mit-app-inventor

CaesarVigenereCipherTemplate

About this app

CaesarVigenereCipherTemplate app is an enhancement to the APCSP Mobile 6.5 Caesar cipher app that includes a Vigenère cipher. Enter text in the Plaintext or Ciphertext fields and use the buttons to encrypt or decrypt. The Caesar ListPicker selects the shift. The Vigenère TextBox sets the key.

Code

This is a template app, with stubs for caesar and vigenere — the actual encrypt / decrypt procedures — as well as shift.

• Abstracting the shift procedure so that it can be called with positive or negative shifts enables it to be used for encrypting and decrypting for both the Caesar cipher and the Vigenère cipher. An approach to implementing a signed shift is to use modulo — however, the modulo of (and remainder of) operation(s) yield results on [0, n - 1], which is not suitable for accessing a list of length n, whose indexes are on [1, n]. shift has…

  Preconditions:

  • letter is of length 1
  • letter is in alphabet

• caesar shifts the cleaned text by a fixed amount (shift), positively or negatively. alphabet establishes the lexicographical order for the letters. caesar is an abstraction that can be called from both the Encrypt and Decrypt Buttons and is an example of the accumulator pattern.
• vigenere shifts the cleaned text by an amount corresponding to the letters of key, positively (if direction is 1) or negatively (if direction is -1). alphabet establishes the lexicographical order for the letters. vigenere is an abstraction that can be called from both the Encrypt and Decrypt Buttons and is an example of the accumulator pattern.

As is usual with APCSP Mobile template apps, there is an existing UX and some pre-written procedures provided. The clean procedure is an example of the accumulator pattern. It returns text with all characters not in alphabet removed.

Designer

All components retain their default properties — except initial Text for the ListPicker is 0 on reset and Width and Height are set to Fill parent... where necessary to center UX components. None of the TextBox components are changed on reset.

Credits

• Mobile CSP is the MIT App Inventor-based curriculum for Advanced Placement® Computer Science Principles.

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