In cryptography, a certificate authority or certification authority (CA), is an entity that issues digital certificates. — Wikipedia
Generally we talk about SSL certificates (based on the original, now obsoleted standard) or TLS certificates based on the currently valid standard. You would know that you'll be using encryption when the URL of the webpage you're visiting starts with https://. Most web browsers indicate encryption by displaying some sort of lock icon.
Using HTTPS and assuming its safe to do so
One of the most famous HTTPS critics is Mr. Harri Hursti.
He is pretty famous computer security expert and has succesfully hacked
a number or things. He is currently works in his own company SafelyLocked.
#SSL is so insecure it’s only worth a post-it note from the NSA. - Harri Hursti in Slush 2013 as relayed by Twitter
During an interview in Finnish newspaper Talouselämä he said: SSL, one of the cornerstones in network security is broken and it cannot be fixed. It cannot be trusted.
SSL certificates provide one thing, and one thing only: Encryption between the two ends using the certificate.
They do not, and never been able to, provide any verification of who is on either end. This is because literally one second after they are issued, regardless of the level of effort that goes into validating who is doing the buying, someone else can be in control of the certificate, legitimately or otherwise.
Now, I understand perfectly well that Verisign and its brethren have made a huge industry out of scamming consumers into thinking that identification is indeed something that a certificate provides; but that is marketing illusion and nothing more. Hokum and hand-waving.
It all comes down to, can you determine that you are using the same crypto key that the server is? The reason for signing certificates and the like is to try to detect when you are being hit with a man-in-the-middle attack. In a nutshell, that attack is when you try to open a connection to your 'known' IP address, say, 220.127.116.11. Even though you are connecting to a 'known' IP address of a server you trust, doesn't mean you can necessarily trust traffic from that IP address. Why not? Because the Internet works by passing data from router to router until your data gets to it's destination. Every router in between is an opportunity for malicious code on that router to re-write your packet, and you'd never know the difference, unless you have some way to verify that the packet is from the trusted server.
The point of a CA-signed certificate is to give slightly stronger verification that you are actually using the key that belongs to the server you are trying to connect to.
Long quote, huh. The short version of that is: Not to assume that if somebody has been verified has somebody or something, that they really are. You can safely assume, that traffic between that party is encrypted, but you really don't know who has the key to decrypt.
If we ignore the dirty details of encryption keys and their exchange during encrypted request initiation and concentrate only on the encryption cipher suite, not all of the cipher suites used are secure enough. There exists pretty good ones, but the worst ones are pretty crappy and cannot be trusted at all.
On March 15th 2011 an affiliate company was used to create a new user
account into Comodo's system. That newly created account was used to
issue a number of certificates: mail.google.com, www.google.com, login.yahoo.com, login.skype.com, addons.mozilla.org and login.live.com. The gain of having those domains is immediately clear. If combined with some other type of attack (most likely DNS cache poisoning)
a victim can be lured into a rogue site for Google, Yahoo, Microsoft
and Mozilla without them ever knowing that all of their data is
compromised. Some of the possible attack vectors include injecting rogue
software updates to victims' computers.
On July 1st 2011 DigiNotar's CA was compromised
by allegedly Iranian crackers. The company didn't even notice the crack
at until 19th of July. At the time there had been a valid certificate
issued to Google in the wild. Again, these false certificates were combined with
some other type of security flaw (most likely a DNS poisoning) and redirected victims to
access a fake "Google" site without them ever known that all the
passwords and transmitted data was leaked.
Both of those cases scream: DON'T use certificates for verification.
But in reality Microsoft Windows, Skype and Mozilla Firefox are built
so that if a certificate on the other end is verified by the connecting
client, all is good. The remote party is valid, trusted and verified.
Remember from earlier: "They do not, and never been able to, provide any verification of who is on either end". Still a lot of software is written to misuse certificates like the examples above do.
About trust in Internet
From the above incidents we learn the following: Who do you trust in Internet? Who would you consider a trustworthy party, so that you'd assume that what the other party says or does is so? Specifically in Internet context, if a party identifies themselves, do you trust that they are who they claim to be? Or if a party says, that they'll take your credit card details and agree to deliver goods which you purchased back to you, do you trust that they'll keep their promise?
Lot of questions, no answers. Trust is a complicated thing after all. Most people would say, that they'll trust major service providers like Google or Microsoft or Amazon, but frown upon smaller ones with no reputable history to prove their actions. The big companies allegedly work with NSA and gladly deliver your details and behavior for US Government agency to study if you're about to commit serious crime. So you choose to trust them, right? No.
Well then, are smaller parties any safer? Nobody really knows. They can be, or then again not.
Regular users really don't realize, that the vendor of their favorite web browser and operating system has chosen for you as an user of their software to trust a number of other parties. Here is a glimpse of the list of trusted certification authorities from Mozilla Firefox:
I personally don't trust a company with AddTrust AB or America Online Inc. or CNNIC or ... the list goes on. I don't know who they are, what they do or why should they be worth my trust. All I know for a fact that they paid Mozilla money to include their root certificate in Mozilla's software product to establish implicit trust between them and me, when I access a website they chose to issue a certificate to. That's a tall order!
There really is no reliable way of identifying the other party you're talking or communicating to in the Internet. Many times I wish there was, but since the dawn of time, Internet has been designed to be an anonymous place and that's the way it stays until drastic changes are introduced into it.
No, The Internet is not broken, despite what Mr. Hursti claims. The Internet is much more than HTTP or encryption or HTTP + encryption. For the purpose of securing HTTP we'll just need some secure method of identification for the other party. Critical applications would be on-line banking and on-line shopping. Optimally all traffic would be encrypted all the time (see HTTP 2.0 details), but it is an overkill after all. Encryption adds extra bytes to transfers and for example most large files don't contain any information that would need to be hidden. When moving credit card data or passwords or such, the amount of bytes transferred is typically small in comparison to downloading system updates or install images.
Also I'd like to point out that to keep a connection secure, it's mostly about the keys. Keeping the keys safe and exchanging keys safely. The actual ongoing encryption during a session can be thought to be secure enough to be trusted. However, a great deal of thought must be put into allowing or disallowing a cipher suite. And still: all that is based on current knowledge. Once a while some really smart people find out that a cipher that was thought to be secure isn't because of a flaw or mathematical breakthrough which renders a cipher suite not secure enough to be used.
We need encrypted HTTP but not the way they implemented it in 1994. It is not the task of Verisign (or any other CA-issuer) to say: "The other party has been verified by us, they are who they claim to be". Also there must be room for longer and longer keys and new cipher suites to keep the secure Internet reliable.