diff --git a/pubspec.yaml b/pubspec.yaml
index 3f784b9..1d3c39a 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -10,7 +10,7 @@ dependencies:
   asn1lib: ^1.5.8
   convert: ^3.1.2
   meta: ^1.16.0
-  pointycastle: ^3.9.1
+  pointycastle: ">=3.9.1 <5.0.0"
   pinenacl: ^0.6.0
 
 dev_dependencies: