Python Script for Encryption and Decryption of files

Question

i was going through one of the python recipes in active state and found the following code. I've come up with the following 4 questions. Will be very grateful for any guidance and explanations.

Q::Im unable to exactly figure out as to why is a "key-random seed" taken from the user at the first place in line "k=long(sys.argv[2])" ? especially since, this value 'k' doesnt seemed to be used in the later part of the code ?

Secondly, in the part of the code for encryption and decryption, what is the exact implication of the statement "bytearray[i]-random.randint(0,255)%256" ? does this mean that the unicode values of each character is shifted or displaced during encryption and re-shifted back to its original value when decrypted ?

Thirdly, does the "bytearray= map(ord, f1.read())" compute the unicode point values of every character in the file ?

Lastly, Since random numbers are used in the encryption and decryption, what factor guarantees that the decryption of the encrypted file will be accurate ? has the seed value "k" got anything to do with this ?

Below is the code that i'm studying.

Will greatly appreciate your guidance,Thanks in advance

# encdec.py

import sys
import random

if len(sys.argv) != 5:
    print "Usage: encdec.py e/d longintkey [path]filename1 [path]filename2"
    sys.exit()

k = long(sys.argv[2]) # key
random.seed(k)

f1 = open( sys.argv[3], "rb")
bytearr = map (ord, f1.read () )
f2 = open( sys.argv[4], "wb" )

if sys.argv[1] == "e": # encryption

    for i in range(len(bytearr)):
        byt = (bytearr[i] + random.randint(0, 255)) % 256
        f2.write(chr(byt))


if sys.argv[1] == "d": # decryption

    for i in range(len(bytearr)):
        byt = ((bytearr[i] - random.randint(0, 255)) + 256 ) % 256
        f2.write(chr(byt))

f1.close()
f2.close()

eumiro · Accepted Answer · 2011-02-07 15:37:06Z

6

random.seed(k) will set the random number generator to some state, that it will deliver the same list of random numbers for the same k.

import random
print [random.randint(0,255) for i in xrange(5)]

returns different numbers each time I run the script.

import random
random.seed(2)
print [random.randint(0,255) for i in xrange(5)]

returns [244, 242, 14, 21, 213] each time I run the script.

That's why if you seed the random numbers generator with a specific value of k, it will give you a very specific list of random numbers which can be used to encode the given file and then decode it.

Second question - yes. It works with one byte at a time, so 256 different values.

Third question - yes, it computes the integer ordinal of a one-character string each time. It is the same like bytearr = [ ord(c) for c in f1.read() ] - read a character at a time and create a list of their values (0 - 255).

Lastly - See above.

And just a pythonic hint for the two loops... if you iterate over an array, you can use

for c in bytearr:

and then use c instead of

for i in range(len(bytearr)):

and using bytearr[i].

So, for example:

for c in bytearr:
    byt = (c + random.randint(0, 255)) % 256
    f2.write(chr(byt))

Your question: when i open the decrypted file using pico, why is that im seeing the strange characters ? shouldn't they be just some random alphabets and numbers ?

No. Look here. There are 2*26=52 letters, 10 digits, some more printable characters and some not printable characters (newline, tab,...). You encode/decode on the 'decimal' value (0-255) and your pico shows you the characters in the 'char' column.

edited Feb 7, 2011 at 15:37

answered Feb 7, 2011 at 15:13

eumiro

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3 Comments

DSM Over a year ago

+1 for speed! Only thing I'd add is that unicode has nothing to do with it: the file's opened in binary mode and processed as such.

Nura Over a year ago

Thanks eumiro ! that solves most of my doubts. Im still unclear about what does the "bytearr = map (ord, f1.read () )" actually do ?

Nura Over a year ago

Thanks @DSM !One more little thing..when i open the decrypted file using pico, why is that im seeing the strange characters ? shouldn't they be just some random alphabets and numbers ?

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