diff --git a/Security.md b/Security.md
index b8cfcb9..3be4d9c 100644
--- a/Security.md
+++ b/Security.md
@@ -6,7 +6,17 @@ This page exists so that there is one page to read to learn about the security g
 
 ## Access Control
 
-The Tahoe distributed filesystem is composed of files and directories.  Each file has a unique and unguessable identifier, called a "CHK-URI".  Possession of this identifier is necessary and sufficient to download, reconstruct, decrypt, and verify the integrity of the file.  If a person is not given the CHK-URI, then they cannot see the 
+The Tahoe distributed filesystem is composed of files and directories.
+
+### Files
+
+#### read access
+
+Each file has a unique and unguessable identifier, called a "CHK-URI", which is derived from the file contents.  Possession of this identifier is necessary and sufficient to download, reconstruct, decrypt, and verify the integrity of the file.  If a person is not given the CHK-URI, then they cannot see the contents of the file.
+
+#### mutation
+
+Files in the Tahoe grid are immutable.  If you upload a file to the grid, and then change part of it and upload it again, then there are now two files in the grid -- the old one and the new one -- and each has a distinct, unique, CHK-URI.
 
 ## Traffic Analysis