IPsec Basics

Security Associations

A security association (SA) is a unidirectional agreement between the VPN participants regarding the methods and parameters to use in securing a communication channel. Full bidirectional communication requires at least two SAs, one for each direction. Through the SA, an IPsec tunnel can provide the following security functions:

• Privacy (through encryption)

• Content integrity (through data authentication)

• Sender authentication and—if using certificates—nonrepudiation (through data origin authentication)

The security functions you employ depend on your needs. If you need only to authenticate the IP packet source and content integrity, you can authenticate the packet without applying any encryption. On the other hand, if you are concerned only with preserving privacy, you can encrypt the packet without applying any authentication mechanisms. Optionally, you can both encrypt and authenticate the packet. Most network security designers choose to encrypt, authenticate, and replay-protect their VPN traffic.

An IPsec tunnel consists of a pair of unidirectional SAs—one SA for each direction of the tunnel—that specify the security parameter index (SPI), destination IP address, and security protocol (Authentication Header [AH] or Encapsulating Security Payload [ESP] employed. An SA groups together the following components for securing communications:

• Security algorithms and keys.

• Protocol mode, either transport or tunnel. Junos OS devices always use tunnel mode. (See Packet Processing in Tunnel Mode.)

• Key-management method, either manual key or AutoKey IKE.

• SA lifetime.

For inbound traffic, Junos OS looks up the SA by using the following triplet:

• Destination IP address.

• Security protocol, either AH or ESP.

• Security parameter index (SPI) value.

For outbound VPN traffic, the policy invokes the SA associated with the VPN tunnel.

Manual Key

With manual keys, administrators at both ends of a tunnel configure all the security parameters. This is a viable technique for small, static networks where the distribution, maintenance, and tracking of keys are not difficult. However, safely distributing manual-key configurations across great distances poses security issues. Aside from passing the keys face-to-face, you cannot be completely sure that the keys have not been compromised while in transit. Also, whenever you want to change the key, you are faced with the same security issues as when you initially distributed it.

AutoKey IKE

When you need to create and manage numerous tunnels, you need a method that does not require you to configure every element manually. IPsec supports the automated generation and negotiation of keys and security associations using the Internet Key Exchange (IKE) protocol. Junos OS refers to such automated tunnel negotiation as AutoKey IKE and supports AutoKey IKE with preshared keys and AutoKey IKE with certificates.

• AutoKey IKE with preshared keys—Using AutoKey IKE with preshared keys to authenticate the participants in an IKE session, each side must configure and securely exchange the preshared key in advance. In this regard, the issue of secure key distribution is the same as that with manual keys. However, once distributed, an autokey, unlike a manual key, can automatically change its keys at predetermined intervals using the IKE protocol. Frequently changing keys greatly improves security, and automatically doing so greatly reduces key-management responsibilities. However, changing keys increases traffic overhead; therefore, changing keys too often can reduce data transmission efficiency.

  A preshared key is a key for both encryption and decryption, which both participants must have before initiating communication.

• AutoKey IKE with certificates—When using certificates to authenticate the participants during an AutoKey IKE negotiation, each side generates a public-private key pair and acquires a certificate. As long as the issuing certificate authority (CA) is trusted by both sides, the participants can retrieve the peer’s public key and verify the peer's signature. There is no need to keep track of the keys and SAs; IKE does it automatically.

Diffie-Hellman Exchange

A Diffie-Hellman (DH) exchange allows participants to produce a shared secret value. The strength of the technique is that it allows participants to create the secret value over an unsecured medium without passing the secret value through the wire. The size of the prime modulus used in each group's calculation differs as shown in the below table. Diffie Hellman (DH) exchange operations can be performed either in software or in hardware. The following Table 1 lists different Diffie Hellman (DH) groups and specifies whether the operation performed for that group is in the hardware or in software.

Table 1: Diffie Hellman (DH) groups and their exchange operations performed

Diffie-Hellman (DH) Group	Prime Module Size
DH Group 1	768-bit
DH Group 2	102-bit
DH Group 5	1536-bit
DH Group 14	2048-bit
DH Group 15	3072-bit
DH Group 16	4096-bit
DH Group 19	256-bit elliptic curve
DH Group 20	384-bit elliptic curve
DH Group 21	521-bit elliptic curve
DH Group 24	2048-bit with 256-bit prime order subgroup

Starting in Junos OS Release 19.1R1, SRX Series Firewalls (except SRX300, SRX320, SRX340, SRX345, SRX380, SRX550HM Series Firewalls) support DH groups 15, 16, and 21.

Starting in Junos OS Release 20.3R1, vSRX Virtual Firewall (vSRX 3.0) instances with junos-ike package installed support DH groups 15, 16, and 21.

We do not recommend the use of DH groups 1, 2, and 5.

Because the modulus for each DH group is a different size, the participants must agree to use the same group.

IPsec Authentication Algorithms (AH Protocol)

The Authentication Header (AH) protocol provides a means to verify the authenticity and integrity of the content and origin of a packet. You can authenticate the packet by the checksum calculated through a Hash Message Authentication Code (HMAC) using a secret key and either MD5 or SHA hash functions.

• Message Digest 5 (MD5)—An algorithm that produces a 128-bit hash (also called a digital signature or message digest) from a message of arbitrary length and a 16-byte key. The resulting hash is used, like a fingerprint of the input, to verify content and source authenticity and integrity.

• Secure Hash Algorithm (SHA)—An algorithm that produces a 160-bit hash from a message of arbitrary length and a 20-byte key. It is generally regarded as more secure than MD5 because of the larger hashes it produces. Because the computational processing is done in the ASIC, the performance cost is negligible.

For more information on MD5 hashing algorithms, see RFC 1321 and RFC 2403. For more information on SHA hashing algorithms, see RFC 2404. For more information on HMAC, see RFC 2104.

IPsec Encryption Algorithms (ESP Protocol)

The Encapsulating Security Payload (ESP) protocol provides a means to ensure privacy (encryption) and source authentication and content integrity (authentication). ESP in tunnel mode encapsulates the entire IP packet (header and payload) and then appends a new IP header to the now-encrypted packet. This new IP header contains the destination address needed to route the protected data through the network. (See Packet Processing in Tunnel Mode.)

With ESP, you can both encrypt and authenticate, encrypt only, or authenticate only. For encryption, you can choose one of the following encryption algorithms:

• Data Encryption Standard (DES)—A cryptographic block algorithm with a 56-bit key.

• Triple DES (3DES)—A more powerful version of DES in which the original DES algorithm is applied in three rounds, using a 168-bit key. DES provides significant performance savings but is considered unacceptable for many classified or sensitive material transfers.

• Advanced Encryption Standard (AES)—An encryption standard which offers greater interoperability with other devices. Junos OS supports AES with 128-bit, 192-bit, and 256-bit keys.

For authentication, you can use either MD5 or SHA algorithms.

Even though it is possible to select NULL for encryption, it has been demonstrated that IPsec might be vulnerable to attack under such circumstances. Therefore, we suggest that you choose an encryption algorithm for maximum security.