All operations

Tap code

Taps for the row, a pause, taps for the column, knocked through a wall.

The grid

Five rows of five: A B C D E, F G H I J, L M N O P, Q R S T U, V W X Y Z. A letter is knocked as its row number, a pause, then its column number. H is two taps, pause, three taps.

There is no K. Twenty-five cells take twenty-five letters, so K goes out as C and the word around it settles which was meant. A letter costs six taps on average and ten at worst, Z being five and five. That is the whole bandwidth of the channel.

A tap is written here as a full stop, or a bullet if that is easier to count. Decoding pairs up runs of taps rather than trusting the spacing, so a transcript with commas, slashes or ragged gaps still comes back.

Hoa Lo prison

Carlyle Harris, an Air Force captain everyone called Smitty, was shot down over North Vietnam on 4 April 1965. He remembered an instructor describing a five-by-five knock alphabet during survival training. In June 1965 he shared a cell in Hoa Lo prison in Hanoi with three other prisoners and taught it to them. Splitting the four up is what spread it, and by August most of the men held there had it.

Communicating was punished, so it went out in whatever form the guards were not watching. Jeremiah Denton worked up a version in coughs, sniffs and sneezes. Everything got compressed, because a sentence took minutes. GN closed a conversation for the night, and GBU, God bless you, is the message loaded above written out in full.

No key at all

The grid is fixed, there is no key, and anyone who notices the knocks arrive in pairs can start counting.

Secrecy was never the job. Prisoners in isolation used it to work out who else was in the building. It carried names that would reach families at home, and it settled what a man would say under interrogation. James Stockdale said the tapping stopped being an exchange of letters and became conversation, carrying elation, sadness and sarcasm. A code that needs nothing but a wall buys a channel when every other channel is gone.

The idea is older than Vietnam. Russian prisoners knocked a Cyrillic version through cell walls in the nineteenth century, and Arthur Koestler gave it to Rubashov in Darkness at Noon in 1940.

The Polybius square is this grid written in digits. The Nihilist cipher came out of the Russian prisons where the knocking started, and Morse code does the same job for a channel that carries two lengths of signal.