| 6.2 |  | IOS Cryptosystem |
| 6.2.5 |  | Data integrity |

| Data integrity is a critical function within a VPN. VPN data is transported over the public Internet. This data could potentially be intercepted and modified. To guard against this, each message has a hash attached to it. A hash is a method of verifying that the contents of a transmission are the same at both ends of the path, similar to a checksum. A hash value is created by a hash function that takes variable input such as a packet and returns a fixed-size string. The hash guarantees the integrity of the original message. If the transmitted hash matches the received hash, the message has not been tampered with. However, if there is no match, the message was altered. Two common hashing algorithms are Message Digest (MD) and Secure Hash Algorithm (SHA). There are several versions of each algorithm.

In the example in the figure, someone is trying to send Terry Smith a check for $10 . At the remote end, Alex Jones is trying to cash the check for $100. As the check progressed through the Internet, it was altered. Both the recipient and dollar amounts were changed. In this case, the hashes did not match. The transaction is no longer valid.