Golden Dawning
@mrexcessive WHA
EKOparty15 cry200 Perfect security
The problem
Description: It is not maybe so perfect. Hints: Use the golden math! Attachment: crypto200.zip Crypto
The solution
So... I don't usually get anywhere with the hard crypto challenges, all that ECC and Prime Number messing about... BUT this one looks like it's an XOR solver, so there is some hope.
The hint seems extremely blatant to me... golden ratio - or Phi...
Check out Numberphile for background http://www.numberphile.com/videos/making_golden_ratio_song.html
well maybe not that specific page... but somewhere on numberphile... Just go check out numberphile generally ... do it now!
OK... back from Numberphile ?
It is an XOR... Phi is a long stream of numbers... Perfect security (in crypto sense) is often descibed as an XOR with a never repeating, never disclosed pattern.
But... the truth will out !
Looking at the zip file:
Archive: zip/crypto200.zip Length Method Size Cmpr Date Time CRC-32 Name -------- ------ ------- ---- ---------- ----- -------- ---- 19402 Defl:N 10634 45% 2015-08-27 00:14 036a7123 output.enc 10456 Defl:N 2533 76% 2015-08-26 03:28 629c8d20 perfect -------- ------- --- ------- 29858 13167 56% 2 files
OK So on the face of it you pass two files as command line arguments to ./perfect
This xors them together and produces binary output as a hex dump
Tested with eg.txt "ABCD" and pad.txt "1234"
The output.enc we have been given ends like this:
0004a50: 3062 3737 3232 6639 6637 3062 6537 3231 0b7722f9f70be721 0004a60: 6366 6563 6239 6230 3231 6638 3431 3265 cfecb9b021f8412e 0004a70: 6531 3363 3166 6538 6662 6161 6631 6635 e13c1fe8fbaaf1f5 0004a80: 3739 3765 3635 3462 3863 3933 6139 3835 797e654b8c93a985 0004a90: 3739 3434 3038 3362 3063 3138 3035 3764 7944083b0c18057d 0004aa0: 3830 6232 6161 3264 3761 6534 6639 3938 80b2aa2d7ae4f998 0004ab0: 6535 3638 3832 6439 3639 6163 3336 3637 e56882d969ac3667 0004ac0: 6530 3032 3965 6531 6264 3334 3237 6661 e0029ee1bd3427fa 0004ad0: 3737 3735 3737 3766 3236 3061 3634 6539 7775777f260a64e9 0004ae0: 3833 3739 6564 6538 3237 3638 3930 3637 8379ede827689067 0004af0: 6436 6638 3964 6137 3535 3238 3135 3464 d6f89da75528154d 0004b00: 3030 3633 6464 3630 3532 3330 3365 3330 0063dd6052303e30 0004b10: 3039 3331 3338 3338 3335 3333 3333 3330 0931383835333330 0004b20: 3337 3336 3332 3333 3330 3335 3335 3336 3736323330353536 0004b30: 3333 3338 3331 3336 3333 3331 3336 3334 3338313633313634 0004b40: 3330 3331 3339 3332 3332 3334 3335 3334 3031393232343534 0004b50: 3335 3330 3333 3332 3335 3337 3336 3335 3530333235373635 0004b60: 3336 3337 3333 3339 3332 3335 3339 3339 3637333932353939 0004b70: 3337 3336 3335 3331 3337 3335 3333 3330 3736353137353330 0004b80: 3338 3330 3331 3334 3332 3337 3331 3336 3830313432373136 0004b90: 3330 3337 3331 3334 3333 3330 3338 3337 3037313433303837 0004ba0: 3331 3338 3338 3336 3332 3338 3335 3339 3138383632383539 0004bb0: 3338 3333 3336 3330 3333 3337 3334 3336 3833363033373436 0004bc0: 3335 3330 3335 3337 3331 3530353731
That sticks out a mile.
The very end is just digits... final three rows for example are a the sequence
313838363238353938333630333734363530353731
Which is Ascii (in hex) for the digit sequence:
1 8 8 6 2 8 5 9 8 3 6 0 3 7 4 6 5 0 5 7 1
So I have one thing to try... does that sequence occur anywhere in an expansion of Phi ?
We need an expansion of Phi... To the Google...
I found 20k digits of golden ratio here http://www.goldennumber.net/phi-million-places/
It turns out that sequence occurs at offset 9700
So... probably... perhaps...
Get the appropriate section of Phi from the downloaded expansion...
Turn output.enc from hex into binary (some of which is hex!)
Turn the expanded Phi section into hex
XOR... and out pops a flag ??
Well it was a bit more involved... the key was 9701 bytes long and repeated once.
Python to do the file mangling:
#!/usr/bin/python
encrypted_fname = "output.enc"
encrypted_bin_fname = "encrypted.bin"
phidata_fname = "Phi20000"
pad_fname = "pad.key"
f = open(phidata_fname,"rb")
phiraw = f.read()
f.close()
#Truncate phi at 19402 / 2 digits
phi = phiraw[:(19402 / 2)]
#Write phi out as pad
f = open(pad_fname,"wb")
f.write(phi)
f.close()
#Read output.enc as hex and turn to binary
f = open(encrypted_fname,"rb")
d = f.read()
f.close()
d = d.decode("hex")
#Write out as encrypted.bin
f = open(encrypted_bin_fname,"wb")
f.write(d)
f.close()
print "Now ls to check lengths then do..."
print "./perfect encrypted.bin pad.key"
And then take the output... and that is a GIF file...
rename and inspect with GIMP.