RE:
> base58-encode: [one-byte network ID][20-byte hash][one-byte address class][3-byte checksum]
How will the code distinguish between the old scheme:
[one-byte-version][20-byte-hash][4-byte-checksum]
and the new?
1 in 256 old addresses will have a first-byte-of-checksum that matches the new address class; I guess the code would do something like:
a) If the 4-byte checksum matches, then assume it is a singlesig address (1 in 2^32 multisig addresses will incorrectly match)
b) If the one-byte-address-class and 3-byte checksum match, then it is a valid p2sh
c) Otherwise, invalid address
The 1 in 2^32 multisig addresses also being valid singlesig addresses makes me think this scheme won't work-- an attacker willing to generate 8 billion or so ECDSA keys could generate a single/multisig collision. I'm not sure how that could be leveraged to their advantage, but I bet they'd find a way.
RE: should it be a BIP: The BIP process is described in BIP 0001, and you're following it perfectly so far:
1) Post a rough draft of the idea here to see if there's any chance it'll be adopted
2) Assuming a positive response and no major flaws: write up a draft BIP
3) Post the draft BIP here, where it can be picked apart.
4) Assuming no major flaws, ask the BIP editor (Amir) for a BIP number
I'd also encourage you to actually implement your idea between steps 3 and 4. But in this particular case, I think an attacker being able to create singlesig/p2sh address collisions counts as a major flaw.
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Gavin Andresen