RTMPE ----- This document is a clean-room specification of the RTMP "Encryption" scheme called RTMPE. It contains industry-standard crypto primitives, ARC4, HMACSHA256 and Diffie-Hellman. The specification was created by reviewing the source code of rtmpdump v1.6. Academic and other discussion is invited. Distribution of this document is unlimited and encouraged. Implementations even more so. More info: http://lkcl.net/rtmp Conventions ----------- data[x:y] means "bytes x through y, inclusive" - like in python x+y on bytes means "append the two byte streams, consecutively" data[x] means "the byte offset by x" - like in python. /* ... */ means comments bigendian32(x) means create 4 bytes in big-endian order, from a 32-bit integer. Constants --------- RTMP_SIG_SIZE = 1536 SHA256DL = 32 /* SHA 256-byte Digest Length */ RandomCrud = { 0xf0, 0xee, 0xc2, 0x4a, 0x80, 0x68, 0xbe, 0xe8, 0x2e, 0x00, 0xd0, 0xd1, 0x02, 0x9e, 0x7e, 0x57, 0x6e, 0xec, 0x5d, 0x2d, 0x29, 0x80, 0x6f, 0xab, 0x93, 0xb8, 0xe6, 0x36, 0xcf, 0xeb, 0x31, 0xae } SWFVerifySig = { 0x1, 0x1 } /* data in quotes does not include quotes as part of data */ GenuineFMSKey = "Genuine Adobe Flash Media Server 001" GenuineFPKey = "Genuine Adobe Flash Player 001" GenuineFMSKeyCrud = GenuineFMSKey + RandomCrud GenuineFPKeyCrud = GenuineFPKey + RandomCrud GetServerDHOffset ----------------- The purpose of this function is to calculate the offset of the Server's Diffie-Hellmann key. Its input is 4 consecutive bytes. offset = byte[0] + byte[1] + byte[2] + byte[3] offset = modulo(offset,632) offset = offset + 8 For sanity, the offset should be no bigger than (767-128) GetServerGenuineFMSKeyDigestOffset ---------------------------------- The purpose of this function is to calculate the offset of the Server's Digest. Input data is 4 consecutive bytes. offset = byte[0] + byte[1] + byte[2] + byte[3] offset = modulo(offset,728) offset = offset + 776 For sanity, the offset should be no bigger than (1535-32) GetClientDHOffset ----------------- The purpose of this function is to calculate the offset of the client's Diffie-Hellmann key. Input data is 4 consecutive bytes. offset = byte[0] + byte[1] + byte[2] + byte[3] offset = modulo(offset,632) offset = offset + 772 For sanity, the offset should be no bigger than (RTMP_SIG_SIZE-128-4) GetClientGenuineFPKeyDigestOffset --------------------------------- The purpose of this function is to calculate the offset of the client's Digest. Input data is 4 consecutive bytes. offset = byte[0] + byte[1] + byte[2] + byte[3] offset = modulo(offset,728) offset = offset + 12 For sanity, the offset should be no bigger than (771-32) Packet Format ------------- The packets consist of a one byte command followed by a 1536 byte message Bytes : Description ------- ----------- 0 Command 1:1536 message of RTMP_SIG_SIZE bytes Client First Exchange --------------------- This is the first packet to be generated. clientsig and clientsig2 are RTMP_SIG_SIZE bytes. serversig and serversig2 are RTMP_SIG_SIZE bytes. Note: Encryption is only supported on versions at least 9.0.115.0 Note: The 0x08 command-byte is not yet known. It is understood to involve further obfuscation of the Client and Server Digests, and is understood to be implemented in Flash 10. Command byte: 0x06 if encrypted 0x08 if further encrypted (undocumented) 0x03 if unencrypted Message: 0:3 32-bit system time, network byte ordered (htonl) 4:7 Client Version. e.g. 0x09 0x0 0x7c 0x2 is 9.0.124.2 8:11 Obfuscated pointer to "Genuine FP" key 12:1531 Random Data, 128-bit Diffie-Hellmann key and "Genuine FP" key. 1532:1535 Obfuscated pointer to 128-bit Diffie-Hellmann key Calculate location of Diffie Hellmann Public Key and create it: dhpkl = GetClientDHoffset(clientsig[1532:1535]) DHPrivateKeyC, DHPublicKeyC = DHKeyGenerate(128) /* 128-bit */ clientsig[dhpkl:dhpkl+127] = DHPublicKeyC Calculate location of Client Digest and create it: /* Note: the SHA digest message is calculated from the bytes of the message, excluding the 32-bytes where the digest itself goes. */ cdl = GetClientGenuineFPKeyDigestOffset(clientsig[8:11]) msg = clientsig[0:cdl-1] + clientsig[cdl+SHA256DL:RTMP_SIG_SIZE-1] clientsig[cdl:cdl+SHA256DL-1] = HMACsha256(msg, GenuineFPKey) First Exchange: Send all 1537 bytes (command + clientsig) to the server; Read 1537 bytes (command + serversig) from the server. Note that the exact circumstances under which "Message Format 1" or "Message Format 2" are utilised is unknown. It is therefore necessary for clients to utilise the SHA verification to determine which of the two message formats is being received (!) Command byte: 0x06 if encrypted - same as client request 0x03 if unencrypted - same as client request Message Format 1: 0:3 32-bit system time, network byte ordered (htonl) 4:7 Server Version. e.g. 0x09 0x0 0x7c 0x2 is 9.0.124.2 8:11 Obfuscated pointer to "Genuine FMS" key 12:1531 Random Data, 128-bit Diffie-Hellmann key and "Genuine FMS" key. 1532:1535 Obfuscated pointer to 128-bit Diffie-Hellmann key Calculate location of Server Digest and compare it: sdl = GetClientGenuineFMSKeyDigestOffset(serversig[8:11]) msg = serversig[0:sdl-1] + serversig[sdl+SHA256DL:RTMP_SIG_SIZE-1] Compare(serversig[sdl:sdl+SHA256DL-1], HMACsha256(msg, GenuineFMSKey)) Calculate location of Server Diffie Hellmann Public Key and get it: dhpkl = GetClientDHoffset(serversig[1532:1535]) DHPublicKeyS = serversig[dhpkl:dhpkl+127] Message Format 2: 0:3 32-bit system time, network byte ordered (htonl) 4:7 Server Version. e.g. 0x09 0x0 0x7c 0x2 is 9.0.124.2 8:767 Random Data and 128-bit Diffie-Hellmann key 768:771 Obfuscated pointer to 128-bit Diffie-Hellmann key 772:775 Obfuscated pointer to "Genuine FMS" key 776:1535 Random Data and "Genuine FMS" key. Calculate location of Server Digest and compare it: sdl = GetServerGenuineFMSKeyDigestOffset(serversig[772:775]) msg = serversig[0:sdl-1] + serversig[sdl+SHA256DL:RTMP_SIG_SIZE-1] Compare(serversig[sdl:sdl+SHA256DL-1], HMACsha256(msg, GenuineFMSKey)) Calculate location of Server Diffie Hellmann Public Key and get it: dhpkl = GetServerDHoffset(serversig[768:771]) DHPublicKeyS = serversig[dhpkl:dhpkl+127] Compute Diffie-Hellmann Shared Secret: The key is only needed if encryption was negotiated. DHSharedSecret = DH(DHPrivateKeyC, DHPublicKeyS) Compute SWFVerification token: If a SWFHash is used, a SWFVerification response will need to be calculated, and returned on-demand to a "ping" request. SWFsize is the size of the SWF file. Note: It is assumed that the reader is familiar enough with RTMP to know what a "ping" is. Where the ordinary ping type is 0x0006, and the pong response is of type 0x0007, an SWF verification ping is of type 0x001a and the SWF verification pong is of type 0x001b. Packet sizes of type 0x001b are 44 bytes: 2 bytes for the type itself and 42 bytes for the SWF verification response. swfvk = serversig[RTMP_SIG_SIZE-SHA256DL:RTMP_SIG_SIZE-1] SWFDigest = SWFVerifySig + bigendian32(SWFsize) + bigendian32(SWFsize) + HMACsha256(SWFHash, swfvk) Initialise ARC4 Send / Receive Keys: The ARC4 keys KeyIn and KeyOut are used to decrypt and encrypt incoming and outgoing data, respectively. KeyIn = ARC4Key(HMACsha256(DHPublicKeyS, DHSharedSecret)[0:15]) KeyOut = ARC4Key(HMACsha256(DHPublicKeyC, DHSharedSecret)[0:15]) Explanation in words: To calculate the ARC4 key for the data received by the client (KeyIn), take the Server's initial 128-bit Diffie-Hellmann Secret (from which the DH Shared Secret was calculated) and calculate the HMACsha256 digest of that server's secret, using the DH Shared Secret as the HMACsha256 key. To calculate the ARC4 key for the data sent by the client (KeyOut), take the Client's initial 128-bit Diffie-Hellmann Secret (from which the DH Shared Secret was calculated) and calculate the HMACsha256 digest of the client's secret, using the DH Shared Secret as the HMACsha256 key. Read Second Exchange: Note: the second response appears to be read directly after the first response, rather than the normal client-server arrangement of interleaving client writes with server sends. Read 1536 bytes (serversig2) from the server. Validate Second Response: If Flash Player version 9 Hand-shaking is not being utilised, then the server will have simply sent a copy of the client's own previous packet back to it. Otherwise, the client verifies the response (the first four bytes of which are likely to be zero if there was a validation error), as follows: digest = HMACsha256(DHPublicKeyC, GenuineFMSKeyCrud) signature = HMACsha256(serversig2[0:RTMP_SIG_SIZE-SHA256DL-1], digest) Compare(signature, serversig2[RTMP_SIG_SIZE-SHA256DL:RTMP_SIG_SIZE-1]) Generate Second Response: clientsig2[0:RTMP_SIG_SIZE] = Random Data digest = HMACsha256(DHPublicKeyS, GenuineFPKeyCrud) signature = HMACsha256(clientsig2[0:RTMP_SIG_SIZE-SHA256DL-1], digest) Send Second Response: Write 1536 bytes (clientsig2) to server. Update ARC4 Keys: If encryption is enabled, then ONLY after the handshaking is completed is the ARC4 keys applied to future communication.