def c15(s):
"""
This function attempts to check whether PKCS#7 padding is being used
properly in the given string
If the padding is wron, it raises an exception
"""
# if PKCS#7 padding were used then the last byte must indicate the same
pad_chr = s[-1]
try:
size = ord(pad_chr)
Flag = True
# the last size bytes must be equal to size for a valid PKCS#7 padding
# if not, we have wrong padding here
for i in range(size):
if ord(s[-i-1]) != size:
Flag = False
break
# raise exception if we don't have proper padding
if Flag == True:
print base.pkcs7_unpad(s)
print s[:-ord(s[-1])]
else:
raise Exception
except:
# raise exception if any of the operations throws an exception
print "exception!"
raise Exception
def c10(filename):
f = open(filename,"r")
data = f.readlines()
f.close()
pad = "\x04"
decoded_data = base.base64_to_hex(''.join(data).strip()).decode('hex')
block_size = 16
key = "YELLOW SUBMARINE"
iv = ''.join(["\x00" for i in range(block_size)])
blocks = [str(decoded_data[i*block_size : (i+1)*block_size]) for i in range(int(len(decoded_data)/block_size))]
op = []
size = len(blocks)
for i in range(size):
pt = base.AES_ECB_decrypt(blocks[i],key)
res = base.equal_size_xor(bytearray(pt),bytearray(iv))
res = base.bytearray_to_ASCII(res)
op.append(res)
iv = blocks[i]
plaintext = base.pkcs7_unpad(''.join(op))
print plaintext