The encrypted diary of prisoner of war Donald Hill is unique. Hill not only encrypted his entries but also disguised them as a collection of mathematical tables. In a self-experiment, I tried to encrypt and hide a text like he did.
Donald Hill (1915-1995), a British military pilot, was stationed in Hong Kong during the Second World War. He was captured by the Japanese and had to spend four years in a prisoner of war camp. Already before his years in captivity, Hill wrote a diary – this was illegal, as British soldiers were not allowed to keep private notes (notes like these might have provided helpful information to the enemy). To hide his writing activities, Hill disguised his diary as a collection of multiplication tables. Before pocket calculators came up, tables like these were quite common. While in captivity, Hill continued to write his journal.
Donald Hill’s diary is the only steganographic diary I have ever seen. It is contained in my encrypted book list (00041). As can be seen in the following scan, the disguise worked very well. These number sequences don’t look like an encrypted text, let alone a diary, at all:
After his return to England, Hill married his fiancé, Pamela. The story of Donald Hill is told in Andro Linklater’s book Code of Love.
Donald Hill never talked about his diary to his wife. He never revealed the encryption system he had used. Only after Hill’s death in 1995, Pamela tried to find out what the strange number sequences were about. She asked mathematics professor Philip Aston from the University of Surry to analyze them. Philip Aston broke the cipher and published a paper about it, which is now available online.
Aston published the whole plaintext of the diary, but only a few pages of the original. A one-to-one match between a ciphertext passage and a diary page is not available. And then, I am not sure, if I have understood all the details of the enciphering process correctly.
To get a better understanding, I decided to take a part of the plaintext provided by Aston and to encrypt it in the way Donald Hill did almost eight decades ago. As Hill encrypted blocks of 1122 letters, I had to select a plaintext passage of this length. I chose the first 1122 letters from the diary entry of December 8th, 1941:
I AM DISTURBED EARLY AS THE COLONIAL SECRETARY RINGS UP TO SAY THAT WAR WITH JAPAN IS IMMINENT HELL THERE GOES MY SLEEP AND I WAKE THE OTHER OFFICERS OVER BREAKFAST WE ARE TOLD THAT WE ARE AT WAR WITH JAPAN WE DASH DOWN TO FLIGHTS JUST IN TIME TO HEAR AN OMINOUS ROAR OF PLANES AND NINE BOMBERS ESCORTED BY OVER THIRTY FIGHTERS APPEAR HEADING OUR WAY THERE’S NO TIME TO DO ANYTHING EXCEPT TO MAN OUR DEFENCE POSTS THE BOMBERS PASS OVERHEAD BUT THE FIGHTERS SWOOP DOWN ON US AND POUR A CONCENTRATED FIRE INTO OUR PLANES WE GIVE THEM ALL WE’VE GOT WHICH IS PRECIOUS LITTLE SOME INDIAN TROOPS GET PANICKY AND RUSH INTO A SHELTER IN THEIR EXCITEMENT THEY FIRE THEIR LEWIS GUN THERE IS A MAD RUSH FOR SAFETY AND BY A MIRACLE NO ONE IS HIT AFTER TWENTY MINUTES OF CONCENTRATED ATTACK BY THE FIGHTERS THE BEESTE WITH BOMBS GOES UP IN SMOKE AND THE TWO WALRUS ARE LEFT BLAZING AND SINK FINALLY THEY MAKE OFF NOT UNSCARRED WE HOPE AND WE INSPECT THE DAMAGE BOTH WALRUS ARE GONE ONE BEESTE IS ABLAZE ANOTHER BADLY DAMAGED LEAVING ONE PLANE INTACT WE ATTEMPT TO PUT OUT THE FIRE PRAYING THAT THE BOMBS WON’T EXPLODE THE BLAZE IS TOO FIERCE AND SHE IS COMPLETELY BURNED WITH TWO RED HOT HEAVY BOMBS AMONGST THE RUINS ONE AIRCRAFT LEFT BUT NO CASUALTIES TO PERSONNEL EIGHT CIVIL MACHINES ARE BURNT OUT INCLUDING THE AMERICAN CLIPPER IN THE AFTERNOON BOMBERS COME OVER AGAIN BOMBING THE DOCK AN
The cipher Hill used is a transposition cipher based on the following keyword consisting of 34 letters (it includes the names of him and his wife):
In the first step, Hill wrote the 1122 letters passage column-wise into a matrix with 34 columns (as 1122 is a multiple of 34, no letters remained), with the keyword standing in the first row of the matrix:
Now, Hill reordered the columns such that the keyword in the first line was ordered alphabetically (if there were several letters of the same kind, they remained in their original order):
Next, Hill read out the cipher text line-wise: DMSTIINSK…
In the last step, Hill transferred each letter to a two-digit number (A=10, B=11, C=12, D=13, …) and wrote down the result in groups of four:
1322 2829 1818 2328 2013 2814 1029 1021 1328 1622
2924 2410 2723 2727 1130 1423 1314 1811 1011 2110
3229 1110 1027 2227 2114 1429 1714 2414 2325 2918
1414 2829 1024 2415 3214 1521 2222 1428 3422 1128
1410 2129 2932 2918 1028 1425 3228 2929 1023 2729
1029 1027 1410 1013 1329 2910 2112 2729 3214 1714
1423 2830 2514 1418 1017 2224 1030 2327 2028 2935
1218 1017 1716 1024 2914 1421 1410 2713 1725 2110
2322 2714 2412 2323 3414 1414 3418 1728 2814 1414
3522 1213 3134 1127 2818 1429 1027 2922 3418 2310
2428 2925 2928 1023 1829 1711 1511 1414 1015 1411
2114 2810 2123 2317 3418 2727 1628 2212 1727 1712
Hill’s encryption procedure resulted in tables like this:
Mind the number group consisting of four zeroes (“0000”) in the middle of the second last line. Hill used these zero-groups as separators between two 1122-letter blocks.
Using such a complex cipher for a diary is quite unusual. Apparently, Hill had enough time to care about his writing activities while in captivity and he was especially cautious. I don’t know what happened to prisoners of war who were caught writing a diary, but the consequences were certainly not very pleasant.
Further reading: A hidden message sent by a prisoner of war in WW1