Tales from the Crypt

Encryption is the two way mechanism that allows a media file to be impossible to read for all circumstances except one. In the d-cinema world there are two different encryption styles for the two different times that a movie is in public hands. One type is when the hard disk comes from the studio. Another type is used during the storage, then play-out of the encrypted movie.

Encryption and Keys

Section 2: Tales from the Crypt

Encryption is the process of hiding the contents of something (for example, a digital movie), so that it cannot be read, understood, or effectively stolen. (A movie that cannot be watched is not something a crook would want to steal.)

An algorithm is a procedure, or set of rules that guide a process which changes something from one state to another. It is most often a calculation. For example – without thinking of the reason why we would want to do this – we might tell a person or computer that for every even number we see we are going to add ‘5’ to it, and for every odd number we will add ‘6’ to it.

Let’s remember that, at one point, each frame of a movie is converted to digital. Instead of different amounts of saturation of dyes on film, we have different digits that define the absolute color of each pixel at each position. If someone could read the values of those pixels, we say that it is in ‘plain text’.

An encryption algorithm is a procedure to take a plain text, which can be read by anyone, and by using encryption techniques turn the plain text into a cyphertext, which can be read by no one.  To do these two things, an encryption algorithm uses keys, which are special codes, as part of the algorithm.

A key, in the encryption world, is code that can either encrypt (make unreadable) a plain text, or decrypt (make readable) a cyphertext.  The key provides the algorithm with “computational complexity”, which  means that, the larger the key, the more complex and secure the encryption will be. Said another way, the larger the key, the larger the number of wrong interpretations or decryptions that could be made. The number of possibilities created by today's encryption keys create numbers so large that a linguist would have trouble naming them.

There are two types of encryption algorithms based on the number of keys used. The first is “symmetric encryption algorithms”, which use the same key for encryption and decryption (one key). The second type is called “asymmetric algorithms”, which use one key for encryption, and a different second key for decryption. One of these two methods of encryption/decryption are chosen depending on the different security requirements, and depending on the situation of the material that needs protecting.

For example, the answer to the question “Why isn’t ‘the best’ encryption used all the time?” must be looked at from those views. The money in my bank account is extremely valuable to me, but it is unlikely that I could remember a PIN number that is 128 or 256 numbers and letters long, especially if that code must be constantly changed multiplying the numerical equivalent of the date and dividing by the time. Perhaps I could use a computer, but imagine that bank decides that their computer and my computer must revalidate the keys every few seconds and that the numbers must be 1,024 digits long, or 2,048 digits long. It becomes obvious that the computation that ensures validation takes more computing power and time than the computers can do efficiently, and it is probably beyond the value of the transaction involved.

For these reasons, different styles of encryption with different levels of complexity are used in our lives. Digital cinema is no different, using different types and levels of encryption at different times of the process of manufacturing and transport and storage of the material.

Symmetric encryption uses the same key for encryption and decryption, so that key must remain secret.  If a symmetric key becomes known to an opponent, the encryption system is compromised .  Key management – protecting the symmetric key, and safely distributing it to the parties involved in a communication – must be done securely. A key could not be exchanged over an open, insecure (that is, un-encrypted) communication path such as a regular phone conversation or an unencrypted email.

There are several types of symmetric encryption systems. Why? Because eventually the power of computers are used to figure out methods of ‘breaking’ a key. So new types evolve. Sometimes old ‘broken’ types are used because they still have value. A key may be considered broken, but it is only broken in a technical sense. For example, in theory someone can break the WPA-2 encryption used on your WiFi system at your house. In reality, you would probably hear the truck of generators supplying the power to the truck full of computers out side your house being used to break the code. It would take several weeks, so if you see that happening, change your key every 10 days just to freak them out.

Some of the names of symmetric encryption systems are: DES, Triple-DES, RC2, RC4 and AES.

Asymmetric encryption uses two keys. The two keys are created at the same time so that they can be used together as a pair. One of the keys is kept secret – it is called the private key. The other key can be shared – it is called the public key. The public key can be known by anyone. People put their public key on their web pages.

Some of the names of asymmetric encryption systems are: RSA and DSA. A famous, and somewhat popular one is PGP. There is a version of that named GPG which has a ‘free software license’ which we’ll probably recommend somewhere later in the studies.

Generally speaking, the public key is used to encrypt, and the private key is used to decrypt. This private key is known to no one other than the key holder. It may seem like magic sometimes, but this fact of a long, unknown private key leads to a couple of interesting uses.

And again, since the ‘white hats’ are always trying to keep in front of the ‘black hats’, different encryption and cryptographic techniques are used together.

The main use of asymmetric key pairs is to allow two parties to safely exchange information without that information being able to be read by an unauthorized third party. For example, a manager at a cinema chain could create a set of keys using PGP. The manager could send their public key to a distributor “in an open”, that is, in an unencrypted message. The distributor could then use the exhibitor’s public key to encrypt a message, and then send a private message to the exhibitor. Except for the person with the private key, there is zero likelihood that anyone can read the message, can break the encryption, even if they have the public key of both parties.

Here is a video that explains this very well. Remember from earlier lessons that VPN is a virtual private Network. In the video the author uses what looks like a straw to indicate a communication line that is secure, protected.

The author also shows the second use of a asymmetric key pair, similar to what we discussed above: that is, to allow one party to authenticate the source of a communication.  For example, let’s say that the theater needs to confirm that a message was sent by a distributor. The distributor creates a message that is encrypted with the distributor’s private key. (Remember, a private key is held by only one entity on the planet.) The theater then takes the distributor’s public key, and decrypts the message. Only a message encrypted with the distributor’s private key could be decrypted with the distributor’s public key, thus “proving” that the distributor sent the message.

You might want to wear some earphones for this clip since the author’s microphone is too far away and there is a lot of echo.

That was pretty good, eh? Take a break, come back, review the above lesson, then watch it again. If you have difficulty here, go to the forums and discuss this with someone, describing where the logic of the lesson falls apart. You might discover that you need to look up something in the glossary. If you do look it up, go to the Course Glossary and put in your definition there.

What is kind of obvious, but not really pointed out, is that these things are done with computers. In the digital cinema case, the messages are not between two people, but rather between two machines. The distributor’s keys and the projector’s or the server’s keys.

So, are you really with us so far?

Okay, this use of the asymmetric keys can be taken a step further.

You will recall that, in the lesson regarding SHA, the SHA hash algorithm can be used to prove that a digital document, such as a movie, has not been modified in any way.  A hashing algorithm can be combined with an asymmetric encryption to create a digital signature.

This is how it works.  A sender uses a hashing algorithm to create a message digest of a message,  then appends the message digest to the end of the message. The sender encrypts the message plus digest with the sender’s private key, and sends the result. The receiver uses the sender’s public key to decrypt the message and the digest. The decrypted digest is called a digital signature; in effect, it authenticates the sender, while at the same time assuring that the message has not been tampered with.

But wait! There is one more security technique that you need to know, because it, too – the digital certificate – is also used with digital cinema.

A digital certificate is a code grouping that contains all kinds of information, including a serial number, the name of the organization which issued the certificate, a public key, a digital signature, and other information. The advantage that a digital certificate provides, is that it can authenticate a sender via a trusted third party, known as a Certificate Authority.

A trusted third party, or Certificate Authority, is an organization that is not part of a two-way communication. The Certificate Authority issues a digital certificate to one party, for example, a web site. A user contacts the web site using a browser, and the browser asks for the digital certificate. The web site passes the digital certificate to the user's browser, and the browser passes it on to the Certificate Authority, the trusted third party. The Certificate Authority, using the public key for the digital signature, then decrypts the signature, which decrypts the message digest for the message. The message is then confirmed, and the web site is authenticated, via a third party, to the web browser.

Last modified: Saturday, 28 May 2011, 3:44 PM