Original text:
Media Distributor.  The Media Distributor decrypts and checks integrity of the hop-by-hop security.  The Media Distributor MAY change some of the RTP header information that would impact the end- <del> to-end integrity.  The original value of any RTP header field that is changed is included in a new RTP header extension called the Original Header Block.  The new RTP packet is encrypted with the hop-by-hop cryptographic algorithm before it is sent.  The receiving endpoint decrypts and checks integrity using the hop-by-hop cryptographic algorithm and then replaces any parameters the Media Distributor changed using the information in the Original Header Block before decrypting and checking the end-to-end integrity. </del>  One can think of the double as a normal SRTP transform for encrypting the RTP in a way where things that only know half of the key, can <del> decrypt and modify part of the RTP packet but not other parts of if </del>  including the media payload. 2. 3. <del> This specification uses a cryptographic context with two parts: an inner (end-to-end) part that is used by endpoints that originate and consume media to ensure the integrity of media end-to-end, and an outer (hop-by-hop) part that is used between endpoints and Media Distributors to ensure the integrity of media over a single hop and to enable a Media Distributor to modify certain RTP header fields. RTCP is also handled using the hop-by-hop cryptographic part.  The RECOMMENDED cipher for the hop-by-hop and end-to-end algorithm is </del>  AES-GCM.  Other combinations of SRTP ciphers that support the procedures in this document can be added to the IANA registry. 4. The Original Header Block (OHB) contains the original values of any <del> modified header fields.  In the encryption process, the OHB is </del>  appended to the RTP payload.  In the decryption process, the receiving endpoint uses it to reconstruct the original RTP header, so that it can pass the proper AAD value to the inner transform. OHB cannot reflect their original values, the receiver will be unable to verify the E2E integrity of the packet. <del> The OHB has the following syntax (in ABNF): </del>  If present, the PT and SEQ parts of the OHB contain the original payload type and sequence number fields, respectively.  The final 5.2. The Media Distributor has the part of the key for the outer (hop-by- <del> hop), but it does not have the part of the key for the (end-to-end) cryptographic algorithm.  The cryptographic algorithm and key used to decrypt a packet and any encrypted RTP header extensions would be the same as those used in the endpoint's outer algorithm and key. </del>  In order to modify a packet, the Media Distributor decrypts the <del> packet, modifies the packet, updates the OHB with any modifications not already present in the OHB, and re-encrypts the packet using the cryptographic using the outer (hop-by-hop) key. </del>  5.3. 6. Unlike RTP, which is encrypted both hop-by-hop and end-to-end using <del> two separate cryptographic key, RTCP is encrypted using only the </del>  outer (hop-by-hop) cryptographic key.  The procedures for RTCP encryption are specified in RFC3711 and this document introduces no additional steps. There are some RTP related extensions that need special consideration to be used by a relay when using the double transform due to the end- to-end protection of the RTP.  The repair mechanism, when used with <del> double, typically operate on the double encrypted data then take the results of theses operations and encrypted them using only the HBH key.  This results in three cryptography operation happening to the repair data sent over the wire. </del>  7.1. When using RTX RFC4588 with double, the cached payloads MUST be the encrypted packets with the bits that are sent over the wire to the other side.  When encrypting a retransmission packet, it MUST be <del> encrypted in packet repair mode. </del>  A typical RTX receiver would decrypt the packet, undo the RTX <del> transformation, then process the resulting packet using the normally by using the steps in decrypt. </del>  7.2. When using RED RFC2198 with double, the primary encoding MAY contain RTP header extensions and CSRC identifiers but non primary encodings <del> can not. </del>  <del> The sender takes encrypted payloads from the cached packets to form </del>  the RED payload.  Any header extensions from the primary encoding are <del> copied to the RTP packet that will cary the RED payload and the other RTP header information such as SSRC, SEQ, CSRC, etc are set to the same as the primary payload.  The RED RTP packet is then encrypted in repair mode and sent. The receiver decrypts the payload to find the RED payload.  Note a media relay can do this decryption as the packet was sent in repair mode that only needs the hop-by-hop key.  The RTP headers and header extensions along with the primary payload and PT from inside the RED payload are used to form the encrypted primary RTP packet which can then be decrypted with double.  The RTP headers (but not header extensions or CSRC) along with PT from inside the RED payload are used for from the non primary payloads.  The time offset information in the RED data MUST be used to adjust the sequence number in the RTP header by using the timestamp offset and packet rate to find a sequence number offset to adjust by.  At this point the non primary </del>  packets can be decrypted with double. Note that Flex FEC I-D.ietf-payload-flexible-fec-scheme is a superset 7.4. When DTMF is sent with RFC4733, it is end-to-end encrypted and the <del> relay can not read it so it can not be used to control the relay. </del>  Other out of band methods to control the relay need to be used instead. While this document only defines a profile based on AES-GCM, it is possible for future documents to define further profiles with <del> different inner and outer crypto in this same framework.  For </del>  example, if a new SRTP transform was defined that encrypts some or all of the RTP header, it would be reasonable for systems to have the option of using that for the outer algorithm.  Similarly, if a new To summarize what is encrypted and authenticated, we will refer to all the RTP fields except headers created by the sender and before <del> the pay load as the initial envelope and the RTP payload information </del>  with the media as the payload.  Any additional headers added by the sender or Media Distributor are referred to as the extra envelope. <del> The sender uses the end-to-end key to encrypts the payload and authenticate the payload + initial envelope which using an AEAD </del>  cipher results in a slight longer new payload.  Then the sender uses the hop-by-hop key to encrypt the new payload and authenticate the <del> initial envelope extra envelope and the new payload. </del>  The Media Distributor has the hop-by-hop key so it can check the authentication of the received packet across the initial envelope, extra envelope and payload data but it can't decrypt the payload as it does not have the end-to-end key.  It can add or change extra envelope information.  It then authenticates the initial plus extra <del> envelope information plus payload with a hop-by-hop key.  This hop- by-hop for the outgoing packet is typically different than the hop- </del>  by-hop key for the incoming packet. The receiver can check the authentication of the initial and extra envelope information from the Media Distributor.  This, along with <del> the OHB, is used to construct a synthetic packet that is should be identical initial envelope plus payload to one the sender created and the receiver can check that it is identical and then decrypt the original payload. </del>  The end result is that if the authentications succeed, the receiver <del> knows exactly what the payload and initial envelope the sender sent, as well as exactly which modifications were made by the Media Distributor and what extra envelope the Media Distributor send.  The </del>  receive does not know exactly what extra envelope the sender sent. <del> It is obviously critical that the intermediary has only the outer (hop-by-hop) algorithm key and not the half of the key for the the inner (end-to-end) algorithm.  We rely on an external key management protocol to assure this property. </del>  Modifications by the intermediary result in the recipient getting two values for changed parameters (original and modified).  The recipient

Reviewer comments:
NAME added few nits to the commit
Oh, one things I think we should add ref to ABNF

Revised text:
Media Distributor.  The Media Distributor decrypts and checks integrity of the hop-by-hop security.  The Media Distributor MAY change some of the RTP header information that would impact the end-  <ins> to-end integrity.  In that case, the original value of any RTP header field that is changed is included in a new RTP header extension called the Original Header Block.  The new RTP packet is encrypted with the hop-by-hop cryptographic algorithm before it is sent.  The receiving endpoint decrypts and checks integrity using the hop-by-hop cryptographic algorithm and then replaces any parameters the Media Distributor changed using the information in the Original Header Block before decrypting and checking the end-to-end integrity. </ins> One can think of the double as a normal SRTP transform for encrypting the RTP in a way where things that only know half of the key, can  <ins> decrypt and modify part of the RTP packet but not other parts, </ins> including the media payload. 2. 3.  <ins> This specification uses a cryptographic context with two parts: The RECOMMENDED cipher for the hop-by-hop and end-to-end algorithm is </ins> AES-GCM.  Other combinations of SRTP ciphers that support the procedures in this document can be added to the IANA registry. 4. The Original Header Block (OHB) contains the original values of any  <ins> modified RTP header fields.  In the encryption process, the OHB is </ins> appended to the RTP payload.  In the decryption process, the receiving endpoint uses it to reconstruct the original RTP header, so that it can pass the proper AAD value to the inner transform. OHB cannot reflect their original values, the receiver will be unable to verify the E2E integrity of the packet.  <ins> The OHB has the following syntax (in ABNF RFC5234): </ins> If present, the PT and SEQ parts of the OHB contain the original payload type and sequence number fields, respectively.  The final 5.2. The Media Distributor has the part of the key for the outer (hop-by-  <ins> hop) cryptographic algorithm, but it does not have the part of the key for the (end-to-end) cryptographic algorithm.  The cryptographic algorithm and key used to decrypt a packet and any encrypted RTP header extensions would be the same as those used in the endpoint's outer algorithm and key. </ins> In order to modify a packet, the Media Distributor decrypts the  <ins> received packet, modifies the packet, updates the OHB with any modifications not already present in the OHB, and re-encrypts the packet using the the outer (hop-by-hop) cryptographic key before transmitting. </ins> 5.3. 6. Unlike RTP, which is encrypted both hop-by-hop and end-to-end using  <ins> two separate cryptographic keys, RTCP is encrypted using only the </ins> outer (hop-by-hop) cryptographic key.  The procedures for RTCP encryption are specified in RFC3711 and this document introduces no additional steps. There are some RTP related extensions that need special consideration to be used by a relay when using the double transform due to the end- to-end protection of the RTP.  The repair mechanism, when used with  <ins> double, typically operates on the double encrypted data and encrypts them using only the HBH key.  This results in three cryptography operation happening to the repair data sent over the wire. </ins> 7.1. When using RTX RFC4588 with double, the cached payloads MUST be the encrypted packets with the bits that are sent over the wire to the other side.  When encrypting a retransmission packet, it MUST be  <ins> encrypted the packet in repair mode. </ins> A typical RTX receiver would decrypt the packet, undo the RTX  <ins> transformation, then process the resulting packet normally by using the steps in decrypt. </ins> 7.2. When using RED RFC2198 with double, the primary encoding MAY contain RTP header extensions and CSRC identifiers but non primary encodings  <ins> cannot. </ins>  <ins> The sender takes encrypted payload from the cached packets to form </ins> the RED payload.  Any header extensions from the primary encoding are  <ins> copied to the RTP packet that will carry the RED payload and the other RTP header information such as SSRC, SEQ, CSRC, etc are set to the same as the primary payload.  The RED RTP packet is then encrypted in repair mode and sent. The receiver decrypts the payload to find the encrypted RED payload. Note a media relay can do this decryption as the packet was sent in repair mode that only needs the hop-by-hop key.  The RTP headers and header extensions along with the primary payload and PT from inside the RED payload (for the primary encoding) are used to form the encrypted primary RTP packet which can then be decrypted with double. The RTP headers (but not header extensions or CSRC) along with PT from inside the RED payload corresponding to the redundant encoding are used to from the non primary payloads.  The time offset and packet rate information in the RED data MUST be used to adjust the sequence number in the RTP header.  At this point the non primary </ins> packets can be decrypted with double. Note that Flex FEC I-D.ietf-payload-flexible-fec-scheme is a superset 7.4. When DTMF is sent with RFC4733, it is end-to-end encrypted and the  <ins> relay can not read it so it cannot be used to control the relay. </ins> Other out of band methods to control the relay need to be used instead. While this document only defines a profile based on AES-GCM, it is possible for future documents to define further profiles with  <ins> different inner and outer algorithms in this same framework.  For </ins> example, if a new SRTP transform was defined that encrypts some or all of the RTP header, it would be reasonable for systems to have the option of using that for the outer algorithm.  Similarly, if a new To summarize what is encrypted and authenticated, we will refer to all the RTP fields except headers created by the sender and before  <ins> the payload as the initial envelope and the RTP payload information </ins> with the media as the payload.  Any additional headers added by the sender or Media Distributor are referred to as the extra envelope.  <ins> The sender uses the end-to-end key to encrypt the payload and authenticate the payload + initial envelope, which using an AEAD </ins> cipher results in a slight longer new payload.  Then the sender uses the hop-by-hop key to encrypt the new payload and authenticate the  <ins> initial envelope, extra envelope and the new payload. </ins> The Media Distributor has the hop-by-hop key so it can check the authentication of the received packet across the initial envelope, extra envelope and payload data but it can't decrypt the payload as it does not have the end-to-end key.  It can add or change extra envelope information.  It then authenticates the initial plus extra  <ins> envelope information plus payload with a hop-by-hop key.  The hop-by- hop key for the outgoing packet is typically different than the hop- </ins> by-hop key for the incoming packet. The receiver can check the authentication of the initial and extra envelope information from the Media Distributor.  This, along with  <ins> the OHB, is used to construct a synthetic packet which should be identical to the initial envelope plus payload to one the sender created and the receiver can check that it is identical and then decrypt the original payload. </ins> The end result is that if the authentications succeed, the receiver  <ins> knows exactly the payload and initial envelope the sender sent, as well as exactly which modifications were made by the Media Distributor and what extra envelope the Media Distributor sent.  The </ins> receive does not know exactly what extra envelope the sender sent.  <ins> It is obviously critical that the intermediary has access to just the outer (hop-by-hop) algorithm key and not the half of the key for the the inner (end-to-end) algorithm.  We rely on an external key management protocol to ensure this property. </ins> Modifications by the intermediary result in the recipient getting two values for changed parameters (original and modified).  The recipient