I am not good in maths, so I am stuck in a small problem.
How can I solve this equation?
$$a^x\bmod n=b$$
for example
$$11^x\bmod 23=15$$
How can I find what is the value of $x$?
2026-02-23 08:36:58.1771835818
How to solve $a^x\bmod n=b$
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This is known as the discrete logarithm problem, and it is believed to be a hard problem in general.
(In fact if it isn't "hard", in an appropriate technical sense, the security of many widely-used cryptographic primitives would break down).
For numbers as small as the one in your example, you can get through with simple trial-and-error: Keep multiplying by $11$ (and reducing modulo $23$) until you reach $15$.
When the numbers get too large for trial-and-error, you're in trouble. There are known methods that are somewhat faster than trying everything, but they're still not fast, and they're not simple to describe at all.