Safe online transactions (Part 2) - Certificate authorities & SSL explained

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Safe online transactions (Part 2)

Certificate authorities

In practice, this problem is solved by one more level of indirection: the CA or certificate authority. A CA issues digital certificates that identify a particular person or entity and the public key used by that person or entity. In essence, a digital certificate is the name (usually a domain name) and the associated public key encrypted by the CA’s private key. You can check the validity of a certificate by decrypting it with the CA’s public key.

But hold on, you may be asking, how do Alice and Bob know the CA’s public key? Can’t Mallory just intercept this and replace with his own public key? Technically yes, but in practice the CA’s public key is provided as a certificate with the browser or as part of the operating system. CA certificates are truly publicly published. You trust that these certificates are valid because they’re delivered to you with your operating system or browser.

Once Alice and Bob buy their digital certificates from a particular CA, they can send them to each other with impunity in essence by trusting the CA. Alice can check Bob’s certificate (and discover his public key) by decrypting it with the CA’s certificate, and vice versa. Once that’s done, they can send each other secure messages ad infinitum.

Description: Certificate authorities


Online banking

Now imagine that Alice is you, 13th is your bank, and you want to take a look at your accounts online and pay some bills. You certainly don’t want Eve to see your account details, and definitely don’t want Mallory to be fiddling with your transactions as you send then to your bank. Before RSA and public key systems, this would have been nigh on impossible: you would have had to securely agree on a large key with your bank. In fact, the bank would have had to agree on(and store) a random-looking key for all of its customers and keep them safe from prying eyes. The bank would have a nigh-on impossible task keeping the world’s Eves and Mallorys from joining as employees and accessing all those private keys.

But with public key cryptosystems, this all becomes feasible. It is the basis of SSL (Secure Socket Layer) and TLS (Transport Layer Security). The latter is the newer version of the former, but everyone still uses the term SSL - although it does look a little different. The first problem is that we normal people don’t (usually) have a private/public key pair and a digital certificate that proves who we are (for a start, certificates are very expensive), so we have to approach things differently.

SSL explained

When you visit a hank’s website, the bank’s server will automatically redirect von to its secure site using the HTTPS protocol before you can log in. This results in your browser and the hank’s site negotiating a secure channel using SSL. This negotiation goes a little like this (note that I’ve simplified it greatly).

The browser sends a message stating what the latest version of SSL it can support and a list of symmetric algorithms it can use.

The web server sends back a message with the version of SSL and the algorithm that will he used. It sends its certificate as well.

 The client verifies the certificate using the known certificates that came with tile browser; in other words, it checks that it has been signed by a trusted CA and that it hasn’t expired.

If the certificate is valid, the browser generates a one-time key for the session, encrypts it with the servers public key (it’s part of the certificate), and sends it to the server.

The server decrypts tile key then uses that key together with the agreed symmetric algorithm for the rest of the session.

Let’s take stock. Your browser is certain that tile server is who it says it is (your browser is trying to access, and the certificate says it’s valid for - anti the CA agrees). The browser has generated a cryptographic key that will be used for one time only: this particular session. It’ll be thrown away after you log out. The key was Sent encrypted with’s public key, which only can decrypt with its private ke3 There are a couple of other messages sent that check your browser and the web server have agreed on the same key (if anything went wrong, the session is dropped).

Once has presented you with a login screen (which will be sent over IITTPS, if the batik knows what it’s doing) and you’ve filled it in, it’ll know who you are. Your id and password will have been sent encrypted over the secure channel that you’ve both established. Eve and Mallory are completely out of the loop.

Spotlight on... Hacking SSL

Recently, Rizzo and Duong showed off a hack against SSL used with a block cipher. There are two main ways to encrypt with a block cipher: encrypt the blocks separately (known as electronic codebook, or ECB), or XOR the previous encrypted block with the current block and then encrypt it (known as cipher-block chaining, or CBC). The latter needs an initialisation vector (IV) - a random, block to XOR with the first plaintext block. The block cipher SSL uses CBC with an IV.

When communicating with a server, data passes back and forth as separate packets. Does the IV for the first block of the next file or packet come from the last encrypted block of the previous file, or do we start afresh with a new IV? SSL uses the first option, which can lead to a small security hole. If the attacker is controlling one side of the channel over which the encrypted data is flowing, he can inject some specially constructed data into the stream using the previous IV. He guesses at the contents of the previous block, constructs his attack block with it, and if the resulting encrypted block Is what he expects, he was correct.

But how does he gain control of one side? Rizzo and Duong used a Java applet that they injected into a web page through a cross-site hack. It isn’t easy, but it shows that good security is hard. SSL was considered good security. Note though that although this hack has been proven, it doesn’t mean that SSL is broken for most transactions on the web. Browser manufacturers are already fixing the problem.




DigiNotar was a certificate authority in the Netherlands. On July 19 2011, hackers accessed DigiNotar’s systems. They created around 30 fake certificates for well-known domains, including,, and

Consider what they could do with a certificate for Well assume they are called Mallory and have governmental powers so they can monitor all internet traffic. Alice tries to connect to Gmail, but would in fact be setting up an SSL session with Mallory’s server (Mallory’s certificate says it is, and this Dutch CA proves it). Mallory’s server would then set up a session with the real Gmail so he became the man in the middle. At that point Mallory could read all of Alice’s emails without her being aware of it.

It’s thought that Mallory in this case was the Iranian government and that it was trying to gain information about dissidents. The solution, once the bogus certificates were discovered, was to renounce DigiNotar as a CA. Its trust was revoked and all certificates it issued were made invalid.

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