ROT13stä

ROT13stä is a variant of the ROT13 cipher designed for text in the Swedish alphabet, which includes the letters Å, Ä, and Ö in addition to the standard 26 Latin letters. In ROT13stä, each letter is moved 13 positions forward within a 29-letter alphabet, wrapping around at the end. Non-letter characters and punctuation are not altered, and the case of letters is preserved.

Alphabet and mapping are defined as follows: A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z, Å, Ä, Ö. To encode, each letter’s index in this sequence is increased by 13 modulo 29. For example, A maps to N, B to O, and C to P. Conversely, decoding generally requires shifting by 16 (since 29 − 13 = 16) to recover the original text. This asymmetric property means the encoding and decoding operations are not identical as they are in the traditional ROT13 over a 26-letter alphabet.

Practical use of ROT13stä is mainly within puzzles, word games, or stylistic text obfuscation where Swedish letters need to be preserved. It is not intended to replace cryptographic standards and provides no security guarantees beyond simple concealment.

Example: the string "ABC" becomes "NOP" under ROT13stä. To revert, "NOP" should be transformed by a 16-step shift to return to "ABC". See also ROT13 for the conventional, 26-letter variant.