diff --git a/lib/resty/hmac.lua b/lib/resty/hmac.lua
index 5143ac0..f7419a9 100644
--- a/lib/resty/hmac.lua
+++ b/lib/resty/hmac.lua
@@ -6,7 +6,7 @@ local ffi_new = ffi.new
 local ffi_str = ffi.string
 local ffi_gc = ffi.gc
 local ffi_typeof = ffi.typeof
-local C = ffi.C
+local C = (ffi.os=="Windows" and ffi.load'libeay32' or ffi.C)
 local setmetatable = setmetatable
 local error = error