Sortof small, portable in compilation sense, one could use GPG or OpenSSL or one of the variants. [1] OpenSSL and GPG should be around for some time. You can also keep the source code [2] around of any version in the event that they break backwards compatibility with the method you used to encrypt your files such as deprecating a protocol or individual cipher.
ls -alh bin/openssl lib/libssl.so.1.1 lib/libcrypto.so.1.1
-r-xr-xr-t 1 root root 616K Jul 26 17:39 bin/openssl
-r-xr-xr-t 1 root root 2.4M Jul 26 17:38 lib/libcrypto.so.1.1
-r-xr-xr-t 1 root root 552K Jul 26 17:38 lib/libssl.so.1.1
[1] -
https://stackoverflow.com/questions/16056135/how-to-use-open...[2] - https://ftp.openssl.org/snapshot/ or rsync://ftp.openssl.org::openssl-ftp/snapshot/