I think something is hidden here, something that may seem useless. GPG key: yaxceburkaukvefinneuvUvjueknykfa

» file trollface
trollface: ELF 32-bit MSB executable, OpenRISC, version 1 (SYSV), statically linked, not stripped

OpenRISC, cool! It's for an open source CPU! Let's see where the flag is hidden...

I had no success running the binary using qemu, although it has support for the or32 arch:

% qemu-or32 ./trollface
PC=00025d2c
R00=00000000 R01=007fe000 R02=007fe000 R03=00002000
R04=007fc000 R05=000292ac R06=00000000 R07=00000000
R08=00000000 R09=000020e4 R10=00000000 R11=00000000
R12=00000000 R13=00000000 R14=00000000 R15=00000000
R16=00000000 R17=00000000 R18=00000000 R19=00000000
R20=00000000 R21=00000000 R22=00000000 R23=00000000
R24=00000000 R25=00000000 R26=00000000 R27=00000000
R28=00000000 R29=00000000 R30=00000000 R31=00000000

Time to download a toolchain - conveniently available in binary form as gcc-or1k-elf-4.8.1-x86_64.tar.bz2 somewhere on the web.

Looking at the disassembly using gdb or objdump, we see a huuuge main function, over 32000 instructions long! However even without knowing much about the architecture, we can see that most instructions don't have any effect:

[...]
   0x00002738 <+32>:    l.addi r16,r16,0
   0x0000273c <+36>:    l.nop 0x4141
   0x00002740 <+40>:    l.ori r19,r19,0x0
   0x00002744 <+44>:    l.addi r2,r0,32
   0x00002748 <+48>:    l.sb 0(r1),r2
   0x0000274c <+52>:    l.nop 0x4141
   0x00002750 <+56>:    l.muli r5,r5,1
   0x00002754 <+60>:    l.nop 0x4141
   0x00002758 <+64>:    l.ori r2,r2,0x0
[...]

I tried filtering them using grep ( grep -Ev "nop|(addi?|subi?|ori?) (r[0-9]+),\\2,(0|r0|0x0)|(muli?) (r[0-9]+),\\5,1"), however over 5800 mostly 'proper' instructions remained, likely autogenerated.

The toolchain comes with an emulator, so we can just run the binary:

» gcc-or1k-elf-4.8.1-x86_64/bin/or1k-elf-run ./trollface
WARNING: l.nop with unsupported code 0x00003066
WARNING: l.nop with unsupported code 0x00004d56
WARNING: l.nop with unsupported code 0x00006752
WARNING: l.nop with unsupported code 0x00004345
WARNING: l.nop with unsupported code 0x00005141
[...]
WARNING: l.nop with unsupported code 0x00004141
WARNING: l.nop with unsupported code 0x00004141
WARNING: l.nop with unsupported code 0x00004141
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                        ...

I had to shorten the output, we get lots of lines complaining about an invalid nop operand. Looks like data! Playing around with it, we find that it is little-endian words that make up a large bas64 string.

%gcc-or1k-elf-4.8.1-x86_64/bin/or1k-elf-run ./trollface 1>/dev/null 2>>(
    cut -d " " -f 6|sed 's/0x0000//g' |
    perl -pe's/(..)(..)\n/chr(hex($2)).chr(hex($1))/e'|
    base64 -d > hidden)

% file hidden
hidden: ERROR: ELF 32-bit MSB executable, OpenRISC, version 1 (SYSV), statically linkederror reading (Invalid argument)

% gcc-or1k-elf-4.8.1-x86_64/bin/or1k-elf-run ./hidden
flag{Have you ever heard something about OpenRISC?}

Yes, we have!