Not JSON

wAAAAAIwAB4AAABhYmNkZWZnaGppa2xtbm9wcXJzdHV2d3h5el97fQAEMQCTAAAAEDAABQAAABAxAAsAAAAQMgAAAAAAEDMABgAAABA0ABsAAAAQNQASAAAAEDYADgAAABA3AA0AAAAQOAAaAAAAEDkADgAAABAxMAAFAAAAEDExABoAAAAQMTIAAAAAABAxMwAaAAAAEDE0AAEAAAAQMTUAEgAAABAxNgAOAAAAEDE3AA0AAAAQMTgAHAAAAAAA

What we are given is some sort of serialized data that is then base64 encoded. Using base64 on the command line, we are only really given one readable string and a series of numbers from 0 to 18. Using xxd we can see that there is hex data stored inbetween each of these numbers. I put just these hex characters into a new file, but also noticed a pattern while doing so. The hex characters matched the positions of the characters in the first string and spell something out. I did this the old school way, manually on the command line.

Another easier way to do this would have been to use CyberChef. Base64 followed by BSON Deserialize would give you the following output, steps to solve being the same.

{
  "0": "abcdefghjiklmnopqrstuvwxyz_{}",
  "1": [
    5,
    11,
    0,
    6,
    27,
    18,
    14,
    13,
    26,
    14,
    5,
    26,
    0,
    26,
    1,
    18,
    14,
    13,
    28
  ]
}