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sas: clarify ECDH process in step 12 #1720

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Feb 26, 2024
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sas: clarify ECDH process in step 12 #1720

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a6b643b
sas: clarify ECDH process in step 12
sumnerevans Jan 30, 2024
7942e2a
sas: use possessive for Alice's device
sumnerevans Feb 22, 2024
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1 change: 1 addition & 0 deletions changelogs/client_server/newsfragments/1720.clarification
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Original file line number Diff line number Diff line change
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Clarify how to perform the ECDH exchange in step 12 of the SAS process.
10 changes: 6 additions & 4 deletions content/client-server-api/modules/end_to_end_encryption.md
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Expand Up @@ -660,10 +660,12 @@ The process between Alice and Bob verifying each other would be:
11. Alice's device receives Bob's message and verifies the commitment
hash from earlier matches the hash of the key Bob's device just sent
and the content of Alice's `m.key.verification.start` message.
12. Both Alice and Bob's devices perform an Elliptic-curve
Diffie-Hellman
(*ECDH(K<sub>A</sub><sup>private</sup>*, *K<sub>B</sub><sup>public</sup>*)),
using the result as the shared secret.
12. Both Alice's and Bob's devices perform an Elliptic-curve Diffie-Hellman using
their private ephemeral key, and the other device's ephemeral public key
(*ECDH(K<sub>A</sub><sup>private</sup>*, *K<sub>B</sub><sup>public</sup>*)
for Alice's device and
*ECDH(K<sub>B</sub><sup>private</sup>*, *K<sub>A</sub><sup>public</sup>*)
for Bob's device), using the result as the shared secret.
13. Both Alice and Bob's devices display a SAS to their users, which is
derived from the shared key using one of the methods in this
section. If multiple SAS methods are available, clients should allow
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