The intended receiver must know both the correct method and the value of
the key that was used, in order to be able to decrypt the message. For commercial
encryption schemes, the method will be public knowledge. Protecting the secrecy of the
key becomes crucial.
Integrity, in the context of communications security, refers to the ability to make
certain that the message received has not been altered in any way and is identical to the
message that was sent. The frame check sequence (FCS) bytes are one example of an
integrity check, but they are not considered secure. The ordinary FCS bytes are not
calculated over the plaintext message and protected by encryption. Instead they are
calculated over the ciphertext, using a known method and sent in the clear (unencrypted).
The FCS bytes help to identify packets that have been accidentally damaged in transit. An
attacker, however, could recalculate the ordinary FCS (for example, to hide their deliberate
alteration of a packet they captured and retransmitted). The harder it is for an attacker to
correctly recalculate the integrity check sequence or security hash function, the more
reliable a test of message integrity it is.
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