commit ef91cd6a595128847c991eb875d105b850d60fcf Author: Nick Mathewson <nickm@torproject.org> Date: Tue Feb 13 09:03:58 2018 -0500 Small tweaks to clarify recognized even more --- tor-spec.txt | 32 ++++++++++++++++---------------- 1 file changed, 16 insertions(+), 16 deletions(-) diff --git a/tor-spec.txt b/tor-spec.txt index 7aba3da..13691b7 100644 --- a/tor-spec.txt +++ b/tor-spec.txt @@ -1443,22 +1443,22 @@ see tor-design.pdf. other nodes in the circuit back to the originator. Commands marked as either can be sent either by the originator or other nodes. - The 'recognized' field is used for a simple indication for if the - cell still encrypted or not. When sending cells the unencrypted - 'recognized' MUST be set to zero. When receiving and decrypting - cells the 'recognized' MAY be zero if we're the endpoint the cell - is destined for (there's a 1/2^32 chance of encrypted bytes - coincidently being zero), and non-zero otherwise. - - When 'recognized' in an unencrypted relay payload is zero the - 'digest' field is computed as the first four bytes of the running - digest of all the bytes that have been destined for this hop of - the circuit or originated from this hop of the circuit, seeded from - Df or Db respectively (obtained in section 5.2 above), and including - this RELAY cell's entire payload (taken with the digest field set to - zero). - - When the 'recognized' field of a RELAY cell is zero, and the digest + The 'recognized' field is used for a simple indication for if the cell + still encrypted or not. When sending cells, the unencrypted 'recognized' + MUST be set to zero. + + When receiving and decrypting cells the 'recognized' will always be + zero if we're the endpoint that the cell is destined for. For cells + that we should relay, the 'recognized' field will usually be nonzero, + but will accidentally be zero with P=2^-32. + + When handling a relay cell, if the 'recognized' in field in a + decrypted relay payload is zero, the 'digest' field is computed as + the first four bytes of the running digest of all the bytes that have + been destined for this hop of the circuit or originated from this hop + of the circuit, seeded from Df or Db respectively (obtained in + section 5.2 above), and including this RELAY cell's entire payload + (taken with the digest field set to zero). If the digest is correct, the cell is considered "recognized" for the purposes of decryption (see section 5.5 above).