Rename fixed notion of time

Rename from "fixed update expiration time" to "fixed update start time",
because we are not actually modifying the expiration time. We are fixing
the system time to the time the update was started.

Signed-off-by: Joshua Lock <[email protected]>

Author: joshuagl

tuf-spec.md

@@ -1092,8 +1092,8 @@ repo](https://github.com/theupdateframework/specification/issues).
   process should not leave clients in an unrecoverable state.
 
   **5.0**. **Record the time at which the update began** as the fixed update
-  expiration time.  Time is fixed at the beginning of the update workflow to
-  allow an application using TUF to effectively pause time, in order to prevent
+  start time.  Time is fixed at the beginning of the update workflow to allow
+  an application using TUF to effectively pause time, in order to prevent
   metadata which is valid at the beginning of an update from expiring during
   the update workflow.
 
@@ -1153,10 +1153,10 @@ repo](https://github.com/theupdateframework/specification/issues).
   * **5.2.8**. **Repeat steps 5.2.1 to 5.2.8**.
 
   * **5.2.9**. **Check for a freeze attack.** The expiration timestamp in the
-  trusted root metadata file MUST be higher than the fixed update expiration
-  time.  If the trusted root metadata file has expired, abort the update
-  cycle, report the potential freeze attack.  On the next update cycle, begin
-  at step 5.1 and version N of the root metadata file.
+  trusted root metadata file MUST be higher than the fixed update start time.
+  If the trusted root metadata file has expired, abort the update cycle,
+  report the potential freeze attack.  On the next update cycle, begin at step
+  5.1 and version N of the root metadata file.
 
   * **5.2.10**. **If the timestamp and / or snapshot keys have been rotated,
   then delete the trusted timestamp and snapshot metadata files.** This is done
@@ -1197,8 +1197,8 @@ used to download the timestamp metadata file is of the fixed form FILENAME.EXT
     timestamp metadata file, abort the update cycle, and report the failure.
 
   * **5.3.3**. **Check for a freeze attack.** The expiration timestamp in the
-  new timestamp metadata file MUST be higher than the fixed update expiration
-  time.  If so, the new timestamp metadata file becomes the trusted timestamp
+  new timestamp metadata file MUST be higher than the fixed update start time.
+  If so, the new timestamp metadata file becomes the trusted timestamp
   metadata file.  If the new timestamp metadata file has expired, discard it,
   abort the update cycle, and report the potential freeze attack.
 
@@ -1243,10 +1243,10 @@ the timestamp metadata file.
   the update cycle, and report the failure.
 
   * **5.4.5**. **Check for a freeze attack.** The expiration timestamp in the
-  new snapshot metadata file MUST be higher than the fixed update expiration
-  time.  If so, the new snapshot metadata file becomes the trusted snapshot
-  metadata file.  If the new snapshot metadata file is expired, discard it,
-  abort the update cycle, and report the potential freeze attack.
+  new snapshot metadata file MUST be higher than the fixed update start time.
+  If so, the new snapshot metadata file becomes the trusted snapshot metadata
+  file.  If the new snapshot metadata file is expired, discard it, abort the
+  update cycle, and report the potential freeze attack.
 
 
   * **5.4.6**. **Persist snapshot metadata.** The client MUST write the file to
@@ -1280,10 +1280,10 @@ snapshot metadata file.
   abort the update cycle, and report the failure.
 
   * **5.5.4**. **Check for a freeze attack.** The expiration timestamp in the
-  new targets metadata file MUST be higher than the fixed update expiration
-  time.  If so, the new targets metadata file becomes the trusted targets
-  metadata file.  If the new targets metadata file is expired, discard it,
-  abort the update cycle, and report the potential freeze attack.
+  new targets metadata file MUST be higher than the fixed update start time.
+  If so, the new targets metadata file becomes the trusted targets metadata
+  file.  If the new targets metadata file is expired, discard it, abort the
+  update cycle, and report the potential freeze attack.
 
   * **5.5.5**. **Persist targets metadata.** The client MUST write the file to
   non-volatile storage as FILENAME.EXT (e.g. targets.json).