diff --git a/.circleci/config.yml b/.circleci/config.yml
index 5a9e1e3..146d5e5 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -5,7 +5,7 @@ version: 2.1
 jobs:
   build-linux:
     docker:
-      - image: circleci/golang:1.12.7
+      - image: circleci/golang:1.13.3
 
     steps:
       - checkout
diff --git a/README.md b/README.md
index 07b202f..c02151f 100644
--- a/README.md
+++ b/README.md
@@ -49,7 +49,7 @@ You may also find other platform-specific wrappers, scripts or tools in the
 If you want to build from source, as opposed to installing one of the pre-built
 packages:
 
-1. Install [Go](https://golang.org) (requires Go 1.12 or later)
+1. Install [Go](https://golang.org) (requires Go 1.13 or later)
 2. Clone this repository
 2. Run `./build`
 
diff --git a/go.mod b/go.mod
index 83c2292..d1a3587 100644
--- a/go.mod
+++ b/go.mod
@@ -17,3 +17,5 @@ require (
 	golang.org/x/sys v0.0.0-20190813064441-fde4db37ae7a
 	golang.org/x/text v0.3.2
 )
+
+go 1.13
diff --git a/src/yggdrasil/tls.go b/src/yggdrasil/tls.go
index 26ecf1d..7212c4d 100644
--- a/src/yggdrasil/tls.go
+++ b/src/yggdrasil/tls.go
@@ -2,6 +2,7 @@ package yggdrasil
 
 import (
 	"bytes"
+	"crypto/ed25519"
 	"crypto/rand"
 	"crypto/tls"
 	"crypto/x509"
@@ -12,8 +13,6 @@ import (
 	"math/big"
 	"net"
 	"time"
-
-	"golang.org/x/crypto/ed25519"
 )
 
 type tcptls struct {