Origin Authentication

IPSec guarantees that the endpoint of the connection is indeed the peer whom they claim to be. This check is performed via authentication during the D-H key agreement protocol. Remember, D-H is used to obtain a shared secret key over an unsecure medium. There are a few ways we can perform authentication for a peer:

• Using a shared secret key

• Using digital signatures

• Using encrypted nonces

Preshared Keys

Both sides agree upon a shared secret key. (This is not one of the derived D-H keys.) What one sides encrypts, the other can decrypt, and it is therefore authentic .

RSA Signatures

RSA signatures , also called digital signatures, allow both sides to obtain a digital certificate, which validates that a public key (remember RSA and DSA) belongs to a certain host. These certificates are exchanged to authenticate the peer using the RSA/DSA algorithm.

RSA uses asymmetric keys for encryption.

RSA Encrypted Nonces

Public/private keys are generated and public keys are passed to peers (not using digital certificates). A nonce (random number) is encrypted using the public key of the peer. The encrypted nonce is sent to the peer, which, using its private key, decrypts the nonce and returns it. Because the peer has the corresponding private key and the nonce is the same, the peer must be authentic.

Digital Signatures

When the hash function is performed and the hash output along with the original data is sent to the peer, the data is subject to man-in-the-middle attacks. The cracker intercepts the hash and data, creates her own data, hashes it using the same algorithm, and sends it to the original receiver. The receiver validates the hash and accepts it. To stop this type of attack, we can use one of the derived D-H shared secret keys to authenticate the data. The data along with the shared secret key is hashed to create the hash output value. Only the peer on the opposite end, who also has the shared secret value, can create the same hash value. Crackers attempting to forge packets will not have the shared key, which means they will never compute a hash that can match the data.

In addition to stopping man-in-the-middle attacks, we can use public-key cryptography to authenticate the hash. The hash is encrypted with the private key of the sender and then sent to the opposite peer. The opposite peer obtains the public key of the sender, decrypts the hash, and then performs the normal hash-checking function to validate the data. Because only one person in the universe has the private key that was used to encrypt the hash, it can be safely assumed that he sent the message.

When we hash data and then encrypt it using our private key, we are creating a digital signature. In essence, we are validating that the hash the peer decrypted (using our public key) could have come from no one else in the universe.

The great thing about digital signatures is that they change for every packet that is sent, but they all can only be unencrypted using the corresponding public key.