We ware given x86-32 binary[0] after disassembling  we 
can see a pile of shit rather than proper code
----
 804845c:       e4 83                   in     al,0x83
 804845e:       f0 55                   lock push ebp
 8048460:       57                      push   edi
 8048461:       e5 53                   in     eax,0x53
 8048463:       81 56 89 0c 45 89 ec    adc    DWORD PTR [esi-0x77],0xec89450c
 804846a:       00 01                   add    BYTE PTR [ecx],al
 804846c:       8b 44 00 30             mov    eax,DWORD PTR [eax+eax*1+0x30]
 8048470:       00 00                   add    BYTE PTR [eax],al
 8048472:       89 24 a1                mov    DWORD PTR [ecx+eiz*4],esp
 8048475:       65 00 84 14 1c c7 c0    add    BYTE PTR gs:[esp+edx*1+0x44c0c71c],al
----

Obviously we have to fix it.
Since the name of challenge is miXer we thought we have to  xor it with
some key to get correct form.
We can guess correct first bytes by based on stand art function prologue

---
	55                   	push   ebp
	89 e5                	mov    ebp,esp
--- 

assuming 4 byte key we start to bruteforce 4th byte... it turns to nothing
Then we start to looking closer to bytes, and realize that all needed bytes are here, only mixed... 
4th bytes should be 1st, 10th second and 6th third. so we have to put rest of the bytes together to get proper prologue/code.
gcc tends to align stack to 16 bytes via 
---
        83 e4 f0             	and    esp,0xfffffff0
---

ok we have 4 bytes left
53 56 57 and 81 
81 is begin of stack allocation - sub esp, xxx
and rest stand for push ebx ; push esi ; push edi
all we need is to put it in correct order, looking thou
some binaries we found this prologue

---
	55                   	push   ebp
	89 e5                	mov    ebp,esp
	57                   	push   edi
	56                   	push   esi
	53                   	push   ebx
	83 e4 f0             	and    esp,0xfffffff0
	81 ec 30 04 00 00    	sub    esp,0x430
---

Lets try it,

---
TEXT_START = 0x8048370
MAIN_BEG = 0x804845c - TEXT_START + 0x00370
MAIN_END = 0x8048830 - TEXT_START + 0x00370
with open('/tmp/miXer.elf.5f96dea48b8c8ab66898e902d3c98b82') as f:
  data = f.read()
code = data[MAIN_BEG:MAIN_END]
data2= list(data)
flip = lambda x: (x[1],x[0])

ctable = dict(map(flip,enumerate(code[:10])))
ctable = [ ctable[c] for c in "\x55\x89\xe5\x57\x56\x53\x83\xe4\xf0\x81"]
for i in range(len(code)/10):
  for j in range(10):
     data2[MAIN_BEG+i*10+j] = code[i*10+ctable[j]]
with open('/tmp/mixfix.bin','w') as f: f.write(''.join(data2))
---
aand check it:
---
[mak@localhost tmp]$ python2 x.py 
[mak@localhost tmp]$ chmod +x mixfix.bin 
[mak@localhost tmp]$ ./mixfix.bin  ; echo
y0ur.f1rst.fl4g
[mak@localhost tmp]$
---
done.

--
mak

-
[0] http://shell-storm.org/repo/CTF/PHDays-Quals-2014/MiXer-2000pts/