1. A method comprising: receiving test data including multiple test strings; encrypting each of the test strings; selecting, for each encrypted test string, a portion of the encrypted test string, wherein the portion of the encrypted test string has a predetermined length equal to a length of a test string; comparing, for each portion of the encrypted test string, the portion of the encrypted test string to a corresponding test string; determining, for each portion of the encrypted test string, whether the portion of the encrypted test string is equivalent to the corresponding test string; assigning, for each portion of the encrypted test string, the portion of the encrypted test string as a replacement string for the corresponding test string based on determining that the portion of the encrypted test string is not equivalent to the corresponding test string; determining, for each replacement string, whether each replacement string is unique relative to other replacement strings; storing each replacement string that is determined to be unique; storing each replacement string that is determined not to be unique; generating, for each replacement string determined not to be unique, an alternate replacement string; storing each alternate replacement string and corresponding test string; and outputting, an alternate replacement string, as a masked string in response to a determination that an input string matches one of the stored test string associated with one of the alternate replacement string.