Here’s how the Union enciphered its messages during the Civil War. Abraham Lincoln sent this dispatch on June 1, 1863:

GUARD ADAM THEM THEY AT WAYLAND BROWN FOR KISSING VENUS CORRESPONDENTS AT NEPTUNE ARE OFF NELLY TURNING UP CAN GET WHY DETAINED TRIBUNE AND TIMES RICHARDSON THE ARE ASCERTAIN AND YOU FILLS BELLY THIS IF DETAINED PLEASE ODOR OF LUDLOW COMMISSIONER

The first word, GUARD, indicates the size of a containing rectangle and the paths on which the words must be laid out to decipher the message. In this case, they’ll go up the first column, down the second, up the fifth, down the fourth, and up the third. Also, just to confuse the Confederates, every eighth word after GUARD is a null and should be discarded. So we get:

FOR     VENUS          LUDLOW   RICHARDSON AND
BROWN   CORRESPONDENTS OF       THE        TRIBUNE
WAYLAND AT             ODOR     ARE        DETAINED
AT      NEPTUNE        PLEASE   ASCERTAIN  WHY
THEY    ARE            DETAINED AND        GET
THEM    OFF            IF       YOU        CAN
ADAM    NELLY          THIS     FILLS      UP

The last steps are to remove THIS FILLS UP, which is only there to fill out the block, and to replace a few code words:

VENUS = colonel
WAYLAND = captured
ODOR = Vicksburg
NEPTUNE = Richmond
ADAM = President of the United States
NELLY = 4:30 p.m.

That gives us the final message:

For Colonel Ludlow,

Richardson and Brown, correspondents of the Tribune, captured at Vicksburg, are detained at Richmond. Please ascertain why they are detained and get them off if you can.

The President, 4:30 p.m.

This system was such a valuable source of breaking news that Lincoln often visited the military telegraph office in the War Department, next to the White House, and would chat with the operators there. One of them, David Homer Bates, who was only 18 when the war started, remembered, “Outside the members of his cabinet and his private secretaries, none were brought into closer or more confidential relations with Lincoln than the cipher-operators, … for during the Civil War the President spent more of his waking hours in the War Department telegraph office than in any other place, except the White House. … His tall, homely form could be seen crossing the well-shaded lawn between the White House and the War Department day after day with unvaried regularity.”

(From David Kahn, The Codebreakers, 1996.)

Secessionist Roger A. Pryor was visiting Fort Sumter just before the outbreak of the Civil War when he accidentally drank a bottle of poison. A Union doctor named Samuel Crawford pumped his stomach, saving his life.

“Some of us questioned the doctor’s right to interpose in a case of this kind,” wrote Union captain Abner Doubleday. “It was argued that if any rebel leader chose to come over to Fort Sumter and poison himself, the Medical Department had no business to interfere with such a laudable intention.”

“The doctor, however, claimed, with some show of reason, that he himself was held responsible to the United States for the medicine in the hospital, and therefore he could not permit Pryor to carry any of it away.”

In 1942, uncertain whether one of its spies had been replaced by a German impersonator, Britain’s Special Operations Executive hit on a clever plan: After a regular radio communication, the British radio operator signed off with HH, short for “Heil Hitler,” the standard farewell among Nazi operators. His counterpart, “Netball,” responded HH automatically, giving himself away.

They confirmed this at the next session:

Netball was several minutes late for his sked (not significant) and signalled ‘q r u’ (‘I have no traffic for London’). Howell replied ‘q t c’ (‘We have a message for you’), and proceeded to transmit it (the message warned Netball never to send less than 150 letters). Howell then signalled ‘HH’, and Netball immediately replied ‘HH’.

‘Right,’ Nick was heard to say to his companion, ‘that’s it then.’

(From Leo Marks, Between Silk and Cyanide, 2001.)

On D-Day, Dwight Eisenhower carried this note in his wallet:

Our landings in the Cherbourg-Havre have failed to gain a satisfactory foothold and I have withdrawn the troops. My decision to attack at this time and place was based upon the best information available. The troops, the air and the Navy did all that bravery and devotion to duty could do. If any blame or fault attaches to the attempt it is mine alone.

He threw it away the next day, but an aide retrieved it. Today it’s in his presidential library.