[syhprum (OP)]

I have visited Bletchley where wartime intercepted messages encoded by 'Enigma' machines were decoded mostly by reverse engineered 'Enigma' machines and primitive electronic computers.
I can well visulise partly decoded messages spewing out of the machines in vast quantitys but how did they know when they had anything meaningful ?.   

[another_someone]

It relied on knowing (or guessing) some part of the message that was sent.

I too visited Bletchley Park this summer, so will have to recall what I can of the visit.

A lot depended on sloppiness on the usage of the machine.  Firstly, I believe in the early days they used to send a pair of 3 character introducer codes on the machine.  The first two character code was a random given code that changed daily, while the second was something the operator would make up on the spot, but some operators would simply repeat the same 3 character code twice, so they would look for a repeated set of 3 letters at the start of the message, and sometimes strike lucky, and when they did, they then had the cypher for a whole collections of machines for that day (probably most machines).

Secondly, many of the machines were used for sending mundane things like weather reports, and so they would look for words in the plaintext that would be common for weather reports, and when they found them, they knew they had the right decryption key.

Thirdly, they would set up situations where they could guess the likely content of the message that would be sent (for instance, sending a given number of bombers to a given town, dropping an exact number of bombs at an exact time, and then listening for the encrypted traffic that would likely be a report of that raid.

Then there were times when our intelligence people fed their intelligence people information, and we simply followed the traffic as that information was reported up the chain.

[syhprum (OP)]

What I was interested in was how was plain text recognised, there must have been reams of useless materiel produced was it examined manually to see if it contained any plain text or was the process mechanised in any way

[another_someone]

What I was interested in was how was plain text recognised, there must have been reams of useless materiel produced was it examined manually to see if it contained any plain text or was the process mechanised in any way

My recollection is as follows:

The whole purpose of the computers was to search for the plain text.

The mechanism was simply a brute force attack on the code, where there were banks of machines that mimicked the function of the enigma machines, the encrypted text was fed in, and very small portions of plain text were used as a match, and the enigma machines were then cycled through all the possible keys until the resultant supposed decrypted text contained the expected plain text.

Sometimes there would be a match on the short stretch of plaintext, but it was mere fluke, and the rest of the decrypted text was still gobbledygook, and then it was a matter of going back for more searches.  That part was manual, but the match for the short stretches was automated.

[lyner]

To be or not to be, that is the gzornnnplatz.