Substitution table
In cryptography, a substitution table assigns letters or groups of letters to certain characters (letters, numbers, special characters) or groups of characters. For encryption purposes , letters in plain text are replaced ( substituted ) with the assigned characters. If the letters are consistently assigned numbers, these can be converted into other values with the help of mathematical operations. When decrypting , substitution tables are used in the last step to convert numbers back into letters.
An example of an encryption method that requires substitution tables is the OTP method (One-Time-Pad, one-time encryption).
Different language-specific tables are used as substitution tables by various secret services, which take into account the frequency of the letters occurring in a language. As an example, a substitution table of the MfS or the NVA of the German Democratic Republic is shown in the picture. The letters A and E are used quite often in the German language, which is why they are at the front of the table.
This table was used in the NVA and the MfS of the GDR for the encryption procedures KORALLE, KOBRA and PYTHON. Agents needed these tables and their OTPs to decode messages sent to them by the number senders.