Cryptography is the science of protecting information with mathematical techniques, ensuring its confidentiality, integrity and authenticity even when it is stored or transmitted over untrusted channels.
How cryptography works
Modern cryptography is built from a few basic primitives:
- Symmetric encryption – the same secret key encrypts and decrypts data; AES (FIPS 197) and ChaCha20 are the standards.
- Public-key cryptography – a key pair with a public and a private key, used for key exchange and digital signatures (RSA, elliptic curves).
- Hash functions – one-way fingerprints of data such as SHA-256, used for integrity checks and password storage.
- Message authentication codes and random number generators – proving that data came from a key holder and producing unpredictable keys.
Protocols combine these primitives: TLS secures websites, Signal secures messaging, and disk encryption protects lost laptops. Following Kerckhoffs’s principle, good algorithms are public and only the key is secret.
Why cryptography matters
Cryptography underpins online banking, software updates, VPNs, passwords and cryptocurrencies. Most real-world failures come not from breaking the mathematics but from mistakes in implementation and use:
- Heartbleed (2014) – a bug in OpenSSL leaked server memory, including private keys;
- a 2008 Debian change crippled OpenSSL’s random number generator, producing predictable keys;
- outdated algorithms such as MD5, SHA-1, DES and RC4 remain in legacy systems (CWE-327).
Quantum computers threaten today’s public-key algorithms, which is why NIST published the first post-quantum cryptography standards (FIPS 203–205) in 2024.
Best practices
- Never design your own algorithms; use well-reviewed libraries and standard protocols.
- Retire deprecated algorithms and plan for crypto-agility.
- Protect keys as carefully as the data they guard.
