A computer-implemented method of replacing a security-relevant unencrypted data string by a placeholder, comprising the steps: providing a plurality of mutually different replacement tables, wherein specified in each of the plurality of replacement tables for each character of the alphabet is precisely one replacement character from the same alphabet and wherein the replacement characters in each of the plurality of replacement tables are all different from each other, receiving the security-relevant unencrypted data string, wherein the data string is formed from a plurality of characters of an alphabet, generating the placeholder replacing the unencrypted data string, with the steps 1. selecting a subset from the plurality of replacement tables, 2. selecting a first substring with at least one character and a second substring with at least one further character from the data string, 3. replacing the at least one character of the first substring by a replacement character specified in a replacement table from the subset, wherein the replacement character depends on the second substring, 4. repeating at least steps 2 and 3 until all characters of the data string were replaced at least twice, outputting the placeholder, wherein selecting the subset from the plurality of replacement tables in step 1 is effected in dependence on a random parameter, wherein the method is characterised in that selecting the subset from the plurality of replacement tables is not dependent on the at least one character of the second substring. providing a plurality of mutually different replacement tables, wherein specified in each of the plurality of replacement tables for each character of the alphabet is precisely one replacement character from the same alphabet and wherein the replacement characters in each of the plurality of replacement tables are all different from each other, receiving the security-relevant unencrypted data string, wherein the data string is formed from a plurality of characters of an alphabet, generating the placeholder replacing the unencrypted data string, with the steps 1. selecting a subset from the plurality of replacement tables, 2. selecting a first substring with at least one character and a second substring with at least one further character from the data string, 3. replacing the at least one character of the first substring by a replacement character specified in a replacement table from the subset, wherein the replacement character depends on the second substring, 4. repeating at least steps 2 and 3 until all characters of the data string were replaced at least twice, 1. selecting a subset from the plurality of replacement tables, 2. selecting a first substring with at least one character and a second substring with at least one further character from the data string, 3. replacing the at least one character of the first substring by a replacement character specified in a replacement table from the subset, wherein the replacement character depends on the second substring, 4. repeating at least steps 2 and 3 until all characters of the data string were replaced at least twice, outputting the placeholder, wherein selecting the subset from the plurality of replacement tables in step 1 is effected in dependence on a random parameter, wherein the method is characterised in that selecting the subset from the plurality of replacement tables is not dependent on the at least one character of the second substring.