Rating:
# CSAW CTF 2016 deedeedee (150) Writeup
> Wow! I can run code at compile time! That's a pretty cool way to keep my flags secret. Hopefully I didn't leave any clues behind...
This challenge uses a program which was most probably written in [D](https://en.wikipedia.org/wiki/D_(programming_language)).
As the description suggests, the program itself just prints a "encrypted" flag whilst the "encryption" possibly was taking place at compile time:
```
676c60677a74326d716c6074325f6c6575347172316773616c6d686e665f68735e6773385e345e3377657379316e327d
```
After some looking around, I figured out that the main function of programs written in D seems to end up as a `_Dmain` symbol.
As neither radare2 nor IDA Pro where able to demangle D function names, I've written a small D program which reads strings from stdin and demangles them if possible:
```d
import core.stdc.stdio : stdin;
import std.stdio;
import std.ascii;
import std.demangle;
int main()
{
string buffer;
bool inword;
int c;
while ((c = fgetc(stdin)) != EOF)
{
if (inword)
{
if (c == '_' || isAlphaNum(c))
buffer ~= cast(char) c;
else
{
inword = false;
write(demangle(buffer), cast(char) c);
}
}
else
{ if (c == '_' || isAlpha(c))
{
inword = true;
buffer.length = 0;
buffer ~= cast(char) c;
}
else
write(cast(char) c);
}
}
if (inword)
write(demangle(buffer));
return 0;
}
```
This produces for example the following output:
```bash
$ ./demangle_names
_D9deedeedee9hexencodeFAyaZAya
immutable(char)[] deedeedee.hexencode(immutable(char)[])
```
In the next step, I searched for functions which contain `deedeedee` in their names. This results in a lot of functions with similar names:
```
> afl~deedeedee
0x0044cde0 1 1020 sym._D9deedeedee7encryptFNaNfAyaZAya
0x0044e370 4 148 sym._D9deedeedee9hexencodeFAyaZAya
0x00451470 4 213 sym._D9deedeedee21__T3encVAyaa3_313131Z3encFNaNfAyaZAya
0x00458280 4 213 sym._D9deedeedee21__T3encVAyaa3_323232Z3encFNaNfAyaZAya
0x00458358 4 213 sym._D9deedeedee21__T3encVAyaa3_333333Z3encFNaNfAyaZAya
0x00458430 4 213 sym._D9deedeedee21__T3encVAyaa3_343434Z3encFNaNfAyaZAya
0x00458508 4 213 sym._D9deedeedee21__T3encVAyaa3_353535Z3encFNaNfAyaZAya
0x004585e0 4 213 sym._D9deedeedee21__T3encVAyaa3_363636Z3encFNaNfAyaZAya
0x004586b8 4 213 sym._D9deedeedee21__T3encVAyaa3_373737Z3encFNaNfAyaZAya
0x00458790 4 213 sym._D9deedeedee21__T3encVAyaa3_383838Z3encFNaNfAyaZAya
0x00458868 4 213 sym._D9deedeedee21__T3encVAyaa3_393939Z3encFNaNfAyaZAya
0x00458940 4 213 sym._D9deedeedee27__T3encVAyaa6_313031303130Z3encFNaNfAyaZAya
0x00458a18 4 213 sym._D9deedeedee27__T3encVAyaa6_313131313131Z3encFNaNfAyaZAya
[...]
0x00471f18 4 213 sym._D9deedeedee33__T3encVAyaa9_343931343931343931Z3encFNaNfAyaZAya
0x00471ff0 4 213 sym._D9deedeedee33__T3encVAyaa9_343932343932343932Z3encFNaNfAyaZAya
0x0
04720c8 4 213 sym._D9deedeedee33__T3encVAyaa9_343933343933343933Z3encFNaNfAyaZAya
0x004721a0 4 213 sym._D9deedeedee33__T3encVAyaa9_343934343934343934Z3encFNaNfAyaZAya
0x00472278 4 213 sym._D9deedeedee33__T3encVAyaa9_343935343935343935Z3encFNaNfAyaZAya
0x00472350 4 213 sym._D9deedeedee33__T3encVAyaa9_343936343936343936Z3encFNaNfAyaZAya
0x00472428 4 213 sym._D9deedeedee33__T3encVAyaa9_343937343937343937Z3encFNaNfAyaZAya
0x00472500 4 213 sym._D9deedeedee33__T3encVAyaa9_343938343938343938Z3encFNaNfAyaZAya
0x004725d8 4 213 sym._D9deedeedee33__T3encVAyaa9_343939343939343939Z3encFNaNfAyaZAya
0x00474a90 1 40 sym._D9deedeedee7__arrayZ
0x00474ab8 1 79 sym._D9deedeedee8__assertFiZv
```
Demangling them shows:
```
> afl~deedeedee[3] | ./demangle_names
sym.pure @safe immutable(char)[] deedeedee.encrypt(immutable(char)[])
sym.immutable(char)[] deedeedee.hexencode(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("111").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("222").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("333").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("444").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("555").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("666").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("777").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("888").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("999").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("101010").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("111111").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("121212").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("131313").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("141414").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("151515").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("161616").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("171717").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("181818").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("191919").enc(immutable(char)[])
[...]
sym.pure @safe immutable(char)[] deedeedee.enc!("485485485").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("486486486").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("487487487").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("488488488").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("489489489").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("490490490").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("491491491").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("492492492").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("493493493").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("494494494").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("495495495").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("496496496").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("497497497").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("498498498").enc(immutable(char)[])
sym.pure @safe immutable(char)[] deedeedee.enc!("499499499").enc(immutable(char)[])
sym.deedeedee.__array
sym.void deedeedee.__assert(int)
```
It seemed that these functions are used to encrypt the flag. That's why I was looking if these functions are used somewhere in the binary:
```
> axt @ sym._D9deedeedee21__T3encVAyaa3_313131Z3encFNaNfAyaZAya
call 0x44cdfe call sym._D9deedeedee21__T3encVAyaa3_313131Z3encFNaNfAyaZAya in sym._D9deedeedee7encryptFNaNfAyaZAya
> axt @ sym._D9deedeedee21__T3encVAyaa3_313131Z3encFNaNfAyaZAya | ./demangle_names
call 0x44cdfe call sym.pure @safe immutable(char)[] deedeedee.enc!("111").enc(immutable(char)[]) in sym.pure @safe immutable(char)[] deedeedee.encrypt(immutable(char)[])
```
Ok, so there is a encrypt function...
```
> pdf | ./demangle_names
```
This function calls the `enc` functions with incrementing number strings ("111", "222", "333", ..., "498498498", "499499499").
If we look at the content the `enc` functions, we'll see that they are all identical, except of using a different string ("111", "222", etc.). Based on the demangled names, I was able to decompile them approximately as follows:
```d
char[] enc(char[] data, string base) {
auto len = cast(char) to!int(base.length);
auto c = cycle(base);
char[] res;
foreach (tup; zip(c, data))
{
res ~= tup[0] ^ tup[1] ^ len;
}
return res;
}
```
By reversing the function calls, I was able to "decrypt" the flag:
```d
import std.range : cycle, zip;
import std.conv : to, hexString;
import std.stdio;
char[] enc(char[] data, string base) {
auto len = cast(char) to!int(base.length);
auto c = cycle(base);
char[] res;
foreach (tup; zip(c, data))
{
res ~= tup[0] ^ tup[1] ^ len;
}
return res;
}
int main() {
auto data = hexString!"676c60677a74326d716c6074325f6c6575347172316773616c6d686e665f68735e6773385e345e3377657379316e327d";
char[] res = data.dup;
for (int i = 499; i >= 0; --i) {
string base = to!string(i);
res = enc(res, base ~ base ~ base);
}
writeln(res);
return 0;
}
```
```
flag{t3mplat3_met4pr0gramming_is_gr8_4_3very0n3}
```
