räckekodning

Räckekodning, often translated as "counting code" or "reciprocal code" in English, is a simple form of cryptography used in some older systems, particularly in certain types of legacy telecommunication equipment. Its core principle is to invert the digits of a numerical message. For example, if the code is based on a ten-digit system (0-9), a '0' might be represented by a '9', a '1' by an '8', a '2' by a '7', and so on, until '9' is represented by '0'. The mapping is typically symmetrical, meaning the inverse of the inverse is the original digit.

The primary purpose of räckekodning was to provide a very basic level of privacy against casual observation rather than robust security. It was designed to make intercepted messages unintelligible to someone not aware of the specific inversion scheme. This method is easily breakable with even minimal cryptanalytic effort, as the pattern is straightforward and the key space is extremely small. It is not considered secure by modern cryptographic standards and is largely obsolete, replaced by far more sophisticated encryption algorithms. While not truly a form of encryption, it served as an early, rudimentary method of obfuscation.