This text offers a comprehensive introduction to cryptology, based on elementary number theory and essential numerical algorithms. It covers core mathematical topics —number theory, algebraic structures, matrices, determinants, and polynomials —through their use in cryptography. Symmetric-key cryptography and practical techniques are introduced, followed by algorithms and computational complexity. Public-key systems based on integer factorization and discrete logarithms, as well as hash functions and digital signatures, are discussed. Advanced topics include elliptic curve and multivariate polynomial cryptography, primality testing, and integer factorization are also explored. The book is self-contained, with all necessary proofs, theory, and tools to analyze cryptographic security and attacks.