C H A P T E R 8
Cryptography
Cipher Methods • Algorithms • Tools • Secure Protocols
Principles of Information Security, 6th Edition | Whitman & Mattord
Learning Objectives
1. Define cryptology, cryptography, and cryptanalysis; explain plaintext, ciphertext, key, key space, work factor
2. Describe cipher methods: substitution, transposition, XOR, one-time pad, book, and running key ciphers
3. Explain symmetric encryption: DES (broken), 3DES (deprecated), AES, Blowfish, RC4 (broken)
4. Explain asymmetric (public key) encryption: RSA, Diffie-Hellman, ECC, El Gamal
5. Describe cryptographic hash functions: MD5 (broken), SHA-1 (broken), SHA-256, SHA-3, HMAC
6. Explain digital signatures, digital certificates, Certificate Authorities, and PKI
7. Understand TLS (versions 1.0–1.3), S/MIME, PGP, SSH, and HTTPS protocols
8. Describe IPSec (AH vs ESP, Transport vs Tunnel mode) and wireless security (WEP→WPA3)
Chapter 8: Cryptography
Objectives
Foundations of Cryptology — Core Terminology
CRYPTOLOGY
Science of both making (cryptography) and breaking (cryptanalysis) secret codes
PLAINTEXT
Original readable message before encryption is applied
CIPHERTEXT
Scrambled, unreadable output produced by encryption
CIPHER
The algorithm used to transform plaintext into ciphertext and back
KEY
Value used by the algorithm to control encryption/decryption
KEY SPACE
All possible key values: 2^n for n-bit key — larger = more secure
WORK FACTOR
Computational effort to break a cipher — measures cryptographic strength
STEGANOGRAPHY
Hiding the EXISTENCE of a message in a carrier — different from encryption
KEY SPACE & BRUTE-FORCE RESISTANCE
40-bit key
~1 trillion
Seconds (WEP — BROKEN)
56-bit key
~72 quadrillion
Hours (DES — BROKEN)
128-bit key
~340 undecillion
Billions of years (AES-128)
256-bit key
~1.16×10^77
Quantum-safe (AES-256)
Chapter 8: Cryptography
Cryptology Foundations
Cipher Methods — Substitution, Transposition, XOR & One-Time Pad
Substitution
Replaces each plaintext element with another according to a rule. Monoalphabetic (Caesar: shift by 3) or Polyalphabetic (Vigenère: keyword determines shift).
Weakness: Frequency analysis breaks monoalphabetic ciphers; Kasiski examination breaks polyalphabetic
Transposition
Rearranges the POSITIONS of plaintext elements without changing them. Rail fence: write diagonally then read rows. Columnar: write in rows, read columns in key order.
Weakness: Letter frequency is preserved; statistical attacks possible on simple transposition
XOR Operation
Fundamental binary operation: output=1 if inputs differ. Reversible: Ciphertext=Plaintext XOR Key; Plaintext=Ciphertext XOR Key. Core of all stream ciphers and most block ciphers.
Weakness: If key is reused (same IV), XOR of two ciphertexts = XOR of plaintexts (crib-dragging)
One-Time Pad (OTP)
Vernam cipher with truly random key, same length as message, used ONLY ONCE. Information-theoretically UNBREAKABLE — ciphertext reveals nothing about plaintext. Impractical due to key distribution.
Weakness: Only weakness: key distribution, reuse, and non-random generation destroy security
Book Cipher
Key derived from an agreed-upon book or text. Page/line/word numbers serve as key. Security depends entirely on secrecy of which book is used.
Weakness: If the book is discovered, entire system is broken
Running Key Cipher
Like Vigenère but uses a long (non-repeating) text as the key — as long as the message. Avoids periodic repetition, defeating Kasiski examination.
Weakness: Security depends on randomness and secrecy of the key text
Chapter 8: Cryptography
Cipher Methods
Symmetric Encryption Algorithms
Symmetric encryption: SAME key for encrypt and decrypt. Fast (100–1000× asymmetric). Key distribution problem: must securely share key before communicating.
DES
BROKEN
Key:
56-bit
Block:
64-bit
1977 NIST standard. 16-round Feistel network. Brute-forced in 56 hrs (1998 EFF). Key space too small.
3DES
Deprecated
Key:
112/168-bit
Block:
64-bit
EDE: Encrypt-Decrypt-Encrypt with 2 or 3 keys. NIST deprecated 2017; disallowed after 2023.
AES-128
✓ Standard
Key:
128-bit
Block:
128-bit
FIPS 197 (2001). Rijndael algorithm. 10 rounds. Efficient in HW and SW. Current minimum standard.
AES-256
✓ Quantum-safe
Key:
256-bit
Block:
128-bit
14 rounds. Highest security. Grover's algorithm: 256→128-bit effective. Quantum-resistant.
RC4
BROKEN
Key:
Variable
Block:
Stream
Ron Rivest stream cipher. Used in WEP and early SSL. RC4 bias attacks (NOMORE). RFC 7465 bans in TLS.
Blowfish
Legacy
Key:
32–448-bit
Block:
64-bit
Bruce Schneier (1993). Patent-free and fast. 64-bit block → SWEET32 birthday attack. Superseded by Twofish.
Chapter 8: Cryptography
Symmetric Algorithms
Asymmetric (Public Key) Encryption
Key pair: PUBLIC key (freely shared) + PRIVATE key (kept secret). What one key encrypts, only the other decrypts.
CONFIDENTIALITY
1
Encrypt with recipient's PUBLIC key
2
Only recipient's PRIVATE key decrypts
3
Anyone can send; only recipient reads
AUTHENTICATION (Digital Signature)
1
Sign with sender's PRIVATE key
2
Anyone with sender's PUBLIC key verifies
3
Only the private key holder could have signed
RSA
Prime factorization
Most widely used. 2048-bit minimum. Key exchange, digital signatures, certificates. Named after Rivest, Shamir, Adleman (1977).
Diffie-Hellman
Discrete logarithm
Key exchange ONLY (not encryption). Invented 1976. First practical solution to key distribution problem. Vulnerable to MitM without authentication.
ECC
Elliptic curves
Elliptic Curve Cryptography. 256-bit ECC ≈ 3072-bit RSA. More efficient — ideal for mobile and IoT. Used in TLS (ECDHE), Bitcoin.
El Gamal
Discrete logarithm
Used for encryption and signatures. Basis for DSA (Digital Signature Standard). Ciphertext ~2× plaintext size. Used in PGP/GPG.
★ Sign with PRIVATE key → verify with PUBLIC key. Encrypt with PUBLIC key → decrypt with PRIVATE key.
Chapter 8: Cryptography
Asymmetric Encryption
Cryptographic Hash Functions
One-way function: any-length input → fixed-length output (digest). Cannot be reversed. Properties: pre-image resistance, collision resistance, avalanche effect.
Algorithm
Output
Status
Notes / Use Case
MD5
128 bits
BROKEN
Collision attacks demonstrated 1996, practical 2004. Non-security checksums only. NEVER for passwords.
SHA-1
160 bits
BROKEN
SHAttered attack (2017) — full collision demonstrated. Banned in certificates. Do NOT use.
SHA-256
256 bits
✓ Standard
SHA-2 family. Current minimum for security applications. TLS, code signing, Bitcoin blockchain.
SHA-384
384 bits
✓ High Sec
SHA-2 family. NSA Suite B. High-security TLS configurations.
SHA-512
512 bits
✓ High Sec
SHA-2 family. Faster than SHA-256 on 64-bit systems.
SHA-3 (Keccak)
224–512b
✓ Modern
Different design (sponge construction vs Merkle-Damgård). NIST 2015. Not widely deployed yet.
bcrypt/Argon2
Variable
✓ Passwords
Deliberately SLOW + salted. Purpose-built for password storage. Use instead of SHA for passwords.
★ MD5 & SHA-1 are BROKEN. Use SHA-256+ for integrity. Use bcrypt/Argon2 for passwords. NEVER store unsalted hashes.
Chapter 8: Cryptography
Hash Functions
Digital Signatures, Certificates & PKI
DIGITAL SIGNATURE PROCESS
SIGN (Sender)
Hash the message → Encrypt hash with PRIVATE key → Attach signature
SEND
Message + digital signature transmitted to recipient
VERIFY (Recipient)
Decrypt signature with sender's PUBLIC key → Compare with hash of received message → MATCH = valid
PKI TRUST HIERARCHY
Root CA
Self-signed; kept OFFLINE for security; trust anchor; signs intermediate CA certs only
Intermediate CA
Online; signs end-entity certificates; if compromised, limited scope revocation
End-Entity Cert
Server (TLS), person (S/MIME), code (signing); contains subject's public key
PKI COMPONENTS
CRL:
Certificate Revocation List — list of revoked certificate serial numbers published by CA
OCSP:
Online Certificate Status Protocol — real-time single-certificate validity checking
RA:
Registration Authority — handles identity verification on behalf of CA
LDAP:
Certificate repository — stores certificates and CRLs for retrieval
Chapter 8: Cryptography
Digital Signatures & PKI
SSL/TLS — Securing Web and Network Communications
TLS VERSION STATUS
SSL 2.0
BROKEN
Multiple critical flaws. Deprecated 2011.
SSL 3.0
POODLE
POODLE attack. Deprecated 2015.
TLS 1.0
Deprecated
BEAST, POODLE; PCI DSS prohibited since 2018.
TLS 1.1
Deprecated
RFC 8996 deprecated. PCI DSS prohibited.
TLS 1.2
Min. Standard
Secure when configured correctly. Avoid weak ciphers.
TLS 1.3
✓ BEST
Forward secrecy mandatory. All weak ciphers removed. Faster.
TLS HANDSHAKE (HYBRID ENCRYPTION)
1
1. Client Hello:
Supported TLS versions + cipher suites
2
2. Server Hello + Cert:
Chosen cipher suite + X.509 certificate
3
3. Key Exchange:
Verify cert; DHE/ECDHE generates session keys (PFS)
4
4. Session Keys:
Both derive symmetric session key from exchange
5
5. Symmetric Data:
All application data encrypted with AES-GCM/ChaCha20
TLS = HYBRID: Asymmetric (RSA/ECDHE) for key exchange + Symmetric (AES) for bulk data.
Chapter 8: Cryptography
TLS/SSL
Email Security — S/MIME vs PGP
S/MIME
Secure/Multipurpose Internet Mail Extensions
Trust: Hierarchical PKI — X.509 Certificates from Certificate Authorities
Sender signs email with own PRIVATE key
Recipient encrypts with sender's PUBLIC key (from X.509 cert)
Both parties need valid X.509 certificates
Built into Outlook, iOS Mail, Apple Mail
✓ Enterprise-friendly; managed PKI; certificate revocation support
✗ Requires certificate from CA; cost; PKI administration overhead
PGP / GPG
Pretty Good Privacy / GNU Privacy Guard
Trust: Web of Trust — users vouch for each other's public keys
Same encrypt/sign operations as S/MIME
Keys generated by users; no CA required
Users exchange public keys directly or via keyserver
GPG is free, open-source OpenPGP implementation
✓ No CA required; free; strong security; suitable for individual use
✗ Decentralized key management; harder for enterprise; requires plugin
Chapter 8: Cryptography
S/MIME & PGP
IPSec — Internet Protocol Security
IPSec: Security at the IP layer (Layer 3). Used for VPNs. Two protocols: AH (auth only) and ESP (encryption + auth). Two modes: Transport and Tunnel.
AH — Authentication Header
Provides: Authentication + Integrity — NO encryption
• Authenticates entire IP packet (including outer header)
• Provides data integrity and source authentication
• Does NOT encrypt — content visible to eavesdroppers
• Incompatible with NAT (covers IP header which NAT modifies)
ESP — Encapsulating Security Payload
Provides: Encryption + Authentication + Integrity
• Encrypts IP payload (data)
• Optional authentication of packet
• NAT-compatible (does not cover outer IP header)
• The most commonly used IPSec protocol in practice
IPSEC MODES
TRANSPORT MODE
Protects: IP PAYLOAD only (data)
Header: Original IP header visible
Use: Host-to-host protection. Two systems communicating directly.
TUNNEL MODE
Protects: ENTIRE original packet
Header: New outer IP header added
Use: Site-to-site VPN. Gateway-to-gateway. Entire packet encrypted.
Chapter 8: Cryptography
IPSec
SSH, HTTPS & Secure Protocol Port Reference
SSH (Secure Shell) (Port TCP 22)
Secure remote administration, file transfer (SFTP/SCP), and port forwarding. Replaced: Telnet (23), FTP (21), rsh/rlogin. Authentication: password or public key (preferred).
HTTPS (Port TCP 443)
HTTP over TLS. Server certificate verifies website identity. All data encrypted. HSTS (HTTP Strict Transport Security) prevents SSL stripping downgrade attacks.
SECURE PROTOCOL PORT REFERENCE
Protocol
Port
Purpose
HTTPS
443
HTTP over TLS — secure web browsing
SMTPS
465 / 587
SMTP over TLS — secure email sending
IMAPS
993
IMAP over TLS — secure email retrieval
POP3S
995
POP3 over TLS — secure email retrieval
SSH / SFTP / SCP
22
Secure shell, secure file transfer, secure copy
FTPS
990 (implicit) / 21 (explicit)
FTP over TLS — secure file transfer
LDAPS
636
LDAP over TLS — secure directory services
DNSSEC
53
DNS with digital signatures (integrity, not encryption)
Chapter 8: Cryptography
SSH & Protocol Ports
Wireless Security Protocols — WEP, WPA, WPA2, WPA3
WEP
1999
COMPLETELY BROKEN
Encryption:
RC4 (40/104-bit)
Key Mgmt:
Static IV (24-bit)
IV too short (24-bit) — reused quickly. RC4 keystream recovery in minutes. Aircrack-ng breaks WEP trivially. NEVER USE.
WPA
2003
DEPRECATED — avoid
Encryption:
TKIP (RC4-based)
Key Mgmt:
Per-packet key mixing
TKIP was interim fix for WEP. RC4 still vulnerable. WPA PSK offline dictionary attacks remain possible. Use WPA2 minimum.
WPA2
2004
CURRENT MINIMUM ✓
Encryption:
CCMP (AES-128)
Key Mgmt:
4-way handshake + PSK
802.11i standard. AES/CCMP is strong. KRACK (2017) affects handshake but patched. WPA2-Enterprise (802.1X) preferred over PSK for corporate.
WPA3
2018
RECOMMENDED BEST ✓✓
Encryption:
GCMP (AES-256)
Key Mgmt:
SAE (Dragonfly)
SAE replaces PSK — eliminates offline dictionary attacks. PMF (Protected Management Frames) mandatory. 192-bit mode for enterprise. Forward secrecy per session.
★ WEP = BROKEN (never use). WPA = deprecated. WPA2 = current minimum. WPA3 = best standard (SAE, PMF).
Chapter 8: Cryptography
Wireless Security
Key Exam Distinctions — Chapter 8 Cryptography
Symmetric vs Asymmetric?
Symmetric: same key, fast, key distribution problem. Asymmetric: key pair (public/private), slow, solves key distribution. Hybrid systems use both.
Sign vs Encrypt key direction?
Sign: PRIVATE key signs, PUBLIC key verifies. Encrypt: PUBLIC key encrypts, PRIVATE key decrypts. Opposite directions — common exam trap.
Hash vs HMAC vs Digital Signature?
Hash: integrity only. HMAC: integrity + authentication (shared key). Digital Signature: integrity + authentication + non-repudiation (key pair).
DES vs AES status?
DES (56-bit) = BROKEN (brute-forceable). 3DES = deprecated. AES-128 = current minimum. AES-256 = quantum-resistant. AES = NIST FIPS 197.
MD5 vs SHA-1 vs SHA-256?
MD5 = BROKEN (collisions 2004). SHA-1 = BROKEN (SHAttered 2017). SHA-256 = current minimum standard. Use bcrypt/Argon2 for passwords.
IPSec AH vs ESP?
AH: authentication/integrity ONLY, no encryption, incompatible with NAT. ESP: encryption + authentication, NAT-compatible, most common. ESP can include AH's functions.
Transport mode vs Tunnel mode?
Transport: encrypts payload only, original IP header visible, host-to-host. Tunnel: encrypts entire original packet, new outer header added, site-to-site VPN.
S/MIME vs PGP trust model?
S/MIME: hierarchical PKI — X.509 certificates from Certificate Authorities. PGP: web of trust — users vouch for each other's public keys. No CA required.
Chapter 8: Cryptography
Key Distinctions
Algorithm & Protocol Quick Reference
Algorithm
Type
Key/Output
Status
Quantum?
Primary Use
DES
Sym Block
56-bit
BROKEN
✗ No
Legacy only — do not use
3DES
Sym Block
112/168-bit
Deprecated
✗ No
Phasing out
AES-128
Sym Block
128-bit
✓ Standard
—
TLS, disk enc, WPA2
AES-256
Sym Block
256-bit
✓ Quantum-safe
—
High security, long-term
RC4
Stream
Variable
BROKEN
✗ No
Banned in TLS (RFC 7465)
Blowfish
Sym Block
32–448-bit
Legacy
✗ No
Not recommended (64-bit block)
RSA-2048
Asymmetric
2048-bit
✓ Min. Std.
—
Key exchange, signatures, certs
ECC P-256
Asymmetric
256-bit
✓ Recommended
—
TLS, mobile, IoT, Bitcoin
Diff-Hellman
Key Exch
2048+
✓ (DHE=PFS)
—
Key exchange in TLS, IPSec
MD5
Hash
128-bit out
BROKEN
✗ No
Checksums only — not security
SHA-1
Hash
160-bit out
BROKEN
✗ No
Do not use — deprecated
SHA-256
Hash
256-bit out
✓ Standard
—
TLS, code signing, Bitcoin
HMAC-SHA256
MAC
256-bit out
✓ Standard
—
TLS record integrity, IPSec
Chapter 8: Cryptography
Algorithm Quick Ref
Exam Tips — Chapter 8 Cryptography
1
DES (56-bit) = BROKEN. 3DES = deprecated. AES-128 = current standard (FIPS 197). AES-256 = quantum-resistant. Never deploy DES.
2
Symmetric: same key, fast, key distribution problem. Asymmetric: key pair, slow, solves key distribution. TLS uses HYBRID: ECDHE (key exchange) + AES (bulk data).
3
Sign with PRIVATE key → verify with PUBLIC key. Encrypt with PUBLIC key → decrypt with PRIVATE key. These use keys in OPPOSITE directions.
4
MD5 = BROKEN (2004). SHA-1 = BROKEN (SHAttered 2017). SHA-256 = current minimum. Argon2/bcrypt for passwords (deliberately slow + salted).
5
HMAC = hash + secret key → integrity AND authentication. Plain hash → integrity only. Digital signature → integrity + auth + non-repudiation.
6
IPSec: AH = auth/integrity only (NO encryption, NAT-incompatible). ESP = encryption + auth (NAT-compatible, most common). Transport = payload only. Tunnel = full packet.
7
TLS 1.0/1.1 = deprecated. TLS 1.2 = minimum. TLS 1.3 = best (forward secrecy mandatory). S/MIME = X.509 PKI. PGP = web of trust. SSH port 22.
8
WEP = BROKEN (24-bit IV). WPA = deprecated (TKIP). WPA2 = current minimum (AES-CCMP). WPA3 = best (SAE eliminates offline dict attacks, PMF mandatory).
Chapter 8: Cryptography
Exam Tips
Key Management & Cryptographic Tools — Supporting Infrastructure
KEY MANAGEMENT LIFECYCLE
1
Generation:
CSPRNG (Cryptographically Secure PRNG). Hardware RNG for high-value keys. Entropy quality critical.
2
Distribution:
Key encapsulation with asymmetric crypto; Diffie-Hellman; secure key exchange protocols.
3
Storage:
Hardware Security Module (HSM); encrypt keys at rest; separate key from encrypted data.
4
Rotation:
Replace before expiry; re-encrypt data with new key; automate rotation where possible.
5
Destruction:
Cryptographic erasure; multiple overwrites; HSM key zeroization; certificate of destruction.
CRYPTOGRAPHIC TOOLS
HSM (Hardware Security Module)
Physical device for secure key generation, storage, and management. Keys never leave the HSM in plaintext. Tamper-resistant. Required for PKI root keys and payment systems.
Certificate Authority Software
Issues and manages digital certificates. Examples: Microsoft AD CS, OpenSSL, Let's Encrypt (free DV certificates for HTTPS).
Key Management Service (KMS)
Cloud-based or on-premise systems managing cryptographic keys at scale. AWS KMS, Azure Key Vault, Google Cloud KMS, HashiCorp Vault.
Password Hashing Tools
bcrypt, scrypt, Argon2 — deliberately slow hash functions with built-in salting for secure password storage. NIST recommends Argon2.
Chapter 8: Cryptography
Key Management & Tools
Hybrid Encryption in Practice — How TLS Works
TLS combines ALL THREE cryptography types: Asymmetric (key exchange + auth) + Symmetric (bulk encryption) + Hash (integrity). This is HYBRID encryption.
1
Authentication
Asymmetric Phase
Server presents X.509 certificate. Client verifies using CA chain. RSA or ECDSA signature proves server identity.
Solves authentication problem — confirms you're talking to the real server
2
Key Exchange (ECDHE)
Asymmetric Phase
Ephemeral Diffie-Hellman generates new session key material for THIS session only. Neither party transmits the actual key.
Provides Perfect Forward Secrecy (PFS) — compromise of server key does not expose past sessions
3
Session Key Derivation
Symmetric Phase
Both sides derive the same symmetric session keys from the DH exchange. AES-128 or AES-256 keys generated.
Fast symmetric keys ready for bulk data — solves the speed problem
4
Record Integrity (HMAC/AEAD)
Hash/MAC Phase
Each TLS record authenticated with HMAC or AEAD (GCM). Prevents tampering with encrypted data in transit.
Detects any modification to encrypted traffic — provides integrity alongside confidentiality
Chapter 8: Cryptography
Hybrid Encryption / TLS
Historical Ciphers & Steganography
HISTORICAL CIPHERS — Foundation Concepts
Caesar Cipher
~50 BC
Shifts each letter by 3 (A→D). Monoalphabetic substitution. Broken by frequency analysis — "E" is most common letter.
Vigenère
1553
Polyalphabetic: keyword determines shift at each position. Resists simple frequency analysis. Broken by Kasiski examination.
Enigma Machine
WWII
German electro-mechanical rotor cipher. Broke by Turing at Bletchley Park using cribs (known plaintext). Changed WWII outcome.
One-Time Pad
1917
Vernam cipher. Theoretically unbreakable if used correctly. Impractical due to key distribution problem (key as long as message).
DES
1977
First US government encryption standard. 56-bit key. Broken by EFF Deep Crack in 56 hours (1998). Replaced by AES (2001).
STEGANOGRAPHY vs CRYPTOGRAPHY
CRYPTOGRAPHY: Hides the CONTENT of a message (message existence is known)
STEGANOGRAPHY: Hides the EXISTENCE of a message inside a carrier (image, audio)
Chapter 8: Cryptography
Historical Ciphers & Steg
Chapter 8 Summary — Key Takeaways
1
Cipher methods: Substitution (replace elements) + Transposition (rearrange positions). XOR is the core operation. One-time pad = theoretically unbreakable.
2
Symmetric encryption: DES (BROKEN/56-bit), 3DES (deprecated), AES-128 (current standard), AES-256 (quantum-resistant). RC4 = BROKEN stream cipher.
3
Asymmetric: RSA (factoring), Diffie-Hellman (key exchange only, discrete log), ECC (256-bit ≈ 3072-bit RSA), El Gamal (basis for DSA/PGP).
4
Hash functions: MD5 (BROKEN), SHA-1 (BROKEN), SHA-256 (current standard). HMAC = hash + key. Digital signature = integrity + auth + non-repudiation.
5
PKI: Root CA (offline) → Intermediate CA → End-entity cert (X.509). CRL = revocation list. OCSP = real-time. S/MIME = PKI. PGP = web of trust.
6
TLS 1.0/1.1 = deprecated. TLS 1.2 = minimum. TLS 1.3 = best (PFS mandatory). Hybrid: ECDHE (key exchange) + AES (encryption) + HMAC (integrity).
7
IPSec: AH = auth only (no encryption, NAT-incompatible). ESP = encryption + auth. Transport mode = payload. Tunnel mode = full packet (site-to-site VPN).
8
WEP = BROKEN (24-bit IV). WPA = deprecated (TKIP). WPA2 = current minimum (AES-CCMP). WPA3 = best (SAE, PMF). SSH port 22. HTTPS port 443.
Principles of Information Security, 6th Edition | Whitman & Mattord | Chapter 8: Cryptography