tl;dr

• Hash length extension to forge an impossible winning board.
• Need to perform a PCO in game.

tl;dr

• This challenge revolves around a hidden power_tower_mod function.
• To obtain the flag, we implement the function ourselves, apply Hensel lifting, collide a CMAC, decrypt an RSA ciphertext and finally solve a subset sum problem.

tl;dr

• ECDSA signing server with biased nonce
• Exploitation by modelling a EHNP instance and using z3 Solver for breaking Mersenne Twister

tl;dr

• Timing-based attack on the double-and-add algorithm to recover secret value d
• Predict pseudo-random value using the NSA backdoor on Dual_EC_DRBG

tl;dr

• CTR bit-flipping attack along with CRC recomputation

tl;dr

• use hash extension to bypass password check on login.
• use RSA properties to recover $q$ and get $p$ and therefore $N= p*q$.
• sign the message given, and get the flag.

tl;dr

• Multiplying the middle element of 3x3 matrix with 9 gets thesum of all elements.
• Adding the least element of 5x4 matrix with the larget and multiplying this result by 10 gets the sum of all elements.
• multiplying any row with 2000 and subtrating that number by 2 will get you the sum.
• Remaing two rows can be filled with the result of subtracting 9 from each digit of a corresponding row .

Official writeup of Crypto challenge “Tabula Recta” from InCTFi 2021

tl;dr

• Retrive first few bytes of table through Matrix calculations, then find which special b-exact key is used.

tl;dr

• Extract higher bits of secret using input manipulation
• Extract lower bits of secret using the highers bits and input manipulation

tl;dr

1. Find the co-relation between variables in the LFSR equation
   1. d == out (75%)
   2. a == b (75%)
   3. c^d == out (75%)
   4. (d!= out) => (c==1) always
2. Solve for the seed using 2000 output bits
3. Try out which among the four possible combinations decrypt the flag