The source code for VeraCrypt EFI bootloader files is available at: https://github.com/veracrypt/VeraCrypt-DCS
VeraCrypt-DCS uses EDK II as its UEFI development environement.
VeraCrypt-DCS is licensed under LGPL: https://github.com/veracrypt/VeraCrypt-DCS/blob/master/LICENSE
Here the steps to build VeraCrypt-DCS (Visual Studio 2010 SP1 should be installed)
* Clone EDK: git clone https://github.com/tianocore/tianocore.github.io.git edk2
* Switch to UDK2015 branche: git checkout UDK2015
* Clone VeraCrypt-DCS as DcsPkg inside edk2 folder: git clone https://github.com/veracrypt/VeraCrypt-DCS.git DcsPkg
* Switch to VeraCrypt_1.18 branche: git checkout VeraCrypt_1.18
* Setup EDK by typing edksetup.bat at the root of folder edk2
* change directoty to DcsPkg and then type setenv.bat.
* change directory to DcsPkg\Library\VeraCryptLib and then type mklinks_src.bat: you will be asked to provide the path to VeraCrypt src folder.
* change directory to DcsPkg and then type dcs_bld.bat X64Rel
* After the build is finished, EFI bootloader files will be present at edk2\Build\DcsPkg\RELEASE_VS2010x86\X64