How to crack such a (under) cipher?

Question

Encryption

Hello everyone,
Today we talked with a friend who is not related to IT, on the topic of encryption. Let's say you need to send an encrypted message, which consists of the text of the English / Russian language, about 100-1000 characters long. Let's say we've never heard of RSA, AES, etc. It offers the following version of symmetric encryption: there is a randomly generated character mapping table, where each character in the input has some one specific character in the output, for example:
a -> /
b -> z
c -> .
and so on.....
hence the string aacb will be encrypted as //.z
So, let's say both parties know the key (character table), only one message needs to be transmitted, which is subsequently intercepted by guys with very serious knowledge and computer power, how theoretically can they decrypt it?
Since I heard about the basics of cryptography, I don’t believe that such an algorithm can be safe, but I don’t have enough knowledge to catch up with why. Can you explain?

Answer 1

an encrypted message that consists of English/Russian text, about 100-1000 characters long...
a variant of symmetric encryption: there is a character mapping table, where each character at the input has one specific character at the output

https://ru.wikipedia.org/wiki/Frequency_analysis

Answer 2

Substitution_cipher
How to crack is described in the same place.

Answer 3

They don't like questions like this here.

Answer 4

Such "ciphers" are "cracked" with a pencil on paper in half an hour in the worst case. I liked it as a child and I wrote a program for the Spectrum that generated them.
Turing printed puzzles with such ciphers in newspapers to select people in Bletchley Park for his team.
It's not a cipher. It's just a puzzle for the yellow rag on the train.