To ensure confidentiality of the data as it traverses the shared or public transit network, the data is encrypted by the sender and decrypted by the receiver. The encryption and decryption processes depend on both the sender and the receiver using a common encryption key.
Intercepted packets sent along the VPN connection in the transit network are unintelligible to anyone who does not have the common encryption key. The length of the encryption key is an important security parameter. You can use computational techniques to determine the encryption key. However, such techniques require more computing power and computational time as the encryption keys get larger. Therefore, it is important to use the largest possible key size to ensure data confidentiality.