However, these can be on different devices. Transactions can be signed on an offline machine. Also, you need just the unspent transactions to sign a new transac

However, these can be on different devices. Transactions can be signed on an offline machine. Also, you need just the unspent transactions to sign a new transaction, not the whole blockchain. Bitcoin Trezor works this way, http://www.bitcointrezor.com/ http://www.bitcointrezor.com/