CertPrep

The LPIC-3 303: Security exam covers cryptography, public key infrastructure, file and network security, PAM, SSH hardening, access control systems (SELinux/AppArmor), and system auditing. This guide provides a command-level deep dive into each objective.

Cryptography Concepts

Symmetric vs Asymmetric Encryption

Type	Key Usage	Algorithms	Performance
Symmetric	Same key for encrypt/decrypt	AES, ChaCha20, Twofish	Fast, suitable for bulk data
Asymmetric	Public/private key pair	RSA, ECDSA, Ed25519	Slow, used for key exchange and signatures
Hybrid	Symmetric key encrypted with asymmetric	TLS, SSH, OpenPGP	Practical combination

OpenSSL Command Reference

# Generate an RSA private key
openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:4096 -out private.key

# Generate an ECDSA key (NIST P-256)
openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:prime256v1 -out ecdsa.key

# Generate a self-signed certificate
openssl req -x509 -new -nodes -key private.key -sha256 -days 365 \
  -subj "/C=DE/ST=Berlin/L=Berlin/O=Example/CN=server.example.com" \
  -out cert.pem

# View certificate details
openssl x509 -in cert.pem -text -noout

# Generate a CSR
openssl req -new -key private.key -out request.csr \
  -subj "/CN=server.example.com"

# Encrypt a file with AES-256
openssl enc -aes-256-cbc -salt -in plain.txt -out encrypted.enc

# Decrypt a file
openssl enc -d -aes-256-cbc -in encrypted.enc -out plain.txt

Public Key Infrastructure (PKI)

Certificate Authority Setup

# Create the CA directory structure
mkdir -p ca/{certs,crl,newcerts,private,requests}
chmod 700 ca/private
echo 1000 > ca/serial
touch ca/index.txt

# Configure openssl-ca.cnf
[ ca ]
default_ca = CA_default

[ CA_default ]
database = ca/index.txt
serial = ca/serial
private_key = ca/private/ca.key.pem
certificate = ca/certs/ca.cert.pem
default_days = 365
default_md = sha256
policy = policy_strict

[ policy_strict ]
countryName = match
stateOrProvinceName = match
organizationName = match
organizationalUnitName = optional
commonName = supplied
emailAddress = optional

# Generate CA key and self-signed certificate
openssl genpkey -algorithm RSA -out ca/private/ca.key.pem -pkeyopt rsa_keygen_bits:4096
openssl req -x509 -new -key ca/private/ca.key.pem -out ca/certs/ca.cert.pem \
  -days 3650 -subj "/C=DE/O=Example CA/CN=Example Root CA"

# Sign a server CSR
openssl ca -config openssl-ca.cnf -in server.csr -out server.crt

# Revoke a certificate
openssl ca -config openssl-ca.cnf -revoke server.crt

# Generate CRL
openssl ca -config openssl-ca.cnf -gencrl -out ca/crl/ca.crl.pem

# Verify CRL
openssl crl -in ca/crl/ca.crl.pem -text -noout

Certificate Path Validation

# Verify certificate chain
openssl verify -CAfile ca.cert.pem -untrusted intermediate.cert.pem server.crt

# Check if certificate is revoked (with CRL)
openssl verify -CAfile ca.cert.pem -CRLfile ca.crl.pem server.crt

File Security

POSIX ACLs

# View ACLs
getfacl /path/to/file

# Set ACL: grant user read/write
setfacl -m u:jdoe:rw /path/to/file

# Set ACL: grant group read/execute
setfacl -m g:developers:rx /path/to/dir

# Remove specific ACL entry
setfacl -x u:jdoe /path/to/file

# Set default ACL for directory (inherited by new files)
setfacl -d -m g:developers:rx /path/to/dir

# Recursive ACL application
setfacl -R -m g:developers:rx /path/to/dir

# Backup and restore ACLs
getfacl -R /path > acls.backup
setfacl --restore=acls.backup

Extended Attributes (xattrs)

# Set extended attribute
setfattr -n user.comment -v "Important document" file.txt

# View extended attributes
getfattr -d file.txt

# Remove extended attribute
setfattr -x user.comment file.txt

# List attributes (shows only attribute names)
attr -l file.txt

# Security extended attributes (SELinux context)
getfattr -n security.selinux file.txt

Immutable and Append-Only Flags

# Make a file immutable (even root cannot modify)
chattr +i /etc/hosts

# Append-only mode (log files)
chattr +a /var/log/auth.log

# View flags
lsattr /etc/hosts

# Remove immutable flag
chattr -i /etc/hosts

Network Security

nftables Configuration

# Create a ruleset file: /etc/nftables.conf
table inet filter {
   chain input {
      type filter hook input priority 0; policy drop;
      ct state established,related accept
      ct state invalid drop
      iif "lo" accept
      ip protocol icmp accept
      tcp dport 22 counter accept
      tcp dport 80 counter accept
      tcp dport 443 counter accept
   }
   chain forward {
      type filter hook forward priority 0; policy drop;
   }
   chain output {
      type filter hook output priority 0; policy accept;
   }
}

# Apply ruleset
nft -f /etc/nftables.conf

# List rules
nft list ruleset

# Add temporary rule
nft add rule inet filter input tcp dport 8080 accept

# Delete rule (get handle first)
nft -a list ruleset
nft delete rule inet filter input handle 3

fail2ban Configuration

# /etc/fail2ban/jail.local
[sshd]
enabled = true
port = ssh
filter = sshd
logpath = /var/log/auth.log
maxretry = 5
bantime = 3600
findtime = 600

# Custom filter: /etc/fail2ban/filter.d/nginx-botsearch.conf
[Definition]
failregex = ^<HOST> - -.*GET.*(?:/wp-admin|/admin|/\.env).*HTTP
ignoreregex =

# Manage jails
fail2ban-client status
fail2ban-client status sshd
fail2ban-client set sshd unbanip 10.0.0.5

PAM (Pluggable Authentication Modules)

PAM Module Types

Module Type	Purpose
auth	Verify user identity (password, biometric, 2FA)
account	Check account validity (expiration, time of day)
session	Set up session environment (mount home, log audit)
password	Update authentication tokens (password change)

PAM Configuration Format

# /etc/pam.d/common-auth
auth    [success=2 default=ignore]  pam_unix.so    nullok_secure
auth    [success=1 default=ignore]  pam_ldap.so    use_first_pass
auth    requisite                   pam_deny.so
auth    required                    pam_permit.so

# /etc/pam.d/common-password
password  requisite  pam_pwquality.so  retry=3 minlen=12
password  [success=1 default=ignore]  pam_unix.so  obscure use_authtok sha512
password  requisite                   pam_deny.so
password  required                    pam_permit.so

# /etc/pam.d/common-session
session  required  pam_env.so
session  required  pam_unix.so
session  optional  pam_ldap.so
session  optional  pam_systemd.so

Control Flags

Flag	Behavior
required	Must succeed; continues checking other modules regardless
requisite	Must succeed; stops immediately on failure
sufficient	If succeeds, skips remaining auth modules
optional	Result matters only if this is the only module
include	Include entire configuration from another file
substack	Like include but success/failure scoped to the substack

PAM Example: Google Authenticator 2FA

# Install
apt-get install libpam-google-authenticator

# Configure user
google-authenticator

# /etc/pam.d/sshd
auth  required  pam_google_authenticator.so
auth  required  pam_unix.so

# /etc/ssh/sshd_config
ChallengeResponseAuthentication yes
AuthenticationMethods publickey,keyboard-interactive

SSH Hardening

Server Configuration (/etc/ssh/sshd_config)

# Cryptographic
HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key
KexAlgorithms sntrup761x25519-sha512@openssh.com,diffie-hellman-group16-sha512
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com

# Authentication
PubkeyAuthentication yes
PasswordAuthentication no
ChallengeResponseAuthentication no
AuthenticationMethods publickey
PermitRootLogin prohibit-password
MaxAuthTries 3
LoginGraceTime 30

# Session
ClientAliveInterval 300
ClientAliveCountMax 2
MaxSessions 10
AllowUsers jdoe admin

# Forwarding
X11Forwarding no
AllowTcpForwarding no
AllowAgentForwarding no

SSH Key Management

# Generate Ed25519 key (recommended)
ssh-keygen -t ed25519 -a 100 -f ~/.ssh/id_ed25519

# Generate RSA key (for legacy systems)
ssh-keygen -t rsa -b 4096 -a 100 -f ~/.ssh/id_rsa

# Copy public key to host
ssh-copy-id -i ~/.ssh/id_ed25519.pub user@host

# Use SSH agent for key forwarding
eval "$(ssh-agent -s)"
ssh-add ~/.ssh/id_ed25519
ssh -A user@host

# SSH tunnel (local port forwarding)
ssh -L 8080:internal-web:80 user@bastion

# SSH tunnel (remote port forwarding)
ssh -R 9090:localhost:3000 user@public-host

Mandatory Access Control: SELinux

SELinux Modes

# Check current mode
getenforce

# Check mode and policy
sestatus

# Change mode temporarily
setenforce 0   # Permissive
setenforce 1   # Enforcing

# Change mode permanently
# /etc/selinux/config
SELINUX=enforcing
SELINUXTYPE=targeted

SELinux Contexts

# View file context
ls -Z /var/www/html/index.html

# View process context
ps -eZ | grep httpd

# Change file context
chcon -t httpd_sys_content_t /var/www/html/index.html

# Restore default context
restorecon -Rv /var/www/html/

# Apply contexts based on policy spec
setfiles /etc/selinux/targeted/contexts/files/file_contexts /var/www

# Set file context persistently (for relabeling)
semanage fcontext -a -t httpd_sys_content_t "/web(/.*)?"
restorecon -Rv /web

SELinux Booleans

# List booleans
getsebool -a

# Check specific boolean
getsebool httpd_can_network_connect

# Set boolean (temporary)
setsebool httpd_can_network_connect on

# Set boolean (permanent)
setsebool -P httpd_can_network_connect on

SELinux Audit Analysis

# View denials
grep AVC /var/log/audit/audit.log

# Analyze with sealert
sealert -a /var/log/audit/audit.log

# Create policy module for a custom application
audit2allow -a -M myapp
semodule -i myapp.pp

AppArmor (Alternative to SELinux)

# Check AppArmor status
apparmor_status

# Set profile mode
aa-enforce /path/to/profile  # Enforcing
aa-complain /path/to/profile # Log-only

# Generate profile for a binary
aa-genprof /usr/bin/myapp

# Update profile based on logs
aa-logprof

# Custom profile: /etc/apparmor.d/usr.bin.myapp
#include <tunables/global>

/usr/bin/myapp {
   #include <abstractions/base>

   /etc/myapp/config r,
   /var/lib/myapp/** rw,
   /tmp/myapp-* rw,
   network inet tcp,
}

System Auditing with auditd

# Install and start
apt-get install auditd audispd-plugins

# Add audit rule
auditctl -w /etc/passwd -p wa -k passwd_changes
auditctl -w /etc/shadow -p wa -k shadow_changes
auditctl -a exit,always -S execve -F uid!>=1000 -k user_commands

# List rules
auditctl -l

# Search audit log
ausearch -k passwd_changes
ausearch -ts today -k user_commands

# Generate report
aureport -l  # Login report
aureport -x  # Command execution report
aureport -k  # Keyed event report

# Make rules persistent
# /etc/audit/rules.d/audit.rules

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