Wallets

The wallet software is used to generate and store cryptographic keys. It performs various useful functions, such as receiving and sending Bitcoin, backing up keys, and keeping track of the balance available. Bitcoin client software usually offers both functionalities: a Bitcoin client and wallet. On disk, the Bitcoin Core client wallets are stored as a Berkeley DB file:

$ file wallet.dat
wallet.dat: Berkeley DB (Btree, version 9, native byte-order)

Private keys are generated by randomly choosing a 256-bit number provided by the wallet software. The rules of generation are predefined and were discussed in Chapter 4, Asymmetric Cryptography. Private keys are used by wallets to sign outgoing transactions. Wallets do not store any coins. In fact, in the Bitcoin network, coins do not exist; instead, only transaction information is stored on the blockchain (more precisely, UTXO, unspent outputs), which are then used to calculate the number of bitcoins.

Fundamentally...