Keshirovaniya form: Pc54EjPvTwvXG8IDAfdM0azKcy+otWnScAGrV4zY9Vo=:A:zg8HaDmFVi+K/5ryDlhAldNUBAKaN685K6dHGZhPdkI=
It's NOT keshirovaniya form. It's base64 from the hashed form. :)
Then I could offer two options:
1. Try hash source string of different known algorithms. The one for which the hash will match a reference will be required. Of course, you could pre-weed out the algorithms that generate the hash of a different size.
2. If it is possible to obtain the hashes for different strings, it would be possible to try to accumulate enough pairs <text> <=> <hash> to build the truth table (in bits), this table to build NDF, minimize it and get the formula, which in this sample gives the same result. There is a nonzero probability that it will work for other couples.
(3). Not taken into account. Reverse-engineering most softiny that generates this hash.