Bitcoin is a peer-to-peer (p2p) electronic cash system that uses a distributed timestamp service to record transactions in a public ledger (called the Blockchain). A critical component of Bitcoin's success is the decentralized nature of its architecture, which does not require or even support the establishment of trusted authorities. Yet the absence of certification creates obstacles to its wider acceptance in e-commerce and official uses. We propose a certification system for Bitcoin that offers: a) an opt-in guarantee to send and receive bitcoins only to/ from certified users; b) control of creation of bitcoins addresses (certified users) by trusted authorities. Our proposal may encourage the adoption of Bitcoin in different scenarios that require an officially recognized currency, such as tax payments-often an integral part of e-commerce transactions.