Designing Access Controls
Make Decisions about Remote Access (VPN)
First, you need to consider how remote users will authenticate. Digital certif- icates provide the greatest security. Passwords (preshared keys) open a couple of vulnerabilities:
■Hackers can attempt to collect the passwords.
IKE addresses this vulnerability by sending the password (preshared key) over a secure tunnel (IKE SA). PPTP with
■With IKE, all users enter the same password (preshared key).
If even one user leaks the password (or leaves it written in an insecure location), the privacy of your network is compromised.
However, PPTP allows users to authenticate to a RADIUS server with their individual usernames and passwords as does IKE’s extended authentica- tion (Xauth).
In addition to ensuring that only legitimate, remote users log in to your private network, you must protect the privacy of the data that these users access. Your next consideration is the security of the encryption algorithm. If you are using IPsec, Authentication Header (AH) does not provide encryption; you should select Encapsulating Security Payload (ESP) or, for even more security, ESP and AH. For the encryption algorithm itself, if possible, use AES encryption or at least 3DES. Although DES and PPTP’s MPPE offer a degree of security, they are subject to some attacks. PPTP, in particular, has evinced some flaws.
Example
When factoring in only security for the VPN, the PCU network administrators have selected the options shown in Table
Table 3-26. Selecting VPN Options Based on Security Needs
Factor | VPN Protocol | Authentication | Encryption | Client | Gateway |
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| Method |
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Security | IPsec with IKE or | Digital certificates | ESP with AES and | Any | Any |
| L2TP/IPsec |
| AH with SHA1 |
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