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05 - Cryptography

Domain 5: Cryptography (9%) - objectives 5.1 to 5.4.

The smallest domain, and the one candidates fear most. It is learnable because the exam asks what each tool is for, not how the mathematics works.

5.1 Reasons and requirements

Cryptography provides confidentiality (encryption), integrity (hashing, MACs), authentication and non-repudiation (digital signatures). Choose it by what you need to protect and why: data classification, legal requirements for protecting personal or payment data, and where the data sits. Data at rest (disks, databases, backups), in transit (networks) and in use (memory) each need a plan; the third is the hardest and is where hardware enclaves help.

Quantum concepts matter for planning. A sufficiently powerful quantum computer would break today's public key algorithms (the ones based on factoring and discrete logarithms), while symmetric algorithms and hashes are weakened but survive with larger sizes. Adversaries can record encrypted traffic today and decrypt it later, so long-lived secrets are at risk first. NIST has standardised post-quantum algorithms; FIPS 203 (August 2024) specifies ML-KEM, a key-encapsulation mechanism meant to resist quantum attack, in three parameter sets (ML-KEM-512, -768 and -1024). For the exam: know the problem and that the answer is migration to post-quantum algorithms, supported by good crypto-agility, meaning the ability to swap algorithms without redesigning systems.

5.2 Cryptography concepts

Symmetric encryption uses one shared key for both directions. It is fast and suits bulk data, but the key distribution problem is hard: how do you get the key to the other side safely? AES, specified in FIPS 197, is the standard: a 128-bit block cipher with key sizes of 128, 192 and 256 bits. Key length is the number that grows strength; a longer key means more brute-force work.

Asymmetric encryption uses a mathematically linked pair: a public key anyone can have and a private key only the owner holds. It solves distribution but is slow, so real systems use it to agree or wrap a symmetric key and then use the symmetric key for the data. This hybrid pattern is behind TLS. Remember the direction of use:

Goal Encrypt or sign with Verify or decrypt with
Confidentiality to Alice Alice's public key Alice's private key
Digital signature by Alice Alice's private key Alice's public key

Hashing turns any input into a fixed-size digest. It is one-way, and a good hash makes changing the input change the digest unpredictably and makes finding two inputs with the same digest (a collision) infeasible. Use hashes for integrity checks and for storing password verifiers. A hash is not encryption and cannot be reversed. Passwords should be hashed with a slow, purpose-built password hashing function and a unique salt per user: a salt is random data mixed in so identical passwords produce different hashes and precomputed (rainbow) tables fail. A pepper is a secret kept outside the database.

Digital signatures hash the message and sign the hash with a private key, giving integrity, authentication and non-repudiation together. A message authentication code (MAC) also gives integrity and authenticity, but with a shared key, so it cannot give non-repudiation: either party could have produced it.

Strength depends on the algorithm, key length and implementation. Cryptanalysis is the study of breaking systems, including brute force, known-plaintext and side-channel attacks, and most real failures come from poor key handling or implementation, not broken mathematics. Never invent your own algorithm; use vetted ones.

5.3 Secure protocols

Match the protocol to the use case, and know the weak ones to retire.

  • TLS secures web and many other connections. TLS 1.3 (RFC 8446, August 2018) is designed to prevent eavesdropping, tampering and message forgery; the RFC page now marks it as superseded by RFC 9846, so check the current document, but the exam principle is unchanged: use current TLS and disable old SSL and early TLS versions.
  • HTTPS is HTTP over TLS. SSH replaces Telnet for remote administration. SFTP and SCP replace FTP.
  • IPsec secures traffic at the network layer, in transport mode (host to host, payload protected) or tunnel mode (gateway to gateway, whole packet protected). It uses IKE to negotiate keys and the AH and ESP protocols, where ESP gives encryption and AH only integrity.
  • S/MIME and PGP protect email content; DNSSEC adds signatures so DNS answers can be verified (integrity, not confidentiality).
  • Limitations: a protocol that encrypts the channel does not protect the data once it arrives, and weak ciphers or expired certificates quietly undo the benefit.

5.4 Public key infrastructure

PKI binds public keys to identities using certificates. A certificate authority (CA) signs certificates after a registration authority verifies the requester. A certificate contains the subject, the public key, validity dates and the issuer's signature; browsers and operating systems trust a set of root CAs, and trust flows down a chain from root through intermediate CAs. Roots are kept offline and rarely used.

When a certificate must be withdrawn before expiry, the CA publishes it on a certificate revocation list (CRL), or answers status queries over OCSP. Checking revocation is what makes a stolen key stop working.

Key management is where PKI succeeds or fails. NIST SP 800-57 Part 1 Rev. 5 covers general key management best practice: the protection each key type needs and the functions involved in managing keys. A key goes through generation, distribution, storage, use, rotation, archival and destruction. Generate keys with good randomness, protect them in a hardware security module or a managed key service, separate duties around them, limit usage periods, back them up securely, and destroy them when retired. Key escrow lets a trusted party hold a copy for recovery. The web of trust is the alternative to CAs, used by PGP, where users vouch for each other's keys.

Key terms

  • Symmetric encryption - encryption in which the same secret key encrypts and decrypts
  • Asymmetric encryption - encryption using a linked public and private key pair
  • Hybrid encryption - using asymmetric cryptography to exchange a key and symmetric cryptography to protect the data
  • Hash function - a one-way function that turns any input into a fixed-size digest
  • Collision - two different inputs producing the same hash digest
  • Salt - random per-user data mixed into a password before hashing so identical passwords differ and precomputed tables fail
  • Digital signature - a hash signed with a private key, giving integrity, authentication and non-repudiation
  • Message authentication code - a keyed checksum giving integrity and authenticity but not non-repudiation
  • Crypto-agility - the ability to replace cryptographic algorithms without redesigning the system
  • Certificate authority - a trusted party that signs certificates binding public keys to identities
  • Certificate revocation list - a CA-published list of certificates withdrawn before their expiry
  • Hardware security module - tamper-resistant hardware that generates and stores keys and performs cryptographic operations
  • Web of trust - a decentralised model in which users vouch for one another's public keys