Colossus Mk 2

  • Editors' rating
    9.5 Spectacular


  • Delivers Cryptanalysis as a Service (CaaS) with unprecedented efficiency
  • Has the potential to shorten world conflicts
  • Highly secure
  • High voltages create exciting work environment


  • Bulky and power-hungry
  • High cost of ownership
  • Requires highly qualified support staff
  • Threatens the limbs of careless operators

From the 'Station Z' archives, April 1 1945...
Innovations in computing and communication are the lifeblood of the industry, and so we are delighted to be given the chance to review Colossus Mk 2, a new device from those industrious inventors at the Post Office Research Station, Dollis Hill.

Colossus Mk 2, installed at the Bletchley Park datacentre: sales are expected to break double figures

Checking in at twelve full-height bays, not including the power supply, Colossus Mk 2 isn't quite as portable as other cryptographic equipment such as the Lorentz SZ40/42 console, with which it's now (after some teething problems) largely compatible. However, head designer Tommy Flowers says this isn't an issue as the Colossus Mk 2 is designed to be used for CaaS (Cryptanalysis as a Service) with input supplied by Y station reception and output distributed by the SCU network.

Colossus Mk2 increases the valve count from 1,500 in Mk 1 to 2,400, resulting in a five-fold performance boost

Top ZDNET Reviews

With 2,400 valves, mostly Mullard EF36 pentodes but including 6J5 triodes, 6V6 and 807 tetrodes and GT1C thyratrons, plus a 25Kbps tape input stream, Colossus Mk 2 is the most capable programmable electronic computer on the market — a claim that's easy to verify as it's the only programmable electronic computer on the market. Not that there's much of a market: Bletchley Park remains the only customer for this product, with global sales expected to peak at around ten units.

(According to Flowers, the Colossus marketing department has encountered some problems in creating and executing on the mind-share campaign he was anticipating. "We had hoped for parallel advertising campaigns on the wireless, the newspapers and in cinemas across EMEA", he explained, "but our marketing director vanished after he'd made the pitch to Churchill. All we've been able to discover is that he might now be involved 'at a deep level' on something called Operation Mincemeat." Station Z did contact the Cabinet Office, and was told: "That blithering idiot wanted to run stuff past Jerry. We were only too happy to help".)

Paper tape input can reach 50Kbps, although careless operators can end up, quite literally, digitally challenged

In tests, Colossus Mk 2 proved quite reliable and reasonably swift: we were pleased to be reading reports on Norwegian coastal defences only days after installing our unit. Flowers says that possible refinements include running two Colossi in parallel for multi-core decryption, and also says that the paper tape can be accelerated to around 50Kbps "providing you don't mind losing a couple of fingers". Operator care is required here, as high-voltage bus lines are dotted liberally around the chassis. That said, we feel that too many concessions to user-friendliness can lead to sloppy workmanship, and thoroughly encourage future designers to continue to make computer operation a sporting affair.

We did have some problems with sticking uniselectors, clock recovery from the punched tape input system and ringing on the Q bus. This proved mostly due to fragments of fruit pudding left by Alan Turing, which came as a surprise to the Colossus designers. They had hitherto had not considered the machine to be quite Turing-compote.

Although, as we said, we think the safety aspects of Colossus are overstated, we remain concerned about the implications for compliance with data security best practice. One reader, Field Marshal Erwin Rommel (@desert_fox), told us he was considering complaining to the European privacy regulators over security infringements by Colossus. Unfortunately, as he was trying to advance on his arguments, he kept getting cut off.

The future for Colossus technology is potentially bright, but perhaps not without some interesting problems. We'd like to see a portable version, and while we understand that interoperability with the cryptography of other major world powers is being actively developed, this may be hindered by the fact that a man with a gun is currently telling me to stop writing or it's...