Cryptography secures protocols—step-by-step procedures followed by people or systems—against adversaries who may observe, alter, steal, or impersonate. It protects confidential messages, identifies allies, and supports security and privacy in everyday technology. section 1.1 section 1.9
A protocol with an obvious weakness
Suppose you leave a sword on the ground, nap under a tree, and recover it later. The steps form a protocol, but anyone can steal the sword during the nap. Cryptography improves protocols by accounting for adversaries and their opportunities to interfere. section 1.1
Cryptography is assembled from primitives that make distinct security claims:
- Symmetric cryptography uses one secret key shared by the relevant participants. section 1.5 section 1.9
- Asymmetric cryptography gives participants different views of keys, such as public and private keys. section 1.5 section 1.9
- Cryptographic primitives include algorithms such as symmetric encryption, key exchange, asymmetric encryption, and digital signatures. section 1.9
- Their main security goals often include confidentiality—protecting privacy—and authentication—verifying authenticity. section 1.9
Theoretical cryptography investigates mathematical and other constructions, while applied or real-world cryptography turns useful ideas into protection for applications. Practical decisions include security level, runtime, and input and output sizes. section 1.6
Cryptography is only one part of a secure system. Bugs in surrounding components may provide easier attack paths, so conservative, well-implemented cryptography needs equally careful system design and testing. section 16.6
Treat cryptography as a toolset for recognizing threats, choosing suitable primitives, and spotting dangerous designs—not as permission to create production cryptosystems casually. Complex systems require expert review, and established implementations are safer than newly invented algorithms. section 1.8 section 16.7
