diff --git a/.travis.yml b/.travis.yml
index 379acf6..6be444c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,18 +5,14 @@ os:
 #  - osx
 
 install:
-  - ./travis-install-lib-sodium.sh
+  - COX_BUILD_DEBUG=1 LIBSODIUM_BUILD_DIR=$HOME/libsodium ./build/libsodium_install.sh
 before_script:
-  - export C_INCLUDE_PATH=$LD_LIBRARY_PATH:`pwd`/sodium/include
-  - export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:`pwd`/sodium/lib
-  - env | egrep PKG
-  - export PKG_CONFIG_PATH=`pwd`/sodium/lib/pkgconfig
-  - env | egrep LD_
-  - env | egrep PKG
-  - pwd
+  - export LIBSODIUM_BUILD_DIR=$HOME/libsodium
+after_failure:
+  - "[ -f libsodium_install.out ] && cat libsodium_install.out"
 cache:
   directories:
-    - sodium
+    - "$HOME/libsodium"
 
 #addons:
 #  apt:
diff --git a/build/env.sh b/build/env.sh
new file mode 100644
index 0000000..65349ff
--- /dev/null
+++ b/build/env.sh
@@ -0,0 +1,34 @@
+#!/bin/sh
+
+# Overridable.
+[ -z "$LIBSODIUM_BUILD_DIR" ] && LIBSODIUM_BUILD_DIR=`pwd`/build
+
+
+# Upgraded from time to time.
+export MIN_LIBSODIUM_VERSION=1.0.18
+export LIBSODIUM_SHA256=6f504490b342a4f8a4c4a02fc9b866cbef8622d5df4e5452b46be121e46636c1
+
+
+[ ! -z "$COX_BUILD_DEBUG" ] && export COX_BUILD_VERBOSE=1
+
+function version { echo "$@" | awk -F. '{ printf("%d%03d%03d%03d\n", $1,$2,$3,$4); }'; }
+
+if `pkg-config libsodium --exists`; then
+	PKG_VER=`pkg-config libsodium --modversion`
+
+	if [ $(version "$PKG_VER") -ge $(version "$MIN_LIBSODIUM_VERSION") ]; then
+		[ ! -z "$COX_BUILD_VERBOSE" ] && echo "Using packaged libsodium."
+	else
+		[ ! -z "$COX_BUILD_VERBOSE" ] echo "System packaged libsodium is too old."
+		export LIBSODIUM_INSTALL=1
+	fi
+else
+	[ ! -z "$COX_BUILD_VERBOSE" ] && echo "Missing libsodium system package."
+	export LIBSODIUM_INSTALL=1
+fi
+
+
+if [ "$LIBSODIUM_INSTALL" = "1" ]; then
+	export LIBSODIUM_INSTALL_PATH="$LIBSODIUM_BUILD_DIR"/libsodium
+	export PKG_CONFIG_PATH="$LIBSODIUM_INSTALL_PATH"/lib/pkgconfig
+fi
diff --git a/build/libsodium_install.sh b/build/libsodium_install.sh
new file mode 100755
index 0000000..32f0421
--- /dev/null
+++ b/build/libsodium_install.sh
@@ -0,0 +1,68 @@
+#!/bin/bash
+# The purpose of this file is to install libsodium when:
+# 1. No packaged version is found.
+# 2. The packaged version is older than the version specified in this repository.
+# pkg-config is used to find the library and determine it's version.
+# You may replace the libsodium library with your own installed version by setting PKG_CONFIG_PATH before compiling.
+
+set -e
+
+# Always use bash.  `dash` doesn't work properly with . includes.  I'm not sure why.
+. ./build/env.sh
+
+#export LIBSODIUM_INSTALL=1
+if [ "$LIBSODIUM_INSTALL" != "1" ]; then
+	[ ! -z "$COX_BUILD_VERBOSE" ] echo "Skipping libsodium build."
+	exit
+fi
+
+
+mkdir -p "$LIBSODIUM_BUILD_DIR"
+cd "$LIBSODIUM_BUILD_DIR"
+
+
+if [ ! -f "$LIBSODIUM_INSTALL_PATH/include/sodium.h" ]; then
+	[ ! -z "$COX_BUILD_DEBUG" ] && set -x
+
+	DIRNAME=libsodium-"$MIN_LIBSODIUM_VERSION"
+	TGZ_FILENAME="$DIRNAME".tar.gz
+
+	if [ ! -f "$TGZ_FILENAME" ]; then
+		wget https://download.libsodium.org/libsodium/releases/"$TGZ_FILENAME"
+#		wget https://download.libsodium.org/libsodium/releases/"$TGZ_FILENAME".minisign
+	fi
+
+	SHA=`openssl sha256 -hex < "$TGZ_FILENAME" | sed 's/^.* //'`
+	if [ "$SHA" != "$LIBSODIUM_SHA256" ]; then
+		echo "SHA256 sum doesn't match."
+		echo "$SHA" != "$LIBSODIUM_SHA256"
+		exit 1
+	fi
+
+	if [ ! -d "$DIRNAME" ]; then
+		tar xfz "$TGZ_FILENAME"
+	fi
+
+
+	cd "$DIRNAME"
+	if [ ! -f ".configure.done" ]; then
+		./configure --prefix="$LIBSODIUM_INSTALL_PATH" --disable-shared
+		touch .configure.done
+	fi
+	if [ ! -f ".make.done" ]; then
+		make
+		touch .make.done
+	fi
+	if [ ! -f ".make.install.done" ]; then
+		make install
+		touch .make.install.done
+	fi
+
+	[ ! -z "$COX_BUILD_VERBOSE" ] && echo "Finished building libsodium."
+else
+#	find "$LIBSODIUM_INSTALL_PATH"
+
+	[ ! -z "$COX_BUILD_VERBOSE" ] && echo "Using already built libsodium."
+fi
+
+exit 0
diff --git a/build/pkg-libs.sh b/build/pkg-libs.sh
new file mode 100755
index 0000000..6fd484c
--- /dev/null
+++ b/build/pkg-libs.sh
@@ -0,0 +1,12 @@
+#!/bin/bash
+
+set -e
+
+# Shard directory passed as first argument when called from lib_sodium.cr
+[ ! -z "$1" ] && cd "$1"
+
+./build/libsodium_install.sh > libsodium_install.out 2>&1
+
+. ./build/env.sh
+
+pkg-config libsodium --libs
diff --git a/shard.yml b/shard.yml
index 565710d..7d0da18 100644
--- a/shard.yml
+++ b/shard.yml
@@ -4,6 +4,10 @@ version: 0.1.0
 authors:
   - Andrew Hamon <andrew@hamon.cc>
 
+development_dependencies:
+  ghshard:
+    github: bcardiff/ghshard
+
 crystal: 0.24.1
 
 license: MIT
diff --git a/src/cox/lib_sodium.cr b/src/cox/lib_sodium.cr
index 11d7312..97c3256 100644
--- a/src/cox/lib_sodium.cr
+++ b/src/cox/lib_sodium.cr
@@ -1,5 +1,5 @@
 module Cox
-  @[Link("sodium")]
+  @[Link(ldflags: "`#{__DIR__}/../../build/pkg-libs.sh #{__DIR__}/../..`")]
   lib LibSodium
     fun sodium_init() : LibC::Int
 
diff --git a/travis-install-lib-sodium.sh b/travis-install-lib-sodium.sh
deleted file mode 100755
index 146b780..0000000
--- a/travis-install-lib-sodium.sh
+++ /dev/null
@@ -1,25 +0,0 @@
-#!/bin/sh
-# The purpose of this file is to install libsodium in
-# the Travis CI environment. We recommend using a
-# package manager.
-
-set -e
-
-LIBSODIUM_INSTALL_PATH=`pwd`/sodium
-
-if [ ! -f "sodium/include/sodium.h" ]; then
-	set -x
-
-	mkdir -p "$LIBSODIUM_INSTALL_PATH"
-	find "$LIBSODIUM_INSTALL_PATH"
-
-	wget https://download.libsodium.org/libsodium/releases/LATEST.tar.gz
-	tar xfz LATEST.tar.gz
-	cd libsodium-stable
-	./configure --prefix="$LIBSODIUM_INSTALL_PATH"
-	make
-	make install
-else
-	echo "using cached libsodium build"
-	find "$LIBSODIUM_INSTALL_PATH"
-fi