Working with Byte string in Python

Question

I have the following code taken from Pycrypto documentation for my python 2.7 . Here goes the code.

from Crypto.Cipher import AES
from Crypto import Random
key = b'Sixteen byte key'
iv = Random.new().read(AES.block_size)
cipher = AES.new(key, AES.MODE_CFB, iv)
msg = iv + cipher.encrypt(b'Attack at dawn')
print msg
msg2 = cipher.decrypt(msg)
print str(msg2)

I tried to get the output of the msg in Sublime console. All i got is :

[Decode error - output not utf-8]
[Decode error - output not utf-8]
[Finished in 0.2s]

What I'm trying to do is encrypt the message text and view its encryption text in Sublime output and decode the message and view the original text. I tried to use msg.encode('utf-8') but fails giving error like:

UnicodeDecodeError: 'ascii' codec can't decode byte 0x8a in position 5: ordinal not in range(128)

How do i get the texts? Any help.

The encrypted text is probably not an ASCII or unicode string. You should format it in a different way, such as with repr() or in hexadecimal, base64, etc. — Dan Getz
– Dan Getz, Commented Jan 22, 2014 at 15:31

user395760 · Accepted Answer · 2014-01-22 15:34:12Z

3

The result of the encryption are arbitrary bits and bytes. You can't view them as text, they don't represent text in any encoding.

You can, however, represent these bytes in text. For example, using hexadecimal or base64. You could also use repr, though this could be misleading: Bytes that happen to match the bytes assigned by ASCII to printable characters will be shown as these characters, even though the encrypted bytes are not in any way an ASCII text.

answered Jan 22, 2014 at 15:34

user395760

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knitti · Accepted Answer · 2014-01-22 15:42:49Z

0

Both msg and msg2 are arbitrary byte strings. Trying to en- or decode these from/to UTF-8 will only succeed when they only contain ASCII bytes.

Additionally the docs for pycrypto say for AESCipher.decrypt():

For MODE_CFB, ciphertext length (in bytes) must be a multiple of segment_size/8.

I can't comment on this (no crypto expert), but in your example ciphertext is 30 bytes.

answered Jan 22, 2014 at 15:42

knitti

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