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Originally Posted by jpm
Cryptography was still extremely primitive in 1883, and Auguste obviously was not around when the Enigma machine was put into use by Germany during WW2. Take a look at how the allied cryptographers broke the Germans communications using that device. They had captured books of keys, but they didn't have the algorithm to use with them and as such, the keys were useless. This was more important with the later versions of the Enigma box that added another code wheel and user configurable jumper connections to strengthen the encryption.
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Auguste Kerckhoff was the first to state the principle, but it still holds true today. Here are a few more sentences from the same page:
"If an algorithm is used in products, it will be reverse engineered. Once-secret algorithms that have been reverse engineered include RC4, all digital cellular encryption algorithms, the DVD and DIVX video encryption algorithms, and the FireWire algorithms. Even algorithms buried deep in military hardware will be reverse engineered: the Enigma during World War II, and just about every NATO and Warsaw Pact algorithm during the Cold War. (We don't know those, but the respective militaries do.) It is a good design to assume that the enemy knows the details of your algorithms, because eventually they will."
I would add to the list of failed secrets the MediaMax CD3 copy protection system that can be circumvented by holding down the Shift key.
An algorithm requires significant investment into hardware or software, but keys are easy to generate, today's computers negotiate a new key for each secure connection. An algorithm may have 2 to the power of 128 or even 256 possible keys, whereas the number of potential algorithms is probably less than a hundred. Here is a "real world" example. A safe may have a million possible combinations. Knowing that a particular safe was made by Chubb won't help you much in opening it. On the other hand, imagine that you find a piece of paper with "combination to the safe 60-28-34" written on it. You may only have to try a handful of safes if you are in a small town. It may not work, the piece of paper may have been dropped by someone from hundreds of miles away who was just passing through, but you have a much better chance if you know the combination and have to find the right safe than the other way around.