Asymmetric ratchet root key

Question

Hi,

In the Signal lecture, on slide 60 (the graphic about the Aymmetric ratchet) we can see how the new keys are derived during a conversation.

What I wondered is: according to the graphic the initial root key is used as parameter for KDF for every new derivation. Is this correct, or will every round use the previous root key?

Answer 1

Hi,

Signal always use the previous key, not the initial.

The slide is incorrect, we will update this at some point.

Cheers,
Sebastian

Answer 2

Hi,

That is a mistake. The previous rk is always used. Otherwise, it would not make sense to compute new rk's at all.