You can view the release notes and news entries to see what has changed in various releases. Older releases can be found in the release archives. All releases are signed with a PGP distribution key.
If this software is useful to you, please consider supporting further development.
Source: tar/gzip (sig) / tar/bzip2 (sig)
Windows DLL installer: 32 bit 64 bit (checksums)
Source: tar/gzip (sig) / tar/bzip2 (sig)
Botan is also available as part of number of third party package sets, though how up to date any particular distro is varies widely.