It's a truism in the crypto business that the old telegraph codes were
for economy, with confidentiality against casual readers a noted and
desirable goal. But I've recently acquired two old codebooks that
have stronger ambitions.
The more interesting one is Slater's Telegraph Code, since
confidentiality is its only goal. I have the 9th Edition, from 1938,
but it appears to be originally from the late 1860's. It encodes 25,000 words,
including "a" and "the". There are no sentences, phrases, etc. Users
are told to convert the plaintext word to a number, transform the
number, and convert back to a new word for transmission. Suggested
transformations include adding or subtracting a shared secret constant,
permuting some of the digits of the code number, and/or regrouping the
digits of a string of code numbers. Clearly not military-grade
security, even for the time, I'd guess; in addition to the rather
simple transforms, it's a one-part code.
Equally interesting is the threat model. I quote from the introduction:
On the 1st February, 1870, the telegraph system throughout the
United Kingdom passes into the hands of the Government, who will
work the lines by Post Office officials. In other words, those
who have hitherto so judiciously and satisfactorily managed the
delivery of our sealed letters will in future be entrusted also
with the transmission and delivery of our open letters in the
shape of telegraphic communications, which will thus be exposed
not only to the gaze of public officials, but from the necessity
of the case must be read by them. Now in large or small
communities (particularly perhaps in the latter) there are alwys
to be found prying spirits, curious as to the affairs of their
neighbours, which they think they can manage so much better than
the parties chiefly interested, and proverbially inclined to
gossip.
It goes on to warn of the need for confidentiality in business
communications, especially when undersea telegraph lines are used.
Equally interesting is the fact that despite the common wisdom that
says that secrecy products didn't sell well, this book survived
for about 70 years -- with my edition being printed on the eve of war.
The other confidentiality code I have is "Sheahan's Telegraphic Cipher
Code", from 1892. It was intended for use by railway labor organizers,
to keep management from knowing what they were up to. It has about
7000 code words.
It's a more conventional telegraph code, in that it includes some
phrases. The general confidentiality scheme is similar to Slater's,
though the only suggested transformation is adding or subtracting
a constant to the code number. Because the plaintext is phrases,
rather than just words, there are separate code words along with the
code numbers; these words are sent, rather than the numeric values.
>From a cryptographic perspective, the most interesting item is
that times, days, and numbers do not have code numbers -- the instructions
say to send just the code words. The compiler was worried about a
known or probable plaintext attack on the offset value used for
superencipherment. There is also a warning against mixing plaintext
with ciphertext, "excepting the name of a person or the name of a town".
There is a cipher alphabet for spelling out words, but it, too,
is not superenciphered.
Some of my other, larger code books could have been used in a similar
fashion, but there's no hint of that in the instructions.
--Steve Bellovin, http://www.research.att.com/~smb (me)
http://www.wilyhacker.com ("Firewalls" book)
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