% gpg, pgp, encryption, signing, gnupg

# ============================================================================
# KEY GENERATION & MANAGEMENT
# ============================================================================

# Generate new GPG key pair
gpg --full-generate-key

# Generate key with specific algorithm (RSA 4096)
gpg --full-generate-key --rsa --rsa-key-size 4096

# Quick generate key (defaults)
gpg --quick-generate-key <email>
$ email: echo "user@example.com"

# List public keys
gpg --list-keys

# List secret/private keys
gpg --list-secret-keys

# List keys with fingerprints
gpg --fingerprint

# Export public key (ASCII armored)
gpg --armor --export <key_id> > public_key.asc
$ key_id: echo -e "user@example.com\nABCD1234"

# Export public key (binary)
gpg --export <key_id> > public_key.gpg
$ key_id: echo "user@example.com"

# Export private key (KEEP SECURE!)
gpg --armor --export-secret-keys <key_id> > private_key.asc
$ key_id: echo "user@example.com"

# Import public key
gpg --import <public_key_file>
$ public_key_file: echo -e "public_key.asc\nfriend_key.gpg"

# Import private key
gpg --import <private_key_file>
$ private_key_file: echo "private_key.asc"

# Delete public key
gpg --delete-key <key_id>
$ key_id: echo "user@example.com"

# Delete private key (caution!)
gpg --delete-secret-key <key_id>
$ key_id: echo "user@example.com"

# Edit key (change expiration, add subkey, etc.)
gpg --edit-key <key_id>
$ key_id: echo "user@example.com"

# ============================================================================
# FILE ENCRYPTION & DECRYPTION
# ============================================================================

# Encrypt file for recipient
gpg --encrypt --recipient <recipient_email> <file>
$ recipient_email: echo "friend@example.com"
$ file: echo -e "secret.txt\ndocument.pdf"

# Encrypt file (ASCII armored output)
gpg --armor --encrypt --recipient <recipient_email> <file>
$ recipient_email: echo "friend@example.com"
$ file: echo "message.txt"

# Encrypt for multiple recipients
gpg --encrypt -r <recipient1> -r <recipient2> <file>
$ recipient1: echo "alice@example.com"
$ recipient2: echo "bob@example.com"
$ file: echo "shared_secret.txt"

# Symmetric encryption (password-based, no key required)
gpg --symmetric <file>
$ file: echo "document.txt"

# Decrypt file
gpg --decrypt <encrypted_file> > <output_file>
$ encrypted_file: echo -e "secret.txt.gpg\nmessage.asc"
$ output_file: echo "decrypted.txt"

# Decrypt to stdout
gpg --decrypt <encrypted_file>
$ encrypted_file: echo "secret.txt.gpg"

# Encrypt and sign file
gpg --encrypt --sign --recipient <recipient> <file>
$ recipient: echo "friend@example.com"
$ file: echo "important.pdf"

# ============================================================================
# SIGNING & VERIFICATION
# ============================================================================

# Sign file (detached signature)
gpg --detach-sign <file>
$ file: echo "document.pdf"

# Sign file (ASCII armored signature)
gpg --armor --detach-sign <file>
$ file: echo "software.tar.gz"

# Sign file (cleartext signature - for text files)
gpg --clearsign <file>
$ file: echo "message.txt"

# Verify detached signature
gpg --verify <signature_file> <original_file>
$ signature_file: echo -e "document.pdf.sig\nsoftware.tar.gz.asc"
$ original_file: echo -e "document.pdf\nsoftware.tar.gz"

# Verify clearsigned file
gpg --verify <signed_file>
$ signed_file: echo "message.txt.asc"

# Sign and encrypt in one step
gpg --sign --encrypt --recipient <recipient> <file>
$ recipient: echo "friend@example.com"
$ file: echo "confidential.txt"

# ============================================================================
# KEY TRUST & WEB OF TRUST
# ============================================================================

# Sign someone's public key (vouch for identity)
gpg --sign-key <key_id>
$ key_id: echo "friend@example.com"

# Set trust level for key
gpg --edit-key <key_id>
# Then: trust → select level (1-5) → quit
$ key_id: echo "friend@example.com"

# Trust levels:
# 1 = Don't know / Won't say
# 2 = Don't trust
# 3 = Marginal trust
# 4 = Full trust
# 5 = Ultimate trust (your own keys)

# List key signatures
gpg --list-sigs <key_id>
$ key_id: echo "user@example.com"

# Check key fingerprint (verify identity)
gpg --fingerprint <key_id>
$ key_id: echo "friend@example.com"

# ============================================================================
# KEYSERVER OPERATIONS
# ============================================================================

# Upload public key to keyserver
gpg --keyserver <keyserver_url> --send-keys <key_id>
$ keyserver_url: echo -e "hkps://keys.openpgp.org\nhkps://keyserver.ubuntu.com"
$ key_id: echo "ABCD1234"

# Search for key on keyserver
gpg --keyserver <keyserver_url> --search-keys <email>
$ keyserver_url: echo "hkps://keys.openpgp.org"
$ email: echo "friend@example.com"

# Receive key from keyserver
gpg --keyserver <keyserver_url> --recv-keys <key_id>
$ keyserver_url: echo "hkps://keys.openpgp.org"
$ key_id: echo "ABCD1234EF567890"

# Refresh keys from keyserver (check for updates/revocations)
gpg --refresh-keys

# Popular keyservers:
# hkps://keys.openpgp.org
# hkps://keyserver.ubuntu.com
# hkps://keys.mailvelope.com

# ============================================================================
# REVOCATION & KEY EXPIRATION
# ============================================================================

# Generate revocation certificate (do this IMMEDIATELY after key creation!)
gpg --output revoke_cert.asc --gen-revoke <key_id>
$ key_id: echo "user@example.com"

# Import revocation certificate (revoke compromised key)
gpg --import revoke_cert.asc

# Upload revoked key to keyserver
gpg --keyserver hkps://keys.openpgp.org --send-keys <key_id>
$ key_id: echo "ABCD1234"

# Change key expiration date
gpg --edit-key <key_id>
# Then: expire → select new expiration → save
$ key_id: echo "user@example.com"

# ============================================================================
# GPG AGENT & CACHING
# ============================================================================

# Start GPG agent
gpg-agent --daemon

# Kill GPG agent
gpgconf --kill gpg-agent

# Reload GPG agent config
gpgconf --reload gpg-agent

# Set passphrase cache timeout (in ~/.gnupg/gpg-agent.conf)
# default-cache-ttl 600
# max-cache-ttl 7200

# Disable passphrase caching
# default-cache-ttl 0
# max-cache-ttl 1

# Clear cached passphrases
gpgconf --reload gpg-agent

# ============================================================================
# EMAIL ENCRYPTION
# ============================================================================

# Encrypt email message
gpg --armor --encrypt --sign --recipient <recipient> message.txt

# Decrypt received email
gpg --decrypt encrypted_email.asc

# Thunderbird + Enigmail
# Install Enigmail extension → Import key → Enable encryption

# Protonmail (built-in PGP)
# Settings → Keys → Import/export GPG keys

# Mutt email client with GPG
# Add to ~/.muttrc:
# set pgp_use_gpg_agent = yes
# set pgp_sign_as = YOUR_KEY_ID

# ============================================================================
# ADVANCED OPTIONS
# ============================================================================

# Specify output file
gpg --output <output_file> --encrypt <input_file>
$ output_file: echo "secret.gpg"
$ input_file: echo "document.txt"

# Encrypt with compression
gpg --compress-algo <algorithm> --encrypt <file>
$ algorithm: echo -e "zip\nzlib\nbzip2"
$ file: echo "large_file.tar"

# Use specific cipher algorithm
gpg --cipher-algo <algorithm> --encrypt <file>
$ algorithm: echo -e "AES256\nAES192\nAES128"
$ file: echo "secret.txt"

# Batch mode (no prompts, for scripts)
gpg --batch --yes --encrypt --recipient <recipient> <file>
$ recipient: echo "friend@example.com"
$ file: echo "automated.txt"

# Verbose output (debugging)
gpg --verbose --encrypt <file>
$ file: echo "test.txt"

# ============================================================================
# BEST PRACTICES & OPSEC
# ============================================================================

# Always generate revocation certificate after key creation
# Store revocation cert in secure location (offline backup)

# Use strong passphrase (20+ characters, random)
# Consider using diceware passphrase

# Set key expiration (1-2 years recommended)
# Renew before expiration, don't let it lapse

# Use subkeys for daily operations
# Keep master key offline

# Backup private keys securely
# Use encrypted USB drive or paper backup

# Verify fingerprints in person when possible
# Don't trust keys from keyservers alone

# Regularly refresh keys from keyservers
# Check for revocations

# Use ASCII armor for email/text transmission
# Use binary for file storage (smaller)

# Don't encrypt to untrusted keys
# Verify recipient identity first

# ============================================================================
# CONFIGURATION FILES
# ============================================================================

# GPG config: ~/.gnupg/gpg.conf
# Example settings:
# default-key YOUR_KEY_ID
# keyserver hkps://keys.openpgp.org
# use-agent
# armor

# GPG agent config: ~/.gnupg/gpg-agent.conf
# default-cache-ttl 600
# max-cache-ttl 7200
# pinentry-program /usr/bin/pinentry-gtk-2

# ============================================================================
# TROUBLESHOOTING
# ============================================================================

# Fix "No public key" error
# Import missing key from keyserver

# Fix "Unusable public key" error
# Trust the key: gpg --edit-key <key_id> → trust

# Reset GPG permissions
chmod 700 ~/.gnupg
chmod 600 ~/.gnupg/*

# Rebuild GPG trust database
gpg --check-trustdb

# Check GPG version
gpg --version