If your interests include cryptography, you have certainly heard about Kerckhoffs’ principles, especially the one stating that the method used to encipher data is known to the opponent, and that security must lie in the choice of key. This does not necessarily implies that the method should be public, but only considered as public during its creation. This was enunciated in the January and February issues of the Journal des sciences militaires, 1883.
This journal is still available and I got a copy of Kerckhoffs’ article from the British Library. An electronic version of both parts is now available.
The first part is also available as PDF document.
Here is an approximate English version of the principles that should apply to a crypto-system:
Acknowledgements: Frank Stajano for his scanner and OCR software. Bruno Liénard who help finishing the corrections on the second part. John Kane. Alejandro López who pointed out few errors in Part II.