CTF write-ups

CTF write-ups written by me. Mostly crypto!

2022

• RCTF 2022
  • Crypto 769 - S2DH - Writeup
    • Cracking SIDH using Castryck-Decru Attack
  • Crypto 645 - IS_THIS_LCG? - Writeup
    • Truncated LCG + EC-LCG with only 7 x-coordinate + Matrix LCG
• LakeCTF 2022
  • Crypto 416 - NeutronMail - Writeup
    • Decrypting PGP message with not so random public PGP keys, factoring with gcd.
• angstromCTF 2022
  • Crypto 110 - logloglog - Writeup
    • Pohlig Hellman Algorithm for groups of prime-power order.
• Plaid CTF 2022
  • Crypto 400 - choreography - Writeup
    • Complementation Slide + Twisted Slide Attack on generalized Feistel Structure.
• LINE CTF 2022
  • Crypto 138 - Baby crypto revisited - Writeup
    • Reduction to ECDSA biased nonce atack.
• zer0pts CTF 2022
  • Crypto 304 - Karen - Writeup
    • Solving Hidden Subset Sum Problem with Nguyen-Stern algorithm.

2021

• POC Cutting Edge CTF 2021
  • Crypto 994 - vpoly - Writeup
    • Solving discrete logarithm problem over finite field.
• InCTF 2021
  • Crypto 925 - Trouble With Pairs - Writeup
    • BLS Signature Forgery.
  • Reversing 100 - find_plut0 - Writeup
    • Simple reversing using z3.

2020

• KAPO 2020
  • Crypto 100 - fixed point revenge - Writeup
    • Affine structure of crc.
  • Crypto 100 - Baby Bubmi - Writeup
    • Extension of knapsack cipher.
  • Crypto 100 - Child Beubmi - Writeup
    • Coppersmith attack on multiprime RSA.
• Crypto CTF 2020
  • Crypto 316 - Fatima - Writeup
    • Solve tiny ECDLP and write inverse functions.
  • Crypto 285 - Complex to Hell - Writeup
    • Brute key matrix using flag oracle.
  • Crypto 142 - One Line Crypto - Writeup
    • Weak prime generation logic for textbook RSA.
  • Crypto 95 - Gambler - Writeup
    • Solve cubic equation over polynomial ring.
  • Crypto 90 - Three Ravens - Writeup
    • Small message with single factor leak.
  • Crypto 55 - Amsterdam - Writeup
    • Binomial coefficient based encoding.
  • Crypto 29 - Trailing Bits - Writeup
    • Trucated plaintext.
• Defenit CTF 2020
  • Crypto 201 - Double Message - Writeup
    • Coppersmith's short pad attack + Franklin-Reiter related message attack.
  • Misc 906 - Dodge - Writeup
    • Automated dodge solver.
  • Misc 298 - Minesweeper - Writeup
    • Automated minesweeper solver.
  • Misc 181 - QR Generator - Writeup
    • QRCode repl.
• Pwn2Win CTF 2020
  • Crypto 246 - Omni Crypto - Writeup
    • Coppersmith's attack on partial p exposure on RSA.
  • Crypto 115 - Androids Encryption - Writeup
    • Leak AES key on PCBC.
• DEF CON CTF Qualifier 2020
  • Crypto 130 - coooppersmith - Writeup
    • Factor n using bivariate coppersmith with leaked info.
  • Reversing 120 - ooobash - Writeup
    • Bash shell reversing.
• PlaidCTF 2020
  • Crypto 350 - MPKC - Writeup
    • Destroy multivariate quadratic polynomial based encyption scheme by linear algebra.
  • Crypto 250 - dyrpto - Writeup
    • Coppersmith's short pad attack + Franklin-Reiter related message attack.
• TAMUctf 2020
  • Crypto 338 - ETERNAL_GAME - Writeup
    • Hash length extension attack.
  • Web 262 - MENTALMATH - Writeup
    • Ajax code injection.
  • Web 122 - FILESTORAGE - Writeup
    • LFI + php session poisoning.
  • Web 50 - PASSWORD_EXTRACTION - Writeup
    • Blind SQL injection.
  • Misc 50 - INSTAGRAM - Writeup
    • JPEG marker recovery.
  • Misc 50 - BLIND - Writeup
    • Reverse shell.
• zer0pts CTF 2020
  • Crypto 636 - dirty laundry - Writeup
    • Paillier cryptosystem with Shamir's secret sharing.
  • Crypto 394 - diysig - Writeup
    • RSA with LSB oracle.
  • Crypto 260 - ROR - Writeup
    • Concat LSB for gaining ciphertext
  • Forensics 100 - Locked KitKat - Writeup
    • Bruteforce android pattern lock
• Pragyan CTF 2020
  • Crypto 150 - AskTheOracle - Writeup
    • Oracle Padding Attack.
• nullcon HackIM 2020
  • Crypto 419 - SecureLinearFunctionEvaluation - Writeup
    • Break Bellare-Micali OT by simple math.
• WhiteHat Grand Pix 06 Quals
  • Crypto 200 - Cryptography 01 - Writeup
    • Simple substitution cipher.

2019

• Christmas CTF 2019
  • Crypto 1000 - hide and seek - Writeup
    • Solving hidden number problem + Coppersmith's attack on partial p exposure on RSA.
  • Crypto 995 - christmas pockets - Writeup
    • Classical knapsack cryptosystem is vulnerable to low density attack.
• HITCON CTF 2019 Quals
  • Crypto 200 - Lost Modulus Again - Writeup
    • Recover n to decrypt the flag since d is given.
  • Crypto 200 - Very Simple Haskell - Writeup
    • Decrypt Naccache-Stern Knapsack problem by directly knowing the private key.
• KAPO 2019
  • Crypto 1 - Lenstra-Lenstra-Lovasz - Writeup
    • Recover dp by using Coppersmith's attack, and recover p.
  • Crypto 1 - Weak-Strong - Writeup
    • ROCA attack.
• CSAW CTF Qualification Round 2019
  • Crypto 400 - Fault Box - Writeup
    • Fault attack on RSA CRT with a slight twist.
  • Crypto 300 - SuperCurve - Writeup
    • Solving ECDLP when order is small.
  • Crypto 100 - count on me - Writeup
    • Misdesigned block cipher leads to information leakage.
• Affinity CTF 2019 Quals
  • Crypto 500 - Epic Poem - Writeup
    • XOR encryption, guessing based on flag format.
  • Crypto 350 - GolanG Heights - Writeup
    • Solve quartic equation to factor n on RSA.
• HackCon 2019
  • Crypto 467 - AgainAndAgainAndAgain - Writeup
    • Multiple encryption with Rabin cryptosystem.
• Crypto CTF 2019
  • Crypto 314 - NSA basement - Writeup
    • Factoring n using gcd, decryption with OAEP on multiprime condition.
  • Crypto 166 - Alone in the dark - Writeup
    • Solving pellian equation.
• ISITDTU CTF 2019 Quals
  • Crypto 304 - Chaos - Writeup
    • Simple substitution cipher.
  • Crypto 100 - Easy RSA 1 - Writeup
    • n and e have similar size, so apply Boneh-Durfee attack.
  • Crypto 919 - Easy RSA 2 - Writeup
    • Crack multiprime RSA with Fermat factorization.
  • Crypto 238 - Old Story - Writeup
    • Base64 encoding with guessing.
  • Crypto 395 - decrypt to me - Writeup
    • Weak prng.
• Facebook CTF 2019
  • Crypto 974 - netscream - Writeup
    • Dual_EC_DRBG with some reversing.
  • Crypto 919 - storagespace - Writeup
    • Order of curve is small, use sage's discrete_log() function.
• DEF CON CTF Qualifier 2019
  • Crypto 182 - tania - Writeup
    • DSA with LCG nonces are broken using LLL, or apply biased nonce attack.
• Harekaze CTF 2019
  • Crypto 200 - Now We Can Play!! - Writeup
    • Simple bruteforcing problem.
  • Crypto 100 - ONCE UPON A TIME - Writeup
    • Hill cipher with randomness and padding.
  • Crypto 200 - Show me your private key - Writeup
    • Order of elliptic curve defined over Zmod(n), where n is composite.

2018

• KAPO 2018
  • Reversing 50 - hdbt - Writeup
    • Calculating inverse over finite field.
  • Reversing 50 - ezbt - Writeup
    • Consecutive XOR for inversion.