| 6.2 |  | IOS Cryptosystem |
| 6.2.7 |  | Digital certificates |

| In modern times, a signed document is notarized with a seal and a signature. In the electronic era, a document is signed using a digital signature. A digital signature, or certificate, is an encrypted hash that is appended to a document. It can be used to confirm the identity of the sender and the integrity of the document.

Digital signatures are based on a combination of public key encryption and secure one-way hash function algorithms. A digital certificate contains information to identify a user or device, such as the name, serial number, company, department or IP address . It also contains a copy of the entity’s public key . A Certificate Authority (CA) signs the certificate. The CA is a third party that is explicitly trusted by the receiver to validate identities and to create digital certificates .

To validate the CA’s signature, the receiver must first know the CA’s public key. Normally this is handled out of band or through an operation done at installation. For instance, most web browsers are configured with the public keys of several CA’s by default.

In the example in Figure , the sender derives a hash and encrypts it with its private key. The encrypted hash, which is a digital signature, is attached to the message and forwarded to the remote end. At the receiver end, the encrypted hash is decrypted using the public key of the sender. If the decrypted hash matches the recomputed hash, the signature is genuine. A digital signature ties a message to a sender. The sender is authenticated. The process is summarized as follows:

- At the transmitting end, the private key generated by the sender is used to encrypt the hash.
- At the receiving end:

- The hash is produced by running the original message through a hash algorithm.
- The hash that was appended to the original message is decrypted using the public key from the sender.

- If the hashes match, the message is signed by private key that was generated by the sender.
- Only a specific private key could have produced the digital signature.

Digital signatures are much more scalable than pre-shared keys. Without digital signatures, keys must be manually shared between each pair of devices. Without certificates, every new device added to the network requires a configuration change on every other device it securely communicates with. However, by using digital certificates, each device is enrolled with a CA. When two devices wish to communicate, they exchange certificates and digitally sign data to authenticate each other. When a new device is added to the network, one simply enrolls that device with a CA, and none of the other devices need modification. When the new device attempts a connection, certificates are automatically exchanged and the device can be authenticated.

The two common digital signature algorithms are RSA and Directory System Agent (DSA). RSA is used commercially and is the most common. DSA is used mostly by U.S. Government agencies.

| Media Activity Demonstration Activity: VPN and IPSec Crossword Puzzle In this activity, complete the VPN and IPSec crossword puzzle.