Crypto on z3r0s

Crypto - aliens

Sun, 10 May 2026 00:00:00 +0000

Crypto Aliens Write-up Link to heading

Challenge Summary Link to heading

We are given a remote service and a local copy of the challenge logic in server.py.

The service asks for a message, applies a custom padding routine, appends a similarly padded flag, and then encrypts the result with AES-ECB.

At first glance this looks annoying rather than breakable, because:

Crypto - BabyEncryption

Sun, 10 May 2026 00:00:00 +0000

Baby encryption Link to heading

You are after an organised crime group which is responsible for the illegal weapon market in your country. As a secret agent, you have infiltrated the group enough to be included in meetings with clients. During the last negotiation, you found one of the confidential messages for the customer. It contains crucial information about the delivery. Do you think you can decrypt it?

Crypto - raining primes

Sun, 10 May 2026 00:00:00 +0000

Raining Primes Write-up Link to heading

Summary Link to heading

The service mixes three ideas:

Prime generation of the form p = a*r + b
A homomorphic-looking key update routine
RSA encryption of an AES-encrypted flag

The design breaks because the same hidden 640-bit prime r is reused everywhere:

every prime returned by option 1
both RSA primes
the update_key() routine

Once r is recovered, the rest of the scheme collapses:

Crypto - the last dance

Sun, 10 May 2026 00:00:00 +0000

To be accepted into the upper class of the Berford Empire, you had to attend the annual Cha-Cha Ball at the High Court. Little did you know that among the many aristocrats invited, you would find a burned enemy spy. Your goal quickly became to capture him, which you succeeded in doing after putting something in his drink. Many hours passed in your agency's interrogation room, and you eventually learned important information about the enemy agency's secret communications. Can you use what you learned to decrypt the rest of the messages?

Crypto - twisted entanglement

Sun, 10 May 2026 00:00:00 +0000

Twisted Entanglement Write-Up Link to heading

Target Link to heading

Host: 154.57.164.77:30486
Flag: HTB{Ek3rT_W4s_s000_b0R1nG_1N_1991_4nD_1_h4t3_Pr0b4b1l1Ty_s0_I_Us3_4_ECC_S33d!}

Vulnerabilities Link to heading

The challenge has two independent weaknesses that chain together cleanly.

1. Invalid-curve scalar multiplication Link to heading

Menu option 1 accepts an arbitrary user point and computes:

public_key = multiply(private_key, point, E)

There is no validation that the input point lies on the original secp256k1 curve. The code only uses a and p inside the EC formulas, so any point on any curve of the form: