This is a read note of Mastering Bitcoin Chapter Ch11: Bitcoin Security.

1 Introduction

Securing bitcoin is challenging because bitcoin is not an abstract reference to value, like a balance in a bank account. Bitcoin is very much like digital cash or gold.

However, bitcoin has capabilities that cash, gold, and bank accounts do not. A bitcoin wallet, containing your keys, can be backed up like any file. It can be stored in multiple copies, even printed on paper for hard-copy backup.

2 Security Principles

The core principle in bitcoin is decentralization and it has important implications for security. A decentralized system like bitcoin pushes the responsibility and control to the users. Because security of the network is based on Proof-of-Work, not access control, the network can be open and no encryption is required for bitcoin traffic.

Two basic principles:

  • Don’t take control of keys away from users
  • Don’t take transactions off the blockchain.

3 Trust Root

Traditional security architecture is based upon a concept called the root of trust, which is a trusted core used as the foundation for the security of the overall system or application.

In bitcoin, the consensus system creates a trusted public ledger that is completely decentralized. A correctly validated blockchain uses the genesis block as the root of trust, building a chain of trust up to the current block. Bitcoin systems can and should use the blockchain as their root of trust.

4 Best Practices

Humans have used physical security controls for thousands of years. By comparison, our experience with digital security is less than 50 years old. Modern general-purpose operating systems are not very secure and not particularly suited to storing digital money. Some best practices are

  • Cold storage: Bitcoin keys are nothing more than long numbers. This means that they can be stored in a physical form, such as printed on paper or etched on a metal coin. A cold storage system is one where the keys are generated on an offline system (one never connected to the internet) and stored offline either on paper or on digital media, such as a USB memory stick.
  • Hardware Wallets: A bitcoin hardware wallet has just one purpose: to hold bitcoin securely. Without general-purpose software to compromise and with limited interfaces, hardware wallets can deliver an almost foolproof level of security to nonexpert users. It is a long term solution.
  • Diversifying Risk: Users should spread the risk among multiple and diverse bitcoin wallets. Prudent users will keep only a small fraction, perhaps less than 5%, of their bitcoin in an online or mobile wallet as “pocket change.” The rest should be split between a few different storage mechanisms, such as a desktop wallet and offline (cold storage).
  • Multisig and Governance: Multisignature addresses secure funds by requiring a minimum number of signatures to make a payment. The signing keys should be stored in a number of different locations and under the control of different people.
  • Survivability: You should consider sharing access details with a trusted relative or lawyer. A more complex survivability scheme can be set up with multi-signature access and estate planning through a lawyer specialized as a “digital asset executor.”