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@@ -0,0 +1,734 @@
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+$Id$
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+
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+ Tor Protocol Specification
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+
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+ Roger Dingledine
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+ Nick Mathewson
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+
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+Note: This document specifies Tor as currently implemented in versions
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+0.1.2.1-alpha and earlier. Current protocol designs are described in
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+tor-spec.txt.
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+
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+0. Preliminaries
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+
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+0.1. Notation and encoding
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+
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+ PK -- a public key.
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+ SK -- a private key.
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+ K -- a key for a symmetric cypher.
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+
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+ a|b -- concatenation of 'a' and 'b'.
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+
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+ [A0 B1 C2] -- a three-byte sequence, containing the bytes with
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+ hexadecimal values A0, B1, and C2, in that order.
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+
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+ All numeric values are encoded in network (big-endian) order.
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+
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+ H(m) -- a cryptographic hash of m.
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+
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+0.2. Security parameters
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+
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+ Tor uses a stream cipher, a public-key cipher, the Diffie-Hellman
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+ protocol, and a hash function.
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+
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+ KEY_LEN -- the length of the stream cipher's key, in bytes.
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+
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+ PK_ENC_LEN -- the length of a public-key encrypted message, in bytes.
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+ PK_PAD_LEN -- the number of bytes added in padding for public-key
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+ encryption, in bytes. (The largest number of bytes that can be encrypted
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+ in a single public-key operation is therefore PK_ENC_LEN-PK_PAD_LEN.)
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+
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+ DH_LEN -- the number of bytes used to represent a member of the
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+ Diffie-Hellman group.
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+ DH_SEC_LEN -- the number of bytes used in a Diffie-Hellman private key (x).
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+
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+ HASH_LEN -- the length of the hash function's output, in bytes.
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+
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+ CELL_LEN -- The length of a Tor cell, in bytes.
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+
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+0.3. Ciphers
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+
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+ For a stream cipher, we use 128-bit AES in counter mode, with an IV of all
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+ 0 bytes.
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+
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+ For a public-key cipher, we use RSA with 1024-bit keys and a fixed
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+ exponent of 65537. We use OAEP padding, with SHA-1 as its digest
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+ function. (For OAEP padding, see
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+ ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-1/pkcs-1v2-1.pdf)
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+
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+ For Diffie-Hellman, we use a generator (g) of 2. For the modulus (p), we
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+ use the 1024-bit safe prime from rfc2409, (section 6.2) whose hex
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+ representation is:
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+
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+ "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08"
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+ "8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B"
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+ "302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9"
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+ "A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6"
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+ "49286651ECE65381FFFFFFFFFFFFFFFF"
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+
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+ As an optimization, implementations SHOULD choose DH private keys (x) of
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+ 320 bits. Implementations that do this MUST never use any DH key more
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+ than once.
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+
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+ For a hash function, we use SHA-1.
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+
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+ KEY_LEN=16.
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+ DH_LEN=128; DH_GROUP_LEN=40.
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+ PK_ENC_LEN=128; PK_PAD_LEN=42.
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+ HASH_LEN=20.
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+
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+ When we refer to "the hash of a public key", we mean the SHA-1 hash of the
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+ DER encoding of an ASN.1 RSA public key (as specified in PKCS.1).
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+
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+ All "random" values should be generated with a cryptographically strong
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+ random number generator, unless otherwise noted.
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+
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+ The "hybrid encryption" of a byte sequence M with a public key PK is
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+ computed as follows:
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+ 1. If M is less than PK_ENC_LEN-PK_PAD_LEN, pad and encrypt M with PK.
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+ 2. Otherwise, generate a KEY_LEN byte random key K.
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+ Let M1 = the first PK_ENC_LEN-PK_PAD_LEN-KEY_LEN bytes of M,
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+ and let M2 = the rest of M.
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+ Pad and encrypt K|M1 with PK. Encrypt M2 with our stream cipher,
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+ using the key K. Concatenate these encrypted values.
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+ [XXX Note that this "hybrid encryption" approach does not prevent
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+ an attacker from adding or removing bytes to the end of M. It also
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+ allows attackers to modify the bytes not covered by the OAEP --
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+ see Goldberg's PET2006 paper for details. We will add a MAC to this
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+ scheme one day. -RD]
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+
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+0.4. Other parameter values
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+
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+ CELL_LEN=512
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+
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+1. System overview
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+
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+ Tor is a distributed overlay network designed to anonymize
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+ low-latency TCP-based applications such as web browsing, secure shell,
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+ and instant messaging. Clients choose a path through the network and
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+ build a ``circuit'', in which each node (or ``onion router'' or ``OR'')
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+ in the path knows its predecessor and successor, but no other nodes in
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+ the circuit. Traffic flowing down the circuit is sent in fixed-size
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+ ``cells'', which are unwrapped by a symmetric key at each node (like
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+ the layers of an onion) and relayed downstream.
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+
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+2. Connections
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+
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+ There are two ways to connect to an onion router (OR). The first is
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+ as an onion proxy (OP), which allows the OP to authenticate the OR
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+ without authenticating itself. The second is as another OR, which
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+ allows mutual authentication.
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+
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+ Tor uses TLS for link encryption. All implementations MUST support
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+ the TLS ciphersuite "TLS_EDH_RSA_WITH_DES_192_CBC3_SHA", and SHOULD
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+ support "TLS_DHE_RSA_WITH_AES_128_CBC_SHA" if it is available.
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+ Implementations MAY support other ciphersuites, but MUST NOT
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+ support any suite without ephemeral keys, symmetric keys of at
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+ least KEY_LEN bits, and digests of at least HASH_LEN bits.
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+
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+ An OP or OR always sends a two-certificate chain, consisting of a
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+ certificate using a short-term connection key and a second, self-
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+ signed certificate containing the OR's identity key. The commonName of the
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+ first certificate is the OR's nickname, and the commonName of the second
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+ certificate is the OR's nickname, followed by a space and the string
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+ "<identity>".
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+
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+ All parties receiving certificates must confirm that the identity key is
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+ as expected. (When initiating a connection, the expected identity key is
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+ the one given in the directory; when creating a connection because of an
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+ EXTEND cell, the expected identity key is the one given in the cell.) If
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+ the key is not as expected, the party must close the connection.
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+
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+ All parties SHOULD reject connections to or from ORs that have malformed
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+ or missing certificates. ORs MAY accept or reject connections from OPs
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+ with malformed or missing certificates.
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+
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+ Once a TLS connection is established, the two sides send cells
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+ (specified below) to one another. Cells are sent serially. All
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+ cells are CELL_LEN bytes long. Cells may be sent embedded in TLS
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+ records of any size or divided across TLS records, but the framing
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+ of TLS records MUST NOT leak information about the type or contents
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+ of the cells.
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+
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+ TLS connections are not permanent. An OP or an OR may close a
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+ connection to an OR if there are no circuits running over the
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+ connection, and an amount of time (KeepalivePeriod, defaults to 5
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+ minutes) has passed.
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+
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+ (As an exception, directory servers may try to stay connected to all of
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+ the ORs -- though this will be phased out for the Tor 0.1.2.x release.)
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+
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+3. Cell Packet format
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+
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+ The basic unit of communication for onion routers and onion
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+ proxies is a fixed-width "cell". Each cell contains the following
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+ fields:
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+
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+ CircID [2 bytes]
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+ Command [1 byte]
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+ Payload (padded with 0 bytes) [CELL_LEN-3 bytes]
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+ [Total size: CELL_LEN bytes]
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+
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+ The CircID field determines which circuit, if any, the cell is
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+ associated with.
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+
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+ The 'Command' field holds one of the following values:
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+ 0 -- PADDING (Padding) (See Sec 6.2)
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+ 1 -- CREATE (Create a circuit) (See Sec 4.1)
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+ 2 -- CREATED (Acknowledge create) (See Sec 4.1)
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+ 3 -- RELAY (End-to-end data) (See Sec 4.5 and 5)
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+ 4 -- DESTROY (Stop using a circuit) (See Sec 4.4)
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+ 5 -- CREATE_FAST (Create a circuit, no PK) (See Sec 4.1)
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+ 6 -- CREATED_FAST (Circuit created, no PK) (See Sec 4.1)
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+ 7 -- HELLO (Introduce the OR) (See Sec 7.1)
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+
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+ The interpretation of 'Payload' depends on the type of the cell.
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+ PADDING: Payload is unused.
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+ CREATE: Payload contains the handshake challenge.
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+ CREATED: Payload contains the handshake response.
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+ RELAY: Payload contains the relay header and relay body.
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+ DESTROY: Payload contains a reason for closing the circuit.
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+ (see 4.4)
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+ Upon receiving any other value for the command field, an OR must
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+ drop the cell.
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+
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+ The payload is padded with 0 bytes.
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+
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+ PADDING cells are currently used to implement connection keepalive.
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+ If there is no other traffic, ORs and OPs send one another a PADDING
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+ cell every few minutes.
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+
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+ CREATE, CREATED, and DESTROY cells are used to manage circuits;
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+ see section 4 below.
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+
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+ RELAY cells are used to send commands and data along a circuit; see
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+ section 5 below.
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+
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+ HELLO cells are used to introduce parameters and characteristics of
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+ Tor clients and servers when connections are established.
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+
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+4. Circuit management
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+
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+4.1. CREATE and CREATED cells
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+
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+ Users set up circuits incrementally, one hop at a time. To create a
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+ new circuit, OPs send a CREATE cell to the first node, with the
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+ first half of the DH handshake; that node responds with a CREATED
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+ cell with the second half of the DH handshake plus the first 20 bytes
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+ of derivative key data (see section 4.2). To extend a circuit past
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+ the first hop, the OP sends an EXTEND relay cell (see section 5)
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+ which instructs the last node in the circuit to send a CREATE cell
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+ to extend the circuit.
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+
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+ The payload for a CREATE cell is an 'onion skin', which consists
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+ of the first step of the DH handshake data (also known as g^x).
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+ This value is hybrid-encrypted (see 0.3) to Bob's public key, giving
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+ an onion-skin of:
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+ PK-encrypted:
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+ Padding padding [PK_PAD_LEN bytes]
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+ Symmetric key [KEY_LEN bytes]
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+ First part of g^x [PK_ENC_LEN-PK_PAD_LEN-KEY_LEN bytes]
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+ Symmetrically encrypted:
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+ Second part of g^x [DH_LEN-(PK_ENC_LEN-PK_PAD_LEN-KEY_LEN)
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+ bytes]
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+
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+ The relay payload for an EXTEND relay cell consists of:
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+ Address [4 bytes]
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+ Port [2 bytes]
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+ Onion skin [DH_LEN+KEY_LEN+PK_PAD_LEN bytes]
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+ Identity fingerprint [HASH_LEN bytes]
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+
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+ The port and address field denote the IPV4 address and port of the next
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+ onion router in the circuit; the public key hash is the hash of the PKCS#1
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+ ASN1 encoding of the next onion router's identity (signing) key. (See 0.3
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+ above.) (Including this hash allows the extending OR verify that it is
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+ indeed connected to the correct target OR, and prevents certain
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+ man-in-the-middle attacks.)
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+
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+ The payload for a CREATED cell, or the relay payload for an
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+ EXTENDED cell, contains:
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+ DH data (g^y) [DH_LEN bytes]
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+ Derivative key data (KH) [HASH_LEN bytes] <see 4.2 below>
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+
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+ The CircID for a CREATE cell is an arbitrarily chosen 2-byte integer,
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+ selected by the node (OP or OR) that sends the CREATE cell. To prevent
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+ CircID collisions, when one OR sends a CREATE cell to another, it chooses
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+ from only one half of the possible values based on the ORs' public
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+ identity keys: if the sending OR has a lower key, it chooses a CircID with
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+ an MSB of 0; otherwise, it chooses a CircID with an MSB of 1.
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+
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+ Public keys are compared numerically by modulus.
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+
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+ As usual with DH, x and y MUST be generated randomly.
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+
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+4.1.1. CREATE_FAST/CREATED_FAST cells
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+
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+ When initializing the first hop of a circuit, the OP has already
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+ established the OR's identity and negotiated a secret key using TLS.
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+ Because of this, it is not always necessary for the OP to perform the
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+ public key operations to create a circuit. In this case, the
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+ OP MAY send a CREATE_FAST cell instead of a CREATE cell for the first
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+ hop only. The OR responds with a CREATED_FAST cell, and the circuit is
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+ created.
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+
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+ A CREATE_FAST cell contains:
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+
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+ Key material (X) [HASH_LEN bytes]
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+
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+ A CREATED_FAST cell contains:
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+
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+ Key material (Y) [HASH_LEN bytes]
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+ Derivative key data [HASH_LEN bytes] (See 4.2 below)
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+
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+ The values of X and Y must be generated randomly.
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+
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+ [Versions of Tor before 0.1.0.6-rc did not support these cell types;
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+ clients should not send CREATE_FAST cells to older Tor servers.]
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+
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+4.2. Setting circuit keys
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+
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+ Once the handshake between the OP and an OR is completed, both can
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+ now calculate g^xy with ordinary DH. Before computing g^xy, both client
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+ and server MUST verify that the received g^x or g^y value is not degenerate;
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+ that is, it must be strictly greater than 1 and strictly less than p-1
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+ where p is the DH modulus. Implementations MUST NOT complete a handshake
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+ with degenerate keys. Implementations MUST NOT discard other "weak"
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+ g^x values.
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+
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+ (Discarding degenerate keys is critical for security; if bad keys
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+ are not discarded, an attacker can substitute the server's CREATED
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+ cell's g^y with 0 or 1, thus creating a known g^xy and impersonating
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+ the server. Discarding other keys may allow attacks to learn bits of
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+ the private key.)
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+
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+ (The mainline Tor implementation, in the 0.1.1.x-alpha series, discarded
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+ all g^x values less than 2^24, greater than p-2^24, or having more than
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+ 1024-16 identical bits. This served no useful purpose, and we stopped.)
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+
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+ If CREATE or EXTEND is used to extend a circuit, the client and server
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+ base their key material on K0=g^xy, represented as a big-endian unsigned
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+ integer.
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+
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+ If CREATE_FAST is used, the client and server base their key material on
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+ K0=X|Y.
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+
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+ From the base key material K0, they compute KEY_LEN*2+HASH_LEN*3 bytes of
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+ derivative key data as
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+ K = H(K0 | [00]) | H(K0 | [01]) | H(K0 | [02]) | ...
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+
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+ The first HASH_LEN bytes of K form KH; the next HASH_LEN form the forward
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+ digest Df; the next HASH_LEN 41-60 form the backward digest Db; the next
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+ KEY_LEN 61-76 form Kf, and the final KEY_LEN form Kb. Excess bytes from K
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+ are discarded.
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+
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+ KH is used in the handshake response to demonstrate knowledge of the
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+ computed shared key. Df is used to seed the integrity-checking hash
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+ for the stream of data going from the OP to the OR, and Db seeds the
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+ integrity-checking hash for the data stream from the OR to the OP. Kf
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+ is used to encrypt the stream of data going from the OP to the OR, and
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+ Kb is used to encrypt the stream of data going from the OR to the OP.
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+
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+4.3. Creating circuits
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+
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+ When creating a circuit through the network, the circuit creator
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+ (OP) performs the following steps:
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+
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+ 1. Choose an onion router as an exit node (R_N), such that the onion
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+ router's exit policy includes at least one pending stream that
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+ needs a circuit (if there are any).
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+
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+ 2. Choose a chain of (N-1) onion routers
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+ (R_1...R_N-1) to constitute the path, such that no router
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+ appears in the path twice.
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+
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+ 3. If not already connected to the first router in the chain,
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+ open a new connection to that router.
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+
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+ 4. Choose a circID not already in use on the connection with the
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+ first router in the chain; send a CREATE cell along the
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+ connection, to be received by the first onion router.
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+
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+ 5. Wait until a CREATED cell is received; finish the handshake
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+ and extract the forward key Kf_1 and the backward key Kb_1.
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+
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+ 6. For each subsequent onion router R (R_2 through R_N), extend
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+ the circuit to R.
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+
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+ To extend the circuit by a single onion router R_M, the OP performs
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+ these steps:
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+
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+ 1. Create an onion skin, encrypted to R_M's public key.
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+
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+ 2. Send the onion skin in a relay EXTEND cell along
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+ the circuit (see section 5).
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+
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+ 3. When a relay EXTENDED cell is received, verify KH, and
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+ calculate the shared keys. The circuit is now extended.
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+
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+ When an onion router receives an EXTEND relay cell, it sends a CREATE
|
|
|
+ cell to the next onion router, with the enclosed onion skin as its
|
|
|
+ payload. The initiating onion router chooses some circID not yet
|
|
|
+ used on the connection between the two onion routers. (But see
|
|
|
+ section 4.1. above, concerning choosing circIDs based on
|
|
|
+ lexicographic order of nicknames.)
|
|
|
+
|
|
|
+ When an onion router receives a CREATE cell, if it already has a
|
|
|
+ circuit on the given connection with the given circID, it drops the
|
|
|
+ cell. Otherwise, after receiving the CREATE cell, it completes the
|
|
|
+ DH handshake, and replies with a CREATED cell. Upon receiving a
|
|
|
+ CREATED cell, an onion router packs it payload into an EXTENDED relay
|
|
|
+ cell (see section 5), and sends that cell up the circuit. Upon
|
|
|
+ receiving the EXTENDED relay cell, the OP can retrieve g^y.
|
|
|
+
|
|
|
+ (As an optimization, OR implementations may delay processing onions
|
|
|
+ until a break in traffic allows time to do so without harming
|
|
|
+ network latency too greatly.)
|
|
|
+
|
|
|
+4.4. Tearing down circuits
|
|
|
+
|
|
|
+ Circuits are torn down when an unrecoverable error occurs along
|
|
|
+ the circuit, or when all streams on a circuit are closed and the
|
|
|
+ circuit's intended lifetime is over. Circuits may be torn down
|
|
|
+ either completely or hop-by-hop.
|
|
|
+
|
|
|
+ To tear down a circuit completely, an OR or OP sends a DESTROY
|
|
|
+ cell to the adjacent nodes on that circuit, using the appropriate
|
|
|
+ direction's circID.
|
|
|
+
|
|
|
+ Upon receiving an outgoing DESTROY cell, an OR frees resources
|
|
|
+ associated with the corresponding circuit. If it's not the end of
|
|
|
+ the circuit, it sends a DESTROY cell for that circuit to the next OR
|
|
|
+ in the circuit. If the node is the end of the circuit, then it tears
|
|
|
+ down any associated edge connections (see section 5.1).
|
|
|
+
|
|
|
+ After a DESTROY cell has been processed, an OR ignores all data or
|
|
|
+ destroy cells for the corresponding circuit.
|
|
|
+
|
|
|
+ To tear down part of a circuit, the OP may send a RELAY_TRUNCATE cell
|
|
|
+ signaling a given OR (Stream ID zero). That OR sends a DESTROY
|
|
|
+ cell to the next node in the circuit, and replies to the OP with a
|
|
|
+ RELAY_TRUNCATED cell.
|
|
|
+
|
|
|
+ When an unrecoverable error occurs along one connection in a
|
|
|
+ circuit, the nodes on either side of the connection should, if they
|
|
|
+ are able, act as follows: the node closer to the OP should send a
|
|
|
+ RELAY_TRUNCATED cell towards the OP; the node farther from the OP
|
|
|
+ should send a DESTROY cell down the circuit.
|
|
|
+
|
|
|
+ The payload of a RELAY_TRUNCATED or DESTROY cell contains a single octet,
|
|
|
+ describing why the circuit is being closed or truncated. When sending a
|
|
|
+ TRUNCATED or DESTROY cell because of another TRUNCATED or DESTROY cell,
|
|
|
+ the error code should be propagated. The origin of a circuit always sets
|
|
|
+ this error code to 0, to avoid leaking its version.
|
|
|
+
|
|
|
+ The error codes are:
|
|
|
+ 0 -- NONE (No reason given.)
|
|
|
+ 1 -- PROTOCOL (Tor protocol violation.)
|
|
|
+ 2 -- INTERNAL (Internal error.)
|
|
|
+ 3 -- REQUESTED (A client sent a TRUNCATE command.)
|
|
|
+ 4 -- HIBERNATING (Not currently operating; trying to save bandwidth.)
|
|
|
+ 5 -- RESOURCELIMIT (Out of memory, sockets, or circuit IDs.)
|
|
|
+ 6 -- CONNECTFAILED (Unable to reach server.)
|
|
|
+ 7 -- OR_IDENTITY (Connected to server, but its OR identity was not
|
|
|
+ as expected.)
|
|
|
+ 8 -- OR_CONN_CLOSED (The OR connection that was carrying this circuit
|
|
|
+ died.)
|
|
|
+
|
|
|
+ [Versions of Tor prior to 0.1.0.11 didn't send reasons; implementations
|
|
|
+ MUST accept empty TRUNCATED and DESTROY cells.]
|
|
|
+
|
|
|
+4.5. Routing relay cells
|
|
|
+
|
|
|
+ When an OR receives a RELAY cell, it checks the cell's circID and
|
|
|
+ determines whether it has a corresponding circuit along that
|
|
|
+ connection. If not, the OR drops the RELAY cell.
|
|
|
+
|
|
|
+ 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
|
|
|
+ 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.
|
|
|
+
|
|
|
+ The OR then decides whether it recognizes the relay cell, by
|
|
|
+ inspecting the payload as described in section 5.1 below. If the OR
|
|
|
+ recognizes the cell, it processes the contents of the relay cell.
|
|
|
+ Otherwise, it passes the decrypted relay cell along the circuit if
|
|
|
+ the circuit continues. If the OR at the end of the circuit
|
|
|
+ encounters an unrecognized relay cell, an error has occurred: the OR
|
|
|
+ sends a DESTROY cell to tear down the circuit.
|
|
|
+
|
|
|
+ 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=N...1,
|
|
|
+ Decrypt with Kb_I. If the payload is recognized (see
|
|
|
+ section 5.1), then stop and process the payload.
|
|
|
+
|
|
|
+ For more information, see section 5 below.
|
|
|
+
|
|
|
+5. Application connections and stream management
|
|
|
+
|
|
|
+5.1. Relay cells
|
|
|
+
|
|
|
+ Within a circuit, the OP and the exit node use the contents of
|
|
|
+ RELAY packets to tunnel end-to-end commands and TCP connections
|
|
|
+ ("Streams") across circuits. End-to-end commands can be initiated
|
|
|
+ by either edge; streams are initiated by the OP.
|
|
|
+
|
|
|
+ The payload of each unencrypted RELAY cell consists of:
|
|
|
+ Relay command [1 byte]
|
|
|
+ 'Recognized' [2 bytes]
|
|
|
+ StreamID [2 bytes]
|
|
|
+ Digest [4 bytes]
|
|
|
+ Length [2 bytes]
|
|
|
+ Data [CELL_LEN-14 bytes]
|
|
|
+
|
|
|
+ The relay commands are:
|
|
|
+ 1 -- RELAY_BEGIN [forward]
|
|
|
+ 2 -- RELAY_DATA [forward or backward]
|
|
|
+ 3 -- RELAY_END [forward or backward]
|
|
|
+ 4 -- RELAY_CONNECTED [backward]
|
|
|
+ 5 -- RELAY_SENDME [forward or backward]
|
|
|
+ 6 -- RELAY_EXTEND [forward]
|
|
|
+ 7 -- RELAY_EXTENDED [backward]
|
|
|
+ 8 -- RELAY_TRUNCATE [forward]
|
|
|
+ 9 -- RELAY_TRUNCATED [backward]
|
|
|
+ 10 -- RELAY_DROP [forward or backward]
|
|
|
+ 11 -- RELAY_RESOLVE [forward]
|
|
|
+ 12 -- RELAY_RESOLVED [backward]
|
|
|
+
|
|
|
+ Commands labelled as "forward" must only be sent by the originator
|
|
|
+ of the circuit. Commands labelled as "backward" must only be sent by
|
|
|
+ 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 in any unencrypted relay payload is always set
|
|
|
+ to 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 4.2 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
|
|
|
+ is correct, the cell is considered "recognized" for the purposes of
|
|
|
+ decryption (see section 4.5 above).
|
|
|
+
|
|
|
+ (The digest does not include any bytes from relay cells that do
|
|
|
+ not start or end at this hop of the circuit. That is, it does not
|
|
|
+ include forwarded data. Therefore if 'recognized' is zero but the
|
|
|
+ digest does not match, the running digest at that node should
|
|
|
+ not be updated, and the cell should be forwarded on.)
|
|
|
+
|
|
|
+ All RELAY cells pertaining to the same tunneled stream have the
|
|
|
+ same stream ID. StreamIDs are chosen arbitrarily by the OP. RELAY
|
|
|
+ cells that affect the entire circuit rather than a particular
|
|
|
+ stream use a StreamID of zero.
|
|
|
+
|
|
|
+ The 'Length' field of a relay cell contains the number of bytes in
|
|
|
+ the relay payload which contain real payload data. The remainder of
|
|
|
+ the payload is padded with NUL bytes.
|
|
|
+
|
|
|
+ If the RELAY cell is recognized but the relay command is not
|
|
|
+ understood, the cell must be dropped and ignored. Its contents
|
|
|
+ still count with respect to the digests, though. [Before
|
|
|
+ 0.1.1.10, Tor closed circuits when it received an unknown relay
|
|
|
+ command. Perhaps this will be more forward-compatible. -RD]
|
|
|
+
|
|
|
+5.2. Opening streams and transferring data
|
|
|
+
|
|
|
+ To open a new anonymized TCP connection, the OP chooses an open
|
|
|
+ circuit to an exit that may be able to connect to the destination
|
|
|
+ address, selects an arbitrary StreamID not yet used on that circuit,
|
|
|
+ and constructs a RELAY_BEGIN cell with a payload encoding the address
|
|
|
+ and port of the destination host. The payload format is:
|
|
|
+
|
|
|
+ ADDRESS | ':' | PORT | [00]
|
|
|
+
|
|
|
+ where ADDRESS can be a DNS hostname, or an IPv4 address in
|
|
|
+ dotted-quad format, or an IPv6 address surrounded by square brackets;
|
|
|
+ and where PORT is encoded in decimal.
|
|
|
+
|
|
|
+ [What is the [00] for? -NM]
|
|
|
+ [It's so the payload is easy to parse out with string funcs -RD]
|
|
|
+
|
|
|
+ Upon receiving this cell, the exit node resolves the address as
|
|
|
+ necessary, and opens a new TCP connection to the target port. If the
|
|
|
+ address cannot be resolved, or a connection can't be established, the
|
|
|
+ exit node replies with a RELAY_END cell. (See 5.4 below.)
|
|
|
+ Otherwise, the exit node replies with a RELAY_CONNECTED cell, whose
|
|
|
+ payload is in one of the following formats:
|
|
|
+ The IPv4 address to which the connection was made [4 octets]
|
|
|
+ A number of seconds (TTL) for which the address may be cached [4 octets]
|
|
|
+ or
|
|
|
+ Four zero-valued octets [4 octets]
|
|
|
+ An address type (6) [1 octet]
|
|
|
+ The IPv6 address to which the connection was made [16 octets]
|
|
|
+ A number of seconds (TTL) for which the address may be cached [4 octets]
|
|
|
+ [XXXX Versions of Tor before 0.1.1.6 ignore and do not generate the TTL
|
|
|
+ field. No version of Tor currently generates the IPv6 format.
|
|
|
+
|
|
|
+ Tor servers before 0.1.2.0 set the TTL field to a fixed value. Later
|
|
|
+ versions set the TTL to the last value seen from a DNS server, and expire
|
|
|
+ their own cached entries after a fixed interval. This prevents certain
|
|
|
+ attacks.]
|
|
|
+
|
|
|
+ The OP waits for a RELAY_CONNECTED cell before sending any data.
|
|
|
+ Once a connection has been established, the OP and exit node
|
|
|
+ package stream data in RELAY_DATA cells, and upon receiving such
|
|
|
+ cells, echo their contents to the corresponding TCP stream.
|
|
|
+ RELAY_DATA cells sent to unrecognized streams are dropped.
|
|
|
+
|
|
|
+ Relay RELAY_DROP cells are long-range dummies; upon receiving such
|
|
|
+ a cell, the OR or OP must drop it.
|
|
|
+
|
|
|
+5.3. Closing streams
|
|
|
+
|
|
|
+ When an anonymized TCP connection is closed, or an edge node
|
|
|
+ encounters error on any stream, it sends a 'RELAY_END' cell along the
|
|
|
+ circuit (if possible) and closes the TCP connection immediately. If
|
|
|
+ an edge node receives a 'RELAY_END' cell for any stream, it closes
|
|
|
+ the TCP connection completely, and sends nothing more along the
|
|
|
+ circuit for that stream.
|
|
|
+
|
|
|
+ The payload of a RELAY_END cell begins with a single 'reason' byte to
|
|
|
+ describe why the stream is closing, plus optional data (depending on
|
|
|
+ the reason.) The values are:
|
|
|
+
|
|
|
+ 1 -- REASON_MISC (catch-all for unlisted reasons)
|
|
|
+ 2 -- REASON_RESOLVEFAILED (couldn't look up hostname)
|
|
|
+ 3 -- REASON_CONNECTREFUSED (remote host refused connection) [*]
|
|
|
+ 4 -- REASON_EXITPOLICY (OR refuses to connect to host or port)
|
|
|
+ 5 -- REASON_DESTROY (Circuit is being destroyed)
|
|
|
+ 6 -- REASON_DONE (Anonymized TCP connection was closed)
|
|
|
+ 7 -- REASON_TIMEOUT (Connection timed out, or OR timed out
|
|
|
+ while connecting)
|
|
|
+ 8 -- (unallocated) [**]
|
|
|
+ 9 -- REASON_HIBERNATING (OR is temporarily hibernating)
|
|
|
+ 10 -- REASON_INTERNAL (Internal error at the OR)
|
|
|
+ 11 -- REASON_RESOURCELIMIT (OR has no resources to fulfill request)
|
|
|
+ 12 -- REASON_CONNRESET (Connection was unexpectedly reset)
|
|
|
+ 13 -- REASON_TORPROTOCOL (Sent when closing connection because of
|
|
|
+ Tor protocol violations.)
|
|
|
+
|
|
|
+ (With REASON_EXITPOLICY, the 4-byte IPv4 address or 16-byte IPv6 address
|
|
|
+ forms the optional data; no other reason currently has extra data.
|
|
|
+ As of 0.1.1.6, the body also contains a 4-byte TTL.)
|
|
|
+
|
|
|
+ OPs and ORs MUST accept reasons not on the above list, since future
|
|
|
+ versions of Tor may provide more fine-grained reasons.
|
|
|
+
|
|
|
+ [*] Older versions of Tor also send this reason when connections are
|
|
|
+ reset.
|
|
|
+ [**] Due to a bug in versions of Tor through 0095, error reason 8 must
|
|
|
+ remain allocated until that version is obsolete.
|
|
|
+
|
|
|
+ --- [The rest of this section describes unimplemented functionality.]
|
|
|
+
|
|
|
+ Because TCP connections can be half-open, we follow an equivalent
|
|
|
+ to TCP's FIN/FIN-ACK/ACK protocol to close streams.
|
|
|
+
|
|
|
+ An exit connection can have a TCP stream in one of three states:
|
|
|
+ 'OPEN', 'DONE_PACKAGING', and 'DONE_DELIVERING'. For the purposes
|
|
|
+ of modeling transitions, we treat 'CLOSED' as a fourth state,
|
|
|
+ although connections in this state are not, in fact, tracked by the
|
|
|
+ onion router.
|
|
|
+
|
|
|
+ A stream begins in the 'OPEN' state. Upon receiving a 'FIN' from
|
|
|
+ the corresponding TCP connection, the edge node sends a 'RELAY_FIN'
|
|
|
+ cell along the circuit and changes its state to 'DONE_PACKAGING'.
|
|
|
+ Upon receiving a 'RELAY_FIN' cell, an edge node sends a 'FIN' to
|
|
|
+ the corresponding TCP connection (e.g., by calling
|
|
|
+ shutdown(SHUT_WR)) and changing its state to 'DONE_DELIVERING'.
|
|
|
+
|
|
|
+ When a stream in already in 'DONE_DELIVERING' receives a 'FIN', it
|
|
|
+ also sends a 'RELAY_FIN' along the circuit, and changes its state
|
|
|
+ to 'CLOSED'. When a stream already in 'DONE_PACKAGING' receives a
|
|
|
+ 'RELAY_FIN' cell, it sends a 'FIN' and changes its state to
|
|
|
+ 'CLOSED'.
|
|
|
+
|
|
|
+ If an edge node encounters an error on any stream, it sends a
|
|
|
+ 'RELAY_END' cell (if possible) and closes the stream immediately.
|
|
|
+
|
|
|
+5.4. Remote hostname lookup
|
|
|
+
|
|
|
+ To find the address associated with a hostname, the OP sends a
|
|
|
+ RELAY_RESOLVE cell containing the hostname to be resolved. (For a reverse
|
|
|
+ lookup, the OP sends a RELAY_RESOLVE cell containing an in-addr.arpa
|
|
|
+ address.) The OR replies with a RELAY_RESOLVED cell containing a status
|
|
|
+ byte, and any number of answers. Each answer is of the form:
|
|
|
+ Type (1 octet)
|
|
|
+ Length (1 octet)
|
|
|
+ Value (variable-width)
|
|
|
+ TTL (4 octets)
|
|
|
+ "Length" is the length of the Value field.
|
|
|
+ "Type" is one of:
|
|
|
+ 0x00 -- Hostname
|
|
|
+ 0x04 -- IPv4 address
|
|
|
+ 0x06 -- IPv6 address
|
|
|
+ 0xF0 -- Error, transient
|
|
|
+ 0xF1 -- Error, nontransient
|
|
|
+
|
|
|
+ If any answer has a type of 'Error', then no other answer may be given.
|
|
|
+
|
|
|
+ The RELAY_RESOLVE cell must use a nonzero, distinct streamID; the
|
|
|
+ corresponding RELAY_RESOLVED cell must use the same streamID. No stream
|
|
|
+ is actually created by the OR when resolving the name.
|
|
|
+
|
|
|
+6. Flow control
|
|
|
+
|
|
|
+6.1. Link throttling
|
|
|
+
|
|
|
+ Each node should do appropriate bandwidth throttling to keep its
|
|
|
+ user happy.
|
|
|
+
|
|
|
+ Communicants rely on TCP's default flow control to push back when they
|
|
|
+ stop reading.
|
|
|
+
|
|
|
+6.2. Link padding
|
|
|
+
|
|
|
+ Currently nodes are not required to do any sort of link padding or
|
|
|
+ dummy traffic. Because strong attacks exist even with link padding,
|
|
|
+ and because link padding greatly increases the bandwidth requirements
|
|
|
+ for running a node, we plan to leave out link padding until this
|
|
|
+ tradeoff is better understood.
|
|
|
+
|
|
|
+6.3. Circuit-level flow control
|
|
|
+
|
|
|
+ To control a circuit's bandwidth usage, each OR keeps track of
|
|
|
+ two 'windows', consisting of how many RELAY_DATA cells it is
|
|
|
+ allowed to package for transmission, and how many RELAY_DATA cells
|
|
|
+ it is willing to deliver to streams outside the network.
|
|
|
+ Each 'window' value is initially set to 1000 data cells
|
|
|
+ in each direction (cells that are not data cells do not affect
|
|
|
+ the window). When an OR is willing to deliver more cells, it sends a
|
|
|
+ RELAY_SENDME cell towards the OP, with Stream ID zero. When an OR
|
|
|
+ receives a RELAY_SENDME cell with stream ID zero, it increments its
|
|
|
+ packaging window.
|
|
|
+
|
|
|
+ Each of these cells increments the corresponding window by 100.
|
|
|
+
|
|
|
+ The OP behaves identically, except that it must track a packaging
|
|
|
+ window and a delivery window for every OR in the circuit.
|
|
|
+
|
|
|
+ An OR or OP sends cells to increment its delivery window when the
|
|
|
+ corresponding window value falls under some threshold (900).
|
|
|
+
|
|
|
+ If a packaging window reaches 0, the OR or OP stops reading from
|
|
|
+ TCP connections for all streams on the corresponding circuit, and
|
|
|
+ sends no more RELAY_DATA cells until receiving a RELAY_SENDME cell.
|
|
|
+[this stuff is badly worded; copy in the tor-design section -RD]
|
|
|
+
|
|
|
+6.4. Stream-level flow control
|
|
|
+
|
|
|
+ Edge nodes use RELAY_SENDME cells to implement end-to-end flow
|
|
|
+ control for individual connections across circuits. Similarly to
|
|
|
+ circuit-level flow control, edge nodes begin with a window of cells
|
|
|
+ (500) per stream, and increment the window by a fixed value (50)
|
|
|
+ upon receiving a RELAY_SENDME cell. Edge nodes initiate RELAY_SENDME
|
|
|
+ cells when both a) the window is <= 450, and b) there are less than
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+ ten cell payloads remaining to be flushed at that edge.
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+
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Nick Mathewson