In any cryptographic system, the key is the secret piece of information that determines how data is transformed by an encryption or decryption algorithm. A single algorithm can be used to encrypt countless different messages, but without the correct key, the ciphertext should remain unintelligible. The strength of a cryptosystem therefore depends heavily on key secrecy and key management.
A cryptosystem is the complete set of components required for secure encryption and decryption: the algorithm(s), keys, and protocols governing their use. A well-designed cryptosystem clearly defines how keys are generated, distributed, stored, and revoked, as well as how encryption and decryption are applied to various data types. While cryptography is the art of creating secure systems, cryptanalysis is the science of breaking them—attempting to uncover the key or plaintext without authorization.
Cryptanalysis can range from exploiting mathematical weaknesses in an algorithm to leveraging implementation flaws or operational mistakes (known as side-channel attacks). Historically, advances in cryptanalysis have driven the evolution of cryptography, forcing weaker algorithms into obsolescence and pushing for stronger, more resilient designs.
Kerckhoffs’s principle
This principle, formulated in the late 19th century, states that the security of a cryptosystem should rely solely on the secrecy of the key—not on the secrecy of the algorithm itself. In other words, even if an adversary knows exactly how the encryption algorithm works, they should not be able to decrypt the data without the key. This is why modern cryptographic algorithms are often publicly available and peer-reviewed; their security comes from mathematical robustness, computational constraints based on sufficient key length, not from obscurity. In the next sections we will learn how different algorithms need different key lengths.