An overview of CTFs I participated in during 2021 can be found here.
Check out the Archive to filter by tags!
What is this CTF category?
Most of modern cryptography deals with the encoding, encryption, and authentication of data traffic. Think of web browsing, bank transfers, communication applications, etc. Our first goal is to have our data encrypted such that only those with the appropriate keys can decrypt it. Secondly, we need to find a way for us to be sure an encrypted message indeed came from whom we think it came from and vice versa. As you can imagine, this proves to be easier said than done. During CTFs, you will be confronted with insecure algorithms proving exactly this. You will have to investigate and make sense of any provided source code, look for potential vulnerabilities, and finally exploit the algorithm to work your way to the flag.
What knowledge do I need right now to get started?
Do not be put off by the maths behind cryptography. Aside from high school maths, a quick introduction into modular arithmetic should be enough to get you started. When you are ready to dive further in, some number and group theory, linear algebra, and elliptic curve arithmetic will do wonders for you.
What about cryptographic knowledge? There are many exotic cryptosystems and even the most common ones have plenty of variants. Therefore it is probably best to learn about these along the way. However, almost all CTFs will include at least some challenges based on, but not limited to, the following cryptographic primitives:
 XOR ciphers, which use only bitwise and/or integer operations,
 RSA, a public key cryptosystem with many potential vulnerabilities,
 AES, a symmetric block cipher with various modes of operation,
 ECC, based on elliptic curve arithmetic with its commonly used signature scheme ECDSA.
The best way to hone your cryptography skills is through practice! Try your luck in beginnerfocused CTFs, look for archived challenges on CTFtime, or check out CryptoHack.
What tools do I need to be successful?
Pen, paper, and Python is really all you need, although some experience with Sage (a more mathfocused extension of Python) might prove useful. Usually all attacks and exploits can be written from scratch well within the duration of the CTF. No external tools are required, although the use of existing thirdparty scripts is generally allowed.
Recent Writeups
My Challenges

idek CTF 2022*  Psychophobia
Cryptography – 495 points (11 solves) – Chall author: Polymero (me)
“WANTED: CRYPTO PSYCHIC FOR SINGLE TIME HIRE! (URGENT)
My signatures are all broken and I need somebody to magically fix them ASAP!” 
UMassCTF 2022  HatMash
Cryptography – 500 pts (1 solve) – Chall author: Polymero (me)
What do you mean “We think you spend too much time with matrices.”? It’s just a hash function, jeez…

HackPack CTF 2022  Repeating Offense
Cryptography – 443 pts (20 solves) – Chall author: Polymero (me)
Onetime oracles using RSA or Paillier are not a great idea due to those slippery mathemagicians… I would like to see them slip their way through RSA AND Paillier! After all, you cannot rob two banks at the same time. … What?

HackPack CTF 2022  P(ai)^3
Cryptography – 469 pts (15 solves) – Chall author: Polymero (me)
Paiaiai… My Paillier scheme seems to be broken and I stored my favourite flag in it. Please help me get it back, will you? Who could have guessed this would ever happen? … Me… I I wrote it… yeah.

K3RN3LCTF 2021  Total Encryption
Cryptography – 500 pts (0 solves) – Chall author: Polymero (me)
“To store our most embarrassing secrets, we created a Remote Secure Armoury protected by layered RSA encryption with XOR blinding. Never again will my friends be able to mock me for my use of words!”

K3RN3LCTF 2021  Tick Tock
Cryptography – 496 pts (6 solves) – Chall author: Polymero (me)
“I chopped up my flag and hid it behind this simple key exchange. Try dlogging your way in if you are brave enough!”

K3RN3LCTF 2021  Shrine of the Sweating Buddha
Cryptography – 500 pts (0 solves) – Chall author: Polymero (me)
“Welcome to the Shrine of the Sweating Buddha. Share the burden of your worries, my child ~~~.”
Hint: share some (7) of your worries and perhaps your fortune will guide you to the flag.

K3RN3LCTF 2021  HADIOR
Cryptography – 499 pts (3 solves) – Chall author: Polymero (me)
“HADIOR will hold the DOOR.”

K3RN3LCTF 2021  Game of Secrets
Cryptography – 500 pts (2 solves) – Chall author: Polymero (me)
“John wants to play a game, a game of secrets. Recover his secret or be encrypted.”

K3RN3LCTF 2021  lightningrod
Reverse Engineering – 499 pts (3 solves) – Chall author: Polymero (me)
“Warning: Weather Control Device detected! ZAP ZAP [insert conscript_death.mp3 here]”
“Note: there is a typo in the flag, sorry >m<.”

K3RN3LCTF 2021  PolyProof
Cryptography – 490 pts (11 solves) – Chall author: Polymero (me)
They asked me to set up a zeroknowledge proof that runs in polynomial time. I don’t know what that means but I assume they want me to use polynomials, right?

K3RN3LCTF 2021  Poly Expo go BRRRRR
Cryptography – 494 pts (9 solves) – Chall author: Polymero (me)
“I’m going to say this again: I did not have sexual relations with that polynomial, Miss Polinsky.”

K3RN3LCTF 2021  Beecryption
Cryptography – 500 pts (2 solves) – Chall author: Polymero (me)
I was watching the bees and it seemed as if they were trying to tell me something… Have I finally gone crazy?!?

K3RN3LCTF 2021  Twizzty Buzzinezz
Cryptography – 100 pts (116 solves) – Chall author: Polymero (me)
“Some bees convinced me to invest in their new cryptosystem. They zzzaid their new XOR keyzztream would revolutionizzze the crypto market. However, they quickly buzzed away so all I have is this weird flyer they dropped. Luckily it has some source code on the back.”
“Have I just really been scammed by some bees??” 
K3RN3LCTF 2021  Objection!
Cryptography – 500 pts (2 solves) – Chall author: Polymero (me)
“Looks like Harry is hoarding his flags again… Maybe he will stop if we can convince him both Alice and Carlo dislike hoarding too. Alice and Carlo, being stereotypical CTF admins, are not responding to your complaints. Guess you will just have to answer for them… Luckily, I managed to secure you a channel to the domain controller of the CTF server.”

K3RN3LCTF 2021  NonSquare Freedom (1 and 2)
Cryptography – 465 pts (21 solves) and 490 pts (11 solves) – Chall author: Polymero (me)
“What can I say, I just like squares.”

K3RN3LCTF 2021  Cozzmic Dizzcovery
Cryptography – 499 pts (3 solves) – Chall author: Polymero (me)
“See that comb over there? It came from that meteorite I mentioned yesterday. Take a look at this, if I send bytes in, different bytes come out! Then there’s this button that seems to just produce random bytes… I’m absolutely stumped :S”

K3RN3LCTF 2021  Ain't no Mountain High Enough
Cryptography – 500 pts (1 solve) – Chall author: Polymero (me)
“Hills are easy to climb, but mountains? Hoho, they sure are something else!”

K3RN3LCTF 2021  18007586237
Cryptography – 437 pts (28 solves) – Chall author: Polymero (me)
“I NEED A PLUMBER ASAP, MY FLAG IS LEAKING ALL OVER THE PLACE!!!”