They broke codes by hand – or rather by brain – not with mechanical calculating machines; and they took pride in their work. Once, Olivia fell asleep at her desk, faced with an intractable cipher she had worked on for nearly a week. No one said anything, and even Clive dropped his incessant humming to near inaudibility. When she finally woke up, Olivia yawned, took her pencil in hand and wrote out the intercept contents in clear, from beginning to end.
Several days later, Clive was working on a batch of intercepts from some newly established outstation when his Morse-code humming made Gavriela’s stomach clench, interrupting her work.
‘Mm, mm-hmm, mm-mm-mm-hmm, mm-mm …’
The solidity of the room seemed to twist apart around her.
That night in her digs, after the long walk through the village in night-time gloom, she sat atop her bed, eyes open but with nothing to see: the lights were off and the blackout curtains kept any stray moonlight from entering. She thought of the classical Greeks and their Italian renaissance counterparts (natural imaginings for someone working alongside Fred and Olivia), and a discussion she had once had with Florian Horst in her days at the ETH.
‘Memory palaces,’ she aloud in the dark, remembering how she had argued that the psychological technique was a waste of vivid imagination that could be put to better creative use than memorizing a list of words you could just as well write down.
Then she lay back to sleep.
In the morning, because of the blackout curtains, it was still pitch black. She sat up, barely conscious, saying: ‘Thank you, Roger.’
As she pulled open the curtains, the tag-end of her dream and her waking words were like frozen CO2: solidity evaporating straight to invisible gas, lost from sight.
I can’t tell the others what I think.
Talk of darkness twisting in odd geometric ways – a darkness visible only to her, her brother, and a Soviet agent called Dmitri Shtemenko who was haunted by it – would surely result in her removal to a quiet ward somewhere. The nine-note auditory analogue, the nine notes hummed yesterday by Clive, might easily be her hallucination, nothing real.
I need to break that code.
Because Clive had been humming while he worked at his desk on the enciphered enemy message.
Which means I need to steal the intercept.
And that was treason, wasn’t it? But she checked that she had blank notepaper and a sharp pencil, before she went down to breakfast.
Gavriela plodded through her day, finally breaking off to go the ladies room at the shift’s end. There, Rosie was powdering her face, her headscarf draped across her shoulders.
‘Hi, Gabby,’ she said. ‘Are you coming along on the bus tonight?’
‘I’m not, er, feeling all that good.’ Gavriela put a hand on her stomach. ‘You know. Anyway, I’m all right walking home later, when the cramp goes.’
‘You want me to stay?’
‘Oh, no. Thank you.’
‘Well, you take care. I’ll let people know not to hang around waiting for you.’
Gavriela slipped back into Hut 27 just as Harry was locking up.
‘Sorry,’ she said. ‘Um … Can I carry on working for a while? I don’t really feel like going home just yet.’
‘Are you all right?’
‘A bit under the weather, if you know what I mean.’
‘Are you—? Oh. Right. I’ll, er, see you in the morning. Toodle-oo.’
She gave a smile that probably looked as ill as she felt – though not from the woman’s trouble she was hinting at. For ten minutes, by the light of the overhead bulb above her desk, she stared at her own intercept decrypt-in-progress, scarcely seeing the letters, never mind thinking about the code.
The hut was very quiet with only her inside. The thing was, no one kept regular hours, and any one of the team could return at any minute. She waited, until she no longer could.
Clive had not been in all day. Fred claimed he was going to be with some Admiralty types for the rest of the week, along with Brian; Gavriela thought he did not really know. In the meantime, everyone had their own intercepts to work on; no one would be attempting to complete Clive’s unfinished tasks.
The truth was, the content that Clive had deciphered so far in this batch had been of little interest, its triviality a contrast to some of the other intercepts, including several direct from the Kaiser Wilhelm Institute regarding the atomic bomb programme.
Was this a remotely sensible thing to be doing?
Remember the music.
Those nine discordant notes that Clive had hummed.
You know it’s the darkness.
She went to Clive’s desk, pulled open the drawer, and extracted the intercept. It seemed to have been several messages in one, after a single 12-character prefix: at least, there was one section that remained enciphered while the rest was now in plain text. The prefix, when broken, assigned the ten-wheel settings (two for each bit of each character) on the sending machine. No British officer had ever seen a German Tunny device, yet the mathematical analysis was certain. A peculiarity of this encryption was that ‘adding’ the key to the plaintext (actually performing a bit-wise transformation) produced the enciphered signal, yet performing the same operation again reproduced the original message en clair.
But not in this case, it seemed. The explanation was probably a misread portion of the signal.
What if it’s been enciphered twice?
More precisely, what if part of the signal was so sensitive it could not even be read by the security-cleared teletype operator? So what if someone had encoded once according to their own scheme – starting with the prefix that sounded like the darkness, the horrible rhythm of EAVI as hummed by Clive – and given that message to the operator, as part of a longer signal, for further enciphering and transmission?
The still-enciphered portion read:
EAVI5 N1BF 961Y0 1N2B6 WRRQY 5N172 B5QUB UN1BU N40BF RBLGB F07B5 N19U2 QTBN9 B27QV QYTQ7 DYQ2H BRN2B 519BQ 5Y17B 271VV 19BRF 1UU19
Gavriela concentrated, stringing the random characters like party streamers among the decorations of her childhood bedroom, in the image constructed in her mind: as vivid, almost, as being there once more. She was feeling odd by the time she had finished; but whenever she closed her eyes, she could see the message in full once more: exactly as on the intercept sheet when she checked in reality. Finally, she slipped the sheet back inside the pile, the pile inside the drawer, and closed up the desk.
A message from the darkness lay enciphered in in her mind.
Back in her own chair, she sat for some time with her eyes closed, walking around the imagined room again and again, until she was sure she could reconstruct it at will. Then she opened her eyes, and got ready to leave.
As she was walking past the courtyard, she looked back and saw Silvester heading for Hut 27. Perhaps he would be carrying out an audit – he was that conscientious – but what she had stolen was only inside her head.
I must be insane.
Yet she knew she was right.
At home, she wrote out the message as memorized, without the first four characters she believed to be an identifying prefix.
5N1B5F961Y01N2B6WRRQ95N172B5QUBUN1BUN40BFRB LGBF07B5N19U2QTBN9B27QVQYTQ7DYQ2HBRN2B519BQ 5Y17B271VV19BRF1UU19
The simplest form of cipher is a monoalphabetic substitution – replacing each letter of the clear message with a different letter. But if this were, say, an Enigma message contained inside a Tunny signal, then the cipher would be far less trivial, most likely polyalphabetic: after each character was encoded, the machine would shift to a new letter-replacement scheme. That type of cipher was nearly impossible to break without a crib: a section of message whose plaintext was known or could be guessed. But this message was a cipher-within-a-cipher, and with luck the enemy had relied on the sophistication of the outer algorithm for the main protection, while the inner cipher was simply to prevent a casual glimpse from German signallers.