Digital Imaging and Communications in Medicine (DICOM)

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11.3.2
Encryption
Chapter 11 DICOM Security
Ancient Romans used to shave their slaves’ heads, write secret messages on
them, and let the hair grow. An ingenious way of implementing data security.
The papal scribes in the first crusades used secret text-encoding tools and had
to be executed after a year of active duty in order to maintain secrecy (share
this with your system administrator). The whole history of human civilization
is filled with attempts to encode secrets and, consequently, equal attempts to
break the codes.
The main outcome of this quest was summarized by Mr. Sherlock Holmes:
“What is invented by one man, can be always understood by some other.” Applied
to our subject of data encryption, this can be restated as: any security
code can be broken. It’s just a matter of time.
Encryption is the process of changing the format of the data to protect its
original content. Essentially, encryption can be viewed as translating your data
into another language (code) that cannot be understood without a special key.
Unlike anonymization, encryption has to be reversible; that is, you can always
translate your data back to the original form without any information loss. The
reversibility of encryption eliminates the entire problem of losing important
data that we faced with anonymization. If you have carefully read the previous
section on DICOM anonymization, you should realize that our attempt to encode
confidential DICOM tags in a unique and consistent way was, in fact, an
initial approach to encryption.
Part PS3.15 of the DICOM standard, released a few of years ago, adopted
the existing data encryption techniques to be used with DICOM data. These
techniques pursue three important goals: encrypting the data, verifying the
data origin, and verifying the data integrity. The DICOM standard still needs
to do a good bit of explaining how encryption can be worked into the entire
DICOM IOD model and encoding. Meanwhile, we can review the conceptual
part shared by all encryption techniques.
11.3.2.1
How it All Works
Encryption of digital data can be compared to lossless image compression: you
reversibly transform the original data into another format, impossible to read,
unless you decode the data back to its original form. Consider something as
simple as patient name, “SMITH^JOE”. If left as is, it can easily be spotted in a
DICOM file or DICOM object (see 11.1) without any particular DICOM software
or skills.
Even if you merely replaced each letter in the patient’s name by the following
letter in the alphabet, you would get something more secure: “TNJUI^KPF”.