Cryptology - Unofficial St. Mary's College of California Web Site

7.10. EXERCISES 127
to Gen. E. K. Smith, the commander of the Conferederacy’s Trans-
Mississippi Department. In the fall of 1964, the southwest campaign was
not going well for the Union. Smith’s forces’ next target (north into Missouri
Raid the west Advance south to New Orleans Fight to regain a
foothold across the Mississippi river) was unknown. Perhaps the cipher
gave this target!
September 30
To Genl. E. K. Smith:
What are you doing to execute the instructions sent you to HCDL-
LVW XMWQIG KM GOEI DMWI JN VAS DGUGUHDMITD. If
success will be more certain you can substitute EJTFKMPG OPG-
EEVT KQFARLF TAG HEEPZZU BBWYPHDN OMOMNQQG.
By which you may effect O TPQGEXYK above that part HJ OPG
KWMCT patrolled by the ZMGRIK GGIUL CW EWBNDLZL.
Jeffn. Davis
The last part of the message mentioned patrolling. Perhaps it referred to
gunboat patrols on the river, the only patrols likely to interest Confederate
high command. So Plum guessed that “that part HG OPB KWMCT
patrolled” stood for “that part of the river patrolled”. Plum wrote “this
meaning occurred to the author, at first sight, and doubtless would to any
one familiar with military affairs in that section.”
He then turned to “By which you may effect O TPQGEZYK above that
part of the river ...” Perhaps “a crossing” He soon had decrypted the
message. Can you (Hint: use Plum’s guesses to help find the keyword.)
(So word division led to complete solution. Why leave in word division 15
During the battle of Vicksburg Grant drove between the forces of Permberton
and Joe Johnston, forcing Johnston into the city, which Grant then
besieged. Johnston telegraphed for reinforcements. Unfortunately, the
cipherer made mistakes, and the telegrapher added his own (confusing R
(− −−) with S (− − −), and I (− −) with a pair of E’s (−)). After Kirby
Smith spent 12 fruitless hours trying to read this message, he finally sent
his chief of staff, Major Cunningham, on horseback around the flank of the
Union armies to retrieve the message directly. By the time Cunningham
reached Johnston, Johnston’s army was completely cut off from Smith.
After this, the Confederates retained word division [Pratt 186–7].
This is not to imply that the North was cryptologically far more advanced
than the South. General Albert Myer, in his A Manual of Signals: for the
use of Signal Officers in the Field, he seemed to imply that in Vigenèretype
systems the keys should be chosen at will by the sender and sent at
the start of the transmission! That is, send both the key and ciphertext!
15 “In 1862 South adopted the centuries-old Vigen’ere as its principal official cipher, the
proceeded to violate its inherent strengths for the time by such practices as retaining plaintext
word length, interspersing plain text and cipher, etc” Ralph E. Weber, [Weber]