Before staring , I’m assuming you have searched about these two questions at some point of time :
I’m sure you’ve gone through some theory part while reading the answers of those so let’s move to the fun part rather than the usual theory here!
Have you heard the story of Doge, Isabelle and Karen? It’s something like this…
One day Isabelle sent a message to Doge asking him for some financial help. She clearly mentioned the amount to be $500 and she also mentioned that she would return the amount as soon as possible.
After recieving the message Doge was shocked to see the amount to be $5,000,000.
Who changed the amount and how? Yes you're right! It was Karen. It's not a big deal to eavesdrop or use any other trick to know
the messages as they didn't use secure way to communicate. Here comes the use of Cryptography. Let's continue the story.
Now that they know about the importance of secure communication, they decided to encrypt their messages while sending and decrypt them while recieving. They have one key each for doing this job.
Doge asks Isabelle for some business papers and mentions the time to be exact 5 PM. Isabelle recieves the message and it was writted that she should meet him at 4 PM.
Who did this now? It's Karen again! She somehow managed to break the encryption and read the message and change it.
Here comes the use of strong and secure protocols so that we have an edge over the adversaries.
The difficulty level of the challenges in a good CTF varies from complete beginner to hard/insane which touches lots of domains in this category.
My advice for complete beginner would be:
There are many available tools to make your life easy while solving CTF Challenges. Some basics tools are:
pycryptodome
labmath
numpy
sympy
gmpy or gmpy2
( install these first libgmp3-dev libmpc-dev
)pwntools
(optional - for remote interaction only)Now, let’s jump to some basics and get our hands dirty with the implementation of everything we learn. I’ll be using python3
for most of them but you can choose anything you prefer.
Some theory part: https://en.wikipedia.org/wiki/Radix
Python3 implementations:
>>> n = 123 # decimal form
>>> bin(n) # decimal to binary
'0b1111011'
>>> int('1111011', base=2) # binary to decimal
123
>>> hex(n) # decimal to hexadecimal
'0x7b'
>>> int('0x7b', base=16) # hexadecimal to decimal
123
>>> oct(n) # decimal to octal
'0o173'
>>> int('0o173',8) # octal to decimal
123
>>>
>>> # Some miscellaneous handy tricks
>>>
>>> a = int.from_bytes(b'test', 'big') # bytes to long integers
>>> a
1952805748
>>> a.to_bytes((a.bit_length()+7)//8, 'big') # long integers to bytes
b'test'
>>> bytes.fromhex(hex(a)[2:]) # does the same
b'test'
Along with this, sometimes we might have to work with other base systems. Let’s take a look at them.
Python3 implementations:
>>> import base64
>>> base64.b64encode(b'secret in base64')
b'c2VjcmV0IGluIGJhc2U2NA=='
>>> base64.b64decode(b'c2VjcmV0IGluIGJhc2U2NA==')
b'secret in base64'
>>> base64.b32encode(b'secret in base32')
b'ONSWG4TFOQQGS3RAMJQXGZJTGI======'
>>> base64.b32decode(b'ONSWG4TFOQQGS3RAMJQXGZJTGI======')
b'secret in base32'
>>> base64.b85encode(b'secret in base85')
b'b7f<4Wpp5EZXjY|b7eR+'
>>> base64.b85decode(b'b7f<4Wpp5EZXjY|b7eR+')
b'secret in base85'
PS There are many other base systems we rarely use.
Thanks for reading! Feel free to reach out on any of the social platforms given below.
The next topic will be #Classical Cryptosystems
Stay tuned! Stay Safe!
Author - rey
(CTF Player @DarkArmy)
Discord - rey#7813
Twitter - https://twitter.com/r3yc0n1c
Github - https://github.com/r3yc0n1c