V I R T U A L P R I V A T E N E T W O R K S
Virtual Private Networks
7-4
Security Appliance User Guide
Version 3R2
7
Figure 7-3: Using Tunnel Mode
The AH protocol provides data integrity, authentication, and anti-replay
protection. The AH protocol uses a secret key and a hash function—
either Message Digest (MD5) or Secure Hash Algorithm-1 (SHA-1)—to
authenticate the packet with a checksum calculation or hash-based
message authentication code (HMAC).
Table 7-1
explains MD5 and SHA-
1.
Table 7-1: MD5 and SHA-1 Description
The ESP protocol ensures privacy (encryption), source authentication,
and content integrity (authentication). ESP includes the ability to
encrypt, encrypt and authenticate, and authenticate only where SHA-1
Hash Function
Description
MD5
A one-way hash function. This hash function
takes variable-length messages and formats
them to a fixed length using a 16-byte key to
produce a 128-bit hash.
SHA-1
A one-way hash function. This hash function
takes variable-length messages and formats
them to a fixed length using a 20-byte key to
produce a 160-bit hash.
