Playfair — 6×6 Variant Cipher Decryption

Challenge

The server provides a randomly generated 36-character alphabet, encrypts a message using a 6×6 Playfair variant, and expects decryption. The plaintext doesn't follow standard picoCTF{...} format.

6×6 Playfair Decryption Rules

• Same Row: Shift each character LEFT (opposite of encryption)
• Same Column: Shift each character UP (opposite of encryption)
• Rectangle: Swap columns (same as encryption)

def decrypt_pair(pair, matrix):
    p1 = get_index(pair[0], matrix)
    p2 = get_index(pair[1], matrix)
    if p1[0] == p2[0]:  # Same row - shift left
        return matrix[p1[0]][(p1[1]-1) % 6] + \
               matrix[p2[0]][(p2[1]-1) % 6]
    elif p1[1] == p2[1]:  # Same column - shift up
        return matrix[(p1[0]-1) % 6][p1[1]] + \
               matrix[(p2[0]-1) % 6][p2[1]]
    else:  # Rectangle - swap columns
        return matrix[p1[0]][p2[1]] + matrix[p2[0]][p1[1]]

Key Insights

• 6×6 matrix expands keyspace to 36 characters (vs traditional 25)
• Padding character is the first character of the alphabet
• Modulo arithmetic handles wraparound for boundary indices
• Automated socket-based interaction required for server challenges

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