Bitcoin is a protocol, not a software, so there is no Bitcoin source code. Bitcoin, the protocol, is the sum of all Bitcoin nodes and interactions. Almost all full nodes help the network by accepting transactions and blocks from other full nodes, validating these transactions and blocks, and relaying them to other complete nodes. The main implementation is Bitcoin Core software and any developer can contribute to the project. All you need is in the GitHub repository. The Bitcoin Core source code is written in C++. By simply looking at the code, you can see that there are about four out of five code types:

  • Straightforward lists of declarations and simple definitions, with one level of indentation at most
  • Conditional logic with many levels of indentation
  • Cryptographic code, varying from compact calculations to big blocks of numbers
  • User interface code, using the Qt framework, which includes patterns of repetition and relatively simple conditional logic in C++ files, as well as some XML files