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. -- -- Gavin Andresen