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I am getting confused with what exactly is the nature of the Signed Prekey (SPK) used in signal protocol. I understand what it is used for, but I think the confusion stems from its name. Is it just the normal key-pair whose public part will get signed and sent to the server as a part of the "key bundle"?

Which means, THAT exact SPK that we send in the bundle as well is just a random public key, and is in no way signed or modified by the IdentityKey itself.

To conclude:

Based on my understanding, this SPK could also be named "Key that WILL get signed but IS NOT signed" Or is there a previous key, lets call it just "PreKey" that gets signed by IdentityKey and thats how we get Signed PreKey?

I sound like a crazy person to myself and it's really hard to even formulate this question. I've read technical paper on signal's website and also whitepapers of signal implementation in WhatsApp but still I'm getting confused how is this SPK obtained?

Hope this doesn't look like non-sense. Thanks

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Signed Prekey is one of the long-term prekeys which is used in X3DH. The public component of Signed Prekey is signed by the Identity private key and sent to the Key Distribution Server as part of key bundle. To make sure that the server doesn't tamper with key bundle, Signed Prekey is signed by the Identity key and Identity key is verified using out of band authentication with the help of safety numbers at some time in future.

One-time prekeys are disposable and deleted from the server every time a contact requests public key bundle from the server. Unlike one-time prekeys, Signed Prekey is not disposable but it is not permanent either. Signed Prekey gets expired in 48 hours and needs to be updated by the client whereas Identity key is permanent until app reinstall or change in device.

X3DH can still be initialised without one-time prekey if all the one-time prekeys are used up which can rarely happen. State of one-time prekeys are checked every 12 hours and if approximately 2/3 of 100 one-time prekeys are used up, they are replenished again. Expiry time of Signed Prekey can be extended to reuse it in future X3DH. The purpose of Signed PreKey is to provide some protection if no one time pre key was given, if a device was taken after the interval the initial shared secret still couldn't be calculated.

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  • $\begingroup$ Thanks for these insights, it is very useful to deeper understanding of Signal protocol, but I feel my question remains unanswered. Before we get SPK, do we initially have some random KeyPair (KP) who's public part gets signed and that whole KP (with signed public part) we now call SPK? $\endgroup$ – Dante Aug 9 '19 at 20:12
  • $\begingroup$ @Dante No. Initially you have nothing. When you request a key bundle, you get public Identity key, public Signed PreKey with its signature and a public one-time prekey. $\endgroup$ – defalt Aug 10 '19 at 4:40
  • $\begingroup$ Alright, I think i got it. SPK is just a random key pair who's public part will be signed and sent along in a key bundle to the server. That's why I said it should be called "key that WILL get signed" it is not signed it self. btw. I'm considering this whole process at the time when user registers with the app, before initiator even requests key bundle from the server. $\endgroup$ – Dante Aug 10 '19 at 21:28

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