Digital Imaging and Communications in Medicine (DICOM)

11.3 Securing the Data 257
This is already huge progress; the real name of the patient is now completely
hidden. You can show this to anyone and they won’t be able to read it, but you
(or your business partner, knowing the next letter encryption rule) can always
decode the original name. This is the essence of data encryption, and the only
major problem with the “next letter” code is that it is too easy to break. There is
another problem: the information about your method may eventually leak into
the public domain, which will immediately expose all of your encrypted data.
You will have to start over.
This is why a set of much more complex and ingenious algorithms has been
developed to encrypt your information in a most unbreakable manner. These
techniques are based on math and number theory, and remain well beyond the
scope of this book. Nevertheless, the essence of modern encryption is simple to
grasp; it is based on something we can call the piggy bank approach (Fig. 80).
It’s easy to put a coin in a piggy bank, and it is impossible to get it out unless
you have a key to a secret opening at the bottom (let’s just assume for a
moment that brute-force hammering will not work). Instead of piggy banks,
most current encryption algorithms rely on number factoring. If I give you two
numbers (A = 4091 and B = 9859) and ask you to multiply them, you will solve
the problem very quickly, if not on paper, then at least with any calculator or
computer. If, on the contrary, I give you a number (40,333,169) and ask you to
find its factors (numbers A and B such that A×B = 40,333,169) the problem becomes
much tougher; there is really no way to find its factors other than trying
all possible combinations of A and B. But if I give you the value of A (a key),
then you’ll be able to find (decrypt) B rather quickly: B = 40,333,169/A.
For large numbers, number multiplication (encryption) and number division
(decryption) can still be done very fast. But number factoring (brute-force
breaking the code without knowing the keys) might take years even on a fast
computer; and this is the whole idea behind contemporary data encryption.
These encryption techniques can very easily encode any data (with the use
of public keys), but getting the data back (decrypting) requires very special
knowledge (a secret private key). Your entire PACS or teleradiology network
can become a security fortress in which data submitted to it is easily hidden
Fig. 80 Piggy-bank security