tor-spec: provide pseudocode for digest check

This commit implements a pseudocode example for the digest in both:
encryption and decryption cases.

The pseudocode itself is a combination of Python code and the Rust
slice type.

Fixes #205

1 files changed, 43 insertions, 0 deletions

diff --git a/tor-spec.txt b/tor-spec.txt
index b42ee26..669851f 100644
--- a/tor-spec.txt
+++ b/tor-spec.txt
@@ -49,6 +49,7 @@ Table of Contents
         5.6. Handling relay_early cells
     6. Application connections and stream management
         6.1. Relay cells
+            6.1.1. Calculating the 'Digest' field
         6.2. Opening streams and transferring data
             6.2.1. Opening a directory stream
         6.3. Closing streams
@@ -1791,6 +1792,48 @@ see tor-design.pdf.
    understood, the cell must be dropped and ignored. Its contents
    still count with respect to the digests and flow control windows, though.
 
+6.1.1. Calculating the 'Digest' field
+
+   The 'Digest' field itself serves the purpose to check if a cell has been
+   fully decrypted, that is, all onion layers have been removed.  Having a
+   single field, namely 'Recognized' is not sufficient, as outlined above.
+
+   When ENCRYPTING a RELAY cell, an implementation does the following:
+
+     # Encode the cell in binary (recognized and digest set to zero)
+     tmp = cmd + [0, 0] + stream_id + [0, 0, 0, 0] + length + data + padding
+
+     # Update the digest with the encoded data
+     digest_state = hash_update(digest_state, tmp)
+     digest = hash_calculate(digest_state)
+
+     # The encoded data is the same as above with the digest field not being
+     # zero anymore
+     encoded = cmd + [0, 0] + stream_id + digest[0..4] + length + data +
+               padding
+
+     # Now we can encrypt the cell by adding the onion layers ...
+
+   When DECRYPTING a RELAY cell, an implementation does the following:
+
+     decrypted = decrypt(cell)
+
+     # Replace the digest field in decrypted by zeros
+     tmp = decrypted[0..5] + [0, 0, 0, 0] + decrypted[9..]
+
+     # Update the digest field with the decrypted data and its digest field
+     # set to zero
+     digest_state = hash_update(digest_state, tmp)
+     digest = hash_calculate(digest_state)
+
+     if digest[0..4] == decrypted[5..9]
+       # The cell has been fully decrypted ...
+
+   The caveat itself is that only the binary data with the digest bytes set to
+   zero are being taken into account when calculating the running digest.  The
+   final plain-text cells (with the digest field set to its actual value) are
+   not taken into the running digest.
+
 6.2. Opening streams and transferring data
 
    To open a new anonymized TCP connection, the OP chooses an open