This message looks very similar to Base64, but with different index table. With some guessing we can create new index table and map it to the standard one for decoding.

```
import base64

encoded = 'гЖзпвшБпвшБзИЗНлШ3ЙлгВБуЩЧНщШЦглИЖЩшб20жа2Ейа2ХшвщоКХ2ФжШЧЙлИЖСпв2НхгмХшЩЦРтИЖ5лгшБиШЧНлИЖЕйШ2ХщвшБцШЧНщг29шЩВБпвшБса3Н7Ш3ХщгЖ9уЧ2И2НЕ9збЗБоШЦЙлгЕ9йНЖ50ЧшС0МЗБедУБ1еР=='

alphabet = 'АБВГДЕЖЗИЙКЛМНОПРСТУФХЦЧШЩабвгдежзийклмнопрстуфхцчшщ0123456789+/'
standard_base = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'

encoded = encoded.translate(str.maketrans(alphabet, standard_base))
decoded = base64.b64decode(encoded)

print(decoded.decode())
```

This outputs:
>this is a secret message from kackers:
We are discovered, new base access password is kks{custom_b64_alphabet_c4nt_$t0p_y0u}

Original writeup (https://github.com/4p3ls1n/write-ups/tree/master/kksctf_open_2019/Анотхер%20месседж%20фром%20бейс).