Comment big-endian format, the unpacking operator, encryptor creation and encryption call

mega/utils.py

@@ -8,25 +8,31 @@
 
 def a32_to_str(a):
     return struct.pack('>%dI' % len(a), *a)
+# > , my guess is that choosing the big-endian and not letting it be native is aiming at making the code independent of the platform, in other words independent of the endianness of the platform: to be checked with @juanriaza https://docs.python.org/3.6/library/struct.html?highlight=struct#byte-order-size-and-alignment
+# https://docs.python.org/2/tutorial/controlflow.html#unpacking-argument-lists just to explain the use of '*' in front of 'a'
 
 
 def aes_cbc_encrypt(data, key):
     encryptor = AES.new(key, AES.MODE_CBC, '\0' * 16)
+    # choice of CBC mode is justified on https://mega.nz/doc 12.10 "A chunk MAC is computed as follows (this is essentially CBC-MAC, which was chosen instead of the more efficient OCB over intellectual property concerns)"
+    # the last argument, the initiatlisation vector is optional http://pythonhosted.org/pycrypto/Crypto.Cipher.AES-module.html#new
     return encryptor.encrypt(data)
+    # length data must be a multiple of 16 http://pythonhosted.org/pycrypto/Crypto.Cipher.blockalgo.BlockAlgo-class.html#encrypt
 
 
 def aes_cbc_encrypt_a32(data, key):
     return str_to_a32(aes_cbc_encrypt(a32_to_str(data), a32_to_str(key)))
 
 
 def str_to_a32(b):
-    if len(b) % 4:  # Add padding, we need a string with a length multiple of 4
+    if len(b) % 4:  # Add padding, we need a string with a length multiple of 4 because the size of an unsigned integer as indicated in the call of the unpack method below is 4 bytes https://docs.python.org/3.6/library/struct.html#format-characters
         b += '\0' * (4 - len(b) % 4)
     return struct.unpack('>%dI' % (len(b) / 4), b)
 
 
 def mpi2int(s):
     return int(binascii.hexlify(s[2:]), 16)
+# returns the string s, truncated of its 2 first caracters as an integer https://docs.python.org/3.6/library/functions.html?highlight=int#int
 
 
 def aes_cbc_decrypt(data, key):