Among several messages sent to Washington on June 27, 1864, Major-General John Foster, Commanding the Department of the South, sent a short note announcing an important break in Federal intelligence gathering at Charleston:
I have to report that the rebel signal code now used by the enemy at Charleston has been deciphered by Sergt. John D. Colvin, of the Signal Corps. For this, which is the result of his constant watchfulness, study, and perseverance for several weeks, night and day, at his station in Fort Strong, I respectfully recommend that he be rewarded by promotion to lieutenant in the Signal Corps, or by a brevet or medal of honor.
Several points of interpretation here. What Sergeant Colvin did is more or less what the signal officers supporting the Army of the Tennessee in Georgia did earlier in the month. From the characters sent in the intercepted messages, patterns emerge. If the word “Jones” or “Johnson” are sent several times a day, that string becomes a possible key. At that point, the process is a real world “Wheel of Fortune” match up. And keep in mind, the Confederates were either using a variation of the key referenced by General P.G.T. Beauregard earlier in the year or a code wheel, with key mappings changed at at intervals.
Captain Henry R. Clum, Chief Signal Officer for the Department of the South, elaborated some on the process in a report of June 30:
About the 1st of the month Sergt. John D. Colvin was stationed at Fort Strong, on Morris Island, with the several codes, heretofore used by the rebels, for the purpose of reading the enemy’s signals if possible. If not successful he was to take down the numbers for the purpose of deciphering them. For nearly two weeks nothing could be made out of their signals, but by persevering he finally succeeded in learning their codes, a copy of which was forwarded you on the 14th instant. Sergeant Colvin is still at Fort Strong, with instructions to telegraph all messages read by him to General Schimmelfennig, who is commanding in that district. Messages have been read by him from Beach Inlet, Battery Bee, and Fort Johnson. Lieutenant Roberts reports that the sergeant has also succeeded in deciphering all but three or four of the letters of the cipher as used by the enemy.
However, before citing this as the Civil War equal of breaking the Japanese code before the battle of Midway, keep in mind the setting. A majority of messages sent around Charleston were by courier. And there was a functioning telegraph between most points around the harbor, including Fort Sumter. What Colvin was able to intercept were somewhat routine in nature. While important at the tactical level, these were not revealing of the broad aspects of Confederate operations at Charleston. (And I will post some examples of that later in July, timing to a 150th.)
Still, Colvin not only broke the Confederate code once, he was able to break it after successive code changes. It is not recorded if the Confederates realized Colvin was successful breaking their code and thus changed the codes, or if the changes were simply a matter of routine. I suspect the latter. Only much later, in the early fall, were the Confederates aware of the Federal code-breaking.
Colvin’s efforts were significant enough to warrant mention by Lieutenant-Colonel William Nicodemus, the Army’s Chief Signal Officer, in a report, covering all operations in all theaters from November 1863 to October 1864, to the Secretary of War:
This man has displayed a remarkable talent and fitness for this branch of the service. Major-General Foster has received such valuable information through his means that he has recommended that he be promoted, or that he receive a brevet or medal. Captain Clum also speaks in the highest terms of him. He is, therefore, respectfully recommended to favorable notice by the department.
Colvin initially enlisted with the 47th Pennsylvania Infantry in September 1861. He transferred to the Signal Corps in the fall of 1863. While Colvin received no medal for his efforts, he was promoted to 2nd Lieutenant, effective February 14, 1865… just in time to see the end of the war at Charleston.
(Citations from OR, Series I, Volume 35, Part I, Serial 65, page 47; Series I, Volume 35, Part II, Serial 66, pages 150-1; Series III, Volume 4, Serial 125, page 826.)