78 lines
2.2 KiB
ReStructuredText
78 lines
2.2 KiB
ReStructuredText
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Proof of work
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=============
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This page describes Bitmessage's Proof of work ("POW") mechanism as it exists in
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Protocol Version 3. In this document, hash() means SHA512(). SHA512 was chosen
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as it is widely supported and so that Bitcoin POW hardware cannot trivially be
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used for Bitmessage POWs. The author acknowledges that they are essentially the
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same algorithm with a different key size.
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Both ``averageProofOfWorkNonceTrialsPerByte`` and ``payloadLengthExtraBytes``
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are set by the owner of a Bitmessage address. The default and minimum for each
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is 1000. (This is the same as difficulty 1. If the difficulty is 2, then this
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value is 2000). The purpose of ``payloadLengthExtraBytes`` is to add some extra
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weight to small messages.
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Do a POW
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--------
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Let us use a ``msg`` message as an example::
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payload = embeddedTime + encodedObjectVersion + encodedStreamNumber + encrypted
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``payloadLength``
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the length of payload, in bytes, + 8
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(to account for the nonce which we will append later)
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``TTL``
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the number of seconds in between now and the object expiresTime.
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.. include:: pow_formula.rst
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::
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initialHash = hash(payload)
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start with ``trialValue = 99999999999999999999``
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also start with ``nonce = 0`` where nonce is 8 bytes in length and can be
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hashed as if it is a string.
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::
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while trialValue > target:
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nonce = nonce + 1
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resultHash = hash(hash( nonce || initialHash ))
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trialValue = the first 8 bytes of resultHash, converted to an integer
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When this loop finishes, you will have your 8 byte nonce value which you can
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prepend onto the front of the payload. The message is then ready to send.
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Check a POW
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-----------
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Let us assume that ``payload`` contains the payload for a msg message (the nonce
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down through the encrypted message data).
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``nonce``
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the first 8 bytes of payload
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``dataToCheck``
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the ninth byte of payload on down (thus it is everything except the nonce)
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::
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initialHash = hash(dataToCheck)
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resultHash = hash(hash( nonce || initialHash ))
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``POWValue``
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the first eight bytes of resultHash converted to an integer
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``TTL``
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the number of seconds in between now and the object ``expiresTime``.
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.. include:: pow_formula.rst
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If ``POWValue`` is less than or equal to ``target``, then the POW check passes.
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