NaCl Page:
https://nacl.cr.yp.to/index.html

libsodium Page:
https://libsodium.org/
https://github.com/jedisct1/libsodium

Download:

wget https://hyperelliptic.org/nacl/nacl-20110221.tar.bz2

verify checksums:
sha512sum 4c031ceffe6a28dc74b46ac003d485531f78de467c802df73c8b22ca53644dabb7d2e3080b7bdd6583f0d07ad76b6d95bc0ffdce319ca2f80ee041e6fe618656 nacl-20110221.tar.bz2
sha256sum 4f277f89735c8b0b8a6bbd043b3efb3fa1cc68a9a5da6a076507d067fc3b3bf8 nacl-20110221.tar.bz2
sha1sum 6007a6aee249f5a534ec53fddfc364601fba9629 nacl-20110221.tar.bz2
md5sum 7efb5715561c3d10dafd3fa97b4f2d20 nacl-20110221.tar.bz2

Unzip and compile:

bunzip2 < nacl-20110221.tar.bz2 | tar -xf -
cd nacl-20110221
time ./do