The ciphertext is base-34 encoded. There aren't many clues that would suggest what the problem is, except analysis of the character set. The characters used are within the lowercase letter set and digits, suggesting the possibility of a number encoded as base >16. The highest character observed is 'x', which is present in base-n encoded data starting with base-34.

As of the most recent patch (posted after OpenCTF ended) FeatherDuster (https://github.com/nccgroup/featherduster) is capable of solving this challenge with its base_n_solver module, which will be run against this sample in autopwn mode:

<span>
$ python featherduster.py <(echo 3xvsw8su9k7xlcq39w38uv33q5u5dt260uwb81mwnjjeiku8emau2788t)
</span>...<edited out for brevity>...



FeatherDuster> autopwn
[+] Analyzing samples...
[+] Messages may be encrypted with a stream cipher or simple XOR.
[!] Individual messages have failed statistical tests for randomness.
[!] This suggests weak crypto is in use.
[!] Consider running single-byte or multi-byte XOR solvers.
Running module: alpha_shift
...<edited out for brevity>...
Running module: base_n_solver
Best answers for sample: 3xvsw8su9k7xlcq39w38uv33q5u5dt260uwb
--------------------------------------------------------------------------------
'w3_u5ed_a_d4rtb0ard_t0_p1ck_th3_b4se'
'\x0bE)\xbf@\x06\x9eE\xd5D\xcb\xf4\xce\x15\xdc\xb1\xf7\x81\xf9\xe8\xaa\x17\x1d\xa5b\x10@\x16\xab\xa5\x870+\xbau\xbf}'
'\x02W\xe5\rUTe\xe2Z\x877\xb1#\x08\x12z?~1`\x91R\xc6\\E8\x1c4hMm/\x17-:R\x10'


<span>FeatherDuster> 
</span>