rev - Entropy Discord

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./entropy-discord: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=3d97bd7da5c93a0d18f7f4c479dca11439d1ff7d, for GNU/Linux 3.2.0, stripped

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      *fd = &unk_47BE0;
      v29 = sub_AEC0;
      s1[0] = &off_528B8;
      s1[1] = &dword_0 + 2;
      v22 = fd;
      v23 = 1u;
      sub_45440(s1);
      byte_55961 = 0;
      signal(5, handler);
      __debugbreak();

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 33 87 08 B8 A2 F5 B8 04 74 D6 0B E7 AE 20 E6 33 E7 F3 5D A5 CD 54 02 28 4B FB E8 7D 23 23 0A 5B 2B 5D 20 46 4C 41 47 3A 20 5B 2B 5D 20 43 6F 6E 67 72 61 74 75 6C 61 74 69 6F 6E 73 21 20 59 6F 75 27 76 65 20 74 61 6D 65 64 20 74 68 65 20 65 6E 74 72 6F 70 79 21 0A 0A 5B 2D 5D 20 54 72 61 6E 73 66 6F 72 6D 61 74 69 6F 6E 20 63 68 65 63 6B 20 66 61 69 6C 65 64

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"""
Decrypt the embedded entropy blob from entropy-discord.
Only the first 30 bytes are encrypted with a Java-style LCG keystream.
Requires: python, z3-solver (pip install z3-solver).
"""

from z3 import *

# Full blob at 0x47ED2; first 30 bytes are ciphertext.
BLOB = bytes(
    [
        0x33,
        0x87,
        0x08,
        0xB8,
        0xA2,
        0xF5,
        0xB8,
        0x04,
        0x74,
        0xD6,
        0x0B,
        0xE7,
        0xAE,
        0x20,
        0xE6,
        0x33,
        0xE7,
        0xF3,
        0x5D,
        0xA5,
        0xCD,
        0x54,
        0x02,
        0x28,
        0x4B,
        0xFB,
        0xE8,
        0x7D,
        0x23,
        0x23,
        0x0A,
        0x5B,
        0x2B,
        0x5D,
        0x20,
        0x46,
        0x4C,
        0x41,
        0x47,
        0x3A,
        0x20,
        0x5B,
        0x2B,
        0x5D,
        0x20,
        0x43,
        0x6F,
        0x6E,
        0x67,
        0x72,
        0x61,
        0x74,
        0x75,
        0x6C,
        0x61,
        0x74,
        0x69,
        0x6F,
        0x6E,
        0x73,
        0x21,
        0x20,
        0x59,
        0x6F,
        0x75,
        0x27,
        0x76,
        0x65,
        0x20,
        0x74,
        0x61,
        0x6D,
        0x65,
        0x64,
        0x20,
        0x74,
        0x68,
        0x65,
        0x20,
        0x65,
        0x6E,
        0x74,
        0x72,
        0x6F,
        0x70,
        0x79,
        0x21,
        0x0A,
        0x0A,
        0x5B,
        0x2D,
        0x5D,
        0x20,
        0x54,
        0x72,
        0x61,
        0x6E,
        0x73,
        0x66,
        0x6F,
        0x72,
        0x6D,
        0x61,
        0x74,
        0x69,
        0x6F,
        0x6E,
        0x20,
        0x63,
        0x68,
        0x65,
        0x63,
        0x6B,
        0x20,
        0x66,
        0x61,
        0x69,
        0x6C,
        0x65,
        0x64,
    ]
)

ENC_LEN = 30  # number of bytes actually encrypted

FLAG_PREFIXES = [b"pctf{", b"PCTF{"]

MOD = 1 << 48
MULT = 0x5DEECE66D
ADD = 0xB


def solve_seed():
    """
    Recover the initial 48-bit state with loose constraints:
    - Plaintext is printable.
    - Contains a flag prefix (case-insensitive pctf{) somewhere in the encrypted region.
    """
    s0 = BitVec("s0", 48)
    s = s0
    solver = Solver()
    plains = []
    for i, c in enumerate(BLOB[:ENC_LEN]):
        s = (s * MULT + ADD) & (MOD - 1)
        ks = Extract(7, 0, s >> 8)  # keystream byte
        p = BitVec(f"p_{i}", 8)
        solver.add(p == c ^ ks)
        plains.append(p)
    # printable constraint
    for p in plains:
        solver.add(p >= 0x20, p <= 0x7e)
    # require a flag prefix somewhere
    hits = []
    for pref in FLAG_PREFIXES:
        for off in range(0, ENC_LEN - len(pref) + 1):
            hits.append(
                And(
                    plains[off] == pref[0],
                    plains[off + 1] == pref[1],
                    plains[off + 2] == pref[2],
                    plains[off + 3] == pref[3],
                    plains[off + 4] == pref[4],
                )
            )
    solver.add(Or(*hits))
    if solver.check() != sat:
        raise RuntimeError("Seed not solved; adjust constraints.")
    model = solver.model()
    seed = model[s0].as_long()
    plain_bytes = [model[p].as_long() for p in plains]
    return seed, bytes(plain_bytes)


def decrypt(seed):
    s = seed
    out = []
    for c in BLOB[:ENC_LEN]:
        s = (s * MULT + ADD) & (MOD - 1)
        ks = (s >> 8) & 0xFF
        out.append(c ^ ks)
    return bytes(out) + BLOB[ENC_LEN:]


if __name__ == "__main__":
    seed, first_part = solve_seed()
    plaintext = decrypt(seed)
    print(f"[+] Seed: 0x{seed:012x}")
    print(f"[+] Decrypted (first {ENC_LEN} bytes): {first_part}")
    print("[+] Full decrypted message:")
    print(plaintext.decode("ascii", errors="replace"))

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[+] Seed: 0x000000002279
[+] Decrypted (first 30 bytes): b'PCTF{iTz_mY_puT3R--My_3nT40PY}'
[+] Full decrypted message:
PCTF{iTz_mY_puT3R--My_3nT40PY}
[+] FLAG: [+] Congratulations! You've tamed the entropy!

[-] Transformation check failed