[Bitcoin-development] Version bytes

[Stefan Thomas]

Hey Pieter,

> Otherwise, we could reset testnet (not actually reset, just
> change its addresses a bit), and simply state odd=testnet, even=realnet.

We could use the XOR hack for now and remove it the next time we reset 
testnet. But I do think the 111 is baggage we want to get rid of. Using 
the lsb as a simple flag is much cleaner.

Cheers,

Stefan


On 7/7/2011 1:15 PM, Pieter Wuille wrote:
> Hello everyone,
>
> after a discussion on IRC, we decided to try to standardize the version bytes
> used by bitcoin for several applications.
>
> There are 3 components that seem meaningful:
> * network? (realnet, testnet, alternate chains?)
> * data class? (address, private key, master key, ...?)
> * version? (real version, per data class defined)
>
> There is no technical reason why different network and different data classes
> would need separate version bytes, but i think it is a good thing to keep
> them from colliding. People will mix them up, and when things are well
> defined, a nice warning message could help a lot ("Oops it seems you entered
> a private key instead of an address!").
>
> So, first of all, there is already one actually used alternate chain, namely
> namecoin, using version byte 52 for addresses. For this reason, i'd like to
> reserve bit 16 in the version byte for "alternate chain". When bit 16 is set,
> everything is up to the network itself, and no further semantics are defined.
>
> When bit 16 isn't set:
>
> Then remains the rest of the network. The problem is that testnet already uses
> version 111, which is not a single bit. We can use a trick though, namely
> choosing bit 1 for testnet, and if bit 1 is set, XOR the rest of the version
> number with 111. Otherwise, we could reset testnet (not actually reset, just
> change its addresses a bit), and simply state odd=testnet, even=realnet.
>
> That leaves use with 6 more bits to play with, namely 128,64,32 and 8,4,2.
> As 128 is already used for private keys, let's use (128,64,32) for data classes,
> and (8,4,2) for versions.
>
> So, in full:
> * Bits 128/64/32 define data class
> ** 0 = address
> ** 32,64,96,160,192 = reserved for future use
> ** 128 = private key
> ** 224 = extended data class, another "data class" byte follows
> * Bit 16 defines "private"
> ** 0 = bitcoin
> ** 16 = alternate chain
> * Bits 8/4/2 define version number
> ** 0 = only thing used for now
> ** 2,4,6,8,10,12 = reserved for future use
> ** 14 = extended version, another version byte follows
> * Bit 1 defines testnet
> ** 0 = realnet
> ** 1 = testnet (possibly using XOR 111, if not reset)
>
> This whole discussion started when Stefan wanted to define a format for master keys from which
> to derive deterministic wallet keys, i suggest using data class 192 for that, leaving the
> lower numbers for more basic data, like public keys.
>
> Any comments?
>