1. A method for performing cryptographic operations using computing equipment, comprising: with the computing equipment, obtaining a plaintext version of a string of characters that have a given format; with the computing equipment, applying a format preserving encryption algorithm to the string to produce an encrypted version of the string that complies with the given format; with the computing equipment, determining whether the encrypted version of the string to which the format preserving encryption algorithm has been applied satisfies given string constraints; and when the encrypted version of the string is determined to satisfy the given string constraints, halting further application of the format preserving encryption algorithm with the computing equipment and using the encrypted string as ciphertext corresponding to the plaintext; and when the encrypted version of the string is determined to not satisfy the given string constraints, applying the format preserving encryption algorithm to the encrypted version of the string with the computing equipment at least one additional time to alter the encrypted version of the string until the encrypted version of the string satisfies the given string constraints and serves as ciphertext corresponding to the plaintext.