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BIP47 is a new secret-sharing protocol which succeeds BIP64 (stealth addresses).

The test vectors are here.

I am stuck on part 2.iv. We multiply S0 = B0 * a0 (024ce8e3b04ea205ff49f529950616c3db615b1e37753858cc60c1ce64d17e2ad8 * 8d6a8ecd8ee5e0042ad0cb56e3a971c760b5145c3917a8e7beaf0ed92d7a520c = 03f5bb84706ee366052471e6139e6a9a969d586e5fe6471a9b96c3d8caefe86fef).

From that we are looking for:

s = HMAC-SHA512(x, o)
"x" is the x value of the secret point
"o" is the outpoint being spent by the first pubkey-exposing input of the transaction.

What does HMAC-SHA512(x, o) mean, first of all? What is o specifically? And how does this factor in with the test vector values?

  • I've just had a thought: is it as simple as being the notation for HMAC-SHA512(passphrase, salt)? – Wizard Of Ozzie Dec 17 '15 at 12:50
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Per the creator of BIP47 (Justus Ranvier), it's RFC2014 notation:

https://en.wikipedia.org/wiki/Hash-based_message_authentication_code#Definition_.28from_RFC_2104.29

https://tools.ietf.org/html/rfc2104

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