Secure Data Storage: SQL Encryption Challenge

Data security is paramount in modern applications. This challenge asks you to implement a basic encryption mechanism within an SQL database to protect sensitive data at rest. You'll be designing and implementing a function that encrypts and decrypts data stored in a specific column, ensuring confidentiality and compliance with security best practices.

Problem Description

You are tasked with creating a SQL function that encrypts and decrypts string data within a database table. The function should accept a string as input and return its encrypted (or decrypted) version. The encryption should utilize a simple Caesar cipher with a configurable shift value. The function must be reversible; decryption should correctly restore the original string. The function should be designed to be easily integrated into SQL INSERT, UPDATE, and SELECT statements.

Key Requirements:

• Encryption Function: Takes a string and a shift value as input, returns the encrypted string.
• Decryption Function: Takes an encrypted string and the same shift value as input, returns the original string.
• Caesar Cipher: Implement a Caesar cipher where each character is shifted by a specified number of positions in the alphabet. Wrap around the alphabet (e.g., shifting 'z' by 1 should result in 'a'). Only alphabetic characters (a-z, A-Z) should be shifted; other characters (numbers, symbols, spaces) should remain unchanged.
• Shift Value: The shift value should be configurable and passed as a parameter to both the encryption and decryption functions.
• SQL Integration: The function should be usable within standard SQL queries.

Expected Behavior:

• The encryption function should consistently encrypt the same input string with the same shift value to the same encrypted string.
• The decryption function should consistently decrypt an encrypted string with the same shift value used for encryption back to the original string.
• Non-alphabetic characters should be preserved during both encryption and decryption.
• The function should handle empty strings gracefully (returning an empty string).

Edge Cases to Consider:

• Empty String: What should happen if the input string is empty?
• Shift Value of 0: What should happen if the shift value is 0?
• Large Shift Values: Consider how large shift values (e.g., greater than 26) should be handled (modulo arithmetic is appropriate).
• Non-ASCII Characters: While the prompt specifies a-z, A-Z, consider how the function might behave with other character sets. (For simplicity, assume only ASCII characters are used in this challenge).

Examples

Example 1:

Input: String = "Hello", Shift = 3
Output: "Khoor"
Explanation: Each letter in "Hello" is shifted forward by 3 positions in the alphabet. 'H' becomes 'K', 'e' becomes 'h', 'l' becomes 'o', and so on.

Example 2:

Input: String = "Khoor", Shift = 3
Output: "Hello"
Explanation: Each letter in "Khoor" is shifted backward by 3 positions in the alphabet, reversing the encryption from Example 1.

Example 3:

Input: String = "This is a test 123!", Shift = 5
Output: "Ymnx nx f yjxy 123!"
Explanation: Only the alphabetic characters are shifted. Spaces, numbers, and punctuation remain unchanged.

Constraints

• Shift Value: The shift value must be an integer between 1 and 25 (inclusive). Values outside this range should be handled appropriately (e.g., using modulo arithmetic).
• String Length: The input string can be up to 255 characters long.
• SQL Dialect: The solution should be reasonably portable across common SQL dialects (e.g., MySQL, PostgreSQL, SQL Server). Avoid dialect-specific functions where possible.
• Performance: The function should encrypt/decrypt strings in a reasonable amount of time (less than 100ms for a 255-character string).

Notes

• Consider using a CASE statement or similar conditional logic to handle different character types (alphabetic vs. non-alphabetic).
• Remember that SQL functions are often limited in their ability to handle complex logic. Keep the implementation as straightforward as possible.
• The Caesar cipher is a simple encryption method and is not suitable for production environments requiring strong security. This challenge is intended to demonstrate basic SQL function creation and data manipulation.
• Think about how to handle uppercase and lowercase letters consistently. You might choose to convert all letters to lowercase before shifting and then back to uppercase, or preserve the original case.
• Pseudocode Example (Conceptual):

FUNCTION encrypt(string, shift):
  encrypted_string = ""
  FOR EACH character IN string:
    IF character IS an uppercase letter:
      encrypted_char = CHAR( (ASCII(character) - ASCII('A') + shift) MOD 26 + ASCII('A'))
    ELSE IF character IS a lowercase letter:
      encrypted_char = CHAR( (ASCII(character) - ASCII('a') + shift) MOD 26 + ASCII('a'))
    ELSE:
      encrypted_char = character
    encrypted_string = encrypted_string + encrypted_char
  RETURN encrypted_string

FUNCTION decrypt(encrypted_string, shift):
  RETURN encrypt(encrypted_string, -shift) // Decryption is just encryption with the negative shift