In this chapter, the conventional cryptography fundamentals are introduced. The chapter starts with basic terminology and cryptographic schemes, including symmetric and asymmetric cryptography, basic ciphers such as substitution and transposition ciphers, and one-time pads. The concepts of secrecy, authentication, and non-repudiation are introduced, followed by various cryptanalytic attacks such as ciphertext-only, know-plaintext, chosen-plaintext, chosen-ciphertext, and adaptive-chosen-plaintext attacks. In the section on information-theoretic approach to cryptography, the concept of perfect security is introduced and compared against the computational security. In the same section, unicity distance is discussed as well as the role of compression in cryptography. Then, one-way functions and one-way hash functions are discussed. The chapter concludes with several relevant practical cryptographic systems, including DES and RSA systems, as well as Diffie-Hellman public key distribution.