C2 Key parity and key check values

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Lesson C2: Odd parity on DES keys, and the key check values (KCV) every key exchange is verified with.

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The opening of the lesson

Keys are typed, read out over the phone, printed on component sheets and loaded into HSMs by people. Two cheap checks catch the mistakes:

Parity catches a mistyped DES key byte before it is used.
Key check values (KCVs) prove that two parties hold the same key without revealing it.

Every key exchange, key ceremony and HSM import you will ever do ends with "the KCV is 08D7B4", or the equivalent for your key.

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