Abstract: A method and apparatus for digital cellular telephone encryption includes a switch when a choice between compatibility security and a higher security is required. Plaintext is converted into ciphertext using a three-stage transformation process. In a first stage, the plaintext is transformed using a secret key to control encrypting by an invertible transformation process. In a second stage, the output of the first stage is transformed by an involutory transformation, which is unkeyed when the compatibility switch is in a compatibility setting and keyed when the compatibility switch is in a secure setting. In a third stage, the output of the second stage is transformed by the inverse of the transformation performed in the first stage. The compatibility switch is used to toggle between compatibility with a less secure encryption standard and a key-based secure encryption.