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1.7 Layers and Cryptography

Encryption and integrity protection are sometimes done on the original message or on each chunk of the message, and if on each chunk, it might be done end-to-end or hop-by-hop. There are interesting tradeoffs and implications of these choices. If done on the original message, it can be protected while being stored, and the infrastructure does not need to even know whether the data it is moving is cryptographically protected. This means that the location where the cryptographically protected message is kept, and the infrastructure for transmitting the message, need not be trusted.

Encryption hop-by-hop can foil traffic analysis, i.e., it hides from eavesdroppers the information about which parties are communicating. Thus it is useful even if encryption is being done at other layers. If done hop-by-hop, the packet switches must be trusted, because by definition of hop-by-hop, the packet switches will see the plaintext.

If done end-to-end as the data is being transmitted, if individual chunks are separately encrypted and integrity protected, then the data that arrives intact can be used, whereas if there's only a single integrity check for the entire message, then any corruption or loss will require retransmitting the entire thing, since (by definition of cryptographically protecting the data as a whole instead of individual chunks) there will be no way to know where the loss/corruption occurred.

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