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Cryptography and PKI

Symmetric and asymmetric encryption, hashing algorithms, digital signatures, TLS/SSL protocol mechanics, and Public Key Infrastructure. Essential building blocks for secure communications and data protection.

Key Facts

  • Symmetric encryption (AES) is fast for bulk data but has the key distribution problem
  • Asymmetric encryption (RSA, ECC) solves key exchange but is slow - used for key exchange and signatures, not bulk data
  • ECC 256-bit provides equivalent security to RSA 3072-bit with shorter keys
  • TLS 1.3 reduced handshake to 1-RTT (from 2-RTT in 1.2), removed weak ciphers, mandates PFS
  • Perfect Forward Secrecy (PFS) means compromising the server's private key does not decrypt past sessions
  • MD5 and SHA-1 are broken - never use for security purposes

Symmetric Encryption

Same key for encryption and decryption: - AES - current standard, 128/192/256-bit keys - ChaCha20 - stream cipher, used in TLS, mobile-friendly (faster without AES-NI) - DES/3DES - legacy, broken/deprecated - Key distribution problem: how to securely share the key between parties

Asymmetric Encryption

Public key + private key pair: - RSA - based on factoring large primes, 2048+ bit keys minimum - ECC (Elliptic Curve) - shorter keys for equivalent security - Diffie-Hellman - key exchange protocol (not encryption itself) - Used for key exchange and digital signatures, not bulk data

Hashing

One-way function producing fixed-size output:

Algorithm Status Use Case
SHA-256/SHA-3 Current standard Integrity verification
MD5 Broken (collisions) Never for security
SHA-1 Deprecated (SHAttered 2017) Legacy only
bcrypt/scrypt/Argon2 Current standard Password hashing (deliberately slow, salted)

Properties: deterministic, avalanche effect, pre-image resistance, collision resistance.

Digital Signatures

  1. Sender hashes the message
  2. Sender encrypts hash with private key = signature
  3. Recipient decrypts signature with sender's public key
  4. Recipient hashes the message independently
  5. Hashes match = authentic and unmodified

PKI (Public Key Infrastructure)

Trust Hierarchy

  • Root CA - self-signed, trusted by OS/browser
  • Intermediate CA - signed by Root CA, signs end-entity certificates
  • End-entity certificate - server/client certificates
  • CRL/OCSP - certificate revocation checking

Certificate contains: public key, subject (domain), issuer, validity period, signature.

Certificate Transparency

Public logs of all issued certificates. Tools: crt.sh, censys.io. Used for: - Detecting mis-issued certificates - Subdomain enumeration during reconnaissance

TLS/SSL

Protocol Versions

  • SSL - all versions deprecated (SSLv3 broken by POODLE attack)
  • TLS 1.0/1.1 - deprecated
  • TLS 1.2 - current widely supported minimum
  • TLS 1.3 - 1-RTT handshake, removed weak ciphers, mandatory PFS

TLS Handshake (1.2)

Client → Server: ClientHello (supported ciphers, random)
Server → Client: ServerHello (chosen cipher, random) + Certificate
Server → Client: Key Exchange parameters
Client → Server: Key Exchange + ChangeCipherSpec
Both: Encrypted application data

TLS Configuration (Nginx)

ssl_protocols TLSv1.2 TLSv1.3;
ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256;
ssl_prefer_server_ciphers on;
ssl_session_cache shared:SSL:10m;

Let's Encrypt

apt install certbot python3-certbot-nginx
certbot --nginx -d example.com -d www.example.com
# Auto-renew via systemd timer: certbot renew

Gotchas

  • Encryption at rest without proper key management is security theater - keys stored next to encrypted data provide no protection
  • TLS termination at a load balancer means traffic between LB and backend is unencrypted unless re-encrypted
  • Certificate pinning prevents MITM but makes certificate rotation painful - use with caution
  • bcrypt has a 72-byte password limit - longer passwords are silently truncated
  • Self-signed certificates encrypt traffic but do not authenticate identity

See Also