There are a lot of question in CISA or CISSP exam that comparing between End to end Encryption versus Link Encryption. Some of the book explain that the main differences are:

- Link encryption encrypts the entire packet, including headers and trailers, and has to be decrypted at each hop.
- End-to-end encryption does not encrypt the headers and trailers, and therefore does not need to be decrypted at each hop.

Encryption systems must be strong, to serve their required purpose. The strength of the encryption system is based on several factors:

Algorithm — Remember that this is the set of instructions used with the cryptographic key to encrypt plaintext data. Not all algorithms are of the same strength. For example, Caesar might have thought his system of encryption was quite strong, but it is seen as relativity insecure today.

Cryptographic key — A user needs the correct key to encrypt or decrypt the information. As an example, when my brother was a teenager, my parents took the key to his car for violating curfew. Without the key, he had no way to use the car. Had he made a copy, access would have still been possible.