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The Caesar cipher is a simple substitution cipher that shifts each letter in the message by a fixed number of positions in the alphabet. This implementation supports the Spanish alphabet including the Ñ character.

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How it works

The Caesar cipher uses modular arithmetic to shift letters:
  1. Convert each letter to a number (A=0, B=1, …, Z=26)
  2. Add the shift value to each number
  3. Apply modulo 27 to wrap around the alphabet
  4. Convert numbers back to letters
This implementation uses 27 characters (A-Z plus Ñ) instead of the traditional 26-letter English alphabet.

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Algorithm explanation

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Encryption formula

E(x) = (x + shift) mod 27
Where:
  • x is the numeric position of the plaintext letter (0-26)
  • shift is the key (default: 8)
  • E(x) is the encrypted letter position

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Decryption formula

D(x) = (x - shift) mod 27

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Implementation details

The implementation is in sustitucion.py and consists of four main functions:

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Alphabet mapping

abecedario = {'A': 0, 'B': 1, 'C': 2, 'D': 3, 'E': 4, 'F': 5, 'G': 6, 'H': 7, 'I': 8, 'J': 9, 'K': 10, 'L': 11, 'M': 12,
              'N': 13, 'Ñ': 14, 'O': 15, 'P': 16, 'Q': 17, 'R': 18, 'S': 19, 'T': 20, 'U': 21, 'V': 22, 'W': 23, 'X': 24, 'Y': 25, 'Z': 26}

abecedario_inverso = {valor: clave for clave, valor in abecedario.items()}

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Core functions

def convertiraNumero(mensaje):
    cifrado = []
    for letra in mensaje.upper():
        if letra in abecedario:
            cifrado.append(abecedario[letra])
    return cifrado

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Usage example

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Basic encryption and decryption

from sustitucion import convertiraNumero, cifrar, descifrar, convertiraLetra

# Original message
mensaje = "HOLA"

# Shift value (key)
desplazamiento = 8

# Encryption process
numeros = convertiraNumero(mensaje)  # [7, 15, 11, 0]
cifrado = cifrar(numeros, desplazamiento)  # [15, 23, 19, 8]
mensaje_cifrado = convertiraLetra(cifrado)  # "OWSI"

print(f"Original: {mensaje}")
print(f"Encrypted: {mensaje_cifrado}")

# Decryption process
numeros_cifrados = convertiraNumero(mensaje_cifrado)
descifrado = descifrar(numeros_cifrados, desplazamiento)
mensaje_descifrado = convertiraLetra(descifrado)

print(f"Decrypted: {mensaje_descifrado}")  # "HOLA"

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Step-by-step example

1

Convert message to numbers

mensaje = "HOLA"
numeros = convertiraNumero(mensaje)
# Result: [7, 15, 11, 0]
# H=7, O=15, L=11, A=0
2

Apply shift encryption

desplazamiento = 8
cifrado = cifrar(numeros, desplazamiento)
# Result: [15, 23, 19, 8]
# (7+8)%27=15, (15+8)%27=23, (11+8)%27=19, (0+8)%27=8
3

Convert back to letters

mensaje_cifrado = convertiraLetra(cifrado)
# Result: "OWSI"
# 15=O, 23=W, 19=S, 8=I

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Parameters

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mensaje
string
required
The plaintext message to encrypt. Only letters A-Z and Ñ are processed; other characters are ignored.
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pos
int
default:"8"
The shift value (key). Can be any integer from 0 to 26. Values outside this range work but are equivalent to pos % 27.

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Key characteristics

Total possible keys

Only 27 different keys (0-26)

Character handling

Converts to uppercase and ignores non-alphabetic characters

Alphabet size

27 characters including Ñ

Encryption speed

O(n) linear time complexity

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Security notes

The Caesar cipher is extremely insecure and trivial to break. Never use it for protecting real data.

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Vulnerabilities

  1. Brute force attack: With only 27 possible keys, an attacker can try all combinations in seconds
  2. Frequency analysis: Letter frequency patterns are preserved, making cryptanalysis easy
  3. Known-plaintext attack: If any part of the plaintext is known, the key is immediately revealed
  4. No key management: The shift value must be securely shared between parties

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Educational value

Despite its weaknesses, the Caesar cipher is excellent for learning:
  • Basic encryption principles
  • Modular arithmetic
  • Why simple substitution is insufficient
  • How frequency analysis works

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Example with Spanish characters

# Message with Ñ character
mensaje = "ESPAÑA"
numeros = convertiraNumero(mensaje)  # [4, 19, 16, 0, 14, 0]
# E=4, S=19, P=16, A=0, Ñ=14, A=0

# Encrypt with shift of 5
cifrado = cifrar(numeros.copy(), 5)  # [9, 24, 21, 5, 19, 5]
mensaje_cifrado = convertiraLetra(cifrado)  # "JYUFSE"

# Decrypt
descifrado = descifrar(cifrado.copy(), 5)  # [4, 19, 16, 0, 14, 0]
mensaje_original = convertiraLetra(descifrado)  # "ESPAÑA"

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Common use cases

  • Teaching basic cryptography concepts
  • Demonstrating substitution ciphers
  • Learning modular arithmetic
  • Understanding cipher weaknesses

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See also

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