Show page

Differences

This shows you the differences between two versions of the page.

--- sasc:laboratoare:01 [2015/02/24 23:28]
127.0.0.1 external edit
+++ sasc:laboratoare:01 [2017/02/21 11:28] (current)
dan.dragan
@@ Line 1: / Line 1: @@
-===== Laboratorul 01. =====
+===== Lab 01 - Introduction =====
+==== Python Crash Course ====
+=== Python Interactive Mode ===
+Python is an easy to learn interactive programming language. Throughout this lab, we'll be using Python 2.7.
+You can run Python code in two ways:
+  * by writing a source file and running it (eventually, with some command line arguments)
+  * by entering commands in interactive mode
+To start interactive mode, simply open a terminal window and run ''python'':
+<code>
+linux$ python
+</code>
+You can now write Python code just as you would in an actual program:
+<code>
+>>> print "Hello, SASC"
+</code>
+To quit interactive mode either press ''Ctrl+D'' or use:
+<code>
+>>> quit()
+</code>
+Interactive mode is useful when you want to quickly try something out (for example, testing decryption for a certain key) without going through the hassle of writing a new source file and running it.
+=== Python Recipe #1 - Hail, Caesar! ===
+In Python, indentation is important; it is used to organize code in blocks. For example, the following C code:
+<code C>
+int foo(int a, int b) {
+    int x;
+    if (a > b) {
+        x = a;
+    } else {
+        x = b;
+    }
+    return x
+}
+</code>
+is (approximately) the same as the following in Python:
+<code Python>
+def foo(a, b):
+    if a > b:
+        x = a
+    else:
+        x = b
+    return x
+</code>
+In this first recipe we'll see how to encrypt a single letter using Caesar's cipher.
+<code Python>
+alphabet='ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+def caesar_enc(letter):
+  if letter < 'A' or letter > 'Z':
+    print 'Invalid letter'
+    return
+  else:
+    return alphabet[(ord(letter) - ord('A') + 3) % len(alphabet)]
+</code>
+Create a new file named caesar.py containing the code above. To test the code, open the interpreter and try the following:
+<code python>
+linux$ python
+>>> from caesar import *
+>>> print alphabet
+>>> alphabet[0]
+>>> ord('A')
+>>> len(alphabet)
+>>> ord('D') - ord('A')
+>>> 28 % 26
+>>> -1 % 26
+>>> caesar_enc('D')
+>>> caesar_enc('Z')
+>>> caesar_enc('B')
+</code>
+=== Python Exercise #1 - Decrypting a letter ===
+Add a ''caesar_dec'' function to ''caesar.py'', which decrypts a single letter encrypted using Caesar's cipher.
+=== Python Recipe #2 - Encrypting a string ===
+We'll now expand our function to take string as input. First, a little intro to Python ''for'' loops. Some examples:
+<code python>
+for i in [1, 2, 3]:
+    print i
+</code>
+In this first example, ''i'' iterates through the values in list ''[1, 2, 3]''. The code is similar to the classical **C** code:
+<code C>
+for (int i = 1; i <= 3; i++) {
+    printf("%d\n", i);
+}
+</code>
+To create lists of integers use the ''range'' function:
+<code python>
+for i in range(3):
+    print i
+</code>
+Execute the code above in the interpreter. Using the range function with two parameters, change the above to print integers 1 through 10.
+In Python, ''for'' can iterate through multiple types of objects, including strings. Try the below:
+<code python>
+for letter in 'ABCD':
+    print letter
+</code>
+We will now use the ''for'' instruction to expand our ''caesar_enc'' function:
+<code Python>
+alphabet='ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+def caesar_enc_string(plaintext):
+    ciphertext = ''
+    for letter in plaintext:
+        ciphertext = ciphertext + caesar_enc(letter)
+    return ciphertext
+</code>
+Test the above by starting a new interpreter; this time, we'll skip the ''from caesar import *'' part by running the source file before starting interactive mode:
+<code Python>
+linux$ python -i caesar.py
+>>> test = 'HELLO'
+>>> test + 'WORLD'
+>>> caesar_enc_string(test)
+</code>
+Another way to run things, which can be very useful in general is to use a main() function and write your program script as follows:
+<code Python 'test_caesar.py'>
+import sys
+import random
+import string
+import operator
+alphabet='ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+def caesar_enc(letter):
+  if letter < 'A' or letter > 'Z':
+    print 'Invalid letter'
+    return
+  else:
+    return alphabet[(ord(letter) - ord('A') + 3) % len(alphabet)]
+def caesar_enc_string(plaintext):
+    ciphertext = ''
+    for letter in plaintext:
+        ciphertext = ciphertext + caesar_enc(letter)
+    return ciphertext
+def main():
+  m = 'BINEATIVENIT'
+  c = caesar_enc_string(m)
+  print c
+if __name__ == "__main__":
+  main()
+</code>
+Then you can simply run the program, or type the following in a terminal:
+<code Python>
+python test_caesar.py
+</code>
+=== Python Exercise #2 - Decrypting a string ===
+Add the corresponding ''caesar_dec_string'' function.
+=== Python Recipe #3 - Shift ciphers ===
+Python allows passing default values to parameters:
+<code Python>
+def foo(a, b = 3):
+    print a, b
+</code>
+<code Python>
+>>> foo(1)
+>>> foo(1, 2)
+</code>
+We can use default parameter values to expand our ''caesar_enc'' function to take the key as an additional parameter, without breaking compatibility with our previous code.
+<code Python>
+def caesar_enc(letter, k = 3):
+    if letter < 'A' or letter > 'Z':
+        print 'Invalid letter'
+        return None
+    else:
+        return alphabet[(ord(letter) - ord('A') + k) % len(alphabet)]
+def caesar_enc_string(plaintext, k = 3):
+    ciphertext = ''
+    for letter in plaintext:
+        ciphertext = ciphertext + caesar_enc(letter, k)
+    return ciphertext
+</code>
+To test the new functions, try the below:
+<code Python>
+linux$ python -i caesar.py
+>>> caesar_enc_string('HELLO')
+>>> caesar_enc_string('HELLO', 0)
+>>> caesar_enc_string('HELLO', 1)
+</code>
+=== Python Exercise #3 - Shift ciphers ===
+Using default parameters, expand your shift cipher decryption functions to support arbitrary keys.
+=== Python Recipe #4 - Format conversions ===
+<code Python>
+import binascii
+def strxor(a, b): # xor two strings (trims the longer input)
+  return "".join([chr(ord(x) ^ ord(y)) for (x, y) in zip(a, b)])
+def hexxor(a, b): # xor two hex strings (trims the longer input)
+  ha = a.decode('hex')
+  hb = b.decode('hex')
+  return "".join([chr(ord(x) ^ ord(y)).encode('hex') for (x, y) in zip(ha, hb)])
+def bitxor(a, b): # xor two bit strings (trims the longer input)
+  return "".join([str(int(x)^int(y)) for (x, y) in zip(a, b)])
+def str2bin(ss):
+  """
+    Transform a string (e.g. 'Hello') into a string of bits
+  """
+  bs = ''
+  for c in ss:
+    bs = bs + bin(ord(c))[2:].zfill(8)
+  return bs
+def hex2bin(hs):
+  """
+    Transform a hex string (e.g. 'a2') into a string of bits (e.g.10100010)
+  """
+  bs = ''
+  for c in hs:
+    bs = bs + bin(int(c,16))[2:].zfill(4)
+  return bs
+def bin2hex(bs):
+  """
+    Transform a bit string into a hex string
+  """
+  return hex(int(bs,2))[2:-1]
+def byte2bin(bval):
+  """
+    Transform a byte (8-bit) value into a bitstring
+  """
+  return bin(bval)[2:].zfill(8)
+def str2int(ss):
+  """
+    Transform a string (e.g. 'Hello') into a (long) integer by converting
+    first to a bistream
+  """
+  bs = str2bin(ss)
+  li = int(bs, 2)
+  return li
+def int2hexstring(bval):
+  """
+    Transform an int value into a hexstring (even number of characters)
+  """
+  hs = hex(bval)[2:]
+  lh = len(hs)
+  return hs.zfill(lh + lh%2)
+def bin2str(bs):
+  """
+    Transform a binary srting into an ASCII string
+  """
+  n = int(bs, 2)
+  return binascii.unhexlify('%x' % n)
+</code>
+=== Python Exercise #4 - Format conversions ===
+Find the plaintext messages for the following ciphertexts knowing that the cipher is the XOR operation (ciphertext = plaintext XOR key) and the key is "abcdefghijkl".
+<code>
+C1 = "000100010001000000001100000000110001011100000111000010100000100100011101000001010001100100000101"
+C2 = "02030F07100A061C060B1909"
+</code>
+<hidden>
+The solution is {{:ic:laboratoare:lab1_sol.zip|here}}.
+</hidden>