https://blog.cryptographyengineering.com/2011/11/10/format-preserving-encryption-or-how-to/
Format-preserving encryption Project University 2018
In cryptography, format-preserving encryption (FPE), refers to encrypting in such a way that the output (the ciphertext) is in the same format as the input (the plaintext). The meaning of "format" varies. Typically only finite domains are discussed, for example:
To encrypt a 16-digit credit card number so that the ciphertext is another 16-digit number. To encrypt an English word so that the ciphertext is another English word. To encrypt an n-bit number so that the ciphertext is another n-bit number (this is the definition of an n-bit block cipher).