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https://github.com/transmission/transmission
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175 lines
6.7 KiB
Text
175 lines
6.7 KiB
Text
######################################################
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### ----- Message Stream Encryption protocol ----- ###
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### specification by Ludde/uau/The_8472/Parg/Nolar ###
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######################################################
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The following protocol describes a transparent wrapper for bidirectional
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data streams (e.g. TCP transports) that prevents passive eavesdroping
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and thus protocol or content identification.
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It is also designed to provide limited protection against active MITM attacks
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and portscanning by requiring a weak shared secret to complete the handshake.
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You should note that the major design goal was payload and protocol obfuscation,
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not peer authentication and data integrity verification. Thus it does not offer
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protection against adversaries which already know the necessary data to establish
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connections (that is IP/Port/Shared Secret/Payload protocol).
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To minimize the load on systems that employ this protocol fast cryptographic
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methods have been chosen over maximum-security algorithms.
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----------------
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- Declarations -
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----------------
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The entire handshake is in big-endian.
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The crypto handshake is transparent to the next upper protocol,
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thus the payload endianness doesn't matter.
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A is the initiator of the underlying transport (e.g. a TCP connection)
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B is the receiver
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##### DH Parameters
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Prime P is a 768 bit safe prime, "0xFFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A63A36210000000000090563"
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Generator G is "2"
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Xa and Xb are a variable size random integers.
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They are the private key for each side and have to be discarded after
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the DH handshake is done. Minimum length is 128 bit. Anything beyond 180 bit
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is not believed to add any further security and only increases the necessary
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calculation time. You should use a length of 160bits whenever possible, lower
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values may be used when CPU time is scarce.
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Pubkey of A: Ya = (G^Xa) mod P
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Pubkey of B: Yb = (G^Xb) mod P
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DH secret: S = (Ya^Xb) mod P = (Yb^Xa) mod P
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P, S, Ya and Yb are 768bits long
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##### Constants/Variables
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PadA, PadB: Random data with a random length of 0 to 512 bytes each
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PadC, PadD: Arbitrary data with a length of 0 to 512 bytes, can be
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used to extend the crypto handshake in future versions.
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Current implementations may choose to set them to 0-length.
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For padding-only usage in the current version they should be zeroed.
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VC is a verification constant that is used to verify whether the other
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side knows S and SKEY and thus defeats replay attacks of the SKEY hash.
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As of this version VC is a String of 8 bytes set to 0x00.
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crypto_provide and crypto_select are a 32bit bitfields.
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As of now 0x01 means plaintext, 0x02 means RC4. (see Functions)
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The remaining bits are reserved for future use.
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The initiating peer A should provide all methods he supports in the bitfield,
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but he may choose only to provide higher encryption levels e.g. if plaintext
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isn't sufficient for it's security needs.
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The responding peer B should set a bit corresponding to the single method
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which he selected from the provided ones.
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Bits with an unknown meaning in crypto_provide and crypto_select
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should be ignored as they might be used in future versions.
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SKEY = Stream Identifier/Shared secret used to drop connections early if we
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don't have a matching stream. It's additionally used to harden the protocol
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against MITM attacks and portscanning.
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Protocols w/o unique stream properties may use a constant.
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Note: For BitTorrent, the SKEY should be the torrent info hash.
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IA = initial payload data from A
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may be 0-sized if you want to wait for the encryption negotation.
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Peer A may buffer up to 65535 bytes before or during the DH handshake to append
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it to the 3rd step. IA is considered as atomic and thus an implementation may
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not expect that anything is handed to the upper layer before IA is completely
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transmitted. Thus there must be no blocking operations within IA.
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Note, Example for Bittorrent:
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After \19Bittorrent protocol + the BT handshake a block occurs since A waits
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for B to send his handshake before A continues to send his bitfield,
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thus IA can only include the prefix + the bt handshake but not the bitfield
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###### Functions
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len(X) specifies the length of X in 2 bytes.
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Thus the maximum length that can be specified is 65535 bytes, this is
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important for the IA block.
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ENCRYPT() is RC4, that uses one of the following keys to send data:
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"HASH('keyA', S, SKEY)" if you're A
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"HASH('keyB', S, SKEY)" if you're B
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The first 1024 bytes of the RC4 output are discarded.
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consecutive calls to ENCRYPT() by one side continue the encryption
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stream (no reinitialization, no keychange). They are only used to distinguish
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semantically seperate content.
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ENCRYPT2() is the negotiated crypto method.
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Current options are:
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0x01 Plaintext. After the specified length (see IA/IB) each side sends unencrypted payload
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0x02 RC4-128. The ENCRYPT() RC4 encoding is continued (no reinitialization, no keychange)
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HASH() is SHA1 binary output (20 bytes)
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###### The handshake "packet" format
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The handshake is seperated into 5 blocking steps.
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1 A->B: Diffie Hellman Ya, PadA
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2 B->A: Diffie Hellman Yb, PadB
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3 A->B: HASH('req1', S), HASH('req2', SKEY) xor HASH('req3', S), ENCRYPT(VC, crypto_provide, len(PadC), PadC, len(IA)), ENCRYPT(IA)
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4 B->A: ENCRYPT(VC, crypto_select, len(padD), padD), ENCRYPT2(Payload Stream)
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5 A->B: ENCRYPT2(Payload Stream)
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Since the length of PadA and PadB are unknown
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B will be able to resynchronize on HASH('req1', S)
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A will be able to resynchronize on ENCRYPT(VC)
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##### Optional early termination conditions
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(should verified before the indicated step is started).
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If a fail-fast behavior is prefered the following conditions can be used to
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disconnect the peer immediately. If less recognizable patterns are preferred
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a peer may wait and disconnect at a later point. If any of these conditions
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are met the handshake can be considered as invalid.
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2 (termination by B)
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if A sent less than 96 Bytes within 30 seconds
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if A sent more than 608 bytes
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3 (termination by A)
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if B sent less than 96 Bytes within 30 seconds
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if B sent more than 608 bytes
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4 (termination by B)
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if A didn't send the correct S hash within 628 bytes after the connection start (synchronisation point)
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if A didn't send a supported SKEY hash after the S hash
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if VC can't be decoded correctly after the SKEY hash
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if none of the crypto_provide options are supported or the bitfield is zeroed
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from here on it's up to the next protocol layer to terminate the connection
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5 (termination by A)
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if VC can't be decoded correctly within 616 bytes after the connection start (synchronisation point)
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if the selected crypto method wasn't provided
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from here on it's up to the next protocol layer to terminate the connection
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