Table of Contents

VPNs
Supported Standards
Order in Which You Configure Your IPSec

Introduction

This chapter provides a brief overview of Virtual Private Networks (VPNs) and information about the Cisco-supported standards for the IP Security Protocol (IPSec), Internet Key Exchange (IKE), IKE Mode Configuration (Config), and certification authority (CA). The order in which Cisco recommends you configure each of the IPSec components is also provided in this chapter.

This chapter includes the following sections:

• VPNs

• Supported Standards

• Order in Which You Configure Your IPSec

VPNs

With IPSec, data can be transmitted across a public network without fear of observation, modification, or spoofing. As part of its security functions, the PIX Firewall provides IPSec standards-based VPN capability. VPNs maintain the same security and management policies as a private network. With a VPN, customers, business partners, and remote users, such as telecommuters, can access enterprise computing resources securely.

Supported Standards

Cisco implements the following standards for the IPSec and IKE features within the PIX Firewall:

• IPSec—IP Security Protocol. IPSec is a framework of open standards that provides data confidentiality, data integrity, and data authentication between participating peers. IPSec provides these security services at the IP layer; it uses IKE to handle negotiation of protocols and algorithms based on local policy and to generate the encryption and authentication keys to be used by IPSec. IPSec can be used to protect one or more data flows between a pair of hosts, between a pair of security gateways, or between a security gateway and a host.

  IPSec is documented in a series of Internet RFCs, all available at http://www.ietf.org/html.charters/ipsec-charter.html. The overall IPSec implementation is guided by "Security Architecture for the Internet Protocol," RFC# 2401.

• Internet Key Exchange (IKE)—A hybrid protocol that implements Oakley and SKEME key exchanges inside the Internet Security Association and Key Management Protocol (ISAKMP) framework. While IKE can be used with other protocols, its initial implementation is with the IPSec protocol. IKE provides authentication of the IPSec peers, negotiates IPSec security associations, and establishes IPSec keys.

IPSec as implemented in PIX Firewall supports the following additional standards:

• AH—Authentication Header. A security protocol that provides data authentication and optional anti-replay services. AH is embedded in the data to be protected (a full IP datagram).

  The AH protocol (RFC# 2402) allows for the use of various authentication algorithms; PIX Firewall has implemented the mandatory MD5-HMAC (RFC# 2403) and SHA-HMAC
(RFC# 2404) authentication algorithms. Used in conjunction with ISAKMP, the AH protocol algorithms. In conjunction with ISAKMP, the ESP protocol provides anti-replay services.

• ESP—Encapsulating Security Payload. A security protocol that provides data privacy services and optional data authentication, and anti-replay services. ESP encapsulates the data to be protected.

ISAKMP—The Internet Security Association and Key Management Protocol. A protocol framework that defines payload formats, the mechanics of implementing a key exchange protocol, and the negotiation of a security association.

Oakley—A key exchange protocol that defines how to derive authenticated keying material.

Skeme—A key exchange protocol that defines how to derive authenticated keying material, with rapid key refreshment.

• DES—Data Encryption Standard (DES) is used to encrypt packet data. IKE implements the 56-bit DES-CBC with Explicit IV standard. See "CBC."

• Triple DES (3DES)—A variant of DES, which iterates three times with three separate keys, effectively doubling the strength of DES.

• CBC—Cipher Block Chaining (CBC) requires an initialization vector (IV) to start encryption. The IV is explicitly given in the IPSec packet.

• Diffie-Hellman—A public-key cryptography protocol which allows two parties to establish a shared secret over an unsecure communications channel. Diffie-Hellman is used within IKE to establish session keys. 768-bit and 1024-bit Diffie-Hellman groups are supported.

• MD5 (HMAC variant)—MD5 (Message Digest 5) is a hash algorithm used to authenticate packet data. HMAC is a variant which provides an additional level of hashing.

• SHA (HMAC variant)—SHA (Secure Hash Algorithm) is a hash algorithm used to authenticate packet data. HMAC is a variant which provides an additional level of hashing.

• RSA signatures—RSA is the public key cryptographic system developed by Ron Rivest, Adi Shamir, and Leonard Adleman. RSA signatures provides non-repudiation.

IKE Extended Authentication (Xauth) is implemented per the IETF draft-ietf-ipsec-isakmp-xauth-04.txt ("extended authentication" draft). This provides this capability of authenticating a user within IKE using TACACS+ or RADIUS.

IKE Mode Configuration is implemented per the IETF draft-ietf-ipsec-isakmp-mode-cfg-04.txt. IKE Mode Configuration provides a method for a security gateway to download an IP address (and other network level configuration) to the VPN client as part of an IKE negotiation.

X.509v3 certificates—Used with the IKE protocol when authentication requires public keys. Certificate support that allows the IPSec-protected network to scale by providing the equivalent of a digital ID card to each device. When two peers wish to communicate, they exchange digital certificates to prove their identities (thus removing the need to manually exchange public keys with each peer or to manually specify a shared key at each peer). These certificates are obtained from a CA. X.509 is part of the X.500 standard by the ITU.

• X.509v3 certificates.

• Public-Key Cryptography Standard #7 (PKCS #7)—A standard from RSA Data Security, Inc. used to encrypt and sign certificate enrollment messages.

• Public-Key Cryptography Standard #10 (PKCS #10)—A standard syntax from RSA Data Security, Inc. for certificate requests.

• RSA Keys—RSA is the public key cryptographic system developed by Ron Rivest, Adi Shamir, and Leonard Adleman. RSA keys come in pairs: one public key and one private key.

Order in Which You Configure Your IPSec

If you will implement interoperability with a CA, Cisco recommends that you perform your IPSec configuration in the following order:

1. CA (see "Configuring CA")

2. IKE (see "Configuring IKE")

3. IPSec (see "Configuring IPSec")

4. (Optional) IKE Extended Authentication—applies only if you are configuring user authentication for remote VPN clients (see "Configuring Extended Authentication" within "Advanced Configurations")

5. (Optional) IKE Mode Configuration—applies only if you are configuring dynamic IP addressing for remote VPN clients (see "Configuring IKE Mode Config (Dynamic IP Address Assignment for VPN Client)" within "Advanced Configurations")

If you will not implement interoperability with a CA, and you will implement IKE, Cisco recommends that you perform your IPSec configuration in the following order:

1. IKE (see "Configuring IKE")

2. IPSec (see "Configuring IPSec")

3. (Optional) IKE Extended Authentication—applies only if you are configuring user authentication for remote VPN clients (see "Configuring Extended Authentication" within "Advanced Configurations")

4. (Optional) IKE Mode Configuration—applies only if you are configuring dynamic IP addressing for remote VPN clients (see "Configuring IKE Mode Config (Dynamic IP Address Assignment for VPN Client)" within "Advanced Configurations")

If you will not implement IKE, see "Configuring IPSec."

Posted: Thu Aug 31 19:50:03 PDT 2000
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