BIP47 - Reusable Payment Codes - compute Shared Secret for Notification Tx

From BIP47 : https://github.com/bitcoin/bips/blob/master/bip-0047.mediawiki
and test vectors : https://gist.github.com/SamouraiDev/6aad669604c5930864bd

I would like to compute the shared secret from the test vectors:

If I follow the BIP47 protocol:

0) It is assumed that Alice can easily obtain Bob's payment code via a suitable method outside the scope of the payment code protocol.

So Alice knows Bob's Payment Code:

Code:

PC_BOB = PM8TJS2JxQ5ztXUpBBRnpTbcUXbUHy2T1abfrb3KkAAtMEGNbey4oumH7Hc578WgQJhPjBxteQ5GHHToTYHE3A1w6p7tU6KSoFmWBVbFGjKPisZDbP97

Then Alice unserialize PC_BOB and can find Bob's pubkey and chain code (path : m/47'/0'/0')

Code:

pubkey : (02) 9d125e1cb89e5a1a108192643ee25370c2e75c192b10aac18de8d5a09b5f48d5
Chain Code : 1db1243aaa57c7fbea3072249c1bd4dab9482b4fee4d25e1c69707e8144dc137

1) Alice select an input...
2.i) Alice selects the private key corresponding to the first exposed public key, of the first pubkey-exposing input, of the transaction:
I tried with

Code:

a0 = 8d6a8ecd8ee5e0042ad0cb56e3a971c760b5145c3917a8e7beaf0ed92d7a520c path m/47'/0'/0'/0

ao is given in the paper. It gave me the wrong shared secret, I also tried with

Code:

a = 0a5c1795378b3ba756efcb5ca47e605c3f4f8bcff99eced897b45a4b051e980d path m/47'/0'/0'

with a wrong result again ("a" can be computed form the raw entropy)
Which value should I use for Alice private key ?
2.ii)
Alice selects the public key associated with Bob's notification address:
This address is given in the paper:

Code:

B = 024ce8e3b04ea205ff49f529950616c3db615b1e37753858cc60c1ce64d17e2ad8 (= B0) path: m/47'/0'/0'/0

2.iii) Alice calculates a secret point:

Code:

S = a B = 736a25d9250238ad64ed5da03450c6a3f4f8f4dcdf0b58d1ed69029d76ead48d

I think the value of B is correct but what should I take for a ?

Topic: BIP47 - Reusable Payment Codes - compute Shared Secret for Notification Tx (Read 609 times)