I am working on a program in Python that sort-of encrypts a message. I have converted the string (which will vary based on user input) to a list and have a number. I want to automatically change each letter in the list with the number. For example, if the number is three, every A changes into a D. I have a dictionary with the vales for letters, such as {"a" : 1}, {"b" : 2}, etc.
I can not seem to get how I could change the letters (without knowing what they are) and possibly in accordance with my dictionary.
What I have so far (dictionary is somewhere else):
def numtolist(n):
seedstring = str(n)
numlist = []
for digit in seedstring:
numlist.append(int(digit))
return numlist
currentnumber = seed^2
newmessage = str()
for letter in messageList:
numtolist(currentnumber)
num1 = numlist[0]
If the transform is as simple as an alphabetic shift you can simply do it by:
if original_character.isupper(): # Determine whether it is upper/lower case
base_a = 'A' # use 'A' as a base for uppercase
else: #
base_a = 'a' # use 'a' as a base for lowercase
orig_char_ascii = ord(original_character) # Get the original ascii value
orig_char_alpha_index = orig_char_ascii - ord(base_a) # Get the `0-25` alphabetic
# index of the character
shift_number = 3 # Set the amount to shift by
new_char_alpha_index = orig_char_alpha_index + shift_number # Add the shift value
new_char_alpha_index = new_char_alpha_index % 26 # Take the modulus to impose
# periodic boundary conditions
# i.e. if you had 'z' + 3
# 'z' + 3 -> 25 + 3 -> 28
# 28 % 26 = 2, 2 -> 'c'
new_char_ascii_index = ord(base_a) + new_char_alpha_index # scale back to the ascii
# value
new_char = chr(new_char_ascii_index)
The basic idea is that each character corresponds to an ascii number which can be gotten by ord (i.e. ord('a') = 97). The chr method reverses this: chr(97) = 'a'.
The method is briefly: you get the ascii value, scale to a 0-25 alphabetic range, add your shift, wrap values that overflow the alphabetic range, scale back to ascii, and get a character back via chr.
You can compact this method a lot, I was verbose in the name of education:
def shift_char(ch,shift):
if not ch.isalpha():
return ch # return non-alphabetic characters as is
is_upper = 'A' <= original_character <= 'Z'
if is_upper:
base_a = 'A'
else:
base_a = 'a'
return chr(ord(base_a) + (ord(ch)-ord(base_a)+shift)%26)
If you wanted to use the same random shift for a whole string you could split this into a couple methods:
def shift_char(ch,shift):
if not ch.isalpha():
return ch # return non-alphabetic characters as is
if ch.isupper():
base_a = 'A'
else:
base_a = 'a'
return chr(ord(base_a) + (ord(ch)-ord(base_a)+shift)%26)
def shift_string(s,shift):
return ''.join(shift_char(i,r) for i in s)
If you want to be able to decode it later you need a reversal method and to store the shift value:
def shift_char(ch,shift):
if not ch.isalpha():
return ch # return non-alphabetic characters as is
if ch.isupper():
base_a = 'A'
else:
base_a = 'a'
return chr(ord(base_a) + (ord(ch)-ord(base_a)+shift)%26)
def encode_string(s,shift):
return ''.join(shift_char(i,r) for i in s), shift # NOTE EXTRA return value
def decode_string(s,shift):
return ''.join(shift_char(i,-shift) for i in s)
# usage
s = 'Hello, StackOverflow!'
enc_s,shift = encode_string(s)
dec_s = decode_string(enc_s,shift)
If your shift is random you can just pass that as the argument:
import random
def shift_char(ch,shift):
if not ch.isalpha():
return ch # return non-alphabetic characters as is
if ch.isupper():
base_a = 'A'
else:
base_a = 'a'
return chr(ord(base_a) + (ord(ch)-ord(base_a)+shift)%26)
def encode_string(s,shift):
return ''.join(shift_char(i,shift) for i in s)
def decode_string(s,shift):
return ''.join(shift_char(i,-shift) for i in s)
# usage
s = 'Hello, StackOverflow!'
shift = random.randint(1,25)
enc_s = encode_string(s,shift)
dec_s = decode_string(enc_s,shift)
or rearrange the methods
import random
def shift_char(ch,shift):
if not ch.isalpha():
return ch # return non-alphabetic characters as is
if ch.isupper():
base_a = 'A'
else:
base_a = 'a'
return chr(ord(base_a) + (ord(ch)-ord(base_a)+shift)%26)
def encode_string(s):
shift = random.randint(1,25)
return ''.join(shift_char(i,shift) for i in s), shift
def decode_string(s,shift):
return ''.join(shift_char(i,-shift) for i in s)
# usage
s = 'Hello, StackOverflow!'
enc_s, shift = encode_string(s)
dec_s = decode_string(enc_s,shift)