Cryptography

Cryptography is the practice and study of techniques for secure communication in the presence of adversarial behavior. It encompasses encryption, hashing, digital signatures, and key exchange — the mathematical foundations that protect data confidentiality, integrity, and authenticity.

Encryption is reversible — it transforms plaintext into ciphertext using a key, and the original data can be recovered with the correct decryption key. Hashing is one-way — it produces a fixed-size fingerprint from input data that cannot be reversed. Encryption protects confidentiality; hashing ensures integrity.

No. MD5 is cryptographically broken — collisions can be generated in seconds, and it is too fast for password hashing. Use bcrypt, scrypt, or Argon2 for passwords. MD5 still has legitimate uses for non-security checksums and file identification.

SHA-256 is a cryptographic hash function in the SHA-2 family that produces a 256-bit (32-byte) digest. It is used in Bitcoin mining, TLS certificates, digital signatures, and data integrity verification. It is currently considered secure against collision and preimage attacks.

Base64 encodes binary data into a printable ASCII string using 64 characters (A-Z, a-z, 0-9, +, /). It is not encryption — it is simply a way to represent binary data in text format. Common uses include embedding images in HTML/CSS, email attachments, and data URLs.

Symmetric encryption uses one shared key for both encryption and decryption (AES, ChaCha20). It is fast but requires secure key exchange. Asymmetric encryption uses a key pair — public key for encryption, private key for decryption (RSA, Ed25519). It solves the key distribution problem but is slower.

A UUID (Universally Unique Identifier) is a 128-bit identifier designed to be unique across all devices and time. Version 4 UUIDs are random and used for database keys, API identifiers, and session tokens. ULIDs and NanoIDs are modern alternatives with different trade-offs.