Skip to content

Froggy CrackMe – Solution

Category: Reverse / CrackMe
Tools: IDA Pro, Python, Z3

Summary

  • Serial format: XXXXXXXX-XXXXXXXX-XXXXXXXX-XXXXXXXX (32 hex digits, 3 dashes).
  • Validation: Name and serial are combined via FNV-1a hashing and a splitmix64-style mix; a single final comparison decides success.

  • Elegant weakness: One conditional jump controls the outcome. Patching that jump makes the program accept any valid-format input.

  • Challenge archive: Download froggy_crackme.zip

  • Solver script: froggy_solver.py (view or download the keygen source)

1. Validation flow (from reversing)

  1. Anti-debug: Reads /proc/self/status, checks TracerPid:. If non-zero → "Debugger detected. The swamp rejects you."
  2. Input: Prompts for Name and Serial (format above).
  3. Serial parsing: 4 groups of 8 hex digits → two 64-bit values:
  4. v17 = (group1 << 32) | group2
  5. v18 = (group3 << 32) | group4
  6. Name hashes (FNV-1a, prime 0x100000001B3):
  7. Name → hash → v29 = hash ^ 0xA3B1957C4D2E1901
  8. Reversed name → hash → v47 = hash ^ 0xC0D0E0F112233445
  9. Empty name: v29 = 0xB7D49ACC3EB31A82, v47 = 0xD4B5EF4161BE37C6
  10. Final check: A chain of splitmix64-style mix and XOR/add uses v29, v47, v17, v18 and compares the result to 0xB9229933597558C9.
  11. In the binary: cmp rcx, rax at 0x2731 (rax = 0xB9229933597558C9), then a conditional jump (failure path).

2. Single path / elegant weakness

Success is decided by one comparison and one conditional jump:

  • 0x2731: cmp rcx, rax (computed value vs 0xB9229933597558C9)
  • 0x2734: conditional jump (e.g. jnz to "Wrong incantation" at 0x27ac)

If the jump is never taken, execution always reaches "The Swamp Gate opens."

Patch (elegant crack)

  • File: froggy_crackme (or your copy).
  • Location: the 2-byte instruction immediately after cmp rcx, rax (the conditional jump at 0x2734).
  • Original: conditional jump (e.g. 75 xx = jnz).
  • Patch: replace those 2 bytes with 90 90 (two NOPs).

Then:

  • Run without a debugger (or with TracerPid cleared).
  • Enter any non-empty name and any serial in the correct format, e.g.
    Name: Froggy
    Serial: 00000000-00000000-00000000-00000000

The program will always open the gate.

3. Finding a valid key (no patch) – Z3 keygen

To get a real name+serial pair that passes the check without patching we model the check in Z3 and solve for the four 32-bit serial parts.

The solver froggy_solver.py does the following:

  1. From the name: Compute FNV-1a hashes for name and reversed name, then apply the known XORs to get v29 and v47.
  2. fmix-like: Reconstruct the binary’s mix (without the final ^ (x >> 32) where applicable) for intermediate values; the final comparison uses final_mix(x) = fmix_like(x) ^ (fmix_like(x) >> 32) compared to TARGET.
  3. Serial as unknowns: The serial is 4 groups of 8 hex digits → four 32-bit values p0, p1, p2, p3 with:
  4. v17 = (p0 << 32) | p1
  5. v18 = (p2 << 32) | p3
  6. Formula: Build the rest of the binary’s formula (v54, v55, v56, v57, v58) from the decompiled logic and add the constraint final_mix(v58) == TARGET.
  7. Z3: Use bit-vector solver (QF_BV) with multiple random seeds and timeouts to find a satisfying assignment for p0..p3, then format the serial as XXXXXXXX-XXXXXXXX-XXXXXXXX-XXXXXXXX.

Usage

Download the solver: froggy_solver.py, then:

python3 froggy_solver.py

Enter a name when prompted (e.g. Froggy). The script prints a valid serial. It tries a 15s timeout first, then 60s if needed.

Dependencies: z3-solver (pip install z3-solver).

Relevant code (excerpt)

Constants are taken from the binary (TARGET, FNV init/prime, XOR constants, fmix P/Q). The core is:

  • fnv1a64(data) – FNV-1a 64-bit hash.
  • fmix_like(x) – mix used for v52, v53, v57 (no final xor with >>32).
  • final_mix(x) – value compared to TARGET: fmix_like(x) ^ (fmix_like(x) >> 32).
  • solve_for_name(name, per_seed_timeout_ms) – builds the Z3 model and returns a serial string or (None, None).

4. Quick reference

Item Value / Location
Target constant 0xB9229933597558C9
Comparison cmp rcx, rax at 0x2731
Patch Replace conditional jump at 0x2734 with 90 90
Success message "The Swamp Gate opens." / "Froggy approves."