Cryptography & PKI

In practice, TLS uses both: asymmetric encryption to exchange a symmetric session key, then symmetric encryption for the actual data — best of both worlds (Rescorla, 2018; Stallings, 2022).

AES-128, AES-192, and AES-256 were standardized as the replacement for DES following the Rijndael block cipher competition (National Institute of Standards and Technology [NIST], 2001). The Diffie-Hellman key exchange protocol solved the foundational key distribution problem (Diffie & Hellman, 1976), while RSA established the practical basis for public-key cryptography and digital signatures (Rivest et al., 1978). ECC provides equivalent security to RSA with much shorter key lengths, making it preferred in constrained environments (Barker, 2020). 3DES is deprecated for new encryption uses (Barker & Roginsky, 2019).

A hash function takes any input and produces a fixed-length output (digest). It is one-way — you cannot reverse it to get the original data. Used to verify integrity, not encrypt (Stallings, 2022; Ferguson et al., 2010).

Salting adds a random value to a password before hashing — defeats rainbow table attacks. Rainbow tables are precomputed hash lookup tables used to crack unsalted passwords (Ferguson et al., 2010; Barker, 2020).

SHA-256 and SHA-512 are part of the SHA-2 family standardized in FIPS 180-4 (NIST, 2015a). SHA-3 uses a fundamentally different sponge-function construction (NIST, 2015b). MD5 was shown to produce collisions in under an hour via differential cryptanalysis (Wang & Yu, 2005), and SHA-1 was subsequently demonstrated to be practically breakable (Stevens et al., 2017). HMAC combines a hash with a shared secret key to provide both integrity and authenticity (NIST, 2008). The CompTIA Security+ SY0-701 exam objectives flag MD5 and SHA-1 as cryptographically broken (CompTIA, 2023).

A digital signature provides three things: Authentication (who sent it), Integrity (wasn't tampered), and Non-repudiation (sender can't deny it) (Diffie & Hellman, 1976; Rivest et al., 1978).

Critical exam rule: You sign with your PRIVATE key (proves it came from you). Others verify with your PUBLIC key. This is the opposite of encryption (where you encrypt with recipient's public key) (Stallings, 2022; CompTIA, 2023). The sender hashes the message, then encrypts the digest with their private key to form the signature; the receiver independently hashes the received message and decrypts the signature with the sender's public key to compare digests (Ferguson et al., 2010).

PKI (Public Key Infrastructure) is the system for issuing, managing, and revoking digital certificates (Cooper et al., 2008; Barker, 2020).

Certificate types: DV (domain-only), OV (org validated), EV (extended validation — green bar). Wildcard cert: *.domain.com — covers all subdomains. SAN: Subject Alternative Name — multiple hostnames on one cert. X.509 v3 is the standard certificate format specifying public key, subject, issuer, validity period, and extensions (Cooper et al., 2008). CRL and OCSP revocation mechanisms are both profiled in RFC 5280, with OCSP providing real-time status rather than a periodically updated list (Cooper et al., 2008; CompTIA, 2023).

TLS (Transport Layer Security) uses asymmetric encryption to establish trust and exchange a session key, then switches to fast symmetric encryption for all data (Rescorla, 2018; McKay & Cooper, 2019).

TLS 1.3, defined in RFC 8446, eliminates static RSA and Diffie-Hellman key exchange, requiring ephemeral key exchange for all sessions, thereby enforcing Perfect Forward Secrecy (Rescorla, 2018). NIST SP 800-52 Rev. 2 mandates TLS 1.2 as the minimum for federal systems and requires support for TLS 1.3 by January 1, 2024 (McKay & Cooper, 2019). TLS 1.0 and 1.1 are deprecated (McKay & Cooper, 2019; CompTIA, 2023).

Key Escrow: A copy of the encryption key is held by a trusted third party — enables recovery if key is lost, but creates a risk if the escrow is compromised (Barker, 2020; Stallings, 2022).

HSM (Hardware Security Module): Physical device that stores and manages cryptographic keys. Tamper-resistant. Used for CA private keys, payment systems (Ferguson et al., 2010; CompTIA, 2023).

Perfect Forward Secrecy (PFS): Each session uses a unique key — even if a long-term key is compromised, past sessions remain safe. Required by TLS 1.3 (Rescorla, 2018). Key stretching algorithms such as PBKDF2 and bcrypt increase the computational cost of brute-force and dictionary attacks against stored password hashes (Ferguson et al., 2010; Barker & Roginsky, 2019).