add `otp.generate_secret()`

functions.lua

@@ -163,4 +163,13 @@ function otp.create_qr_png(data)
     assert(#png_data == width*height)
 
     return minetest.encode_png(width, height, png_data, 2)
+end
+
+function otp.generate_secret()
+    local buf = minetest.sha1("" .. math.random(10000), true)
+    local s = ""
+    for i=1,20 do
+        s = s .. string.char(string.byte(buf, i))
+    end
+    return s
 end

functions.spec.lua

@@ -57,4 +57,10 @@ mtt.register("otp.create_qr_png", function(callback)
     f:write(png)
     f:close()
     callback()
+end)
+
+mtt.register("otp.generate_secret", function(callback)
+    local s = otp.generate_secret()
+    assert(#s == 20)
+    callback()
 end)