- Nicholson, Ian
- Ship histories and stories, WWII operations
- RAN Ships
- HMAS Sydney II
- September 2000 edition of the Naval Historical Review (all rights reserved)
British concern about the secret war callsign challenge and reply system – obviously its proneness to both error and compromise – is evident in Admiralty dockets (files) and signals from late 1941. Ironically, Captain Burnett was one who had complained when, in September 1941 he reported that, on being challenged, three merchant ships failed to observe Admiralty procedure. Sydney’s subsequent loss when confronting an unidentified ship must have added significantly to the Admiralty’s concern, but it took over six months to fully appreciate that the secret challenge callsigns had at least been partly compromised.
Warnings of the dangers involved in approaching unidentified ships and the tactics and procedures to be used were repromulgated in Admiralty General Message (AGM 1618A) on 16 December 1941 soon after interrogation of Kormoran survivors had been reported to London. Instructions were modified and amplified by further messages on 14 March, 11 June and 11 August 1942. The latter followed supersession of the secret war callsign challenge system on 15 July 1942 when the Admiralty finally concluded that it was probably generally compromised.
The Germans evidently knew much more about UK challenge and reply methods than they admitted, even after the war, just as they were familiar with the QQQQ alarm signal, also used by them as a ruse. Before leaving Germany in December 1940 Detmers was issued with a copy of the Admiralty’s General Instructions for British Merchant Ships which described such procedures. Challenge and reply was not activated until later that month so may not have been included in the edition of the handbook acquired by the Germans then, but further books captured in January and April 1941 would have spelt it out in detail. Captain Detmers and others emphatically denied that they knew anything about the twin letter challenge (which was the middle two letters of the supposed ship’s secret callsign, with the correct response being the outer two letters of the same 4-letter group). Gill and other authors too readily accepted the lie, the former even adding that the German statement was corroborated by the fact that they could quote the actual letters `IK’ which Sydney would have transmitted and which they professed not to understand! They certainly were aware of the challenge procedure and it now seems evident that they also knew the correct reply ‘IP’, generated from Straat Malakka’s secret callsign `IIKP’. It was vital for the Germans to avoid any disclosure of this intelligence coup and their best course was to deny all knowledge of the challenge and reply during interrogation. After all, few if any besides Detmers would have been aware of it, most of the crew even being ignorant of their basic Straat Malakka disguise.
However, in the above quoted PRO papers is a German report of the action which was with documents carried by the Kormoran’s doctor during a 1943 prisoner exchange. In it the 2-letter group (challenge) transmitted by Sydney was said to be `not understood, but thought to possibly be the second and third letters of Straat Malakka’s secret 4-letter call-sign.’ How extraordinary, considering that they professed to know nothing of the system at the time of the action! Papers carried home by Dr Habben on repatriation were likely to be intercepted by the Allies, so obviously any reference to the secret callsign had to be very circumspect, but at the same time it is likely that their words were meant to convey much more. So the comment about the possible source of the 2nd and 3rd letters needed to appear comparatively innocuous to the Allies and also give a false indication of how little the Germans knew of the challenge and reply system. As the procedure for it was actually well known to both Kormoran and the German authorities, and they were each well aware of the others’ knowledge of it, they clearly aimed at some hiddenmeaning, – probably that their raider had both the challenge and the reply for the Straat Malakka. (This is necessarily the author’s interpretation – are there any others? Their lie damns them anyway.)
A modified allied challenge system was introduced and based on transposition of the merchant ship’s callsign letters, with a final single letter acknowledgement by the warship. Then, in early 1943 a streamlined plotting and checking system called CHECKMATE came into force (AFO’S 216/43), reducing dependence on challenge and reply.