since it is invalid already anyway. This means a man in the middle can impersonate the original server with the original certificate and stolen private key and the client has no way to detect this by checking revocation.
I have a few questions about digital certificates:
1) As far as I understand, clients are able to verify server digital certificates, because clients contain a set of well known CA self-signed certificates on their machine. Is that enough for the whole verification to be complete or are there some additional remote calls that need to be made by the client to the actual CA (wherever it lives)? I read some things about certificate revocation (CRL and OCSP) which seemed to indicate that remote calls are necessary at some point or another.
2) When validating a server certificate what happens exactly on client-side? I've always explained it to myself that based on the CA in the server certificate the client finds the corresponding CA self-signed certificate on it's machine and uses the public key from it to decrypt the signature in the server certificate (so no remote calls here). The decrypted result is as far as I understand a digest of the server's public key. So the client hashes the public key in the server certificate to match it to the digest from the last step. If both match, then everything is ok and the client and server can exchange a symmetric key for further communication. Is that correct?
1) Technically, no, there are no remote calls required. Of course, it's a little more complex than that - if you're using OCSP, then yes, you will need to reach an OCSP server and it must be a remote call. If you use CRLs, you will need to make remote calls to fetch the CRL from the CRL Distribution Point [CDP] in the cert; but if the CRL is fetched and cached locally, and is not expired, then the cert checks are all local.
2) You are correct, the client-side 'chains to a trusted root'; there are more checks performed including key usage, date range checks and, as you noted, verifying the signatures of the certs. The final check is to make sure the root is available at the client.
I hope that helps.
I have a short question: why do I need a SSL certificate (I mean only the certificate not the SSL connection)?
In my case Google Chrome deteced, that the connection is encrypted and secure, but everything is red because I created the certificate by myself.
Why I need a SSL certificate, if the connection is secure?
Just because traffic to 192.168.xxx.xxx doesn't leave the boundary of your network doesn't mean that it's safe.
Especially if you have BYODs attached to the network (and even if not, you don't want to be a hard shell with a juicy interior), someone can bring a compromised laptop or phone, attach it to the network, and a virus can intercept everything going on the network (see firesheep).
So you have to assume that the network is malicious - treat your LAN as if it were the internet.
So now the question goes back - why can't I rely on a self-signed certificate (both on a local network as well as the internet)?
Well, what are you protecting against? TLS (SSL) protects against two things:
Interception - even if I MITM you (I become your router), I can't read what you're sending and receiving (so I can't read your Credit Card numbers or password)
Spoofing - I can't inject code between you and the server.
So how does it work?
I connect to the server and get a certificate signed by a CA. This CA is considered trusted by the browser (they have to go through all kinds of audits to get that trust, and they get evicted if they break it). They verify that you control the server and then sign your public key.
So when the client gets the signed public key from the server, he knows he's going to encrypt a message that only the destination server can decrypt, as the MITM wouldn't be able to substitute his own public key for the server's (his public key wouldn't be signed by a CA).
Now you can communicate securely with the server.
What would happen if the browser would accept any SSL cert (self signed)?
Remember how the browser can tell the official cert from a fake MITM cert? By being signed by a CA. If there's no CA, there's literally no way for the browser to know if it's talking to the official server or a MITM.
So self-signed certs are a big no-no.
What you can do, though, is you can generate a cert and make it a "root" cert (practically, start your own CA for your internal computers). You can then load it into your browsers CA store and you'll be able to communicate through SSL without having to go through something like letsencrypt (which, by the way, is how enterprise network monitoring tools work).
In cryptography, a certificate authority or certification authority
(CA) is an entity that issues digital certificates. A digital
certificate certifies the ownership of a public key by the named
subject of the certificate. This allows others (relying parties) to
rely upon signatures or on assertions made about the private key that
corresponds to the certified public key. A CA acts as a trusted third
party—trusted both by the subject (owner) of the certificate and by
the party relying upon the certificate. The format of these
certificates is specified by the X.509 standard.
(from https://en.wikipedia.org/wiki/Certificate_authority)
You are not a trusted CA. Basically, if you sign your own certificate then there is no one that is able to vouch that the server is truly what it is. If you had a valid, trusted third party vouch for you then the certificate would be "valid."
Having a self-signed certificate doesn't necessarily mean that the website is dangerous, its just that the identity of the server can't be verified and thus it is more risky for the vistor.
Self-created or Self Signing Certificate are not trusted by all browsers. As we know at this time all browsers are more strict towards security. Let’s be clear about something right up front, the browsers do not trust you. Period.
It may seem harsh but it’s just a fact, browsers’ jobs are to surf the internet while protecting their users and that requires them to be skeptical of everyone or everything.
The browsers do, however, trust a small set of recognized Certificate Authorities. This is because those CA’s follow certain guidelines, make available certain information are regular partners with the browsers. There’s even a forum, called the CA/B forum, where the CA’s and Browsers meet to discuss baseline requirements and new rules that all CA’s must abide to continue being recognized.
It’s highly regulated.
And you are not a part of the CA/B forum.
The better option is to obtain an SSL Certificate from a trusted certificate authority.
Here's what you need to know about a Self Signed SSL Certificate
I have a portofolio website runing on a IIS Windows server if that matters.But some people complained that they get "website unsafe" when navigating the website.I personaly didnt get that error , and I tried the website on other diveces and they didnt get it either.
Could have something to do with SSL Certificate ? I didn't bought one ,but I have a self signed certificate according to ssl checker
.Do I need to buy a trusted SSL Certificate ? Or is there another problem ?
On my website i have a "Contact us" page with a web form that users should fill with name,email...
EDIT: I don't know if it's ok to post the website link here, if it's needed let me know .
EDIT: Link to website here.
This is a general problem with self-signed certificates, as the visitors of you website, or their browser, are not able to verify the identity of your server. The reason for this is, that there is no Certification Authority that signed it, thus the browser does not have a (root) certificate that is in the chain of trust linked to your certificate.
This problem with self-signed certificates is well explained in this post
The risks are for the client. The point of the SSL server certificate is that it is used by the client to know the server public key, with some level of guarantee that the key indeed belongs to the intended server. The guarantee comes from the CA: the CA is supposed to perform extensive verification of the requester identity before issuing the certificate.
When a client (the user and his Web browser) "accepts" a certificate which has not been issued by one of the CA that the client trusts (the CA which were embedded in Windows by Microsoft), then the risk is that the client is currently talking to a fake server, i.e. is under attack. Note that passive attacks (the attacker observes the data but does not alter it in any way) are thwarted by SSL regardless of whether the CA certificate was issued by a mainstream CA or not.
On a general basis, you do not want to train your users to ignore the scary security warning from the browser, because this makes them vulnerable to such server impersonation attacks (which are not that hard to mount, e.g. with DNS poisoning). On the other hand, if you can confirm, through some other way, that the certificate is genuine that one time, then the browser will remember the certificate and will not show warnings for subsequent visits as long as the same self-signed certificate is used. The newly proposed Convergence PKI is an extension of this principle. Note that this "remembered certificate" holds as long as the certificate is unchanged, so you really want to set the expiry date of your self-signed certificate in the far future (but not beyond 2038 if you want to avoid interoperability issues).
It shall be noted that since a self-signed certificate is not "managed" by a CA, there is no possible revocation. If an attacker steals your private key, you permanently lose, whereas CA-issued certificates still have the theoretical safety net of revocation (a way for the CA to declare that a given certificate is rotten). In practice, current Web browser do not check revocation status anyway.
How does SSL work?
Where is the certificate installed on the client (or browser?) and the server (or web server?)?
How does the trust/encryption/authentication process start when you enter the URL into the browser and get the page from the server?
How does the HTTPS protocol recognize the certificate? Why can't HTTP work with certificates when it is the certificates which do all the trust/encryption/authentication work?
Note: I wrote my original answer very hastily, but since then, this has turned into a fairly popular question/answer, so I have expanded it a bit and made it more precise.
TLS Capabilities
"SSL" is the name that is most often used to refer to this protocol, but SSL specifically refers to the proprietary protocol designed by Netscape in the mid 90's. "TLS" is an IETF standard that is based on SSL, so I will use TLS in my answer. These days, the odds are that nearly all of your secure connections on the web are really using TLS, not SSL.
TLS has several capabilities:
Encrypt your application layer data. (In your case, the application layer protocol is HTTP.)
Authenticate the server to the client.
Authenticate the client to the server.
#1 and #2 are very common. #3 is less common. You seem to be focusing on #2, so I'll explain that part.
Authentication
A server authenticates itself to a client using a certificate. A certificate is a blob of data[1] that contains information about a website:
Domain name
Public key
The company that owns it
When it was issued
When it expires
Who issued it
Etc.
You can achieve confidentiality (#1 above) by using the public key included in the certificate to encrypt messages that can only be decrypted by the corresponding private key, which should be stored safely on that server.[2] Let's call this key pair KP1, so that we won't get confused later on. You can also verify that the domain name on the certificate matches the site you're visiting (#2 above).
But what if an adversary could modify packets sent to and from the server, and what if that adversary modified the certificate you were presented with and inserted their own public key or changed any other important details? If that happened, the adversary could intercept and modify any messages that you thought were securely encrypted.
To prevent this very attack, the certificate is cryptographically signed by somebody else's private key in such a way that the signature can be verified by anybody who has the corresponding public key. Let's call this key pair KP2, to make it clear that these are not the same keys that the server is using.
Certificate Authorities
So who created KP2? Who signed the certificate?
Oversimplifying a bit, a certificate authority creates KP2, and they sell the service of using their private key to sign certificates for other organizations. For example, I create a certificate and I pay a company like Verisign to sign it with their private key.[3] Since nobody but Verisign has access to this private key, none of us can forge this signature.
And how would I personally get ahold of the public key in KP2 in order to verify that signature?
Well we've already seen that a certificate can hold a public key — and computer scientists love recursion — so why not put the KP2 public key into a certificate and distribute it that way? This sounds a little crazy at first, but in fact that's exactly how it works. Continuing with the Verisign example, Verisign produces a certificate that includes information about who they are, what types of things they are allowed to sign (other certificates), and their public key.
Now if I have a copy of that Verisign certificate, I can use that to validate the signature on the server certificate for the website I want to visit. Easy, right?!
Well, not so fast. I had to get the Verisign certificate from somewhere. What if somebody spoofs the Verisign certificate and puts their own public key in there? Then they can forge the signature on the server's certificate, and we're right back where we started: a man-in-the-middle attack.
Certificate Chains
Continuing to think recursively, we could of course introduce a third certificate and a third key pair (KP3) and use that to sign the Verisign certifcate. We call this a certificate chain: each certificate in the chain is used to verify the next certificate. Hopefully you can already see that this recursive approach is just turtles/certificates all the way down. Where does it stop?
Since we can't create an infinite number of certificates, the certificate chain obviously has to stop somewhere, and that's done by including a certificate in the chain that is self-signed.
I'll pause for a moment while you pick up the pieces of brain matter from your head exploding. Self-signed?!
Yes, at the end of the certificate chain (a.k.a. the "root"), there will be a certificate that uses it's own keypair to sign itself. This eliminates the infinite recursion problem, but it doesn't fix the authentication problem. Anybody can create a self-signed certificate that says anything on it, just like I can create a fake Princeton diploma that says I triple majored in politics, theoretical physics, and applied butt-kicking and then sign my own name at the bottom.
The [somewhat unexciting] solution to this problem is just to pick some set of self-signed certificates that you explicitly trust. For example, I might say, "I trust this Verisign self-signed certificate."
With that explicit trust in place, now I can validate the entire certificate chain. No matter how many certificates there are in the chain, I can validate each signature all the way down to the root. When I get to the root, I can check whether that root certificate is one that I explicitly trust. If so, then I can trust the entire chain.
Conferred Trust
Authentication in TLS uses a system of conferred trust. If I want to hire an auto mechanic, I may not trust any random mechanic that I find. But maybe my friend vouches for a particular mechanic. Since I trust my friend, then I can trust that mechanic.
When you buy a computer or download a browser, it comes with a few hundred root certificates that it explicitly trusts.[4] The companies that own and operate those certificates can confer that trust to other organizations by signing their certificates.
This is far from a perfect system. Some times a CA may issue a certificate erroneously. In those cases, the certificate may need to be revoked. Revocation is tricky since the issued certificate will always be cryptographically correct; an out-of-band protocol is necessary to find out which previously valid certificates have been revoked. In practice, some of these protocols aren't very secure, and many browsers don't check them anyway.
Sometimes an entire CA is compromised. For example, if you were to break into Verisign and steal their root signing key, then you could spoof any certificate in the world. Notice that this doesn't just affect Verisign customers: even if my certificate is signed by Thawte (a competitor to Verisign), that doesn't matter. My certificate can still be forged using the compromised signing key from Verisign.
This isn't just theoretical. It has happened in the wild. DigiNotar was famously hacked and subsequently went bankrupt. Comodo was also hacked, but inexplicably they remain in business to this day.
Even when CAs aren't directly compromised, there are other threats in this system. For example, a government use legal coercion to compel a CA to sign a forged certificate. Your employer may install their own CA certificate on your employee computer. In these various cases, traffic that you expect to be "secure" is actually completely visible/modifiable to the organization that controls that certificate.
Some replacements have been suggested, including Convergence, TACK, and DANE.
Endnotes
[1] TLS certificate data is formatted according to the X.509 standard. X.509 is based on ASN.1 ("Abstract Syntax Notation #1"), which means that it is not a binary data format. Therefore, X.509 must be encoded to a binary format. DER and PEM are the two most common encodings that I know of.
[2] In practice, the protocol actually switches over to a symmetric cipher, but that's a detail that's not relevant to your question.
[3] Presumable, the CA actually validates who you are before signing your certificate. If they didn't do that, then I could just create a certificate for google.com and ask a CA to sign it. With that certificiate, I could man-in-the-middle any "secure" connection to google.com. Therefore, the validation step is a very important factor in the operation of a CA. Unfortunately, it's not very clear how rigorous this validation process is at the hundreds of CAs around the world.
[4] See Mozilla's list of trusted CAs.
HTTPS is combination of HTTP and SSL(Secure Socket Layer) to provide encrypted communication between client (browser) and web server (application is hosted here).
Why is it needed?
HTTPS encrypts data that is transmitted from browser to server over the network. So, no one can sniff the data during transmission.
How HTTPS connection is established between browser and web server?
Browser tries to connect to the https://payment.com.
payment.com server sends a certificate to the browser. This certificate includes payment.com server's public key, and some evidence that this public key actually belongs to payment.com.
Browser verifies the certificate to confirm that it has the proper public key for payment.com.
Browser chooses a random new symmetric key K to use for its connection to payment.com server. It encrypts K under payment.com public key.
payment.com decrypts K using its private key. Now both browser and the payment server know K, but no one else does.
Anytime browser wants to send something to payment.com, it encrypts it under K; the payment.com server decrypts it upon receipt. Anytime the payment.com server wants to send something to your browser, it encrypts it under K.
This flow can be represented by the following diagram:
I have written a small blog post which discusses the process briefly. 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."
Mehaase has explained it in details already. I will add my 2 cents to this series. I have many blogposts revolving around SSL handshake and certificates. While most of this revolves around IIS web server, the post is still relevant to SSL/TLS handshake in general. Here are few for your reference:
SSL Handshake and IIS
Client certificate Authentication in SSL Handshake
Do not treat CERTIFICATES & SSL as one topic. Treat them as 2 different topics and then try to see who they work in conjunction. This will help you answer the question.
Establishing trust between communicating parties via Certificate Store
SSL/TLS communication works solely on the basis of trust. Every computer (client/server) on the internet has a list of Root CA's and Intermediate CA's that it maintains. These are periodically updated. During SSL handshake this is used as a reference to establish trust. For exampe, during SSL handshake, when the client provides a certificate to the server. The server will try to cehck whether the CA who issued the cert is present in its list of CA's . When it cannot do this, it declares that it was unable to do the certificate chain verification. (This is a part of the answer. It also looks at AIA for this.) The client also does a similar verification for the server certificate which it receives in Server Hello.
On Windows, you can see the certificate stores for client & Server via PowerShell. Execute the below from a PowerShell console.
PS Cert:> ls Location : CurrentUser StoreNames : {TrustedPublisher, ClientAuthIssuer, Root, UserDS...}
Location : LocalMachine StoreNames : {TrustedPublisher,
ClientAuthIssuer, Remote Desktop, Root...}
Browsers like Firefox and Opera don't rely on underlying OS for certificate management. They maintain their own separate certificate stores.
The SSL handshake uses both Symmetric & Public Key Cryptography. Server Authentication happens by default. Client Authentication is optional and depends if the Server endpoint is configured to authenticate the client or not. Refer my blog post as I have explained this in detail.
Finally for this question
How does the HTTPS protocol recognize the certificate? Why can't HTTP work with certificates when it is the certificates which do all the trust/encryption/authentication work?
Certificates is simply a file whose format is defined by X.509 standard. It is a electronic document which proves the identity of a communicating party.
HTTPS = HTTP + SSL is a protocol which defines the guidelines as to how 2 parties should communicate with each other.
MORE INFORMATION
In order to understand certificates you will have to understand what certificates are and also read about Certificate Management. These is important.
Once this is understood, then proceed with TLS/SSL handshake. You may refer the RFC's for this. But they are skeleton which define the guidelines. There are several blogposts including mine which explain this in detail.
If the above activity is done, then you will have a fair understanding of Certificates and SSL.