When we think of VPNs, often our first thought is that of encryption of the user data. But adversaries or those intent on reading the data could However an attacker could record a conversation and then replay the responses between to participants. What we need to do is to be able to ensure the source of the data is genuine, and that is where digital signatures and certificates comes in.
To construct a Digital Signature, public key encryption systems must be in place. The construction of the Digital Signature entails applying a hash function to the message by concatenation of the message with a known secret key and then applying a mathematical function which will produce a fixed length output known as the digest. The digest is then encrypted with the public decryption key which produces a signature that can be appended to the message to verify that the message is from the genuine source.
The receiver recalculates the hash function and compared with the signature after applying the public key. If the two match, then because only the originator could have known the hash function and the private key, the message must be genuine.
Message Digest algorithms use Hash functions to map many potential inputs to each of a large number of outputs. What is normally produced is a fixed length field, typically a few hundred bits in length. A secret key is shared between sender and receiver and by concatenating this with a message for transfer, the digest is produced.
MD5 (Message Digest 5) is probably the most common hash function used, and it produces a 128 bit digest which is often appended to the header before the packet is transmitted. Any change in the message will cause the digest to change, and even the source and destination IP addresses can be used together with the message contents when creating the digest, which validates the addresses.
Another popular hashing algorithm is SHA (Secure Hash Algorithm) that produces a 160 bit digest ensuring greater security than MD5.
It doesn't matter how long the digest is, an identical digest will always result for an identical packet. But anyone wishing to attack the system could monitor exchanges and determine which packets sent in what ever order would result in some known result. This result could therefore be reproduced by replay of the messages. This is known as a collision attack.
HMAC (Hash-based Message Authentication Code) can be used to combat collision attacks by including two calculated values know as ipid and opid, which are initially calculated using the secret key for the first packet and recalculated for subsequent packets. The values are stored after each packet and recovered for use in the calculation of the digest for the next packet. This ensures that the digest is always different even for identical packets.
A Digital Certificate is produced using some known information such as name, address, mother's maiden name, house number, National Insurance number, or indeed anything. This information is appended to the public key and then used as part of the hash function to create the digest which is then encrypted using the private key through a secure encryption system such as RSA or AES.
A Digital Certificate can be validated by passing it through the public encryption process with the public key for the user to yield the digest. This can be compared with the calculation of the digest from the claimed identity of the user and their public key. If the two calculations yield the same result then the certificate is valid. Digital certificates are appended to messages to verify the authenticity of the source of the message.