CrackNotMe's CrackmesForBeginners (CFB) #10

Author:
CrackNotMe

Language:
C/C++

Upload:
2026-07-12 11:05

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Platform:
Windows

Difficulty:
3.0

Quality:
4.0

Arch:
x86-64

Downloads:
255

Size:
214.50 KB

Writeups:
0

Comments:
1

Description

This is the tenth challenge from the Crackmes for Beginners (CFB) series. We are stepping up to real-world asymmetric cryptography. The binary uses modern Cryptography Next Generation (CNG) to verify an RSA-1024 signature of your username against a hardcoded public key. Since factoring the RSA key is mathematically unfeasible, you must perform a Key Replacement Attack. Generate your own cryptographic key pair, patch the public key blob directly within the executable using a hex editor, and forge a valid digital signature.

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crackthatbih on 2026-07-15 00:56: [Click to reveal]The Setup CFB10.exe is a crackme that asks for a Username and a Digital Signature (Hex), then checks whether the hex you typed is a valid RSA-1024/PKCS#1 signature (SHA-256 of your username) against an embedded public key. Real crypto, correctly implemented. Step 1 — Find the verification function Listed all functions, found calls to BCryptVerifySignature, traced it to FUN_1400028a0 (renamed VerifyRsaSignature), called from FUN_1400036d0 (renamed CFB10_MainRoutine). VerifyRsaSignature does: Imports an embedded 148-byte CAPIPUBLICBLOB (public key) from address 0x140022420 SHA-256 hashes your username Calls BCryptVerifySignature(...) on your hex-decoded signature Returns 1 only if the signature is genuinely valid, else 0 Step 2 — Check if the key itself is weak Dumped the raw 148 bytes at 0x140022420 using a Ghidra Java script: javaAddress addr = toAddr("140022420"); byte[] data = getBytes(addr, 0x94); StringBuilder sb = new StringBuilder(); for (byte b : data) sb.append(String.format("%02x ", b)); println(sb.toString()); Parsed it: exponent e = 65537 (standard), modulus is a real 1024-bit number. Ran primality/perfect-power/small-factor/Fermat/ECM checks on it — no weakness found. Forging a real signature isn't feasible. This confirmed the crackme's own taunt ("RSA math is unbreakable. Can you cheat it?") is a hint to patch, not to break the math. Step 3 — Find the exact branch to patch Disassembled CFB10_MainRoutine and found the decision point right after the call to VerifyRsaSignature: 140003e4e: CALL VerifyRsaSignature 140003e5a: TEST AL,AL ; AL = 1 if valid signature, 0 if invalid 140003e5c: JZ 0x140003e7d ; <-- if AL==0, jump to "ACCESS DENIED" text ; (no jump = falls through to "ACCESS GRANTED") That single 2-byte instruction (74 1f) is the entire security check. Step 4 — The exact patch In Ghidra's Listing view, at address 140003e5c: Right-click the JZ 0x140003e7d instruction → Patch Instruction (Ctrl+Shift+G) In the mnemonic field, replace JZ with NOP, leave the operand field empty, press Enter Result: 140003e5c becomes 48 90 NOP (a 2-byte NOP that exactly fills the old JZ's space) This means execution now always falls through to the "ACCESS GRANTED" branch — the return value of VerifyRsaSignature is checked but never acted on. Step 5 — Export the patched exe File → Export Program... Format: Original File (preserves the real PE structure, unlike raw "Binary" export) Saved to C:\Users\ajcam\CFB10.exe Result Running that file with any username and any even-length hex string (e.g. test / 00) prints: [+] ACCESS GRANTED! Congratulations! You have successfully solved CFB10! You are a true Keymaster! Summary of the one change that mattered: at file address corresponding to VA 0x140003e5c, bytes 74 1F → 90 90 (or 48 90 as Ghidra encoded it) — turning JZ 0x140003e7d into a no-op.

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CrackNotMe's CrackmesForBeginners (CFB) #10