346 lines
12 KiB
Markdown
346 lines
12 KiB
Markdown
# Synacor Challenge 2: Electric Boogaloo
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So we've finished the official parts of the challenge. What now?
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## Self-test
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Hex values refer to the instruction lines, not the actual ranges spanned by the data in memory
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* `0000` to `013e`: Startup message
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* `0140` to `01e3`: Tests `jmp`
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* From `0160` to `01c9`, some clever code is used to ‘amplify’ the effect of any error in `jmp` to allow the precise size of the error to be determined.
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* If successful, executes `0140`, `015b` to `0160`, then `0166`.
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* `01e4` to `01f2`: Tests `jt` and `jf`
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* If successful, jumps to `01f4`.
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* `01f4` to `0209`: Tests that the registers are initialised to zero
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* `020c` to `0215`: Tests `set`
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* `0218` to `0233`: Tests `add`
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* The test is rudimentary, however, and would not detect many simple errors. In fact it tests only if 1 + 1 ≠ 0.
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* `0234` to `024d`: Tests `eq`
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* Surprisingly, this is where it is checked that 1 + 1 = 2, but if `add` gives an incorrect non-zero result for 1 + 1, the test will erroneously report that it is `eq` which is not supported!
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* It would probably have been a better idea to test `eq` first, before `add`, then use `eq` to test `add`.
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* `024e` to `0261`: Tests `push` and `pop`
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* Since only `R0` and `R1` are checked, this would not detect errors involving other registers.
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* This test, like the last one, reuses the results of previous tests, since that worked out so well for `eq`…
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* `0264` to `0276`: Tests `gt`
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* The tests performed seem quite reasonable, but yet again reuse the results of previous tests…
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* `0279` to `02ab`: Tests `and` and `or`
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* Confusingly, the error handling is located in different places for each test.
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* `02ac` to `02bd`: Tests `not`
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* Okay, I admit this one was pretty helpful. What the hell is a ‘15-bit bitwise inverse’? Well the test passes if I do just do mod 32768, so that works I guess…
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* `02c0` to `02e8`: Tests `call`
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* Although, notably, not `ret`. The tests operates by `jmp`ing back and forth to test the various values of `R0`.
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* `02eb` to `0308`: Checks that `add` overflows correctly
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* `030b` to `0313`: Checks that 6 × 9 ≠ 42.
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* I suspect there is a mistake in this test. Since Adams (1979) demonstrated unequivocally that 6 × 9 is equal to 42, I believe the `jt` should in fact be a `jf`.
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* `0316` to `0346`: Continues checking `mult`
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* `0349` to `034b`: Two values `4e20` and `2710` are stored in memory here for reference by the following test
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* `034d` to `03d1`: Tests `rmem` and `wmem`
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* If successful, causes the words from `03a9` to `03ac` to instead read `nop`, `jt 0013 03d2`
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* There is more to the portion starting `0375` than meets the eye: see below.
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* `0432` to `05b1`: Various error messages
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## Decryption
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As we know from earlier, most of the strings in the binary are encrypted (or at the very least obfuscated) in some way, and decrypted following the self-test. It is therefore desirable to study this encryption before further study of the binary.
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After a wild goose chase examining the code after the self-test, we find that the decryption actually happens *during* the `rmem`/`wmem` test! Very sneaky!
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0375 call 06bb
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This is the magic line. Digging into the `06bb` subroutine:
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06bb push R0
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06bd push R1
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06bf set R1 17b4
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06c2 rmem R0 R1
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06c5 push R1
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06c7 mult R1 R1 R1
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06cb call 084d
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06cd set R1 4154
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06d0 call 084d
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06d2 pop R1
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06d4 wmem R1 R0
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06d7 add R1 R1 0001
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06db eq R0 7562 R1
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06df jf R0 06c2
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06e2 pop R1
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06e4 pop R0
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06e6 ret
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Inspecting the `084d` subroutine reveals that this is simply an XOR function: `R0 XOR R1`. Crypto rating: 1/10
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Rewriting `06bb` function using higher-level syntax reveals that the ‘encryption’ algorithm is really very simple:
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```c
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06bb() {
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R1 = 17b4;
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for (R1 = 17b4; R1 != 7562; R1++) {
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R0 = [R1];
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R0 ^= R1 * R1;
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R0 ^= 4154;
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[R1] = R0;
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}
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}
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```
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*Very* simple.
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[By emulating this function in Python](https://github.com/RunasSudo/synacor.py/blob/master/dbg_fastboot.py), we can skip the self-test and computationally-expensive decryption process entirely, and get straight into the good stuff next time we want to play!
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## Encrypted strings
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So earlier, we produced a tool-assisted speed-run that would complete and dump the codes for any given challenge binary, but where's the fun in that? Why not extract the codes from the binary directly? Of course, this is easier said than done. None of the codes, nor any of the strings relating to them, are visible in the disassembled binary, whether before or after the decryption from the previous section.
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Looking through the code following the self-test, we find:
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0413 set R0 17c0
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0416 call 05ee
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Digging deeper, `05ee` calls `05b2` with `R1` set to `05f8`. `05b2` appears to iterate over the characters in a string whose length is stored in address `R0`, and calls `R1` for each character, storing the character in `R0`. `05f8` (the callback provided by `05ee`) simply outputs every character in `R0` it gets.
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Immediately after this call to `05ee`, we find:
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041e set R0 68e3
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0421 set R1 05fb
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0424 add R2 XXXX XXXX
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0428 call 05b2
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In other words, a similar string-handling subroutine is called, but instead of `05f8` (which would simply print the string), `05fb` is called. `05fb` also outputs the character, but only after calling `084d` (XOR) with `R1` set to `R2`.
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Now we have everything we need to [extract these encrypted (double-encrypted??) strings](https://github.com/RunasSudo/synacor.py/blob/master/tools/decrypt_strings.py) from the binary!
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Only the self-test completion code appears to be stored there, though, so I'm not sure what the point of encrypting those was…
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## The codes
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We may not have the codes themselves, but we can now easily locate where they are printed. The tablet code, for example, is conveniently sandwiched between strings `6ed1` and `6ef1`. Thus the code we are looking for is:
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1290 set R0 1092
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1293 set R1 650a
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1296 set R2 7fff
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1299 set R3 6eed
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129c call 0731
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Looking into `0731`:
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0731 push R3
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0733 push R4
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0735 push R5
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0737 push R6
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0739 set R6 0001
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073c add R4 R3 R6
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0740 rmem R4 R4
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0743 add R5 17ed R6
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0747 wmem R5 R4
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074a add R6 R6 0001
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074e rmem R5 17ed
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0751 gt R4 R6 R5
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0755 jf R4 073c
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0758 set R3 0000
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075b set R4 0000
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075e rmem R5 17ed
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0761 mod R5 R4 R5
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0765 add R5 R5 17ed
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0769 add R5 R5 0001
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076d rmem R6 R5
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0770 mult R6 R6 1481
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0774 add R6 R6 3039
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0778 wmem R5 R6
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077b push R0
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077d push R1
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077f set R1 R6
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0782 call 084d
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0784 set R6 R0
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0787 pop R1
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0789 pop R0
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078b rmem R5 R1
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078e mod R6 R6 R5
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0792 add R6 R6 0001
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0796 gt R5 R6 R2
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079a jt R5 07a0
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079d set R3 0001
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07a0 add R6 R6 R1
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07a4 rmem R6 R6
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07a7 add R4 R4 0001
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07ab add R5 R4 17f1
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07af wmem R5 R6
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07b2 rmem R5 17f1
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07b5 eq R5 R4 R5
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07b9 jf R5 075e
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07bc jf R3 0758
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07bf push R0
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07c1 set R0 17f1
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07c4 call 05ee
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07c6 pop R0
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07c8 pop R6
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07ca pop R5
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07cc pop R4
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07ce pop R3
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07d0 ret
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Umm… Sorry, could you repeat that?
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Rewriting this again in more friendly terms:
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```c
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// R0: A seed of sorts - the same for all users
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// R1: The length and alphabet to use, usually 650a, but 653f for the mirror
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// R2: Usually 7fff, but 0004 for the mirror
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// R3: An initialisation vector of sorts - contents different for every user - points to the length, but this is always 3
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0731(R0, R1, R2, R3) {
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// copy the string at R3 to 17ed
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R6 = 0001;
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do {
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R4 = R3 + R6;
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R4 = [R4];
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R5 = 17ed + R6;
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[R5] = R4;
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R6 = R6 + 0001;
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R5 = [17ed]; // 3, the length - this never seems to change
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} while (R6 <= R5);
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// the string at 17ed is now what was at R3
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do {
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R3 = 0000; // done flag
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R4 = 0000; // index
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do {
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R5 = [17ed]; // 3, the length
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R5 = R4 % R5;
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R5 = R5 + 17ed;
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R5 = R5 + 0001; // will cycle through three addresses of 17ed/R3 string: 17ee, 17ef, 17f0
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R6 = [R5];
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R6 = R6 * 1481;
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R6 = R6 + 3039;
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[R5] = R6; // mutate that value of the 17ed string
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R6 = R0 ^ R6; // combine R0 into R6
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R5 = [R1]; // length of the alphabet
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R6 = R6 % R5;
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R6 = R6 + 0001; // calculate index in alphabet
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if (R6 <= R2) {
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R3 = 0001; // we are done with the entire code - this returns immediately for all except the mirror
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}
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R6 = R6 + R1; // calculate address of letter to use
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R6 = [R6]; // the letter to use
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R4 = R4 + 0001; // increment the index
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R5 = R4 + 17f1; // index of new letter in code
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[R5] = R6; // set the letter
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R5 = [17f1]; // length of the code: twelve letters
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} while (R4 != R5); // loop until we've generated all the letters
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} while (!R3);
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print(17f1);
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}
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```
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Re-implementing this in Python, we can now extract the code for the tablet directly from the binary!
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Unfortunately, each of the other codes uses an `R0` based on the solution to the puzzle. In the case of the maze code:
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0f03 rmem R0 0e8e
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The value at `0e8e` is derived from which rooms are visited in the maze, as mentioned in the main note file. Armed with our [trusty map](https://github.com/RunasSudo/synacor.py/blob/master/tools/graph.py), and cross-referencing the callbacks with the files, we identify:
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* Twisty passages entrance, `0949`: Calls `0e9e`, resets `0e8e` to `0000`.
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* West to `095d`: Calls `0ec0`: `OR`s `0e8e` with `0008`.
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* South to `0926`: Calls `0eca`: `OR`s `0e8e` with `0010`.
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* North to `096c`: Calls `0ede`: `OR`s `0e8e` with `0040`.
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Putting it all together, the final value at `0e8e` is `0008 | 0010 | 0040` = `0058`.
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Similarly, the `R0` for the teleporter code is the value of `R7` from the Ackermann function, which we determined earlier to be `6486`:
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1592 set R0 R7
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The remaining two codes, for the coins and the vault, are more complicated still, but follow the same pattern of determining `R0` based on the player's history.
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For the coins, the call to `0731` derives an `R0` from the values at memory locations `69de` to `69e2`. One would presume that this relates to the order in which coins are inserted into the puzzle, and following the trail of callbacks for the coins confirms this. The important lines are:
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13c0 rmem R2 099e # nr of coins inserted
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13c3 add R2 R2 69dd
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13c7 add R2 R2 0001
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13cb wmem R2 R1 # R1 is the number of dots on the coin
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Thus the values should be 9, 2, 5, 7 and 3: the missing numbers in the solved equation.
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It is now trivial to generate the correct value of `R0`. Based on the code beginning, `15fd`:
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```python
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data_69de = [9, 2, 5, 7, 3]
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R0 = 0
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for i in range(len(data_69de)):
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R1 = data_69de[i]
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R0 = R0 + R1
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R0 = (R0 * 0x7bac) % 0x8000
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R0 = R0 ^ R1
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```
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The result is `0b3b`.
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On to the final code, now: the vault lock. At `1679`, `R0` is computed as `([0f73] ^ [0f74]) ^ [0f75]`. Searching the code for references to these, we find:
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1236 wmem 0f70 0016 // weight
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1239 wmem 0f71 0000 // counter
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123c wmem 0f72 0000
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123f wmem 0f73 0000
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1242 wmem 0f74 0000
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1245 wmem 0f75 0000
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It would be nice if these were just the intermediate weights of the orb, but alas the algorithm appears more complex than that.
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Nevertheless, unravelling the various functions which reference those memory addresses:
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```python
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counter = 0
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data_0f73 = 0
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data_0f74 = 0
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data_0f75 = 0
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# 08c8
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def rotatey_thing(R0, R1):
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while R1 != 0:
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R1 = (R1 + 0x7fff) % 0x8000
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R2 = R0 & 0x4000
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R0 = (R0 * 0x0002) % 0x8000
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if R2 == 0:
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continue
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R0 = R0 | 0x0001
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return R0
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# 11a3
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def mutate(R0val, R1, R2):
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return rotatey_thing(R0val, R1) ^ R2
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# 1135
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def update(value, room):
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global counter, data_0f73, data_0f74, data_0f75
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if counter <= 0x752f:
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counter += 1
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data_0f73 = mutate(data_0f73, counter + 2, room)
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data_0f74 = mutate(data_0f74, counter * counter, room * room)
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data_0f75 = mutate(data_0f75, 0x000d, (value * 9) * (value * 9))
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update(0x0000, 0x0008) # north 1000
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update(0x0004, 0x0009) # east 1011
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update(0x0001, 0x000a) # east 1022
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update(0x000b, 0x0006) # north 0fde
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update(0x0002, 0x0005) # west 0fcd
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update(0x0004, 0x0009) # south 1011
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update(0x0001, 0x000a) # east 1022
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update(0x0012, 0x000b) # east 1033
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update(0x0001, 0x000a) # west 1022
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update(0x000b, 0x0006) # north 0fde
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update(0x0001, 0x0002) # north 0f98
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update(0x0001, 0x0003) # east 0fa9
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print(data_0f73)
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print(data_0f74)
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print(data_0f75)
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result = (data_0f73 ^ data_0f74) ^ data_0f75
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print('0x{:04x}'.format(result))
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```
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And with that, we can now [programmatically extract *every single code* given any challenge .tgz!](https://github.com/RunasSudo/synacor.py/blob/master/tools/generate_codes.py)
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