A vacuum tube computer, now termed a first-generation computer, is a computer that uses vacuum tubes for logic circuitry. Although superseded by second generation, transistorized computers, vacuum tube computers continued to be built into the 1960s. These computers were mostly one-of-a-kind designs.
During World War II, special purpose vacuum tube digital computers such as Colossus were used to break German and Japanese ciphers. The military intelligence gathered by these systems was essential to the Allied war effort.
Each Colossus used between 1,600 and 2,400 vacuum tubes. The existence of the machine was kept secret and the public was unaware of its application until the 1970s.
Also during the war, electro-mechanical binary computers were being developed by Konrad Zuse. The German military establishment during the war did not prioritize computer development. An experimental electronic computer circuit with around 100 tubes was developed in 1942, but destroyed in an air raid.
Colossus
The Colossus computers were used to help decipher intercepted radio teleprinter messages that had been encrypted using an unknown device. Intelligence information revealed that the Germans called the wireless teleprinter transmission systems "Sägefisch" (sawfish). This led the British to call encrypted German teleprinter traffic "Fish", and the unknown machine and its intercepted messages "Tunny" (tunafish).
The Colossus computers were used to help decipher intercepted radio teleprinter messages that had been encrypted using an unknown device. Intelligence information revealed that the Germans called the wireless teleprinter transmission systems "Sägefisch" (sawfish). This led the British to call encrypted German teleprinter traffic "Fish", and the unknown machine and its intercepted messages "Tunny" (tunafish).
German and Japanese ciphers
The Lorenz SZ40, SZ42a and SZ42b were German rotor stream cipher machines used by the German Army during World War II. They were developed by C. Lorenz AG in Berlin. The model name SZ was derived from Schlüssel-Zusatz, meaning cipher attachment. The instruments implemented a Vernam stream cipher.
Gilbert Vernam was an AT&T Bell Labs research engineer who, in 1917, invented a cipher system that used the Boolean "exclusive or" (XOR) function, symbolised by ⊕. This is represented by the following "truth table", where 1 represents "true" and 0 represents "false".
| Input | A ⊕ B | |
|---|---|---|
| A | B | |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
Other names for this function are: Not equal (NEQ), modulo 2 addition (without 'carry') and modulo 2 subtraction (without 'borrow').
Vernam's cipher is a Symmetric-key algorithm, i.e. the same key is used both to encipher plaintext to produce the ciphertext and to decipher ciphertext to yield the original plaintext:
- plaintext ⊕ key = ciphertext
and:
- ciphertext ⊕ key = plaintext
This produces the essential reciprocity that allows the same machine with the same settings to be used for both encryption and decryption.
Vernam's idea was to use conventional telegraphy practice with a paper tape of the plaintext combined with a paper tape of the key. Each key tape would have been unique (a one-time tape), but generating and distributing such tapes presented considerable practical difficulties. In the 1920s four men in different countries invented rotor cipher machines to produce a key stream to act instead of a tape. The 1940 Lorenz SZ40/42 was one of these.
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