tor-spec: Rewrite the Routing Relay Cells section

The section now consists of:
* forward encryption at the client
* forward decryption at ORs
* backward encryption at the end (exit)
* backward decryption at the client

Part of 26860.

1 files changed, 48 insertions, 16 deletions

diff --git a/tor-spec.txt b/tor-spec.txt
index db32093..364505f 100644
--- a/tor-spec.txt
+++ b/tor-spec.txt
@@ -1353,18 +1353,35 @@ see tor-design.pdf.
 
 5.5. Routing relay cells
 
-   When an OR receives a RELAY or RELAY_EARLY cell, it checks the cell's
-   circID and determines whether it has a corresponding circuit along that
-   connection.  If not, the OR drops the cell.
+5.5.1. Circuit ID Checks
 
-   Otherwise, if the OR is not at the OP edge of the circuit (that is,
-   either an 'exit node' or a non-edge node), it de/encrypts the payload
+   When a node wants to send a RELAY or RELAY_EARLY cell, it checks the cell's
+   circID and determines whether the corresponding circuit along that
+   connection is still open. If not, the node drops the cell.
+
+   When a node receives a RELAY or RELAY_EARLY cell, it checks the cell's
+   circID and determines whether it has a corresponding circuit along
+   that connection. If not, the node drops the cell.
+
+5.5.2. Forward Direction
+
+   The forward direction is the direction that CREATE cells are sent.
+
+5.5.2.1. Routing from the Origin
+
+   When a relay cell is sent from an OP, the OP encrypts the payload
+   with the stream cipher as follows:
+      OP sends relay cell:
+         For I=N...1, where N is the destination node:
+            Encrypt with Kf_I.
+         Transmit the encrypted cell to node 1.
+
+5.5.2.2. Relaying Forward at Onion Routers
+
+   When a forward relay cell is received by an OR, it decrypts the payload
    with the stream cipher, as follows:
-        'Forward' relay cell (same direction as CREATE):
-            Use Kf as key; decrypt.
-        'Back' relay cell (opposite direction from CREATE):
-            Use Kb as key; encrypt.
-   Note that in counter mode, decrypt and encrypt are the same operation.
+      'Forward' relay cell:
+         Use Kf as key; decrypt.
 
    The OR then decides whether it recognizes the relay cell, by
    inspecting the payload as described in section 6.1 below.  If the OR
@@ -1374,14 +1391,29 @@ see tor-design.pdf.
    encounters an unrecognized relay cell, an error has occurred: the OR
    sends a DESTROY cell to tear down the circuit.
 
+   For more information, see section 6 below.
+
+5.5.3. Backward Direction
+
+   The backward direction is the opposite direction from CREATE cells.
+
+5.5.3.1. Relaying Backward at Onion Routers
+
+   When a backward relay cell is received by an OR, it encrypts the payload
+   with the stream cipher, as follows:
+      'Backward' relay cell:
+         Use Kb as key; encrypt.
+
+5.5.3. Routing to the Origin
+
    When a relay cell arrives at an OP, the OP decrypts the payload
    with the stream cipher as follows:
-         OP receives data cell:
-            For I=1...N,
-                Decrypt with Kb_I.  If the payload is recognized (see
-                section 6..1), then stop and process the payload.
-
-   For more information, see section 6 below.
+         OP receives relay cell from node 1:
+            For I=1...N, where N is the final node on the circuit:
+                Decrypt with Kb_I.
+                If the payload is recognized (see section 6.1), then:
+                    The sending node is I.
+                    Stop and process the payload.
 
 5.6. Handling relay_early cells