Proof of work¶

This page describes Bitmessage’s Proof of work (“POW”) mechanism as it exists in Protocol Version 3. In this document, hash() means SHA512(). SHA512 was chosen as it is widely supported and so that Bitcoin POW hardware cannot trivially be used for Bitmessage POWs. The author acknowledges that they are essentially the same algorithm with a different key size.

Both averageProofOfWorkNonceTrialsPerByte and payloadLengthExtraBytes are set by the owner of a Bitmessage address. The default and minimum for each is 1000. (This is the same as difficulty 1. If the difficulty is 2, then this value is 2000). The purpose of payloadLengthExtraBytes is to add some extra weight to small messages.

Do a POW¶

Let us use a msg message as an example:

payload = embeddedTime + encodedObjectVersion + encodedStreamNumber + encrypted

payloadLength: the length of payload, in bytes, + 8 (to account for the nonce which we will append later)
TTL: the number of seconds in between now and the object expiresTime.

$target = \frac{2^{64}}{{\displaystyle nonceTrialsPerByte (payloadLength + payloadLengthExtraBytes + \frac{ TTL (payloadLength + payloadLengthExtraBytes)}{2^{16}}) }}$

initialHash = hash(payload)

start with trialValue = 99999999999999999999

also start with nonce = 0 where nonce is 8 bytes in length and can be hashed as if it is a string.

while trialValue > target:
    nonce = nonce + 1
    resultHash = hash(hash( nonce || initialHash ))
    trialValue = the first 8 bytes of resultHash, converted to an integer

When this loop finishes, you will have your 8 byte nonce value which you can prepend onto the front of the payload. The message is then ready to send.

Check a POW¶

Let us assume that payload contains the payload for a msg message (the nonce down through the encrypted message data).

nonce: the first 8 bytes of payload
dataToCheck: the ninth byte of payload on down (thus it is everything except the nonce)

initialHash = hash(dataToCheck)

resultHash = hash(hash( nonce || initialHash ))

POWValue: the first eight bytes of resultHash converted to an integer
TTL: the number of seconds in between now and the object expiresTime.

$target = \frac{2^{64}}{{\displaystyle nonceTrialsPerByte (payloadLength + payloadLengthExtraBytes + \frac{ TTL (payloadLength + payloadLengthExtraBytes)}{2^{16}}) }}$

If POWValue is less than or equal to target, then the POW check passes.