Cryptography Reference
In-Depth Information
19.2 Getting In Sync
Most of the algorithms in this topic depend, in one form or other,
ongettingthecompletefile.Themessagemaybehiddeninasmall
fraction of the file, but everything is available for extraction. If the
hidden bits are supposed to be located 14 pixels from the left and 212
from the bottom, that spot can be found because the entire frame is
available.
There are many cases when the entire file isn't around. Photos
are routinely cropped before being reused. Only a short clip of a
sound file or a movie may be available. Anyone turning on a radio
at a particular moment may start receiving the signal at any arbitrary
place in the stream.
Unfortunately, some of the more extreme algorithms in this topic
rely heavily on absolute synchronization to function correctly. Most
effective encryption algorithms change their process throughout the
file, encrypting the
+1
th
byte. Some
of the algorithms for hiding information use a subset of the data and
rely heavily on the boundaries to define those pixels.
Synchronizing a bit stream between a sender and a receiver is not
hard to do, but it is a bit tricky to do in an efficient manner. The
simplest trick is to introduce a special synchronization character and
only use it at the beginning or end of a word. The easiest example is
unary
or base one encoding, where a 1 is used as the boundary and
the number of 0s encodes the value. So a message of
2
th
byte differently from the
n
n
,
3
,
2
would
look like
1001000100
.
Morse code uses two different-sized pauses in the signal stream.
Short pauses break up the dits and the dashes, while pauses that are
three times as long break up the letters. The gap between the three
dots in an S (...) should be one third the size of the gap between the
last dot and the beginning of the next letter. The factor of 3 is just a
convention and many people probably deviate.
Dedicating an entire character like 1 to synchronization in an al-
gorithm is expensive. Can you imagine transmitting the yearly bud-
get of the United States government (currently about $3.1 trillion) in
unary? While the approach is basic, it's not easy to extend in the dig-
ital world, where there are only two real characters. Other characters
are just composed of blocks of 0s or 1s and it's not simple to find the
beginning and end of the block.
