Encryption_gpg

Chapter 37: Encryption and GPG

Overview

Encryption is the cornerstone of Linux system security. This chapter covers symmetric and asymmetric encryption, GPG key management, disk encryption with LUKS, filesystem-level encryption, and practical production implementations. Mastery of these topics is essential for DevOps and SRE roles securing sensitive data.

36.1 Encryption Fundamentals

Types of Encryption

┌─────────────────────────────────────────────────────────────────────────┐
│                        ENCRYPTION TYPES                                 │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                         │
│  SYMMETRIC                  ASYMMETRIC                                 │
│  ┌─────────┐               ┌─────────┐     ┌─────────┐                 │
│  │   AES   │               │   RSA   │     │  ECC   │                 │
│  │   256   │               │ 4096-bit│     │ 384-bit│                 │
│  └────┬────┘               └────┬────┘     └────┬────┘                 │
│       │                        │               │                       │
│       └───────────┬────────────┴───────────────┘                       │
│                   │                                                    │
│           ┌───────▼───────┐                                             │
│           │  Same Key    │  Fast, for bulk data                        │
│           │  Encrypt/    │  (files, disks)                              │
│           │  Decrypt     │                                             │
│           └─────────────┘                                             │
│                                                                         │
│  HYBRID (Common Practice)                                              │
│  ┌─────────────────────────────────────────────────────────┐          │
│  │  1. Generate random symmetric key (AES)                 │          │
│  │  2. Encrypt data with symmetric key (fast)               │          │
│  │  3. Encrypt symmetric key with recipient's public key    │          │
│  │  4. Send both encrypted key + encrypted data              │          │
│  └─────────────────────────────────────────────────────────┘          │
│                                                                         │
└─────────────────────────────────────────────────────────────────────────┘

Key Sizes and Strength

Algorithm	Key Size	Use Case	Security Level
AES-128	128-bit	Legacy systems	Adequate
AES-256	256-bit	Standard	Very Strong
RSA-2048	2048-bit	Signatures, key exchange	Standard
RSA-4096	4096-bit	High security	Very Strong
ECC-256	256-bit	Mobile/embedded	Equivalent to RSA-3072
ChaCha20	256-bit	Modern, TLS	Very Strong

35.2 GPG (GNU Privacy Guard)

GPG Architecture

┌─────────────────────────────────────────────────────────────────────────┐
│                          GPG ECOSYSTEM                                  │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                         │
│   KEY GENERATION                                                        │
│   ┌────────────────────────────────────────────────────────────┐       │
│   │  ┌──────────────┐         ┌──────────────┐                 │       │
│   │  │ Master Key   │         │  Subkeys     │                 │       │
│   │  │ (Certification)       │ (Signing/     │                 │       │
│   │  │ - Sign other keys     │  Encryption/  │                 │       │
│   │  │ - Create subkeys      │  Authentication│                 │       │
│   │  └──────────────┘         └──────────────┘                 │       │
│   └────────────────────────────────────────────────────────────┘       │
│                                                                         │
│   KEYRING STRUCTURE                                                     │
│   ~/.gnupg/                                                             │
│   ├── pubring.kbx     (Public keys + signatures)                       │
│   ├── pubring.gpg     (Legacy public keyring)                          │
│   ├── secring.gpg     (Secret keys - DEPRECATED)                       │
│   ├── private-keys-v1.d/  (Secret key stubs for gpg-agent)             │
│   ├── gpg.conf        (Options)                                        │
│   ├── gpg-agent.conf  (Agent options)                                  │
│   └── trustdb.gpg     (Ownertrust values)                              │
│                                                                         │
│   OPERATIONS                                                            │
│   ┌────────────────────────────────────────────────────────────┐       │
│   │  ENCRYPT                    DECRYPT                        │       │
│   │  ┌─────────┐                ┌─────────┐                     │       │
│   │  │Message  │                │ Encrypted│                    │       │
│   │  │  File   │──►Encrypt──►   │  Data   │──►Decrypt──►Message │       │
│   │  └─────────┘   with RSA     └─────────┘   with RSA  └─────────┘     │
│   │       │                    (needs private key)                     │
│   │       │                                                         │       │
│   │  ┌────▼────┐                ┌────▼────┐                         │       │
│   │  │Public   │                │Private  │                         │       │
│   │  │Key of   │                │Key of   │                         │       │
│   │  │Receiver │                │Receiver │                         │       │
│   │  └─────────┘                └─────────┘                         │       │
│   └────────────────────────────────────────────────────────────┘       │
│                                                                         │
│   SIGNING VERIFICATION                                                  │
│   ┌────────────────────────────────────────────────────────────┐       │
│   │  SIGN                   VERIFY                              │       │
│   │  ┌─────────┐            ┌─────────┐                         │       │
│   │  │Message/ │──►Sign──►  │Signature│──►Verify──► Valid/Invalid│       │
│   │  │  File   │  with RSA  │  (.sig) │  with public key        │       │
│   │  └─────────┘   Private   └─────────┘                         │       │
│   └────────────────────────────────────────────────────────────┘       │
│                                                                         │
└─────────────────────────────────────────────────────────────────────────┘

Key Generation Best Practices

# ============================================================
# PRODUCTION KEY GENERATION - Step by Step
# ============================================================

# Step 1: Configure GPG for better security
mkdir -p ~/.gnupg
chmod 700 ~/.gnupg

cat > ~/.gnupg/gpg.conf << 'EOF'
# Use modern algorithms
default-preference-list SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES168
BZIP2 ZLIB ZIP Uncompressed

# Modern keyserver
keyserver hkps://keyserver.ubuntu.com

# Security options
no-tty
no-emit-version
no-comments

# Fast options (if needed for automation)
# use-agent
EOF

cat > ~/.gnupg/gpg-agent.conf << 'EOF'
default-cache-ttl 600
max-cache-ttl 3600
allow-loopback-pinentry
EOF

# Step 2: Generate master key (Certification only)
gpg --full-generate-key
# Select:
# 1. RSA and RSA (default)
# 2. 4096 bits
# 3. Key does not expire (or set reasonable expiry)
# 4. Your name
# 5. Your email
# 6. Passphrase (use a strong, unique password)

# Step 3: Add subkeys for daily use
gpg --edit-key your@email.com

# In gpg> interactive mode:
# addkey
#   4. RSA (sign only) - 4096 bits
#   Save
# addkey
#   5. RSA (encrypt only) - 4096 bits
#   Save
# addkey
#   6. Authentication - 4096 bits
#   Save
# quit

# Step 4: Backup your keys (CRITICAL!)
# Export master key (keep secure!)
gpg --armor --export-secret-keys your@email.com > master-key-backup.asc
gpg --armor --export-secret-subkeys your@email.com > subkeys-backup.asc

# Store these securely (encrypted USB, secure location)
# DELETE the secret keys from your machine after backup!

# Step 5: Export public key for distribution
gpg --armor --export your@email.com > public-key.asc
gpg --send-keys your@email.com  # Upload to keyserver

Key Management Operations

# ============================================================
# KEY MANAGEMENT COMMANDS
# ============================================================

# List keys
gpg --list-keys                      # Public keys
gpg --list-secret-keys              # Private keys
gpg --list-signatures                # Signatures on keys
gpg --fingerprint your@email.com    # Fingerprint (short form)

# Keyserver operations
gpg --keyserver keyserver.ubuntu.com --search-keys "search term"
gpg --keyserver keyserver.ubuntu.com --recv-key KEYID
gpg --keyserver keyserver.ubuntu.com --send-key KEYID
gpg --refresh-keys                   # Update all keys from keyserver

# Key editing
gpg --edit-key your@email.com
# Commands: list, uid, key, addkey, delkey, revuid, sign, check, quit

# Change passphrase
gpg --change-passphrase your@email.com

# Expire key
gpg --edit-key your@email.com
# gpg> expire
# gpg> save

# Sign a key (establishing trust)
gpg --sign-key other@email.com       # Sign with your key
gpg --lsign-key other@email.com      # Local signature (not exported)

# Export/Import
gpg --armor --export your@email.com > public.asc
gpg --armor --export-secret-keys your@email.com > private.asc
gpg --import keyfile.asc
gpg --import --import-filter import-clean-sigs keyfile.asc

# Delete keys
gpg --delete-secret-keys your@email.com  # First delete secret
gpg --delete-keys your@email.com          # Then public

File Encryption/Decryption

# ============================================================
# FILE ENCRYPTION - Multiple Methods
# ============================================================

# Method 1: Encrypt for specific recipient
gpg -e -r recipient@email.com file.txt
# Output: file.txt.gpg

# Method 2: Encrypt with compression
gpg -z 9 -e -r recipient@email.com largefile.tar
# -z 9 = maximum compression

# Method 3: Encrypt with symmetric cipher (password-based)
gpg -c sensitive.doc
# Prompts for passphrase
# Output: sensitive.doc.gpg

# Method 4: Encrypt + sign (both encryption and authenticity)
gpg -e -s -u sender@email.com -r recipient@email.com message.txt

# Method 5: ASCII armor output (for email/text)
gpg -a -e -r recipient@email.com file.txt
# Output: file.txt.asc (ASCII format)

# Decryption
gpg -d file.txt.gpg                    # Decrypt to stdout
gpg -d -o output.txt file.txt.gpg       # Decrypt to file
gpg file.txt.gpg                        # Interactive decryption

# Symmetric decryption
gpg -d symmetric.gpg

# Batch decryption (for scripts)
echo "passphrase" | gpg --batch --yes --passphrase-fd 0 -d file.gpg

Digital Signatures

# ============================================================
# SIGNING AND VERIFICATION
# ============================================================

# Binary signature (creates .sig file)
gpg --sign document.pdf
# Output: document.pdf.sig

# Clear text signature (for email - visible signature)
gpg --clearsign message.txt
# Output: message.txt.asc
# Contains the message + signature in ASCII

# Detached signature (separate signature file)
gpg --armor --detach-sign document.pdf
# Output: document.pdf.asc
# Use when you don't want to modify original file

# Sign with specific key
gpg -u sender@email.com --sign document.txt

# Multiple signatures
gpg --sign --local-user key1@email.com \
       --sign --local-user key2@email.com document.txt

# Verification
gpg --verify file.txt.sig              # Verify binary signature
gpg --verify file.txt.asc               # Verify ASCII/armored signature
gpg --verify document.pdf.asc document.pdf  # Verify detached signature

# Verbose verification (shows key details)
gpg --verify --verbose file.txt.asc

Web of Trust

┌─────────────────────────────────────────────────────────────────────────┐
│                         WEB OF TRUST                                    │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                         │
│   Trust Levels:                                                         │
│   ┌─────────────────────────────────────────────────────────────┐     │
│   │  ultimate  (u)  - You trust this key completely            │     │
│   │                   (usually your own keys)                   │     │
│   │  full      (f)  - You trust this key fully                 │     │
│   │  marginal  (m)  - You trust this key somewhat              │     │
│   │  never     (n)  - You do not trust this key                │     │
│   │  unknown   (o)  - No trust decision made                   │     │
│   └─────────────────────────────────────────────────────────────┘     │
│                                                                         │
│   Trust Path Example:                                                  │
│                                                                         │
│        [YOU]                                                            │
│          │                                                              │
│          │ signs                                                        │
│          ▼                                                              │
│   ┌─────────────┐     signs         ┌─────────────┐                    │
│   │   Friend A  │ ───────────────►   │   Friend B  │                    │
│   │  (full)     │                     │  (marginal) │                    │
│   └─────────────┘                     └─────────────┘                   │
│                                                 │                        │
│                              signs               │                        │
│                                                 ▼                        │
│                                          ┌─────────────┐                 │
│                                          │   Target    │                 │
│                                          │   Key       │                 │
│                                          └─────────────┘                 │
│                                                                         │
│   You trust Target if:                                                 │
│   - At least one path with "full" trust, OR                           │
│   - At least 3 paths with "marginal" trust                             │
│                                                                         │
└─────────────────────────────────────────────────────────────────────────┘

# Trust management commands
gpg --edit-key your@email.com
# gpg> trust
# Select trust level (1-5)
# gpg> quit

GPG Agent and Caching

# ============================================================
# GPG-AGENT CONFIGURATION
# ============================================================

# Configuration file: ~/.gnupg/gpg-agent.conf

cat > ~/.gnupg/gpg-agent.conf << 'EOF'
# Cache TTL settings
default-cache-ttl 3600          # Default: 600 seconds
max-cache-ttl 86400            # Maximum: 1 day

# PIN entry
pinentry-program /usr/bin/pinentry-gnome3
allow-loopback-pinentry

# Agent lifecycle
# gpg-agent --daemon           # Start agent
# gpgconf --kill gpg-agent     # Stop agent

# Check agent status
gpg-connect-agent /bye
gpg-agent --homedir ~/.gnupg --daemon

# Reload agent
gpg-connect-agent RELOADAGENT /bye

35.3 LUKS (Linux Unified Key Setup)

LUKS Architecture

┌─────────────────────────────────────────────────────────────────────────┐
│                        LUKS DISK ENCRYPTION                             │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                         │
│   Physical Disk Layout:                                                │
│   ┌──────────────────────────────────────────────────────────────┐    │
│   │  LUKS Header (1-2MB)    │       Encrypted Data               │    │
│   │  ┌──────────────────┐   │   ┌─────────────────────────────┐   │    │
│   │  │ Magic String     │   │   │                             │   │    │
│   │  │ (LUKS version)   │   │   │   dm-crypt / dm-luks       │   │    │
│   │  │ Key Slots (8)    │   │   │   (AES-XTS, plain/luks2)   │   │    │
│   │  │ - PBKDF2 params  │   │   │                             │   │    │
│   │  │ - Salt           │   │   │   (Encrypted User Data)    │   │    │
│   │  │ - Key Material   │   │   │                             │   │    │
│   │  └──────────────────┘   │   └─────────────────────────────┘   │    │
│   └──────────────────────────────────────────────────────────────┘    │
│                                                                         │
│   Key Slots (8 maximum):                                               │
│   ┌─────────────────────────────────────────────────────────────┐     │
│   │ Slot 0 │ Slot 1 │ Slot 2 │ Slot 3 │ Slot 4 │ Slot 5 │ Slot 6 │Slot 7│
│   │Active  │Active  │ Empty  │ Empty  │ Empty  │ Empty  │ Empty │Empty │
│   └─────────────────────────────────────────────────────────────┘     │
│   Each slot can hold a different passphrase                           │
│   Allows: key rotation, multiple users, backup passphrases             │
│                                                                         │
│   Encryption Workflow:                                                │
│   ┌─────────────────────────────────────────────────────────────┐     │
│   │  1. PBKDF2 derives key from passphrase + salt               │     │
│   │  2. Master Key (stored in header) decrypted temporarily      │     │
│   │  3. Master Key used to encrypt/decrypt disk data             │     │
│   │  4. Master Key held in kernel keyring (not in header)       │     │
│   └─────────────────────────────────────────────────────────────┘     │
│                                                                         │
└─────────────────────────────────────────────────────────────────────────┘

LUKS Operations

# ============================================================
# LUKS DISK ENCRYPTION - Complete Guide
# ============================================================

# Prerequisites
sudo apt install cryptsetup  # Debian/Ubuntu
sudo yum install cryptsetup # RHEL/CentOS

# Step 1: Prepare the device
lsblk                                    # List block devices
sudo parted /dev/sdb                     # Partition if needed
sudo mkfs.ext4 /dev/sdb1                 # Create filesystem first
# OR create partition: sudo parted /dev/sdb mklabel gpt mkpart primary 0% 100%

# Step 2: Initialize LUKS container
sudo cryptsetup luksFormat /dev/sdb1

# WARNING: This ERASES all data!
# Enter YES (uppercase)
# Enter and confirm passphrase (use strong passphrase!)

# With custom options:
sudo cryptsetup luksFormat --type luks2 \
    --cipher aes-xts-plain64 \
    --key-size 512 \
    --hash sha512 \
    --iter-time 5000 \
    --pbkdf argon2id \
    /dev/sdb1

# LUKS2 is the default since cryptsetup 2.1.0
# Argon2id is recommended (memory-hard, GPU resistant)

# Step 3: Open the container
sudo cryptsetup luksOpen /dev/sdb1 secure_volume
# Creates device mapper: /dev/mapper/secure_volume

# Step 4: Create filesystem
sudo mkfs.ext4 /dev/mapper/secure_volume

# Step 5: Mount and use
sudo mkdir /mnt/encrypted
sudo mount /dev/mapper/secure_volume /mnt/encrypted

# Write some test data
echo "Sensitive data" | sudo tee /mnt/encrypted/secret.txt

# Step 6: Unmount and close
sudo umount /dev/mapper/secure_volume
sudo cryptsetup luksClose secure_volume

# ============================================================
# LUKS MANAGEMENT
# ============================================================

# View LUKS header info
sudo cryptsetup luksDump /dev/sdb1
# Shows: version, cipher, key size, UUID, key slots, etc.

# Add a new passphrase
sudo cryptsetup luksAddKey /dev/sdb1
# Prompts for existing passphrase, then new passphrase

# Remove a passphrase
sudo cryptsetup luksRemoveKey /dev/sdb1
# Must have at least one remaining passphrase!

# Change passphrase
sudo cryptsetup luksChangeKey /dev/sdb1
# Can also change specific slot: luksChangeKey -S 0 /dev/sdb1

# Add key to specific slot
sudo cryptsetup luksAddKey -S 1 /dev/sdb1

# Remove key from specific slot
sudo cryptsetup luksRemoveKey -S 1 /dev/sdb1

# Check which slots are used
sudo cryptsetup luksDump /dev/sdb1 | grep -A 10 "Key Slots"

# Backup LUKS header (CRITICAL for recovery!)
sudo cryptsetup luksHeaderBackup /dev/sdb1 --header-backup-file luks_header.img
# Store securely - with this, attacker can try to crack your passphrase

# Restore LUKS header
sudo cryptsetup luksHeaderRestore /dev/sdb1 --header-backup-file luks_header.img

# Resume suspended container (after system sleep)
sudo cryptsetup luksResume /dev/mapper/secure_volume

LUKS with Keyfile

# ============================================================
# AUTOMOUNT WITH KEYFILE (For servers/automation)
# ============================================================

# Method 1: Create random keyfile
sudo dd if=/dev/urandom of=/root/luks.key bs=4096 count=1
sudo chmod 600 /root/luks.key

# Add keyfile to LUKS slot
sudo cryptsetup luksAddKey /dev/sdb1 /root/luks.key

# Method 2: Add keyfile to specific slot
sudo cryptsetup luksAddKey -S 2 /dev/sdb1 /root/luks.key

# Method 3: Configure /etc/crypttab (auto-unlock at boot)
# /etc/crypttab:
# secure_volume /dev/sdb1 /root/luks.key luks,discard

# Method 4: Using systemd (modern approach)
# /etc/systemd/system/mnt-secure.mount
[Unit]
Description=Encrypted Secure Volume

[Mount]
What=/dev/mapper/secure_volume
Where=/mnt/secure
Type=ext4
Options=defaults

# /etc/fstab:
# /dev/mapper/secure_volume /mnt/secure ext4 defaults 0 2

# Create systemd service to unlock at boot
# /etc/systemd/system/luks-unlock.service
[Unit]
Description=Unlock LUKS volume
Requires=systemd-remount-fs.service
After=systemd-remount-fs.service

[Service]
Type=oneshot
ExecStart=/usr/bin/cryptsetup luksOpen /dev/sdb1 secure_volume
ExecStart=/usr/bin/mount /dev/mapper/secure_volume /mnt/secure
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

# Enable: sudo systemctl enable luks-unlock.service

LUKS Performance Tuning

# ============================================================
# LUKS PERFORMANCE OPTIMIZATION
# ============================================================

# Benchmark encryption options
sudo cryptsetup benchmark

# Example output:
# KDF, 5 iterations, 64 memory, threads: 4
# PBKDF2-sha256      362k
# PBKDF2-sha512      337k
# argon2id           170k  (memory-hard, slower but more secure)

# Recommended options for new containers:
sudo cryptsetup luksFormat --type luks2 \
    --cipher aes-xts-plain64 \  # XTS mode for disk encryption
    --key-size 512 \            # 512-bit = 256-bit effective
    --hash sha512 \             # For key derivation
    --iter-time 5000 \          # ms for PBKDF2 (adjust for security)
    --pbkdf argon2id \          # Better than PBKDF2
    /dev/sdb1

# For older systems (LUKS1):
sudo cryptsetup luksFormat --type luks1 \
    --cipher aes-xts-plain64 \
    --key-size 512 \
    --hash sha512 \
    --iter-time 3000 \
    --pbkdf pbkdf2 \
    /dev/sdb1

# Check if hardware encryption is available
cat /proc/crypto | grep -i aes
ls /dev/crypto  # Should exist if available

# Check CPU for AES-NI support
grep -o aes /proc/cpuinfo | head -1
# Or: lscpu | grep -i aes

LUKS Emergency Recovery

# ============================================================
# EMERGENCY RECOVERY PROCEDURES
# ============================================================

# Scenario 1: System won't boot - LUKS password not accepted
# Boot from rescue media, then:

# Check if LUKS container is accessible
sudo cryptsetup luksDump /dev/sdX1

# If header corrupted, restore from backup
sudo cryptsetup luksHeaderRestore /dev/sdX1 --header-backup-file /path/to/backup.img

# Scenario 2: Forgotten passphrase - but you have backup
# If you have another working slot:
sudo cryptsetup luksOpen /dev/sdX1 recovery

# Scenario 3: Header completely corrupted
# Without backup, data is LOST - this is why backups are critical!
# Only cryptographically secure erasure can recover

# Scenario 4: Suspended container won't resume
# After system crash/suspend
sudo dmsetup remove /dev/mapper/secure_volume
sudo cryptsetup luksOpen /dev/sdX1 secure_volume

# Scenario 5: Check LUKS integrity
sudo cryptsetup luksDump /dev/sdX1
# Check "Failed authentication counter" - too many failures = lockout

# Emergency: Wipe LUKS container completely
sudo cryptsetup luksErase /dev/sdX1
# Or: sudo wipefs -a /dev/sdX1

35.4 Filesystem-Level Encryption

eCryptfs

# ============================================================
# ECryptfs - Stacked Filesystem Encryption
# ============================================================

# Install
sudo apt install ecryptfs-utils

# Create encrypted directory
mkdir -p ~/Private
sudo mount -t ecryptfs ~/Private ~/Private
# Select: passphrase, aes, plain, no signature, no encryption of filenames

# Using the automated tool
ecryptfs-setup-private
# Creates ~/.Private and ~/Private
# ~/.Private contains encrypted data
# ~/Private is the decrypted view when mounted

# Mount manually with options
sudo mount -t ecryptfs \
    -o key=passphrase,ecryptfs_cipher=aes,ecryptfs_key_bytes=32,\
ecryptfs_passthrough=n,ecryptfs_enable_filename_crypt=y \
    /home/user/.Private /home/user/Private

# Unmount (hides data)
ecryptfs-umount-private

# Add signature to keyring for auto-mount
ecryptfs-add-passphrase --fnek
# Add to /etc/ecryptfs/rc.local or pam_ecryptfs

fscrypt (Modern Alternative)

# ============================================================
# fscrypt - Native Linux Encryption (Kernel 4.2+)
# ============================================================

# Install
sudo apt install fscrypt libpam-fscrypt

# Enable on filesystem
sudo tune2fs -O encrypt /dev/sdX1
# Or at mount: mount -o encrypt /dev/sdX1 /mnt

# Setup
fscrypt setup
fscrypt setup root          # For root user
fscrypt setup other_user    # For other users

# Create encrypted directory
mkdir -p ~/encrypted
fscrypt encrypt ~/encrypted

# Check status
fscrypt status
fscrypt status ~/encrypted

# Lock/Unlock
fscrypt lock ~/encrypted
fscrypt unlock ~/encrypted

35.5 Swap Encryption

# ============================================================
# ENCRYPTED SWAP
# ============================================================

# Method 1: Using systemd (recommended)
# /etc/fstab:
# /dev/mapper/cryptswap none swap sw 0 0

# /etc/systemd/system/cryptswap.service:
[Unit]
Description=Encrypted Swap
After=swap.target

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/sbin/cryptsetup open --type plain /dev/sda2 cryptswap --key-file /root/swap.key
ExecStart=/sbin/mkswap /dev/mapper/cryptswap
ExecStop=/sbin/cryptsetup close cryptswap

[Install]
WantedBy=multi-user.target

# Create keyfile
dd if=/dev/urandom of=/root/swap.key bs=4096 count=1
chmod 600 /root/swap.key

# Enable
sudo systemctl enable cryptswap.service
sudo systemctl start cryptswap.service

# Method 2: Using /etc/crypttab (legacy)
# /etc/crypttab:
# cryptswap /dev/sda2 /dev/urandom swap,cipher=aes-xts-plain64,size=512

35.6 OpenSSL for TLS/SSL

# ============================================================
# OPENSSL - TLS CERTIFICATES
# ============================================================

# Generate private key
openssl genrsa -out server.key 2048
openssl genrsa -out server.key 2048  # With encryption
openssl genrsa -aes256 -out server.key 2048

# Generate CSR
openssl req -new -key server.key -out server.csr
openssl req -new -key server.key -out server.csr \
    -subj "/C=US/ST=State/L=City/O=Organization/CN=example.com"

# Self-signed certificate (for testing)
openssl req -x509 -days 365 -key server.key -in server.csr -out server.crt
openssl req -x509 -days 365 -newkey rsa:2048 -keyout server.key -out server.crt

# Verify certificate
openssl x509 -in server.crt -text -noout
openssl verify -CAfile ca.crt server.crt

# Extract info
openssl x509 -in server.crt -noout -subject
openssl x509 -in server.crt -noout -issuer
openssl x509 -in server.crt -noout -dates
openssl x509 -in server.crt -noout -fingerprint

# Convert formats
openssl x509 -in cert.pem -outform DER -out cert.der
openssl x509 -in cert.der -inform DER -outform PEM -out cert.pem

# Generate DH parameters
openssl dhparam -out dhparam.pem 2048

# Test SSL connection
openssl s_client -connect example.com:443
openssl s_client -connect example.com:443 -showcerts
openssl s_client -connect example.com:443 -verify_return_error

# Check cipher suites
openssl ciphers -v 'HIGH:!aNULL:!MD5'

35.7 Production Encryption Checklist

# ============================================================
# PRODUCTION ENCRYPTION CHECKLIST
# ============================================================

# 1. Disk Encryption
# [ ] All sensitive data disks encrypted with LUKS
# [ ] LUKS header backed up and stored securely
# [ ] Keyfiles protected (permissions 600)
# [ ] Multiple LUKS passphrases for recovery

# 2. Filesystem Encryption
# [ ] Home directories encrypted (fscrypt/ecryptfs)
# [ ] Database files encrypted
# [ ] Log files containing sensitive data encrypted

# 3. Network Encryption
# [ ] All production traffic uses TLS 1.2+
# [ ] Strong cipher suites only
# [ ] Certificates from trusted CA
# [ ] Certificate expiration monitoring

# 4. Key Management
# [ ] GPG keys backed up securely
# [ ] Expiration dates set on all keys
# [ ] Revocation certificates generated
# [ ] Key rotation policy in place

# 5. Swap and Temp
# [ ] Swap encrypted
# [ ] /tmp on encrypted volume if sensitive data
# [ ] /dev/shm permissions restricted

# 6. Monitoring
# [ ] Failed decryption attempts logged
# [ ] LUKS authentication failures monitored
# [ ] Certificate expiration alerts

# 7. Recovery
# [ ] Documented recovery procedures
# [ ] Test recovery process
# [ ] Multiple authorized individuals for recovery

35.8 Interview Questions

┌─────────────────────────────────────────────────────────────────────────┐
│                     ENCRYPTION INTERVIEW QUESTIONS                      │
├─────────────────────────────────────────────────────────────────────────┤
                                                                         │
Q1: Explain the difference between symmetric and asymmetric encryption.  │
                                                                         │
A1:                                                                       │
- Symmetric: Same key for encryption/decryption (AES, ChaCha20)         │
  - Pros: Fast, efficient for bulk data                                  │
  - Cons: Key distribution problem                                       │
                                                                         │
- Asymmetric: Public key encrypts, private key decrypts (RSA, ECC)     │
  - Pros: Solves key distribution, enables signatures                   │
  - Cons: Slow, computationally intensive                               │
                                                                         │
- Hybrid: Encrypt data with symmetric key, encrypt key with            │
  asymmetric key (standard practice for file encryption)                │
                                                                         │
─────────────────────────────────────────────────────────────────────────┤
                                                                         │
Q2: How does GPG implement the Web of Trust?                             │
                                                                         │
A2:                                                                       │
- Each user can sign other users' public keys                            │
- Trust levels: ultimate > full > marginal > never > unknown            │
- A key is considered trusted if:                                        │
  - One path with full trust, OR                                         │
  - Three paths with marginal trust                                      │
- Reduces dependency on centralized CAs                                 │
- More complex but decentralized                                         │
                                                                         │
─────────────────────────────────────────────────────────────────────────┤
                                                                         │
Q3: Describe the LUKS encryption structure and key slots.              │
                                                                         │
A3:                                                                       │
- LUKS header contains:                                                  │
  - Magic string, version, cipher info                                   │
  - 8 key slots (each can hold encrypted master key)                   │
  - PBKDF2 parameters for key derivation                                │
- Master key encrypts actual data (via dm-crypt)                       │
- Different passphrases can unlock different slots                      │
- Allows: key rotation, backup passphrases, multiple users              │
- Header backup critical for recovery                                    │
                                                                         │
─────────────────────────────────────────────────────────────────────────┤
                                                                         │
Q4: What are the differences between LUKS1 and LUKS2?                    │
                                                                         │
A4:                                                                       │
| Feature        | LUKS1           | LUKS2                    |          │
|----------------|-----------------|--------------------------|          │
| Max slots      | 8               | 8 (unlimited internally) |          │
| Header size    | 512KB fixed     | 16MB default             |          │
| KDF            | PBKDF2          | Argon2id, PBKDF2         |          │
| Integrity      | None            | dm-integrity             |          │
| Reencryption   | Offline only    | Online reencryption      |          │
| Performance    | Good            | Better (more iterations) |          │
                                                                         │
─────────────────────────────────────────────────────────────────────────┤
                                                                         │
Q5: How would you recover data from a corrupted LUKS container?         │
                                                                         │
A5:                                                                       │
1. First, backup the entire device (dd if=/dev/sdX of=backup.img)      │
2. Try to read header: cryptsetup luksDump /dev/sdX                    │
3. If header corrupted, restore from backup:                            │
   cryptsetup luksHeaderRestore /dev/sdX --header-backup-file backup   │
4. Try each key slot: cryptsetup luksOpen --test-crypto /dev/sdX       │
5. If header completely lost and no backup - data is UNRECOVERABLE     │
6. Use grep -a /dev/sdX for "LUKS" magic string to find header         │
                                                                         │
─────────────────────────────────────────────────────────────────────────┤
                                                                         │
Q6: Explain the difference between dm-crypt and dm-luks.                │
                                                                         │
A6:                                                                       │
- dm-crypt: Plain device encryption, no LUKS header                    │
  - No key slots, single passphrase                                     │
  - Can be faster (no header I/O)                                        │
  - Harder to verify correct passphrase                                  │
  - No key rotation                                                      │
                                                                         │
- dm-luks: LUKS wrapper around dm-crypt                                 │
  - Has LUKS header with key slots                                       │
  - Multiple passphrases                                                 │
  - Key management features                                              │
  - Default choice for most use cases                                    │
                                                                         │
─────────────────────────────────────────────────────────────────────────┤
                                                                         │
Q7: What is Argon2id and why is it preferred over PBKDF2?               │
                                                                         │
A7:                                                                       │
- Argon2id is winner of Password Hashing Competition (2015)             │
- Memory-hard: Uses significant RAM to compute hash                     │
- GPU-resistant: Hard to parallelize on GPU clusters                    │
- Better security margin than PBKDF2                                     │
- Recommended for new LUKS2 containers                                   │
                                                                         │
─────────────────────────────────────────────────────────────────────────┤
                                                                         │
Q8: How do you secure a Linux server's sensitive data at rest?          │
                                                                         │
A8:                                                                       │
1. Encrypt system disk (LUKS) - prevents physical theft                │
2. Encrypt data drives with separate LUKS containers                   │
3. Use filesystem-level encryption for user directories                 │
4. Encrypt swap and /tmp                                                │
5. Store encryption keys in hardware security modules (HSM)             │
6. Implement proper key management: rotation, backup, access control  │
7. Enable full-disk encryption on laptops/mobile devices                │
                                                                         │
─────────────────────────────────────────────────────────────────────────┤
                                                                         │
Q9: What are the security considerations when using keyfiles?           │
                                                                         │
A9:                                                                       │
- Keyfile is just as secure as the passphrase it replaces              │
- If keyfile is compromised, entire encryption is compromised          │
- Best practices:                                                        │
  - Use strong randomness: dd if=/dev/urandom (not /dev/random)        │
  - Restrict permissions: chmod 600 keyfile                             │
  - Store separately from encrypted data                                │
  - Use TPM for automatic unlock without exposed keyfile                │
  - Encrypt keyfile itself with another layer of protection             │
                                                                         │
─────────────────────────────────────────────────────────────────────────┤
                                                                         │
Q10: Explain the difference between file encryption and disk encryption.│
                                                                         │
A10:                                                                      │
- Disk/Full-disk encryption:                                            │
  - Encrypts entire block device                                         │
  - All data at rest protected                                           │
  - Transparent to applications                                          │
  - LUKS, dm-crypt, VeraCrypt                                           │
                                                                         │
- File-level encryption:                                                │
  - Encrypts individual files/directories                              │
  - Can have different keys for different files                        │
  - eCryptfs, fscrypt, GPG-encrypted files                             │
  - Portable - encrypted file can be moved                              │
                                                                         │
- Hybrid approach often used in production                              │
                                                                         │
└─────────────────────────────────────────────────────────────────────────┘

Quick Reference

# GPG Commands
gpg --full-generate-key              # Generate key pair
gpg --list-keys                      # List public keys
gpg --list-secret-keys               # List private keys
gpg -e -r recipient file.txt         # Encrypt
gpg -d file.txt.gpg                  # Decrypt
gpg --sign file.txt                  # Sign
gpg --verify file.sig                # Verify signature
gpg --armor --export email > key.asc # Export public key
gpg --import key.asc                 # Import key

# LUKS Commands
cryptsetup luksFormat /dev/sdX1              # Create LUKS
cryptsetup luksOpen /dev/sdX1 name           # Open container
cryptsetup luksClose name                    # Close container
cryptsetup luksDump /dev/sdX1                # Show header info
cryptsetup luksAddKey /dev/sdX1              # Add passphrase
cryptsetup luksRemoveKey /dev/sdX1           # Remove passphrase
cryptsetup luksHeaderBackup /dev/sdX1 -f backup.img  # Backup header

# OpenSSL Commands
openssl genrsa -out key.pem 2048             # Generate key
openssl req -new -key key.pem -out csr.pem   # Generate CSR
openssl x509 -req -days 365 -in csr.pem -signkey key.pem -out cert.pem  # Self-signed
openssl x509 -in cert.pem -text -noout      # View certificate
openssl s_client -connect host:443         # Test connection

# fscrypt Commands
fscrypt setup                               # Initialize
fscrypt encrypt directory                   # Encrypt directory
fscrypt lock directory                      # Lock directory
fscrypt unlock directory                    # Unlock directory

Summary

GPG: Asymmetric encryption for secure communication, signing, and key management
Web of Trust: Decentralized trust model through key signatures
LUKS: Standard for Linux disk encryption with key slots and secure key derivation
LUKS2 vs LUKS1: LUKS2 offers Argon2id, integrity protection, and online reencryption
Filesystem Encryption: eCryptfs and fscrypt for directory-level encryption
Production: Combine disk encryption, key management, and monitoring

Next Chapter

Chapter 36: System Logging - rsyslog, syslog-ng

Last Updated: February 2026