The License of Crypto++ is somewhat unusual amongst open source projects. A distinction is made between the library as a compilation (i.e., collection), which is copyrighted by Wei Dai, and the individual files in it, which are public domain (with the exception of mars.cpp which is subject to its own license, included in that file).
The library is copyrighted as a compilation in order to place certain disclaimers (regarding warranty, export, and patents) in the license and to keep the attributions and public domain declarations intact when Crypto++ is distributed in source code form. The fact that individual files are public domain means that legally you can place code segments, entire files, or small sets of files (up to the limit set by fair use) into your own project and do anything you want with them without worrying about the copyright.
Because one purpose of the project is to act as a repository of public domain (not copyrighted) cryptographic source code, the code in Crypto++ was either written specifically for this project by its contributors and placed in the public domain, or derived from other sources that are public domain (again with the exception of mars.cpp).