synacor.py/electric_boogaloo.md

Synacor Challenge 2: Electric Boogaloo

So we've finished the official parts of the challenge. What now?

Self-test

Hex values refer to the instruction lines, not the actual ranges spanned by the data in memory

• 0000 to 013e: Startup message
• 0140 to 01e3: Tests jmp
  • 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.
  • If successful, executes 0140, 015b to 0160, then 0166.
• 01e4 to 01f2: Tests jt and jf
  • If successful, jumps to 01f4.
• 01f4 to 0209: Tests that the registers are initialised to zero
• 020c to 0215: Tests set
• 0218 to 0233: Tests add
  • The test is rudimentary, however, and would not detect many simple errors. In fact it tests only if 1 + 1 ≠ 0.
• 0234 to 024d: Tests eq
  • 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!
  • It would probably have been a better idea to test eq first, before add, then use eq to test add.
• 024e to 0261: Tests push and pop
  • Since only R1 and R2 are checked, this would not detect errors involving other registers.
  • This test, like the last one, reuses the results of previous tests, since that worked out so well for eq…
• 0264 to 0276: Tests gt
  • The tests performed seem quite reasonable, but yet again reuse the results of previous tests…
• 0279 to 02ab: Tests and and or
  • Confusingly, the error handling is located in different places for each test.
• 02ac to 02bd: Tests not
  • 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…
• 02c0 to 02e8: Tests call
  • Although, notably, not ret. The tests operates by jmping back and forth to test the various values of R1.
• 02eb to 0308: Checks that add overflows correctly
• 030b to 0313: Checks that 6 × 9 ≠ 42.
  • 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.
• 0316 to 0346: Continues checking mult
• 0349 to 034b: Two values 4e20 and 2710 are stored in memory here for reference by the following test
• 034d to 03d1: Tests rmem and wmem
  • If successful, causes the words from 03a9 to 03ac to instead read nop, jt 0013 03d2
  • There is more to the portion starting 0375 than meets the eye: see below.
• 0432 to 05b1: Various error messages

Decryption

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.

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!

0375 call 06bb

This is the magic line. Digging into the 06bb subroutine:

06bb push R1
06bd push R2
06bf set  R2 17b4
06c2 rmem R1 R2
06c5 push R2
06c7 mult R2 R2 R2
06cb call 084d
06cd set  R2 4154
06d0 call 084d
06d2 pop  R2
06d4 wmem R2 R1
06d7 add  R2 R2 0001
06db eq   R1 7562 R2
06df jf   R1 06c2
06e2 pop  R2
06e4 pop  R1
06e6 ret

Inspecting the 084d subroutine reveals that this is simply an XOR function: R1 XOR R2. Crypto rating: 1/10

Rewriting 06bb function using higher-level syntax reveals that the ‘encryption’ algorithm is really very simple:

06bb() {
	R2 = 17b4;
	for (R2 = 17b4; R2 != 7562; R2++) {
		R1 = [R2];
		R1 ^= R2 * R2;
		R1 ^= 4154;
		[R2] = R1;
	}
}

Very simple.

By emulating this function in Python, 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!

Encrypted strings

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.

Looking through the code following the self-test, we find:

0413 set  R1 17c0
0416 call 05ee

Digging deeper, 05ee calls 05b2 with R2 set to 05f8. 05b2 appears to iterate over the characters in a string whose length is stored in address R1, and calls R2 for each character, storing the character in R1. 05f8 (the callback provided by 05ee) simply outputs every character in R1 it gets.

Immediately after this call to 05ee, we find:

041e set  R1 68e3
0421 set  R2 05fb
0424 add  R3 XXXX XXXX
0428 call 05b2

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 R2 set to R3.

Now we have everything we need to extract these encrypted (double-encrypted??) strings from the binary!

Only the self-test completion code appears to be stored there, though, so I'm not sure what the point of encrypting those was…