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rfc:rfc7425

Independent Submission M. Thornburgh Request for Comments: 7425 Adobe Category: Informational December 2014 ISSN: 2070-1721

           Adobe's RTMFP Profile for Flash Communication

Abstract

 This memo describes how to use Adobe's Secure Real-Time Media Flow
 Protocol (RTMFP) to transport the video, audio, and data messages of
 Adobe Flash platform communications.  Aspects of this application
 profile include cryptographic methods and data formats, flow metadata
 formats, and protocol details for client-server and peer-to-peer
 communication.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This is a contribution to the RFC Series, independently of any other
 RFC stream.  The RFC Editor has chosen to publish this document at
 its discretion and makes no statement about its value for
 implementation or deployment.  Documents approved for publication by
 the RFC Editor are not a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7425.

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.
 This document may not be modified, and derivative works of it may not
 be created, except to format it for publication as an RFC or to
 translate it into languages other than English.

Thornburgh Informational [Page 1] RFC 7425 Adobe RTMFP for Flash Communication December 2014

Table of Contents

 1. Introduction ....................................................3
 2. Terminology .....................................................4
 3. Common Syntax Elements ..........................................4
 4. Cryptography Profile ............................................5
    4.1. Default Session Key ........................................5
    4.2. Diffie-Hellman Groups ......................................6
    4.3. Certificates ...............................................6
         4.3.1. Format ..............................................6
         4.3.2. Fingerprint .........................................7
         4.3.3. Options .............................................7
                4.3.3.1. Hostname ...................................8
                4.3.3.2. Accepts Ancillary Data .....................8
                4.3.3.3. Extra Randomness ...........................8
                4.3.3.4. Supported Ephemeral Diffie-Hellman Group ...9
                4.3.3.5. Static Diffie-Hellman Public Key ...........9
         4.3.4. Authenticity .......................................10
         4.3.5. Signing and Verifying Messages .....................10
                4.3.5.1. Options ...................................11
                         4.3.5.1.1. Simple Password ................11
         4.3.6. Glare Resolution ...................................13
         4.3.7. Session Override ...................................13
    4.4. Endpoint Discriminators ...................................13
         4.4.1. Format .............................................14
         4.4.2. Options ............................................14
                4.4.2.1. Required Hostname .........................15
                4.4.2.2. Ancillary Data ............................15
                4.4.2.3. Fingerprint ...............................16
         4.4.3. Certificate Selection ..............................16
         4.4.4. Canonical Endpoint Discriminator ...................17
    4.5. Session Keying Components .................................18
         4.5.1. Format .............................................19
         4.5.2. Options ............................................19
                4.5.2.1. Ephemeral Diffie-Hellman Public Key .......20
                4.5.2.2. Extra Randomness ..........................20
                4.5.2.3. Diffie-Hellman Group Select ...............21
                4.5.2.4. HMAC Negotiation ..........................21
                4.5.2.5. Session Sequence Number Negotiation .......22
    4.6. Session Key Computation ...................................23
         4.6.1. Public Key Selection ...............................23
                4.6.1.1. Initiator and Responder Ephemeral .........23
                4.6.1.2. Initiator Ephemeral and Responder Static ..23
                4.6.1.3. Initiator Static and Responder Ephemeral ..24
                4.6.1.4. Initiator and Responder Static ............24
         4.6.2. Diffie-Hellman Shared Secret .......................24
         4.6.3. Packet Encrypt/Decrypt Keys ........................25
         4.6.4. Packet HMAC Send/Receive Keys ......................25

Thornburgh Informational [Page 2] RFC 7425 Adobe RTMFP for Flash Communication December 2014

         4.6.5. Session Nonces .....................................26
         4.6.6. Session Sequence Number ............................26
    4.7. Packet Encryption .........................................27
         4.7.1. Cipher .............................................27
         4.7.2. Format .............................................27
         4.7.3. Verification .......................................29
                4.7.3.1. Simple Checksum ...........................30
                4.7.3.2. HMAC ......................................30
                4.7.3.3. Session Sequence Number ...................31
 5. Flash Communication ............................................31
    5.1. RTMP Messages .............................................31
         5.1.1. Flow Metadata ......................................32
         5.1.2. Message Mapping ....................................34
    5.2. Flow Synchronization ......................................35
    5.3. Client-to-Server Connection ...............................36
         5.3.1. Connecting .........................................36
         5.3.2. Server-to-Client Return Control Flow ...............37
         5.3.3. setPeerInfo Command ................................37
         5.3.4. Set Keepalive Timers Command .......................39
         5.3.5. Additional Flows for Streams .......................40
                5.3.5.1. To Server .................................40
                5.3.5.2. From Server ...............................40
                5.3.5.3. Closing Stream Flows ......................41
         5.3.6. Closing the Connection .............................41
         5.3.7. Example ............................................42
    5.4. Direct Peer-to-Peer Streams ...............................43
         5.4.1. Connecting .........................................43
         5.4.2. Return Flows for Stream ............................43
         5.4.3. Closing the Connection .............................44
 6. IANA Considerations ............................................44
    6.1. RTMFP URI Scheme Registration .............................44
 7. Security Considerations ........................................46
 8. References .....................................................47
    8.1. Normative References ......................................47
    8.2. Informative References ....................................49
 Acknowledgements ..................................................49
 Author's Address ..................................................49

1. Introduction

 Adobe's Secure Real-Time Media Flow Protocol (RTMFP) [RFC7016] is a
 general-purpose transport service for real-time media and bulk data
 in IP networks, and it is suited to client-server and peer-to-peer
 (P2P) communication.  RTMFP provides a generalized framework for
 securing its communications according to the needs of its
 application.

Thornburgh Informational [Page 3] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 The Flash platform comprises the Flash runtime (including Flash
 Player) from Adobe Systems Incorporated, communication servers such
 as Adobe Media Server, and interoperable clients and servers provided
 by other parties.
 Real-time streaming network communication for the Flash platform of
 video, audio, and data typically uses Adobe's Real-Time Messaging
 Protocol (RTMP) [RTMP] messages.  RTMP messages were originally
 designed to be transported over RTMP Chunk Stream in TCP [RTMP];
 however, other transports (such as the one described in this memo)
 are possible.
 This memo specifies the syntax and semantics for transporting RTMP
 messages over RTMFP, and it extends Flash communication semantics to
 include direct P2P communication.  This memo further specifies a
 concrete Cryptography Profile for RTMFP tailored to the application
 and cryptographic needs of Flash platform client-server and P2P
 communications.
 These protocols and profiles were developed by Adobe Systems
 Incorporated and are not the product of an IETF activity.

2. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 [RFC2119].
 "HMAC" means the Keyed-Hash Message Authentication Code (HMAC)
 algorithm [RFC2104].
 "HMAC-SHA256" means HMAC using the SHA-256 Secure Hash Algorithm
 [SHA256] [RFC6234].
 "HMAC-SHA256(K, M)" means the calculation of the HMAC-SHA256 of
 message M using key K.

3. Common Syntax Elements

 Definitions of types and structures in this specification use
 traditional text diagrams paired with procedural descriptions using a
 C-like syntax.  The C-like procedural descriptions SHALL be construed
 as definitive.

Thornburgh Informational [Page 4] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 Structures are packed to take only as many bytes as explicitly
 indicated.  There is no 32-bit alignment constraint, and fields are
 not padded for alignment unless explicitly indicated or described.
 Text diagrams may include a bit ruler across the top; this is a
 convenience for counting bits in individual fields and does not
 necessarily imply field alignment on a multiple of the ruler width.
 Unless specified otherwise, reserved fields SHOULD be set to 0 by a
 sender and MUST be ignored by a receiver.
 The procedural syntax of this specification defines correct and
 error-free encoded inputs to a parser.  The procedural syntax does
 not describe a fully featured parser, including error detection and
 handling.  Implementations MUST include means to identify error
 circumstances, including truncations causing elementary or composed
 types not to fit inside containing structures, fields, or elements.
 Unless specified otherwise, an error circumstance SHALL abort the
 parsing and processing of an element and its enclosing elements.
 This memo uses the elementary and composed types described in
 Section 2.1 of RFC 7016.  The definitions of that section are
 incorporated by reference as though fully set forth here.

4. Cryptography Profile

 RTMFP defines a general security framework but delegates specifics,
 such as packet encryption ciphers and key agreement algorithms, to an
 application-defined Cryptography Profile.
 This section defines the RTMFP Cryptography Profile for Flash
 platform communication.

4.1. Default Session Key

 RTMFP uses a Default Session Key and associated default cipher
 configuration during session startup handshaking, where session-
 specific keys and ciphers are negotiated.
 The default cipher is the Advanced Encryption Standard [AES] with
 128-bit keys operating in Cipher Block Chaining [CBC] mode, as
 described in Section 4.7.1.  The Default Session Key is the 16 bytes
 of the string "Adobe Systems 02" encoded in UTF-8 [RFC3629]:
         Hex: 41 64 6F 62 65 20 53 79 73 74 65 6D 73 20 30 32
 The Default Session Key uses checksum mode for packet verification
 and does not use session sequence numbers (Section 4.7.3).

Thornburgh Informational [Page 5] RFC 7425 Adobe RTMFP for Flash Communication December 2014

4.2. Diffie-Hellman Groups

 Implementations conforming to this profile MUST support Diffie-
 Hellman [DH] modular exponentiation (MODP) group 2 (1024 bits) as
 defined in [RFC7296], and SHOULD support Diffie-Hellman MODP group 5
 (1536 bits) and group 14 (2048 bits) as defined in [RFC3526].
 Implementations MAY support additional groups.

4.3. Certificates

 This section defines the certificate format for this Cryptography
 Profile, and the mapping to the abstract properties and semantics for
 RTMFP endpoint identities.

4.3.1. Format

 A certificate in this profile is encoded as a sequence of zero or
 more RTMFP Options and Markers (Section 2.1.3 of RFC 7016).  The
 first marker (if any) in the certificate separates the canonical
 section of the certificate from the remainder.  Some options are
 ignored if they occur outside of the canonical section (that is,
 after the first marker).

Thornburgh Informational [Page 6] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 +~~~/~~~/~~~+   +~~~/~~~/~~~+~~~~~+~~~/~~~/~~~+   +~~~/~~~/~~~+
 | L \ T \ V |...| L \ T \ V |  0  | L \ T \ V |...| L \ T \ V |
 +~~~/~~~/~~~+   +~~~/~~~/~~~+~~~~~+~~~/~~~/~~~+   +~~~/~~~/~~~+
 ^                           ^  ^  ^                           ^
 |  Zero or more non-empty   |  |  |   Zero or more Options    |
 |         Options           |  |  +------  or Markers  -------+
 |                           |  |
 +---  Canonical Section  ---+  +---- First Marker
                                      (if present)
 struct certificate_t
 {
     canonicalStart = remainder();
     canonicalEnd = remainder();
     markerFound = false;
     while(remainder() > 0)
     {
         option_t option :variable*8;
         if(0 == option.length)
             markerFound = true;
         else if(!markerFound)
             canonicalEnd = remainder();
     };
     canonicalSectionLength = canonicalStart - canonicalEnd;
 } :variable*8;

4.3.2. Fingerprint

 A certificate's fingerprint is the SHA-256 hash [SHA256] of the
 canonical section of the certificate (that is, the hash of the first
 canonicalSectionLength bytes of the certificate).
 The certificate's fingerprint is also called the "peer ID".

4.3.3. Options

 This section lists options that can appear in a certificate.  The
 following option type codes are defined:
 0x00:    Hostname (must be in canonical section) (Section 4.3.3.1)
 0x0a:    Accepts Ancillary Data (must be in canonical section)
          (Section 4.3.3.2)
 0x0e:    Extra Randomness (Section 4.3.3.3)

Thornburgh Informational [Page 7] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 0x15:    Supported Ephemeral Diffie-Hellman Group (must be in
          canonical section) (Section 4.3.3.4)
 0x1d:    Static Diffie-Hellman Public Key (must be in canonical
          section) (Section 4.3.3.5)
 An implementation MUST ignore a certificate option type that is not
 understood.

4.3.3.1. Hostname

 This option gives an optional hostname for the endpoint.  This option
 MUST be ignored if is not in the canonical section.  This option MUST
 NOT occur more than once in a certificate.
 +-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
 |   length    \ |     0x00    \ |         hostname              |
 +-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
 struct hostnameCertOptionValue_t
 {
     uint8_t hostname[remainder()];
 } :remainder()*8;

4.3.3.2. Accepts Ancillary Data

 This option indicates that the endpoint will accept an Endpoint
 Discriminator encoding an Ancillary Data option (Section 4.4.2.2).
 This option MUST be ignored if it is not in the canonical section.
 +-------------/-+-------------/-+
 |   length    \ |     0x0a    \ |
 +-------------/-+-------------/-+

4.3.3.3. Extra Randomness

 This option can be used to add extra entropy or randomness to a
 certificate that doesn't have any other cryptographic pseudorandom
 members (such as a public key).  This option is typically used so
 that endpoints using ephemeral Diffie-Hellman keying can have a
 unique certificate fingerprint.

Thornburgh Informational [Page 8] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 +-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
 |   length    \ |     0x0e    \ |       extra randomness        |
 +-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
 struct extraRandomnessCertOptionValue_t
 {
     uint_t extraRandomness[remainder()];
 } :remainder()*8;

4.3.3.4. Supported Ephemeral Diffie-Hellman Group

 This option specifies a Diffie-Hellman group ID that is supported for
 ephemeral keying.  This option MUST be ignored if it is not in the
 canonical section.  This option may occur more than once in the
 certificate; each instance indicates an additional group that is
 supported for key agreement.
 +-------------/-+-------------/-+-------------/-+
 |   length    \ |     0x15    \ |   group ID  \ |
 +-------------/-+-------------/-+-------------/-+
 struct ephemeralDHGroupCertOptionValue_t
 {
     vlu_t groupID :variable*8;
 } :variable*8;
 The presence of this option means that the certificate uses ephemeral
 Diffie-Hellman public keys only.  The certificate MUST NOT contain a
 Static Diffie-Hellman public key (Section 4.3.3.5).

4.3.3.5. Static Diffie-Hellman Public Key

 This option specifies a Diffie-Hellman group ID and static public key
 in that group.  This option MUST be ignored if it is not in the
 canonical section.  This option MAY occur more than once in the
 certificate; however, this option SHOULD NOT occur more than once for
 each group ID.  The behavior for specifying more than one public key
 per group ID is not defined.

Thornburgh Informational [Page 9] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 +-------------/-+-------------/-+-------------/-+
 |   length    \ |     0x1d    \ |   group ID  \ |
 +-------------/-+-------------/-+-------------/-+
 +------------------------------------------------------------------+
 |                  Diffie-Hellman Public Key                       |
 +------------------------------------------------------------------/
 struct staticDHPublicKeyCertOptionValue_t
 {
     vlu_t   groupID :variable*8;
     uintn_t publicKey :remainder()*8; // network byte order
 } :remainder()*8;
 The presence of this option means that the certificate uses static
 Diffie-Hellman public keys only.  The certificate MUST NOT contain
 any Supported Ephemeral Diffie-Hellman Group options
 (Section 4.3.3.4).

4.3.4. Authenticity

 This profile does not use a public key infrastructure, nor are there
 signing keys present in certificates.  Therefore, any properly
 encoded certificate is considered authentic according to Section 3.2
 of RFC 7016.
 A certificate containing a static public key can only be used
 successfully for session communication if the holder of the
 certificate actually holds the private key associated with the public
 key.  Authenticity of an identity and its peer ID (Section 4.3.2)
 having a certificate containing a static public key is implied by
 successful encrypted communication with the associated endpoint
 (Section 4.6).
 See Section 7 for further discussion of security issues related to
 identities.

4.3.5. Signing and Verifying Messages

 RTMFP Initiator Initial Keying and Responder Initial Keying messages
 have a field for the sender's digital signature of the keying
 parameters (Sections 2.3.7 and 2.3.8 of RFC 7016).  In this profile,
 the signature field of those messages is encoded as a sequence of
 zero or more RTMFP Options.

Thornburgh Informational [Page 10] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 +~~~/~~~/~~~~~~~+               +~~~/~~~/~~~~~~~+
 | L \ T \   V   |...............| L \ T \   V   |
 +~~~/~~~/~~~~~~~+               +~~~/~~~/~~~~~~~+
 ^                                               ^
 +-------------  Zero or more Options  ----------+
 struct initialKeyingSignature_t
 {
     while(remainder() > 0)
         option_t option :variable*8;
 } :remainder()*8;
 If a signer has no signature options to send, it MAY encode a
 signature as a UTF-8 capital "X" (hex 58) or as empty.  A verifier
 MUST interpret a malformed signature field or a signature field
 consisting only of a UTF-8 capital "X" as though it was empty.
 If a verifier does not require a signature, it SHALL consider any
 signature field (including an empty or malformed one) to be valid.  A
 verifier MAY require a signature comprising one or more non-empty
 options that are valid according to their respective types.
 This profile does not use a public key infrastructure, nor are there
 signing keys present in certificates.  Section 4.3.5.1.1 defines a
 simple ID/password credential system.

4.3.5.1. Options

 This section lists options that can appear in an RTMFP Initial Keying
 signature field.  The following option type code is defined:
 0x1d:  Simple Password (Section 4.3.5.1.1)
 Future or derived profiles may define additional signature field
 options and semantics; therefore, a verifier SHOULD ignore option
 types that are not understood.

4.3.5.1.1. Simple Password

 This option encodes a password identifier (such as a user name, or an
 application-specific or implementation-specific selector) and an HMAC
 over the signed parameters using the identified password as the HMAC
 key.  This option can occur more than once (for example, to allow
 interoperation between a current and a previous version of an
 implementation using implementation-specific passwords).

Thornburgh Informational [Page 11] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 To support the versioning use case, a verifier SHOULD ignore a Simple
 Password option encoding an unrecognized password identifier.  A
 verifier SHOULD treat the entire signature as invalid if any Simple
 Password option encodes a recognized password identifier with an
 invalid password HMAC.
  0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
 +-------------/-+-------------/-+
 |   length    \ |     0x1d    \ |
 +-------------/-+-------------/-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                           hmacSHA256                          |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
 |                           passwordID                          |
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
 struct simplePasswordSignatureOptionValue_t
 {
     uint8_t hmacSHA256[32];
     uint8_t passwordID[remainder()];
 } :remainder()*8;
 hmacSHA256:  HMAC-SHA256(K, M), where K is the password associated
    with passwordID, and M is the signed parameters.
 passwordID:  The identifier (such as a user name) for the password
    used as the HMAC key.

Thornburgh Informational [Page 12] RFC 7425 Adobe RTMFP for Flash Communication December 2014

4.3.6. Glare Resolution

 Glare occurs when two endpoints initiate a session each to the other
 concurrently.
 Compare the near end's certificate to the far end's with a binary
 lexicographic comparison, one byte at a time, up to the length of the
 shorter certificate.  At the first corresponding byte from each
 certificate that is different, the certificate having the differing
 byte (treated as an unsigned 8-bit integer) with the lower value is
 ordered before the other certificate.  If the certificates are not
 the same length and they are identical up to the length of the
 shorter certificate, then the shorter certificate is ordered before
 the longer.
 The near end prevails as the Initiator in case of glare if its
 certificate is ordered before, or is identical to, the certificate of
 the far end.  Otherwise, the near end's certificate is ordered after
 the far end's certificate, and the near end assumes the role of
 Responder.

4.3.7. Session Override

 A new incoming session overrides an existing session only if the
 certificate for the new session is identical to the certificate for
 the existing session.

4.4. Endpoint Discriminators

 This section describes the Endpoint Discriminator (EPD) (Section 3.2
 of RFC 7016) format and semantics for this Cryptography Profile, and
 the mapping to RTMFP's abstract certificate and identity selection
 semantics.

Thornburgh Informational [Page 13] RFC 7425 Adobe RTMFP for Flash Communication December 2014

4.4.1. Format

 An EPD in this profile is encoded as a sequence of zero or more RTMFP
 Options.
 +~~~/~~~/~~~~~~~+               +~~~/~~~/~~~~~~~+
 | L \ T \   V   |...............| L \ T \   V   |
 +~~~/~~~/~~~~~~~+               +~~~/~~~/~~~~~~~+
 ^                                               ^
 +-------------  Zero or more Options  ----------+
 struct endpointDiscriminator_t
 {
     while(remainder() > 0)
         option_t option :variable*8;
 } :remainder()*8;

4.4.2. Options

 This section lists options that can appear in an EPD.  The following
 option type codes are defined:
 0x00:  Required Hostname (Section 4.4.2.1)
 0x0a:  Ancillary Data (Section 4.4.2.2)
 0x0f:  Fingerprint (Section 4.4.2.3)
 The use of these options for selecting certificates is described in
 Section 4.4.3.
 An implementation MUST ignore EPD option types that are not
 understood.

Thornburgh Informational [Page 14] RFC 7425 Adobe RTMFP for Flash Communication December 2014

4.4.2.1. Required Hostname

 This option indicates the hostname to match against the certificate's
 Hostname option (Section 4.3.3.1).
 +-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
 |   length    \ |     0x00    \ |         hostname              |
 +-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
 struct hostnameEPDOptionValue_t
 {
     uint8_t hostname[remainder()];
 } :remainder()*8;
 This option MUST NOT occur more than once in an EPD.

4.4.2.2. Ancillary Data

 In this profile, this option indicates the server Uniform Resource
 Identifier (URI) [RFC3986] encoded in UTF-8 to which a client is
 connecting on this session, for example,
 "rtmfp://server.example.com/app/instance".
 +-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
 |   length    \ |     0x0a    \ |       ancillary data          |
 +-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
 struct ancillaryDataEPDOptionValue_t
 {
     uint8_t ancillaryData[remainder()];
 } :remainder()*8;
 This option MUST NOT occur more than once in an EPD.

Thornburgh Informational [Page 15] RFC 7425 Adobe RTMFP for Flash Communication December 2014

4.4.2.3. Fingerprint

 This option indicates the 256-bit (32-byte) fingerprint
 (Section 4.3.2) of a certificate.
  0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
 +-------------/-+-------------/-+
 |   length    \ |     0x0f    \ |
 +-------------/-+-------------/-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                          fingerprint                          |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 struct fingerprintEPDOptionValue_t
 {
     uint8_t fingerprint[32];
 } :256;
 This option MUST NOT occur more than once in an EPD.

4.4.3. Certificate Selection

 This section describes the REQUIRED method of determining whether an
 EPD selects a certificate.
 An EPD MUST contain at least one of Fingerprint, Required Hostname,
 or Ancillary Data options to select any certificate.
 A Fingerprint EPD option selects or rejects a certificate no matter
 what other options are present.
 Without a Fingerprint option, a Required Hostname EPD option, if
 present, REQUIRES an identical Hostname option in the certificate.

Thornburgh Informational [Page 16] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 Without a Fingerprint option, an Ancillary Data EPD option, if
 present, REQUIRES that the certificate has an Accepts Ancillary Data
 option.
 if EPD contains a Fingerprint option:
     if certificate.fingerprint == option.fingerprint:
         certificate is selected. stop.
     else:
         certificate is not selected. stop.
 else:
     if EPD contains a Required Hostname option:
         if certificate contains a Hostname option:
             if certificate.hostname != option.hostname:
                 certificate is not selected. stop.
         else:
             certificate is not selected. stop.
     if EPD contains an Ancillary Data option:
         if certificate doesn't have an Accepts Ancillary Data option:
             certificate is not selected. stop.
     else if EPD does not contain a Required Hostname option:
         certificate is not selected. stop.
     certificate is selected. stop.
   Figure 1: Algorithm to Test Whether an EPD Selects a Certificate

4.4.4. Canonical Endpoint Discriminator

 In this profile, a Canonical Endpoint Discriminator (Section 3.2 of
 RFC 7016) contains only a Fingerprint option (Section 4.4.2.3) and no
 other options.  The option length and type code MUST be encoded as
 1-byte VLUs, even though VLU encoding allows those fields to be
 encoded in an arbitrary number of bytes.  That is, the Canonical
 Endpoint Discriminator MUST be exactly 34 bytes long, with a length
 field of 0x21 encoded as one byte, a type code of 0x0f encoded as one
 byte, and 32 bytes of fingerprint.

Thornburgh Informational [Page 17] RFC 7425 Adobe RTMFP for Flash Communication December 2014

  0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     0x21      |     0x0f      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                          fingerprint                          |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 + - - - - - - - + - - - - - - - + - - - - - - - + - - - - - - - +
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 struct canonicalEndpointDiscriminator_t
 {
     uint8_t length = 0x21;
     uint8_t type = 0x0f;
     uint8_t fingerprint[32];
 } :272;

4.5. Session Keying Components

 This section describes the format of the Session Key Initiator
 Component of the Initiator Initial Keying RTMFP chunk and the Session
 Key Responder Component of the Responder Initial Keying RTMFP chunk
 (Sections 2.3.7 and 2.3.8 of RFC 7016).  The Initiator and Responder
 Session Keying Components have the same format.

Thornburgh Informational [Page 18] RFC 7425 Adobe RTMFP for Flash Communication December 2014

4.5.1. Format

 A Session Keying Component in this profile is encoded as a sequence
 of zero or more RTMFP Options.
 +~~~/~~~/~~~~~~~+               +~~~/~~~/~~~~~~~+
 | L \ T \   V   |...............| L \ T \   V   |
 +~~~/~~~/~~~~~~~+               +~~~/~~~/~~~~~~~+
 ^                                               ^
 +-------------  Zero or more Options  ----------+
 struct sessionKeyingComponent_t
 {
     while(remainder() > 0)
         option_t option :variable*8;
 } :remainder()*8;

4.5.2. Options

 This section lists options that can appear in a Session Keying
 Component.  The following option type codes are defined:
 0x0d:  Ephemeral Diffie-Hellman Public Key (Section 4.5.2.1)
 0x0e:  Extra Randomness (Section 4.5.2.2)
 0x1d:  Diffie-Hellman Group Select (Section 4.5.2.3)
 0x1a:  HMAC Negotiation (Section 4.5.2.4)
 0x1e:  Session Sequence Number Negotiation (Section 4.5.2.5)
 An implementation MUST ignore a session keying component option type
 that is not understood.

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4.5.2.1. Ephemeral Diffie-Hellman Public Key

 This option specifies a Diffie-Hellman group ID and public key in
 that group.  This option MUST NOT be sent if the sender's certificate
 has a static Diffie-Hellman public key.  This option MUST be sent if
 the sender's certificate does not have a static Diffie-Hellman public
 key.  This option MUST NOT be sent more than once.
 +-------------/-+-------------/-+-------------/-+
 |   length    \ |     0x0d    \ |   group ID  \ |
 +-------------/-+-------------/-+-------------/-+
 +------------------------------------------------------------------+
 |                  Diffie-Hellman Public Key                       |
 +------------------------------------------------------------------/
 struct ephemeralDHPublicKeyKeyingOptionValue_t
 {
     vlu_t   groupID :variable*8;
     uintn_t publicKey :remainder()*8; // network byte order
 } :remainder()*8;

4.5.2.2. Extra Randomness

 This option can be used to add extra entropy or randomness to a
 keying component, particularly when the sender uses a static public
 key.  When used for that purpose, the extra randomness SHOULD be
 cryptographically strong pseudorandom bytes not less than 16 bytes
 (for cryptographically significant entropy) and not more than 64
 bytes (the length of a SHA-256 input block) in length.  The extra
 randomness serves as a salt when computing the session keys
 (Section 4.6).
 +-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
 |   length    \ |     0x0e    \ |       extra randomness        |
 +-------------/-+-------------/-+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
 struct extraRandomnessKeyingOptionValue_t
 {
     uint_t extraRandomness[remainder()];
 } :remainder()*8;

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4.5.2.3. Diffie-Hellman Group Select

 This option is sent by the Initiator to specify which Diffie-Hellman
 group to use for key agreement.  The Initiator MUST send this option
 when it advertises a static Diffie-Hellman public key in its
 certificate and MUST NOT send this option if it sends an ephemeral
 Diffie-Hellman public key.  This option MUST NOT be sent more than
 once.
 +-------------/-+-------------/-+-------------/-+
 |   length    \ |     0x1d    \ |   group ID  \ |
 +-------------/-+-------------/-+-------------/-+
 struct staticDHGroupSelectKeyingOptionValue_t
 {
     vlu_t   groupID :variable*8;
 } :variable*8;

4.5.2.4. HMAC Negotiation

 This option is used to negotiate sending and receiving of an HMAC
 field for packet verification.
                                 |0 1 2 3 4 5 6 7|
 +-------------/-+-------------/-+-+-+-+-+-+-+-+-+-------------/-+
 |             \ |             \ |         |S|S|R|             \ |
 |   length    / |     0x1a    / |   rsv   |N|O|E|  hmacLength / |
 |             \ |             \ |         |D|R|Q|             \ |
 +-------------/-+-------------/-+-+-+-+-+-+-+-+-+-------------/-+
 struct hmacNegotiationKeyingOptionValue_t
 {
     uintn_t reserved :5;          // rsv
     bool_t  willSendAlways :1;    // SND
     bool_t  willSendOnRequest :1; // SOR
     bool_t  request :1;           // REQ
     vlu_t   hmacLength :variable*8;
 } :variable*8;
 willSendAlways:  If set, the sender will send an HMAC on packets in
    this session.
 willSendOnRequest:  If set, the sender will send an HMAC on packets
    in this session if the other end sets the request flag in its HMAC
    Negotiation.

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 request:  If set, the sender would very much like the receiver to
    send an HMAC on its packets.  If the other end doesn't send an
    HMAC on its packets, the session can fail.
 hmacLength:  If the sender negotiates to send an HMAC on its packets,
    the HMAC field will be this many bytes long.  This value MUST be
    between 4 and 32 inclusive, or 0 if and only if willSendAlways and
    willSendOnRequest are clear.
 The handshake operational semantics for this option are described in
 Section 4.6.4.

4.5.2.5. Session Sequence Number Negotiation

 This option is used to negotiate sending and receiving of the Session
 Sequence Number field for packet verification.
                                 |0 1 2 3 4 5 6 7|
 +-------------/-+-------------/-+-+-+-+-+-+-+-+-+
 |             \ |             \ |         |S|S|R|
 |   length    / |     0x1e    / |   rsv   |N|O|E|
 |             \ |             \ |         |D|R|Q|
 +-------------/-+-------------/-+-+-+-+-+-+-+-+-+
 struct sseqNegotiationKeyingOptionValue_t
 {
     uintn_t reserved :5;          // rsv
     bool_t  willSendAlways :1;    // SND
     bool_t  willSendOnRequest :1; // SOR
     bool_t  request :1;           // REQ
 } :8;
 willSendAlways:  If set, the sender will send a session sequence
    number in packets in this session.
 willSendOnRequest:  If set, the sender will send a session sequence
    number in packets in this session if the other end sets the
    request flag in its Session Sequence Number Negotiation.
 request:  If set, the sender would very much like the receiver to
    send a session sequence number in its packets.  If the other end
    doesn't send a session sequence number in its packets, the session
    can fail.
 The handshake operational semantics for this option are described in
 Section 4.6.6.

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4.6. Session Key Computation

 This section describes how to compute the cryptographic keys and
 other settings for packet encryption and verification.
 The Session Key Near Component (SKNC) means the keying component sent
 by the near end of the session; that is, it is the Session Key
 Initiator Component at the Initiator and the Session Key Responder
 Component at the Responder.
 The Session Key Far Component (SKFC) means the keying component sent
 by the far end of the session; that is, it is the Session Key
 Responder Component at the Initiator and the Session Key Initiator
 Component at the Responder.

4.6.1. Public Key Selection

 This section enumerates the public key selection methods for all
 possible combinations of static or ephemeral public key modes for
 each endpoint according to their certificate options (Section 4.3.3).

4.6.1.1. Initiator and Responder Ephemeral

 The Initiator and Responder list one or more Supported Ephemeral
 Diffie-Hellman Group options (Section 4.3.3.4) in their certificates.
 The Initiator sends exactly one Ephemeral Diffie-Hellman Public Key
 option (Section 4.5.2.1) in its Session Key Initiator Component,
 which selects one group from among those supported by the Responder
 and Initiator.  Responder sends exactly one Ephemeral Diffie-Hellman
 Public Key option in its Session Key Responder Component, in the same
 group as indicated by the Initiator.

4.6.1.2. Initiator Ephemeral and Responder Static

 The Responder lists one or more Static Diffie-Hellman Public Key
 options (Section 4.3.3.5) in its certificate.  The Initiator lists
 one or more Supported Ephemeral Diffie-Hellman Group options in its
 certificate.  The Initiator sends exactly one Ephemeral Diffie-
 Hellman Public Key option in its Session Key Initiator Component,
 which selects one group from among those supported by the Responder
 and Initiator and the corresponding public key for the Responder.
 Responder uses its public key from the indicated group, and sends
 only an Extra Randomness option (Section 4.5.2.2) in its Session Key
 Responder Component to salt the session keys.

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4.6.1.3. Initiator Static and Responder Ephemeral

 The Responder lists one or more Supported Ephemeral Diffie-Hellman
 Group options in its certificate.  The Initiator lists one or more
 Static Diffie-Hellman Public Key options in its certificate.  The
 Initiator sends exactly one Diffie-Hellman Group Select option
 (Section 4.5.2.3) in its Session Key Initiator Component, which
 selects one group from among those supported by the Responder and
 Initiator and the corresponding public key for the Initiator, plus an
 Extra Randomness option to salt the session keys.  The Responder
 sends an Ephemeral Diffie-Hellman Public Key option in its Session
 Key Responder Component in the same group as indicated by the
 Initiator.

4.6.1.4. Initiator and Responder Static

 The Initiator and Responder each list one or more Static Diffie-
 Hellman Public Key options in their certificates.  The Initiator
 sends exactly one Diffie-Hellman Group Select option in its Session
 Key Initiator Component, which selects one group and corresponding
 public keys from among those supported by the Responder and
 Initiator, and an Extra Randomness option to salt the session keys.
 The Responder sends an Extra Randomness option in its Session Key
 Responder Component to add its own salt to the session keys.

4.6.2. Diffie-Hellman Shared Secret

 To be acceptable, a Diffie-Hellman public key MUST have all of the
 following properties:
 o  Be at least 16777216 (2^24);
 o  Be at most the group's prime modulus minus 16777216;
 o  Have at least 16 "1" bits;
 o  Have at least 16 "0" bits, not including leading zeros.
 An endpoint MUST NOT complete to an S_OPEN session with a far
 endpoint using a public key that is not acceptable according to these
 criteria.
 Once the group and corresponding public key of the far end is
 determined, the far end's public key and the near end's private key
 are combined according to Diffie-Hellman [DH] to compute the Diffie-
 Hellman Shared Secret, an integer.

Thornburgh Informational [Page 24] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 In the following sections, DH_SECRET means the Diffie-Hellman Shared
 Secret encoded as a byte-aligned unsigned integer in network byte
 order with no leading zero bytes.  For example, if the shared secret
 is 4886718345, DH_SECRET would be the five bytes:
                          Hex: 01 23 45 67 89

4.6.3. Packet Encrypt/Decrypt Keys

 Packets are encrypted using a symmetric cipher, such as the Advanced
 Encryption Standard [AES].  Distinct keys are used for sending and
 receiving packets.  Each end's sending (encrypt) key is the other
 end's receiving (decrypt) key.
 The raw keys computed in this section for encryption and decryption
 are transformed in a manner specific to the cipher with which they
 are to be used.  In this profile, AES-128 is the only currently
 defined cipher.  For this cipher, the first 128 bits (16 bytes) of
 the 256-bit output of the calculation are taken to be the AES-128
 key.
    Set ENCRYPT_KEY = HMAC-SHA256(DH_SECRET, HMAC-SHA256(SKFC, SKNC));
    Set DECRYPT_KEY = HMAC-SHA256(DH_SECRET, HMAC-SHA256(SKNC, SKFC));
 The full 256 bits of ENCRYPT_KEY and DECRYPT_KEY are used in the
 computations in the following sections.

4.6.4. Packet HMAC Send/Receive Keys

 Packets can be verified that they were not corrupted or modified by
 appending an HMAC to the packet.  Whether to use an HMAC or a simple
 checksum is determined during the initial keying phase using the HMAC
 Negotiation option (Section 4.5.2.4).  Distinct HMAC keys are used
 for sending and receiving packets.  Each end's sending key is the
 other end's receiving key, and vice versa.
    Set HMAC_SEND_KEY = HMAC_SHA256(DH_SECRET, ENCRYPT_KEY);
    Set HMAC_RECV_KEY = HMAC_SHA256(DH_SECRET, DECRYPT_KEY);
 If an endpoint sets the willSendAlways flag in its HMAC Negotiation
 option, then it MUST send an HMAC on packets it sends with this
 session key.

Thornburgh Informational [Page 25] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 If an endpoint's willSendAlways flag is clear but its
 willSendOnRequest flag is set, then it MUST send an HMAC on packets
 it sends with this session key if and only if the other endpoint's
 request flag is set.
 If a sending endpoint's willSendAlways and willSendOnRequest flags
 are clear, then the receiving endpoint SHOULD reject that keying
 component if the receiving endpoint is configured to require the
 sending endpoint to send HMAC.
 If HMAC is negotiated to be used, the corresponding hmacLength MUST
 be between 4 and 32 inclusive.
 If HMAC is negotiated not to be used, a simple checksum is used for
 packet verification.
 The Default Session Key uses the simple checksum and does not use
 HMAC.

4.6.5. Session Nonces

 Session nonces are per-session, cryptographically strong secret
 values known only to the two endpoints of the session.  They can be
 used for application-layer cryptographic challenges (such as signing
 or password verification).  These nonces are a convenience being pre-
 shared and pre-agreed-upon in a secure manner during the initial
 keying handshake.
 Each end's near nonce is the other end's far nonce, and vice versa.
    Set NEAR_NONCE = HMAC_SHA256(DH_SECRET, SKNC);
    Set FAR_NONCE = HMAC_SHA256(DH_SECRET, SKFC);

4.6.6. Session Sequence Number

 Duplicate packets can be detected and rejected by using an optional
 session sequence number inside the encrypted packets.  The session
 sequence number is a monotonically increasing unbounded integer and
 does not wrap.  Session sequence numbers SHOULD start at zero and
 SHOULD increment by one for each packet sent using that session key.
 Implementations MUST handle session sequence numbers with no less
 than 64 bits of range.
 If an endpoint's willSendAlways flag in its Session Sequence Number
 Negotiation option (Section 4.5.2.5) is set, then it MUST send a
 session sequence number in packets it sends with this session key.

Thornburgh Informational [Page 26] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 If an endpoint's willSendAlways flag is clear but its
 willSendOnRequest flag is set, then it MUST send a session sequence
 number on packets it sends with this session key if and only if the
 other endpoint's request flag is set.
 If a sending endpoint's willSendAlways and willSendOnRequest flags
 are clear, then the receiving endpoint SHOULD reject that keying
 component if the receiving endpoint is configured to require the
 sending endpoint to send session sequence numbers.
 The Default Session Key does not use session sequence numbers.

4.7. Packet Encryption

 This section describes the concrete syntax and operational semantics
 of RTMFP packet encryption for this Cryptography Profile.

4.7.1. Cipher

 This profile defines AES-128 [AES] in CBC [CBC] mode as the only
 cipher.  Extensions to this profile can specify and negotiate
 additional ciphers and modes by defining certificate and keying
 component options and associated semantics.
 For AES-128-CBC, the initialization vector (IV) for each packet is 16
 zero bytes.  The IV is not included in the packet.

4.7.2. Format

 The Encrypted Packet is the encryptedPacket field of an RTMFP
 Multiplex packet (Section 2.2.2 of RFC 7016); that is, the portion of
 the Multiplex packet following the scrambled session ID.  The
 Encrypted Packet has the following format:

Thornburgh Informational [Page 27] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 +----------------+     +----------------+~~~~~~~~~~~~~~~~~~~~~~~+
 |  CBC Block 1   | ... |  CBC Block N   |     truncatedHMAC     |
 +----------------+     +----------------+~~~~~~~~~~~~~~~~~~~~~~~+
 ^                                       ^                       ^
 |     Zero or more AES-128 chained      | hmacLength bytes long |
 +--------    cipher blocks   -----------+---  (may be zero)  ---+
 struct flashProfileEncryptedPacket_t
 {
     if(HMAC is being used)
         hmacLength = negotiated length;
     else
         hmacLength = 0;
     struct
     {
         iv[16 bytes] = { 0 };
         blockCount = 0;
         while((remainder() > hmacLength) && (remainder() >= 16))
         {
             uint8_t cbcBlock[16];
             blockCount++;
         }
     } chainedCipherBlocks :variable*16*8;
     if(HMAC is being used)
     {
         if(remainder() == hmacLength)
             uint8_t truncatedHMAC[hmacLength];
         else
             packetVerificationFailed();
     }
     else if(remainder() > 0)
         packetVerificationFailed();
 } :encryptedPacket.length*8;
 cbcBlock:  The next AES-128-CBC block.
 chainedCipherBlocks:  The concatenation of every cipher block in the
    packet (over which the HMAC is computed).
 truncatedHMAC:  If HMAC was negotiated to be used (Section 4.5.2.4),
    this field is set to the first negotiated hmacLength bytes of the
    HMAC of the chainedCipherBlocks.
 The plaintext data before encryption or after decryption has the
 following format:

Thornburgh Informational [Page 28] RFC 7425 Adobe RTMFP for Flash Communication December 2014

  0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
 +~~~~~~~~~~~~~/~+
 | SSEQ (opt.) \ |
 +~~~~~~~~~~~~~/~+
 +~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+
 |        Checksum (opt.)        |
 +~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
 |                        Plain RTMFP Packet                     |
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
 struct flashProfilePlainPacket_t
 {
     if(session sequence numbers being used)
         vlu_t sessionSequenceNumber :variable*8; // SSEQ
     if(HMAC not being used)
         uint16_t checksum;
     packet_t plainRTMFPPacket :variable*8;
 } :chainedCipherBlocks.blockCount*16*8;
 sessionSequenceNumber:  If session sequence numbers were negotiated
    to be used (Section 4.6.6), this field is present and is the VLU
    session sequence number of this packet.
 checksum:  If HMAC was not negotiated to be used, this field is
    present and is the simple checksum (Section 4.7.3.1) of the
    remaining bytes of this structure.
 plainRTMFPPacket:  The (plain, unencrypted) RTMFP Packet
    (Section 2.2.4 of RFC 7016) plus any necessary padding.
 When assembling this structure and prior to calculating the checksum
 (if present), if the structure's total length is not an integer
 multiple of 16 bytes (the AES cipher block size), pad the end of
 plainRTMFPPacket with as many bytes having a value of 0xff as are
 needed to bring the structure's total length to an integer multiple
 of 16 bytes.  The receiver's RTMFP Packet parser (Section 2.2.4 of
 RFC 7016) will consume this padding.

4.7.3. Verification

 In RTMFP, the Cryptography Profile is responsible for packet
 verification.  In this profile, packets are verified with an HMAC or
 a simple checksum, depending on the configuration of the endpoints,
 and optionally verified against replay or duplication using session
 sequence numbers.  The simple checksum is inside the encrypted
 packet, so it becomes essentially a 16-bit cryptographic checksum.

Thornburgh Informational [Page 29] RFC 7425 Adobe RTMFP for Flash Communication December 2014

4.7.3.1. Simple Checksum

 The simple checksum is the 16-bit ones' complement of the 16-bit
 ones' complement sum of all 16-bit (2 bytes in network byte order)
 words to be checked.  If there are an odd number of bytes to be
 checked, then for purposes of this checksum, treat the last byte as
 the lower 8 bits of a 16-bit word whose upper 8 bits are 0.  This is
 also known as the "Internet Checksum" [RFC1071].
 When present, the checksum is calculated over all bytes of the
 plaintext packet starting after the checksum field through the end of
 the plain packet.  It cannot be calculated until the plain packet is
 padded, if necessary, to bring its length to an integer multiple of
 16 bytes (the AES cipher block size).  The session sequence number
 field, if present, and the checksum field itself are not included in
 the checksum.
 On receiving a packet being verified with a checksum: calculate the
 checksum over all the bytes of the plaintext packet following the
 checksum field and compare the checksum to the value in the checksum
 field.  If they match, the packet is verified; if they do not match,
 the packet is corrupt and MUST be discarded as though it was never
 received.

4.7.3.2. HMAC

 When present, the HMAC field is the last hmacLength bytes of the
 packet and is calculated over all of the encrypted cipher blocks of
 the packet preceding the HMAC field.  The value of the HMAC field is
 the first hmacLength bytes of the HMAC-SHA256 of the checked data,
 using the computed HMAC keys (Section 4.6.4) and negotiated
 hmacLength (Section 4.5.2.4).  Note each endpoint independently
 specifies the length of the HMAC it will send via its hmacLength
 field.
 When an endpoint has negotiated to send an HMAC, it encrypts the data
 blocks, computes the HMAC over the encrypted data blocks using its
 HMAC_SEND_KEY, and appends the first hmacLength bytes of that hash
 after the final encrypted data block.
 When an endpoint has negotiated to receive an HMAC, the endpoint
 computes the HMAC over the encrypted data blocks using its
 HMAC_RECV_KEY and then compares the first receive hmacLength bytes of
 the computed HMAC to the HMAC field in the packet.  If they are
 identical, the packet is verified; if they are not identical, the
 packet is corrupt and MUST be discarded as though it was never
 received.

Thornburgh Informational [Page 30] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 HMAC and simple checksum verification are mutually exclusive.

4.7.3.3. Session Sequence Number

 Session sequence numbers are used to detect and reject a packet that
 was duplicated in the network or replayed by an attacker and to
 ensure the first chained cipher block of every packet is unique, in
 lieu of a full-block initialization vector.  Sequence numbers start
 at zero, increase by one for each packet sent in the session, do not
 wrap, and do not repeat.
 When session sequence numbers are negotiated to be used, the receiver
 MUST allow for packets to be reordered in the network by up to at
 least 32 sequence numbers; note, however, that reordering by more
 than three packets can trigger loss detection and retransmission by
 negative acknowledgement, just as with TCP, and is therefore not
 likely to occur in the real Internet.
 [RFC4302], [RFC4303], and [RFC6479] describe Anti-Replay Window
 methods that can be employed to detect duplicate sequence numbers.
 Other methods are possible.
 Any packet received having a session sequence number that was already
 seen in that session, either directly or by being less than the
 lowest sequence number in the Anti-Replay Window, is a duplicate and
 MUST be discarded as though never received.

5. Flash Communication

 The Flash platform uses RTMP [RTMP] messages for media streaming and
 communication.  This section describes how to transport RTMP messages
 over RTMFP flows and additional messages and semantics unique to this
 transport.

5.1. RTMP Messages

 An RTMP message comprises a virtual header and a payload.  The
 virtual header comprises a Message Type, a Payload Length, a
 Timestamp, and a Stream ID.  The format of the payload is dependent
 on the type of message.
 An RTMP message is mapped onto a lower transport layer, such as RTMP
 Chunk Stream [RTMP] or RTMFP.  RTMP messages were initially designed
 along with, and for transport on, RTMP Chunk Stream.  This design
 constrains the possible values of RTMP message header fields.  In
 particular:

Thornburgh Informational [Page 31] RFC 7425 Adobe RTMFP for Flash Communication December 2014

    Message Type is 8 bits wide, and is therefore constrained to
    values from 0 to 255 inclusive;
    Payload Length is 24 bits wide, so messages can be at most
    16777215 bytes long;
    Timestamp is 32 bits wide, so timestamps range from 0 to
    4294967295 and wrap around;
    Stream ID is 24 bits wide, and is therefore constrained to values
    from 0 to 16777215 inclusive.
 RTMP Chunk Stream Protocol Control messages (message types 1, 2, 3,
 5, and 6) are not used when transporting RTMP messages in RTMFP
 flows.  Messages of those types SHOULD NOT be sent and MUST be
 ignored.

5.1.1. Flow Metadata

 All messages in RTMFP are transported in flows.  In this profile, an
 RTMFP flow for RTMP messages carries the messages for exactly one
 RTMP Stream ID.  Multiple flows can carry messages for the same
 Stream ID; for example, the video and audio messages of a stream
 could be sent on separate flows, allowing the audio to be given
 higher transmission priority.
 The User Metadata for flows in this profile begins with a distinct
 signature to distinguish among different kinds of flows.  The User
 Metadata for a flow used for RTMP messages begins with the two-
 character signature "TC".
 The Stream ID is encoded in the flow's User Metadata so that it
 doesn't need to be sent with each message.
 The sender can have a priori knowledge about the kind of media it
 intends to send on a flow and its intended use and can give the
 receiver a hint as to whether messages should be delivered as soon as
 possible or in their original queuing order.  For example, the sender
 might be sending real-time, delay-sensitive audio messages on a flow,
 and hint that the receiver should take delivery of the messages on
 that flow as soon as they arrive in the network, to reduce the end-
 to-end latency of the audio.
 The receiver can choose to take delivery of messages on flows as soon
 as they arrive in the network or in the messages' original queuing
 order.  A receiver that chooses to take delivery of messages as soon
 as they arrive in the network MUST be prepared for the messages to

Thornburgh Informational [Page 32] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 arrive out-of-order.  For example, a receiver may choose not to
 render a newly received audio message having a timestamp earlier than
 the most recently rendered audio timestamp.
 The sender can choose to abandon a message that it has queued in a
 flow before the message has been delivered to the receiver.  For
 example, the sender may abandon a real-time, delay-sensitive audio
 message that has not been delivered within one second, to avoid
 spending transmission resources on stale media that is no longer
 relevant.
 Note: A gap will cause a delay at the receiver of at least one round-
 trip time if the receiver is taking delivery of messages in original
 queuing order.
  0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+~~~~~~~~~~~~~/~+
 |               |               |         |S|r|R|             \ |
 |   0x54  'T'   |   0x43  'C'   |   rsv   |I|s|X|   streamID  / |
 |               |               |         |D|v|I|             \ |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+~~~~~~~~~~~~~/~+
 struct RTMPMetadata_t
 {
     uint8_t signature[2] == { 'T', 'C' };
     uintn_t reserved1       :5; // rsv
     bool_t  streamIDPresent :1; // SID
     uintn_t reserved2       :1; // rsv
     uintn_t receiveIntent   :1; // RXI
         // 0: original queuing order, 1: network arrival order
     if(streamIDPresent)
         vlu_t   streamID        :variable*8;
 } :variable*8;
 signature:  Metadata signature for RTMP message flows, being the two
    UTF-8 coded characters "TC".
 streamIDPresent:  A boolean flag indicating whether the streamID
    field is present.  In this profile, this flag MUST be set.
 receiveIntent:  A hint by the sender as to the best order in which to
    take delivery of messages from the flow.  A value of zero
    indicates a hint that the flow's messages should be received in
    the order they were originally queued by the sender (that is, in
    ascending sequence number order); a value of one indicates a hint
    that the flow's messages should be received in the order they
    arrive in the network, even if there are sequence number gaps or
    reordering.  Network arrival order is typically hinted for live,

Thornburgh Informational [Page 33] RFC 7425 Adobe RTMFP for Flash Communication December 2014

    delay-sensitive flows, such as for audio media.  To take delivery
    of a message as soon as it arrives in the network: receive it from
    the receiving flow's RECV_BUFFER as soon as it becomes complete
    (Section 3.6.3.3 of RFC 7016), and remove it from the RECV_BUFFER.
    Section 3.6.3.3 of RFC 7016 describes how to take delivery of
    messages in original queuing order.
 streamID:  If the streamIDPresent flag is set, this field is present
    and is the RTMP stream ID to which the messages in this flow
    belong.  In this profile, this field MUST be present.
 A receiver SHOULD reject an RTMP message flow if its streamIDPresent
 flag is clear.  This profile doesn't define a stream mapping for this
 case.
 Derived or composed profiles can define additional flow types and
 corresponding metadata signatures.  A receiver SHOULD reject a flow
 having an unrecognized metadata signature.

5.1.2. Message Mapping

 This section describes the format of an RTMP message (Section 5.1) in
 an RTMFP flow.
  0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+
 |  messageType  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           timestamp                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         messagePayload                        |
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/
 struct RTMPMessage_t
 {
     uint8_t  messageType;
     uint32_t timestamp;
     uint8_t  messagePayload[remainder()];
 } :flowMessageLength*8;
 messageType:  The RTMP Message Type;
 timestamp:  The RTMP Timestamp, in network byte order;
 messagePayload:  The payload of the RTMP message;
 payload length:  The RTMP message payload length is inferred from the
    length of the RTMFP message;

Thornburgh Informational [Page 34] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 Stream ID:  The Stream ID for this message is taken from the metadata
    of the flow on which this message was received.

5.2. Flow Synchronization

 RTMFP flows are independent and have no inter-flow ordering
 guarantee.  RTMP was designed for transport over a single, reliable,
 strictly ordered byte stream.  Some RTMP message semantics take
 advantage of this ordering; for example, a Stream EOF User Control
 event must not be processed until after all media messages for the
 corresponding stream have been received.  Flow Synchronization
 messages provide a barrier to align message delivery across flows
 when required by RTMP semantics.
 A Flow Synchronization message is coded as a User Control event
 message (Type 4) having Event Type 34.  Message timestamps are
 ignored and MAY be set to 0.
  0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+
 |       4       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           timestamp                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         eventType = 34        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             syncID                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             count                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 struct flowSyncUserControlMessagePayload_t
 {
     uint16_t eventType = 34;
     uint32_t syncID;
     uint32_t count;
 } :10*8;
 eventType:  The RTMP User Control Message Event Type.  Flow
    Synchronization messages have type 34 (0x22);
 syncID:  The identifier for this barrier;
 count:  The number of flows being synchronized by syncID.  This field
    MUST be at least 1 and SHOULD be at least 2.

Thornburgh Informational [Page 35] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 On receipt of a Flow Synchronization message, a receiver SHOULD
 suspend receipt of further messages on that flow until count Flow
 Synchronization messages (including this one) with the same syncID
 have been received on flows in the same flow association tree.
 Example: Consider flows F1 and F2 in the same NetConnection carrying
 messages M, and let Sync(syncID,count) denote a Flow Synchronization
 message.
                                     |                |
           F1: M1  M2  M4  Sync(8,2) | Sync(13,2).....| M7
                                     |                |
           F2:   M3  Sync(8,2).......| M5  Sync(13,2) | M6
                                     |                |
                                 Barrier 8        Barrier 13
            Figure 2: Example Flow Synchronization Barriers
 Flow Synchronization messages form a delivery barrier to impart at
 least a partial message ordering across flows.  In this example,
 message M5 comes after M1..4 and before M6..7; however, M3 could be
 delivered before or after any of M1, M2, or M4, and M6 could come
 before or after M7.
 Flow Synchronization can cause a priority inversion; therefore, it
 SHOULD NOT be used except when necessary to preserve RTMP ordering
 semantics.

5.3. Client-to-Server Connection

 The client connects to a server.  The connection comprises one main
 control flow in each direction from client to server and from server
 to client for NetConnection messages, and zero or more flows in each
 direction for NetStream media messages.  NetStream flows may come and
 go naturally over time according to media transport needs.  An
 exception on a NetConnection control sending flow indicates the
 closure by the other end of the NetConnection and all associated
 NetStreams.
 The client MUST NOT use the same client certificate for more than one
 server connection; that is, a client's peer ID MUST NOT be reused.

5.3.1. Connecting

 The client desires a connection to a server having an RTMFP URI, for
 example, "rtmfp://server.example.com/app/instance".  The client
 gathers one or more initial candidate addresses for the server named
 in the URI (for example, by using the Domain Name System (DNS)

Thornburgh Informational [Page 36] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 [RFC1035]).  The client creates an EPD having an Ancillary Data
 option (Section 4.4.2.2) encoding the URI.  The client initiates an
 RTMFP session to the one or more candidate addresses using the EPD.
 When the session transitions to the S_OPEN state, the client opens a
 new flow in that session for Stream ID 0 and Receive Intent 0
 "original queuing order".  This is the client's NetConnection main
 control flow.  The client sends an RTMP "connect" command on the flow
 and waits for a response or exception.

5.3.2. Server-to-Client Return Control Flow

 The server, on accepting the client's NetConnection control flow, and
 receiving and accepting the "connect" command, opens one or more
 return flows to the client having Stream ID 0 and associated to the
 control flow from the client.  Flows for Stream ID 0 are the server's
 NetConnection control flows.  The server sends a "_result" or
 "_error" transaction response for the client's connect command.
 When the client receives the first return flow from the server for
 Stream ID 0 and associated to the client's NetConnection control
 flow, the client assumes that flow is the canonical return
 NetConnection control flow from the server, to which all new client-
 to-server flows should be associated.
 On receipt of a "_result" transaction response on Stream ID 0 for the
 client's connect command, the connection is up.
 The client MAY open additional return control flows to the server on
 Stream ID 0, associated to the server's canonical NetConnection
 control flow.

5.3.3. setPeerInfo Command

 The "setPeerInfo" command is sent by the client to the server over
 the NetConnection control flow to inform the server of candidate
 socket addresses through which the client might be reachable.  This
 list SHOULD include all directly connected interface addresses and
 proxy addresses except as provided below.  The list MAY be empty.
 The list need not include the address of the server, even if the
 server is to act as an introducer for the client.  The list SHOULD
 NOT include link-local or loopback addresses.
 This command is sent as a regular RTMP NetConnection command; that
 is, as an RTMP Type 20 Command Message or an RTMP Type 17 Command
 Extended Message on Stream ID 0.  A Type 20 Command Message SHOULD be
 used if the object encoding negotiated during the "connect" and

Thornburgh Informational [Page 37] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 "_result" handshake is AMF0 [AMF0], and a Type 17 Command Extended
 Message SHOULD be used if the negotiated object encoding is AMF3
 [AMF3].
 Note: A Type 20 Command Message payload is a sequence of AMF objects
 encoded in AMF0.
 Note: A Type 17 Command Extended Message payload begins with a format
 selector byte, followed by a sequence of objects in a format-specific
 encoding.  At the time of writing, only format 0 is defined;
 therefore, the format selector byte MUST be 0.  Format 0 is a
 sequence of AMF objects, each encoded in AMF0 by default; AMF3
 encoding for an object can be selected by prefixing it with an
 "avmplus-object-marker" (0x11) as defined in [AMF0].
 To complete the RTMFP NetConnection handshake, an RTMFP client MUST
 send a setPeerInfo command to the server after receiving a successful
 response to the "connect" command.
 (
     "setPeerInfo", // AMF String, command name
     0.0,  // AMF Number, transaction ID
     NULL, // AMF Null, no command object
     ...   // zero or more AMF Strings, each an address
 )
 Each listed socket address includes an IPv4 or IPv6 address in
 presentation format and a UDP port number in decimal, separated by a
 colon.  Since the IPv6 address presentation format uses colons, IPv6
 addresses are enclosed in square brackets [RFC3986].
                      (
                          "setPeerInfo",
                          0.0,
                          NULL,
                          "192.0.2.129:50001",
                          "[2001:db8:1::2]:50002"
                      )
                 Figure 3: Example setPeerInfo Command
 A server SHOULD assume that the client is behind a Network Address
 Translator (NAT) if and only if the observed far endpoint address of
 the session for the flow on which this command was received does not
 appear in the setPeerInfo address list.

Thornburgh Informational [Page 38] RFC 7425 Adobe RTMFP for Flash Communication December 2014

5.3.4. Set Keepalive Timers Command

 The server can advise the client to set or change the client's
 session keepalive timer periods for its connection to the server and
 for its P2P connections.  The server MAY choose keepalive periods
 based on static configuration, application- or deployment-specific
 circumstances, whether the client appears to be behind a NAT, or for
 any other reason.
 The Set Keepalive Timers command is sent by the server to the client
 on Stream ID 0 as a User Control event message (Type 4) having Event
 Type 41.  Message timestamps are ignored and MAY be set to 0.
  0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+
 |       4       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           timestamp                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         eventType = 41        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    serverKeepalivePeriodMsec                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     peerKeepalivePeriodMsec                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 struct setKeepaliveUserControlMessagePayload_t
 {
     uint16_t eventType = 41;
     uint32_t serverKeepalivePeriodMsec;
     uint32_t peerKeepalivePeriodMsec;
 } :10*8;
 eventType:  The RTMP User Control Message Event Type.  Set Keepalive
    Timers messages have type 41 (0x29);
 serverKeepalivePeriodMsec:  The keepalive period, in milliseconds,
    that the client is advised to set on its RTMFP session with the
    server;
 peerKeepalivePeriodMsec:  The keepalive period, in milliseconds, that
    the client is advised to use on its RTMFP sessions with any peer
    that is not the server.

Thornburgh Informational [Page 39] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 The client MUST define minimum values for these keepalive periods,
 below which it will not set them, regardless of the values in this
 message.  The minimum keepalive timer periods SHOULD be at least five
 seconds.  The client MAY define maximum values for these keepalive
 periods, above which it will not set them.
 On receipt of this message from the server, a client SHOULD set its
 RTMFP server and peer keepalive timer periods to the indicated values
 subject to the client's minimum and maximum values.  The server MAY
 send this message more than once, particularly if conditions that it
 uses to determine the timer periods change.

5.3.5. Additional Flows for Streams

 The client or server opens additional flows to the other side to
 carry messages for any stream.  Additional flows are associated to
 the canonical NetConnection control flow from the other side.
       Client                                            Server
       ------>--C2S-Control-Flow------------------------->--+
                                                            |
          +--<------------------------S2C-Control-Flow---<--+
          |                                                 |
          |  <------------------------S2C-Stream-Flow-1--<--+
          |                                  :              |
          |  <------------------------S2C-Stream-Flow-M--<--+
          |
          +-->--C2S-Stream-Flow-1------------------------>
          |               :
          +-->--C2S-Stream-Flow-N------------------------>
     Figure 4: Schematic Flow Association Tree for a NetConnection

5.3.5.1. To Server

 Additional flows from the client to the server for stream messages
 are opened with the Stream ID for that stream and associated in
 return to the server's canonical NetConnection control flow.
 The client MAY create as many flows as desired for any Stream ID
 (including Stream ID 0) at any time.

5.3.5.2. From Server

 Additional flows from the server to the client for stream messages
 are opened with the Stream ID for that stream, and associated in
 return to the client's NetConnection control flow.

Thornburgh Informational [Page 40] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 The server MAY create as many flows as desired for any Stream ID
 (including Stream ID 0) at any time.

5.3.5.3. Closing Stream Flows

 Either end MAY close a sending flow that is not for Stream ID 0 at
 any time with no semantic meaning for the stream.
 At any time, either end MAY reject a receiving flow that is not one
 of the other end's NetConnection control flows.  No flow exception
 codes are defined by this profile, so the receiving end SHOULD use
 exception code 0 when rejecting the flow.  The sending end, on
 notification of any exception for a stream flow, SHOULD NOT open a
 new flow to take the rejected flow's place for transport of messages
 for that stream.  If an end rejects any flow for a stream, it SHOULD
 reject all the flows for that stream, otherwise Flow Synchronization
 messages (Section 5.2) that were in flight could be discarded and
 some flows might become or remain stuck in a suspended state.

5.3.6. Closing the Connection

 The client or server can signal an orderly close of the connection by
 closing its NetConnection control sending flows and all stream
 sending flows.  The other end, on receiving a close/complete
 notification for the canonical NetConnection control receiving flow,
 closes its sending flows.  When both ends observe all receiving flows
 have closed and completed, the connection has cleanly terminated.
 Either end can abruptly terminate the connection by rejecting the
 NetConnection control receiving flows or by closing the underlying
 RTMFP session.  On notification of any exception on a NetConnection
 control sending flow, the end seeing the exception knows the other
 end has terminated abruptly, and can immediately close all sending
 and receiving flows for that connection.

Thornburgh Informational [Page 41] RFC 7425 Adobe RTMFP for Flash Communication December 2014

5.3.7. Example

               Client                    Server
                 |IHello (EPD:anc=URI)     |
             -+- |------------------------>|
              |  |                         |
              |  |       RHello (RCert:anc)|
        RTMFP |  |<------------------------|
       Session|  |                         |
        Hand- |  |IIKeying                 |
        shake |  |------------------------>|
              |  |                         |
              |  |                 RIKeying|
             -+- |<------------------------|
                 |                         |
             -+- |"connect" command        |
       (Str.ID=0)|-CFlow-0---------------->|
              |  |                         |
              |  |       "_result" response|
        RTMP  |  |<----------------SFlow-0-|(Str.ID=0,
       Connect|  |                         | Assoc=CFlow-0)
        Hand- |  |"setPeerInfo" command    |
        shake |  |-CFlow-0---------------->|
             -+- |                         |
                 |"createStream" command   |
             -+- |-CFlow-0---------------->|
              |  |                         |
              |  |     "_result" (str.ID=5)|
              |  |<----------------SFlow-0-|
              |  |                         |
              |  |"play" command           |
       (Str.ID=5,|-CFlow-1---------------->|
   Assoc=SFlow-0)|                         |
              |  | StreamBegin User Control|
              |  |<----------------SFlow-1-|(Str.ID=5,
              |  |                         | Assoc=CFlow-0)
              |  |  (RTMP stream events)   |
   Streaming  |  |<----------------SFlow-1-|
              |  |                         |
              |  |        Audio Data       |
              |  |<----------------SFlow-2-|(Str.ID=5,
              |  |                         | Assoc=CFlow-0)
              |  |        Video Data       |
              |  |<----------------SFlow-3-|(Str.ID=5,
              |  |            :            | Assoc=CFlow-0)
                 |            :            |
           Figure 5: Example NetConnection Message Exchange

Thornburgh Informational [Page 42] RFC 7425 Adobe RTMFP for Flash Communication December 2014

5.4. Direct Peer-to-Peer Streams

 Clients can connect directly to other clients for P2P streaming and
 data exchange.  A client MAY have multiple separate P2P NetStreams
 with a peer in one RTMFP session, each a separate logical connection.
 P2P NetStreams are unidirectional, initiated by a subscriber (the
 side issuing the "play" command) to a publisher.  The subscribing
 peer has a control flow to the publisher.  The publisher has zero or
 more return flows to the subscriber associated to the subscriber's
 control flow, for the stream media and data.

5.4.1. Connecting

 A client desires to subscribe directly to a stream being published in
 P2P mode by a publishing peer.  The client learns the peer ID of the
 publisher and the stream name through application-specific means.
 If the client does not already have an RTMFP session with that peer
 ID, it initiates a new session, creating an EPD containing a
 Fingerprint option (Section 4.4.2.3) for the publisher's peer ID and
 using the server session's DESTADDR as the initial candidate address
 for the session to the peer.  The server acts as an Introducer
 (Section 3.5.1.6 of RFC 7016), using forward and redirect messages to
 help the client and the peer establish a session.
 When an S_OPEN session exists to the desired peer, the client creates
 a new independent flow to that peer.  The flow MUST have a non-zero
 Stream ID.  The client sends an RTMP "play" command over the flow,
 giving the name of the desired stream at the publisher.  This flow is
 the subscriber's control flow.

5.4.2. Return Flows for Stream

 The publisher, on accepting a new flow not indicating a return
 association with any of its sending flows and having a non-zero
 Stream ID, receives and processes the "play" command.  If and when
 the request is acceptable to the publisher, it opens one or more
 return flows to the subscribing peer, associated to the subscriber's
 control flow and having the same Stream ID.  The publisher sends a
 StreamBegin User Control message, appropriate RTMP status events, and
 the stream media over the one or more return flows.
 The subscriber uses the return association of the media flows to the
 subscriber control flow to determine the stream to which the media
 belongs.

Thornburgh Informational [Page 43] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 The publisher MAY open any number of media flows for the stream and
 close them at any time.  The opening and closing of media flows has
 no semantic meaning for the stream, except that the opening of at
 least one flow and the reception of at least one media message or a
 StreamBegin User Control message indicates that the publisher is
 publishing the requested stream to the subscriber.
       Subscriber                                     Publisher
       ------>--Subscriber-Control-Flow------------------>--+
                                                            |
             <------------------Publisher-Stream-Flow-1--<--+
                                            :               |
             <------------------Publisher-Stream-Flow-N--<--+
 Figure 6: Schematic Flow Association Tree for a P2P Direct Connection

5.4.3. Closing the Connection

 Either end can close the stream by closing or rejecting the
 subscriber's control flow.  The publisher SHOULD close and unpublish
 to the subscriber on receipt of a close/complete of the control flow.
 The subscriber SHOULD consider the stream closed on notification of
 any exception on the control flow.

6. IANA Considerations

 This memo specifies option type code values for Certificate fields
 (Section 4.3.3), Endpoint Discriminator fields (Section 4.4.2), and
 Session Keying Component fields (Section 4.5.2).  It also specifies a
 flow metadata signature (Section 5.1.1).  The type code values and
 signatures for this profile are assigned and maintained by Adobe, and
 therefore require no action from IANA.

6.1. RTMFP URI Scheme Registration

 This memo describes use of an RTMFP URI scheme (Section 4.4.2.2,
 Section 5.3.1, Figure 5).  Per this section, the "rtmfp" URI scheme
 has been registered by IANA.
 The syntax and semantics of this URI scheme are described using the
 Augmented Backus-Naur Form (ABNF) [RFC5234] rules from RFC 3986.

Thornburgh Informational [Page 44] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 URI scheme name:  rtmfp
 Status:  provisional
 URI scheme syntax:
    rtmfp-uri-scheme = "rtmfp:"
                     / "rtmfp://" host [ ":" port ] path-abempty
 URI scheme semantics:  The first form is used in the APIs of some
    implementations to indicate instantiation of an RTMFP client
    according to this memo, but without connecting to a server.  Such
    an instantiation might be used for pure peer-to-peer
    communication.
    The second form provides location information for the server to
    which to connect and optional additional information to pass to
    the server.  The only operation for this URI form is to connect to
    a server (initial candidate address(es) for which are named by
    host and port) according to Section 5.3.  The UDP port for initial
    candidate addresses, if not specified, is 1935.  If the host is a
    reg-name, the initial candidate address set SHOULD comprise all
    IPv4 and IPv6 addresses to which reg-name resolves.  The semantics
    of path-abempty are specific to the server.  Connections are made
    using RTMFP as specified by this memo.
 Encoding considerations:  The path-abempty component represents
    textual data consisting of characters from the Universal Character
    Set.  This component SHOULD be encoded according to Section 2.5 of
    RFC 3986.
 Applications/protocols that use this URI scheme name:  The Flash
    runtime (including Flash Player) from Adobe Systems Incorporated,
    communication servers such as Adobe Media Server, and
    interoperable clients and servers provided by other parties, using
    RTMFP according to this memo.
 Interoperability considerations:  This scheme requires use of RTMFP
    as defined by RFC 7016 in the manner described by this memo.
 Security considerations:  See Security Considerations (Section 7) in
    this memo.
 Contact:  Michael Thornburgh, Adobe Systems Incorporated,
    <mthornbu@adobe.com>.
 Author/Change controller:  Michael Thornburgh, Adobe Systems
    Incorporated, <mthornbu@adobe.com>.

Thornburgh Informational [Page 45] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 References:
    Thornburgh, M., "Adobe's Secure Real-Time Media Flow Protocol",
    RFC 7016, November 2013.
    This memo.

7. Security Considerations

 Section 4 details the cryptographic aspects of this profile.
 This profile does not define or use a Public Key Infrastructure
 (PKI).  Clients SHOULD use static Diffie-Hellman keys in their
 certificates (Section 4.3.3.5).  Clients MUST create a new
 certificate with a distinct fingerprint for each new NetConnection
 (Section 5.3).  These constraints make client identities ephemeral
 but unable to be forged.  A man-in-the-middle cannot successfully
 interpose itself in a connection to a target client addressed by its
 fingerprint/peer ID if the target client uses a static Diffie-Hellman
 public key.
 Servers can have long-lived RTMFP instances, so they SHOULD use
 ephemeral Diffie-Hellman public keys for forward secrecy.  This
 allows server peer IDs to be forged; however, clients do not connect
 to servers by peer ID, so this is irrelevant.
 When a client connects to a server, the client will accept the
 response of any endpoint claiming to be "a server".  It is assumed
 that an attacker that can passively observe traffic on a network
 segment can also inject its own packets with any source or
 destination and any payload.  An attacker can trick a client into
 connecting to a rogue server or man-in-the-middle, either by
 observing Initiator Hello packets from the client and responding
 earliest with a matching Responder Hello or by using tricks such as
 DNS spoofing or poisoning to direct a client to connect directly to
 the rogue.  A TCP-based transport would be vulnerable to similar
 attacks.  Since there is no PKI, this profile gives no guarantee that
 the client has actually connected to the desired server, versus a
 rogue or man-in-the-middle.  In circumstances where assurance is
 required that the connection is directly to the desired server, the
 client can use the Session Nonces (Section 4.6.5) to challenge the
 server, for example, over a different channel having acceptable
 security properties (such as an HTTPS) to transitively establish the
 server's identity and verify that the end-to-end communication is
 private and authentic.
 When session sequence numbers (Section 4.7.3.3) are not used, it is
 possible for an attacker to use traffic analysis techniques and
 record encrypted packets containing the start of a new flow, and

Thornburgh Informational [Page 46] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 later to replay those packets after the flow has closed, which can
 look to the receiver like a brand new flow.  In circumstances where
 this can be detrimental, session sequence numbers SHOULD be used.
 Replay of packets for existing flows is not detrimental as the
 receiver detects and discards duplicate flow sequence numbers, and
 flow sequence numbers do not wrap or otherwise repeat.
 Packet encryption uses CBC with the same (null) initialization vector
 for each packet.  This can reveal to an observer whether two packets
 contain identical plaintext.  However, the maximum-length RTMFP
 common header and User Data or Data Acknowledgement header, including
 flow sequence number, always fit within the first 16-byte cipher
 block, so each initial cipher block for most packets will already be
 unique even if timestamps are suppressed.  Sending identical messages
 in a flow uses unique flow sequence numbers, so cipher blocks will be
 unique in this case.  Keepalive pings and retransmission of lost data
 can result in identical cipher blocks; however, traffic analysis can
 also reveal likely keepalives or retransmissions, and retransmission
 only occurs as a result of observable network loss, so this is
 usually irrelevant.  In circumstances where any identical cipher
 block is unacceptable, session sequence numbers SHOULD be used as
 they guarantee each initial cipher block will be unique.
 Packet verification can use a 16-bit simple checksum
 (Section 4.7.3.1).  The checksum is inside the encrypted packet, so
 for external packet modifications the checksum is equivalent to a
 16-bit cryptographic digest.  In circumstances where this is
 insufficient, HMAC verification (Section 4.7.3.2) SHOULD be used.

8. References

8.1. Normative References

 [AES]      National Institute of Standards and Technology, "Advanced
            Encryption Standard (AES)", FIPS PUB 197, November 2001,
            <http://csrc.nist.gov/publications/fips/fips197/
            fips-197.pdf>.
 [AMF0]     Adobe Systems Incorporated, "Action Message Format -- AMF
            0", December 2007, <http://www.adobe.com/go/spec_amf0>.
 [AMF3]     Adobe Systems Incorporated, "Action Message Format -- AMF
            3", January 2013, <http://www.adobe.com/go/spec_amf3>.
 [CBC]      Dworkin, M., "Recommendation for Block Cipher Modes of
            Operation", NIST Special Publication 800-38A, December
            2001, <http://csrc.nist.gov/publications/nistpubs/800-38a/
            sp800-38a.pdf>.

Thornburgh Informational [Page 47] RFC 7425 Adobe RTMFP for Flash Communication December 2014

 [DH]       Diffie, W. and M. Hellman, "New Directions in
            Cryptography", IEEE Transactions on Information Theory, V.
            IT-22, n. 6, June 1977.
 [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
            Hashing for Message Authentication", RFC 2104, February
            1997, <http://www.rfc-editor.org/info/rfc2104>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC3526]  Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
            Diffie-Hellman groups for Internet Key Exchange (IKE)",
            RFC 3526, May 2003,
            <http://www.rfc-editor.org/info/rfc3526>.
 [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
            10646", STD 63, RFC 3629, November 2003,
            <http://www.rfc-editor.org/info/rfc3629>.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66, RFC
            3986, January 2005,
            <http://www.rfc-editor.org/info/rfc3986>.
 [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008,
            <http://www.rfc-editor.org/info/rfc5234>.
 [RFC6234]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
            (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011,
            <http://www.rfc-editor.org/info/rfc6234>.
 [RFC7016]  Thornburgh, M., "Adobe's Secure Real-Time Media Flow
            Protocol", RFC 7016, November 2013,
            <http://www.rfc-editor.org/info/rfc7016>.
 [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
            Kivinen, "Internet Key Exchange Protocol Version 2
            (IKEv2)", STD 79, RFC 7296, October 2014,
            <http://www.rfc-editor.org/info/rfc7296>.
 [RTMP]     Adobe Systems Incorporated, "Real-Time Messaging Protocol
            (RTMP) specification", December 2012,
            <http://www.adobe.com/go/spec_rtmp>.

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 [SHA256]   National Institute of Standards and Technology, "Secure
            Hash Standard", FIPS PUB 180-4, March 2012,
            <http://csrc.nist.gov/publications/fips/fips180-4/
            fips-180-4.pdf>.

8.2. Informative References

 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, November 1987,
            <http://www.rfc-editor.org/info/rfc1035>.
 [RFC1071]  Braden, R., Borman, D., Partridge, C., and W. Plummer,
            "Computing the Internet checksum", RFC 1071, September
            1988, <http://www.rfc-editor.org/info/rfc1071>.
 [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302, December
            2005, <http://www.rfc-editor.org/info/rfc4302>.
 [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
            4303, December 2005,
            <http://www.rfc-editor.org/info/rfc4303>.
 [RFC6479]  Zhang, X. and T. Tsou, "IPsec Anti-Replay Algorithm
            without Bit Shifting", RFC 6479, January 2012,
            <http://www.rfc-editor.org/info/rfc6479>.

Acknowledgements

 Special thanks go to Glenn Eguchi, Matthew Kaufman, and Adam Lane for
 their contributions to the design of this profile.
 Thanks to Philipp Hancke, Kevin Igoe, Paul Kyzivat, and Milos
 Trboljevac for their detailed reviews of this memo.

Author's Address

 Michael C. Thornburgh
 Adobe Systems Incorporated
 345 Park Avenue
 San Jose, CA  95110-2704
 United States
 Phone: +1 408 536 6000
 EMail: mthornbu@adobe.com
 URI:   http://www.adobe.com/

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