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The Secrets of the Unicorn


By Sune Christian Pedersen

The encryption principle of "Monoalphabetic Substitution" has been used (at least) since Julius Cesar’s days. It is also called Cesar code. One alphabet is replaced – substituted - with another. The letter A may for example be B, B may be C, or IBM may be HAL as in "2001: A Space Odyssey". Encryption of this type was used in Europe for centuries, e.g. in the 17th century by the traitor Corfitz Ulfeldt – in initiated circles known under his cover name of "The Unicorn" – and by his wife, Leonora Christina.


The daughter of King Christian the Fourth, Leonora Christina, had not managed to get properly rid of a letter from Corfitz Ulfeldt when she was arrested in Dover, England, in July 1663. The arrest must have come as a surprise to her as under normal circumstances the letter would undoubtedly have been burned. Now she had to make do with tearing it to pieces and "throw it into the latrine" as she later wrote in her "Memory of Woe" while imprisoned in the Blue Tower. Leonora had gone to London to demand repayment of a loan which the English King Charles the Second had contracted with Corfitz. But pressured by the Danish King, Frederik the Third, Charles the Second had her arrested, and his men had to pick up the much damaged and no doubt malodorous letter from the soil tub. At the interrogation she was confronted with the tattered letter. Why did she tear it up, one of the interrogators, Count Rantzau, wanted to know. "Because", she answered, "there were ciphers in it and although it was of no importance, I feared nevertheless that it would arouse suspicion".

At that time the couple had fallen from grace: Corfitz had usurped immense power during his many years as Danish seneschal, but 10 years earlier he had been forced away from the acme of power by accusations of corruption and misconduct in office. He then went into Swedish service and had obtained great influence on the Swedish king, e.g. during the war in 1657-60. But the Swedes also mistrusted the scheming nobleman and in 1660 he was found guilty in high treason in Sweden. Back in Denmark the now mentally weakened Ulfeldt began to revolve plans of rebellion against the Danish King. He imagined this rebellion instigated in coalition with the Electoral Prince of Brandenburg whom he contacted during a stay at a health resort in Brügge, but the Electoral Prince informed Frederik the Third discretely about the traitor’s plans and for that reason the king issued the warrant against Corfitz and Leonora.

The Code Language of the Ulfeldt’s
When the couple and their allies wrote each other, they used a monoalphabetic cipher code in which each letter was replaced with a sign. If there were passages of a letter they wanted to be sure nobody read, they were written in the mysterious ciphers. In the torn letter there was for instance a short passage where Corfitz had written: "Nevertheless, it strikes me as glaringly bad (hereafter cipher text) and I know that they are short (cipher text) where ever it shall come from." We get the feeling that this may have to do with money, but the rest is impossible to guess. Leonora was convinced that "the ciphers had succeeded in full". At first she was confirmed in her belief when Rantzau asked her what the ciphers meant. "I have not got the key and I cannot decipher it by heart," she answered. Nobody believed her.

Thereafter the interrogators began to ask where Leonora’s jewels were. She answered that they had been sold long ago. Another cipher text originating from the couple’s confinement in Malmoe earlier tells, however, a different story. Here Leonora had made a list of her valuable belongings. She noted her belongings in clear text, but where and with whom they were kept, she wrote, of course, in code. Wisely enough, as later on the list was confiscated and sent to Copenhagen. The King’s men could read about things like "a brass-mounted chest in a sealed wooden case embroidered with diamonds and other precious stones" and "a small box of curiosities". But where were all these delightful things hidden?

Zacharias Lund
What the Ulfeldt’s did not know was that in his service the Danish King had one of the leading code-breakers of the world to whom all the intercepted letters were forwarded. So the King soon knew what Ulfeldt described as "glaringly bad" in the letter from Dover: "that our children in Rome are shouting unchastely for money. And I know that they are short and now they shall indeed get some,where ever it shall come from". Leonora was confronted with this passage during the interrogation when Rantzau revealed the he knew "that our sons had written from Rome … I handed the letter back to Count Rantzau without saying a word," she writes in her "Memory of Woe".

Zacharias Lund was the name of the man who disclosed the secrets of the "Unicorn" Corfitz Ulfeldt. Born in 1608, the son of a priest, he was later known as a poet, mathematician, and headmaster of Herlufsholm School. In 1657, when the Swedish King with Corfitz Ulfeldt in his train launched an offensive against Denmark, he was employed as secretary of the Danish chancellery where the deciphering tasks ended up on his desk. During the following 8 years he broke the code of more than 50 encrypted letters between the Swedish King, his generals, envoys, and allies; an impressive amount at the time. Rumours of the efficient Danish code-breaker must have spread as later the Electoral Prince of Brandenburg sent an intercepted letter to the code-breaker of the Danish King to have it deciphered.

The basic method of deciphering monoalphabetic substitution codes is called frequency analysis. As certain letters appear more often than others, it is possible to combine a deciphering by starting to insert frequently used letters as S and E in the encrypted text instead of the most frequently appearing ciphers and slowly proceed with further combinations. Frequency analyses was already used by Arabic pundits in the 9th century and came to Europe in the 13th century: Nevertheless, diplomats and commanders continued to write their letters using the obsolete monoalphabetic substitution that was in fact dangerous to them as it might provide them with a false sense of security. As was e.g. the case with Leonora when she wrote her list of where her gold and jewels were packed away. They were hidden as follows:

A: With the lady Margrete Rantzau (referring to valuables on the previous page)
B: What has been placed with Birrete
1. A brass-mounted chest in a sealed wooden case, embroidered with diamonds and other precious stones.
2. A silver chest sealed and embroidered with diamonds and saphires.
3. A small box of curiosities.
C: Margrethe Hundorf brought them thither (implying that Margrete Hundorf brought the things to above-mentioned Birrete).
4. A common clay chest of letters.
5. A sealed, embroidered wooden chest of gold, signed (i.e. carrying some kind of mark).
6. Two sealed bags, signed, each containing 500 rix-dollars.
D: Rasmus Qvining brought them thither (meaning Rasmus Qvining brought the things to Birrete).
7. A large, white, round box of curiosities.

The persons whose names were ciphered were the Scanian noblewomen Margrethe Rantzau at Skabersjoe, Birgitte Deichmann, daughter of the burgomaster of Malmoe (Birrete), Margrethe Hohendorff (Hundorf) at Roenneholm, and Rasmus Qvining who is unknown to posterity. On the other hand, it is not known how many of the objects the Danish King managed to confiscate. Scania was now in Swedish hands. But some chests containing securities and other things from the Ulfeldt’s were confiscated from Birgitte Deichmann in Copenhagen in the autumn of 1661 so the code-breaker’s disclosure was hardly in vain.

Footnotes:
The creator of a cipher alphabet may counteract the frequency analyses by choosing more than one cipher to symbolize the most frequent letters of the alphabet or by so-called "silent ciphers" which mean nothing, but which may confuse the code-breaker.

Besides ciphers the Ulfeldt’s used secret names and star signs as person codes. The venus sign was e.g. a somewhat uninventive symbol of Leonora whilst the toponym Carthago symbolized the nobleman Hannibal Sehested. This confused Leonora’s interrogators who clearly sensed that the toponyms mentioned in the torn letter were covering other names. Corfitz mentioned for instance practices of drinking which did certainly not apply for the Swiss towns he was writing about. Where was he hiding?


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