0.5 The little bit of maths you need

Four ideas. Nothing more. Each is re-explained where used.

Idea 1 — Bits, bytes, and "key size"

A bit is 0 or 1. A byte is 8 bits (values 0–255).

A 128-bit key is 128 bits of secret — 2^128 possible values, which is about 3.4 × 10^38. That number is the entire reason symmetric crypto works:

   2^128 = 340,282,366,920,938,463,463,374,607,431,768,211,456

   A machine trying 1,000,000,000,000 (10^12) keys per second
   would need about 10^19 years to try them all.
   The universe is about 1.4 x 10^10 years old.

Brute force is not a threat to a 128-bit key. Every real attack works around the key, not through it. Remember that; it reframes everything.

Idea 2 — XOR

Exclusive-or. The one operation you must know.

   0 XOR 0 = 0        Rule: output 1 when the inputs DIFFER.
   0 XOR 1 = 1
   1 XOR 0 = 1
   1 XOR 1 = 0

   The magic property:   (A XOR B) XOR B = A

   plaintext  0110 1001
   key        1010 1100   XOR
              ---------
   ciphertext 1100 0101

   ciphertext 1100 0101
   key        1010 1100   XOR
              ---------
   plaintext  0110 1001    <-- back to the original

XOR with a key encrypts; XOR with the same key decrypts. That's the core of every stream cipher.

Idea 3 — Modular arithmetic ("clock arithmetic")

   On a 12-hour clock:  9 + 5 = 2       because 14 mod 12 = 2

   "mod n" means: divide by n, keep the remainder.
      17 mod 5 = 2        100 mod 7 = 2        6 mod 6 = 0

Public-key crypto works in enormous clocks — numbers hundreds of digits long. That's the only reason it's hard to reverse.

Idea 4 — One-way functions

A function that's easy forwards and impractical backwards.

   EASY:   multiply 1,048,573 x 1,048,583  = 1,099,483,725,059
   HARD:   given 1,099,483,725,059, find the two factors

   With 300-digit numbers, "hard" means "no known method finishes
   before the sun burns out".

All of cryptography is built on one-way functions. Hashing is one-way. RSA relies on factoring being one-way. Diffie–Hellman relies on discrete logarithms being one-way.

That's genuinely all

If those four made sense, you can read every chapter here. Where deeper maths appears it's marked optional and you can skip it.