Cryptography Reference
In-Depth Information
Information Hiding
At the minimum, any encrypted message leaks the information that the
content of the message is valuable enough to protect, fueling in turn the
desire to break through its protections. Kerckhoffs notes in his 1883 survey
of the field that “in the 17th century, the simple fact of having corre-
sponded in secret characters was still considered an aggravating circum-
stance by English tribunals.”
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Thus, in many cases, it may be easier, more
appropriate, or more efficient to hide information than to encrypt it.
For these reasons, methods for
hiding information
(steganography) were
often more developed than methods for
scrambling
information (cipher
and codes). In the former, the sender leaves the message in the clear, but
hides it in a way known only to the recipient and invisible to others. In
the latter, a transformation is applied to the linguistic code itself so that it
becomes meaningless to whoever does not have the information necessary
to invert the transformation (i.e., the key). With steganography, one is
confident that the adversary cannot discover that a message has been
transmitted at all. With ciphers, one is confident that even though the
adversary is able to access a scrambled message, the method is strong
enough to withstand attempts to decode it.
Steganographic techniques embed a message within a
cover signal
or
cover text
, taking advantage of their properties to avoid detection. The
message may be encoded in the visual, auditory, linguistic, or physical
domains using a wide range of techniques, including invisible inks, using
only some of the letters of a message, such as the first letter of every word,
or the least significant bits of a digital image. Information hiding may also
be used for authentication: in the seventeenth and eighteenth centuries,
publishers of logarithmic tables deliberately introduced errors in the least
significant digits of some entries so that any copy of the tables could be
traced back to its legitimate producer. The practice is still relied on today
by database and mailing list vendors who insert bogus entries to identify
customers who try to resell their product.
Historically, information hiding and cryptography evolved in an inti-
mate relationship, and methods from either approach were traditionally
discussed together. Kerckhoffs's 1883 survey begins by noting that “it is
under the rubric:
steganography
,
cipher
, or
secret writing
that certain ency-
clopedic dictionaries provide information which relate to cryptography.”
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It is thus worth remarking that contemporary cryptography textbooks
