TODO

• refactor everything into a proper class ✓
• better prime number generation ✓
• implement the following:
  • Setting: the alphabet will have 27 characters, the blank and the 26 letters of the English alphabet. ✓
  • Generates a public key and a private key. The public key will be randomly generated in the required interval. ✓
    • Plaintext message units are blocks of k letters, whereas ciphertext message units are blocks of k+1 letters. The plaintext is completed with blanks, when necessary. ✓
      • We must have 27^k < n < 27^k+1
  • Using the public key, encrypts a given plaintext. ✓
    • there will be a plaintext validation - check that all characters of initial message belong to the alphabet ✓
  • Using the private key, decrypts a given ciphertext. ✓
    • there will be a ciphertext validation - check if original string is the same as decrypted string ✓