Symmetric vs Asymmetric Cryptography
Cryptography Fundamentals
Understanding the difference between symmetric and asymmetric cryptography is crucial for IPsec implementation and security analysis.
Symmetric Cryptography
Symmetric cryptography uses the same key for both encryption and decryption. It's fast and efficient but requires secure key distribution.
Advantages
- Speed: Very fast encryption/decryption
- Efficiency: Low computational overhead
- Scalability: Suitable for large data volumes
- Resource Usage: Minimal CPU and memory
Disadvantages
- Key Distribution: Challenge of secure key sharing
- Key Management: n(n-1)/2 keys for n users
- Non-repudiation: Cannot prove message origin
- Initial Setup: Requires pre-shared secrets
Asymmetric Cryptography
Asymmetric cryptography uses a pair of keys: a public key for encryption and a private key for decryption. It solves the key distribution problem but is computationally expensive.
Advantages
- Key Distribution: No need for shared secrets
- Scalability: Only 2n keys for n users
- Non-repudiation: Digital signatures possible
- Authentication: Can verify sender identity
Disadvantages
- Speed: 100-10,000x slower than symmetric
- Resource Usage: High CPU and memory requirements
- Key Size: Requires much larger key sizes
- Complexity: More complex implementation
IPsec Hybrid Approach
| Component | Cryptography Type | Purpose | Reason |
|---|---|---|---|
| IKE Key Exchange | Asymmetric | Initial authentication & key agreement | Solves key distribution problem |
| Data Encryption | Symmetric | Bulk data protection | Performance and efficiency |
| Digital Signatures | Asymmetric | Authentication and non-repudiation | Prove identity and message integrity |
Cryptographic Algorithms
Symmetric Encryption Algorithms
IPsec supports various symmetric encryption algorithms for protecting data confidentiality.
AES (Advanced Encryption Standard)
AES-128
Key Size: 128 bits
Performance: Excellent
Security Level: 128-bit
Recommended: Yes
AES-192
Key Size: 192 bits
Performance: Good
Security Level: 192-bit
Recommended: Yes
AES-256
Key Size: 256 bits
Performance: Good
Security Level: 256-bit
Recommended: Yes
Legacy Algorithms
Deprecated Algorithms
These algorithms should be avoided in new deployments due to security vulnerabilities:
- DES: 56-bit key, easily broken
- 3DES: Triple DES, slow and limited security
- RC4: Stream cipher with known weaknesses
- Blowfish: 64-bit block size vulnerability
Encryption Modes
| Mode | Description | Parallel Processing | Error Propagation | IPsec Usage |
|---|---|---|---|---|
| CBC | Cipher Block Chaining | Decryption only | Single block | Traditional ESP |
| CTR | Counter Mode | Full parallel | None | Modern ESP |
| GCM | Galois/Counter Mode | Full parallel | None | AEAD ESP |
Hash Functions
Integrity Protection
Hash functions provide data integrity by creating a unique "fingerprint" of the message. Any change to the data will result in a completely different hash value.
Hash Function Properties
Required Properties
- Deterministic: Same input always produces same output
- Fixed Output: Always produces same-size hash
- Fast Computation: Quick to calculate
- Avalanche Effect: Small input change = big output change
Security Properties
- Pre-image Resistant: Hard to find input for given hash
- Second Pre-image Resistant: Hard to find different input with same hash
- Collision Resistant: Hard to find two inputs with same hash
Common Hash Algorithms
SHA-256
Output Size: 256 bits
Security Level: 128-bit
Performance: Good
Status: Secure
SHA-384
Output Size: 384 bits
Security Level: 192-bit
Performance: Good
Status: Secure
SHA-512
Output Size: 512 bits
Security Level: 256-bit
Performance: Good
Status: Secure
SHA-1
Output Size: 160 bits
Security Level: Broken
Performance: Fast
Status: Avoid
HMAC (Hash-based Message Authentication Code)
HMAC combines a hash function with a secret key to provide both integrity and authentication.
HMAC Construction
HMAC(K, m) = H((K ⊕ opad) || H((K ⊕ ipad) || m))
Where:
- K = secret key
- m = message
- H = hash function (e.g., SHA-256)
- opad = outer padding (0x5c repeated)
- ipad = inner padding (0x36 repeated)
- || = concatenation
- ⊕ = XOR operationDigital Certificates
PKI Authentication
Digital certificates provide scalable authentication for IPsec by using Public Key Infrastructure (PKI) to verify identities without requiring pre-shared keys.
Certificate Components
| Field | Description | Example |
|---|---|---|
| Subject | Identity of certificate owner | CN=vpn.company.com |
| Issuer | Certificate Authority that signed cert | CN=Company Root CA |
| Public Key | Subject's public key | RSA 2048-bit key |
| Validity Period | Certificate lifetime | Valid from/to dates |
| Digital Signature | CA's signature over certificate | RSA-SHA256 signature |
Certificate Validation Process
IPsec Certificate Authentication Benefits
Advantages
- Scalability: No pre-shared keys needed
- Security: Strong cryptographic authentication
- Management: Centralized certificate authority
- Revocation: Can revoke compromised certificates
- Non-repudiation: Digital signatures provide proof
Considerations
- Complexity: Requires PKI infrastructure
- Management: Certificate lifecycle management
- Overhead: Larger certificate exchanges
- Dependencies: Relies on CA availability
- Cost: CA services and management tools
Certificate-based IKE Configuration Example
# Cisco ASA Certificate Authentication
crypto ca trustpoint COMPANY-CA
enrollment url http://ca.company.com/certsrv
subject-name CN=ASA-VPN-Gateway,OU=IT,O=Company
keypair-name VPN-KEYPAIR
crypto ikev2 policy 10
encryption aes-256
integrity sha256
group 14
prf sha256
crypto ikev2 profile CERT-PROFILE
match identity remote any
identity local dn
authentication remote rsa-sig
authentication local rsa-sig
pki trustpoint COMPANY-CA