Monoalphabetic Cipher and Inverse Written in Python

This entry is part 6 of 7 in the series Computer Network Security

Here is my implementation of a Monoalphabetic Cipher written with a python dictionary:
monoalpha = {
    'a': 'm',
    'b': 'n',
    'c': 'b',
    'd': 'v',
    'e': 'c',
    'f': 'x',
    'g': 'z',
    'h': 'a',
    'i': 's',
    'j': 'd',
    'k': 'f',
    'l': 'g',
    'm': 'h',
    'n': 'j',
    'o': 'k',
    'p': 'l',
    'q': 'p',
    'r': 'o',
    's': 'i',
    't': 'u',
    'u': 'y',
    'v': 't',
    'w': 'r',
    'x': 'e',
    'y': 'w',
    'z': 'q',
    ' ': ' ',

inverse_monoalpha = {}
for key, value in monoalpha.iteritems():
    inverse_monoalpha[value] = key

message = "This is an easy problem"
encrypted_message = []
for letter in message:
    encrypted_message.append( monoalpha[letter.lower()] )

print ''.join( encrypted_message )
The encrypted output: uasi si mj cmiw lokngch

Now we may use the inverse cipher to decrypt a message, “rmij’u uamu xyj”

encrypted_message = "rmij'u uamu xyj"
decrypted_message = []
for letter in encrypted_message:
        decrypted_message.append( inverse_monoalpha[letter] )
    except KeyError:
        decrypted_message.append( letter )

print ''.join( decrypted_message )
Decrypted message: wasn't that fun
Series NavigationWhy does a Hash provide better message integrity then an Internet checksum?Block cipher lab

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