Internet DRAFT - draft-ietf-perc-double
draft-ietf-perc-double
Network Working Group C. Jennings
Internet-Draft P. Jones
Intended status: Standards Track R. Barnes
Expires: March 1, 2020 Cisco Systems
A. Roach
Mozilla
August 29, 2019
SRTP Double Encryption Procedures
draft-ietf-perc-double-12
Abstract
In some conferencing scenarios, it is desirable for an intermediary
to be able to manipulate some parameters in Real Time Protocol (RTP)
packets, while still providing strong end-to-end security guarantees.
This document defines a cryptographic transform for the Secure Real
Time Protocol (SRTP) that uses two separate but related cryptographic
operations to provide hop-by-hop and end-to-end security guarantees.
Both the end-to-end and hop-by-hop cryptographic algorithms can
utilize an authenticated encryption with associated data (AEAD)
algorithm or take advantage of future SRTP transforms with different
properties.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 1, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Cryptographic Context . . . . . . . . . . . . . . . . . . . . 4
3.1. Key Derivation . . . . . . . . . . . . . . . . . . . . . 5
4. Original Header Block . . . . . . . . . . . . . . . . . . . . 5
5. RTP Operations . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Encrypting a Packet . . . . . . . . . . . . . . . . . . . 7
5.2. Relaying a Packet . . . . . . . . . . . . . . . . . . . . 8
5.3. Decrypting a Packet . . . . . . . . . . . . . . . . . . . 9
6. RTCP Operations . . . . . . . . . . . . . . . . . . . . . . . 10
7. Use with Other RTP Mechanisms . . . . . . . . . . . . . . . . 11
7.1. RTP Retransmission (RTX) . . . . . . . . . . . . . . . . 11
7.2. Redundant Audio Data (RED) . . . . . . . . . . . . . . . 11
7.3. Forward Error Correction (FEC) . . . . . . . . . . . . . 12
7.4. DTMF . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8. Recommended Inner and Outer Cryptographic Algorithms . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 13
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
10.1. DTLS-SRTP . . . . . . . . . . . . . . . . . . . . . . . 14
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
12.1. Normative References . . . . . . . . . . . . . . . . . . 15
12.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Encryption Overview . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
Cloud conferencing systems that are based on switched conferencing
have a central Media Distributor device that receives media from
endpoints and distributes it to other endpoints, but does not need to
interpret or change the media content. For these systems, it is
desirable to have one cryptographic key that enables encryption and
authentication of the media end-to-end while still allowing certain
information in the header of a Real Time Protocol (RTP) packet to be
changed by the Media Distributor. At the same time, a separate
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cryptographic key provides integrity and optional confidentiality for
the media flowing between the Media Distributor and the endpoints.
The framework document [I-D.ietf-perc-private-media-framework]
describes this concept in more detail.
This specification defines a transform for the Secure Real Time
Protocol (SRTP) that uses the AES-GCM algorithm [RFC7714] to provide
encryption and integrity for an RTP packet for the end-to-end
cryptographic key as well as a hop-by-hop cryptographic encryption
and integrity between the endpoint and the 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-to-end integrity. In that
case, the original value of any RTP header field that is changed is
included in an "Original Header Block" that is added to the packet.
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.
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
decrypt and modify part of the RTP packet but not other parts,
including the media payload.
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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Terms used throughout this document include:
o Media Distributor: A device that receives media from endpoints and
distributes it to other endpoints, but does not need to interpret
or change the media content (see also
[I-D.ietf-perc-private-media-framework])
o end-to-end: The path from one endpoint through one or more Media
Distributors to the endpoint at the other end.
o hop-by-hop: The path from the endpoint to or from the Media
Distributor.
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o Original Header Block (OHB): An octet string that contains the
original values from the RTP header that might have been changed
by a Media Distributor.
3. Cryptographic Context
This specification uses a cryptographic context with two parts:
o 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
o 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
AES-GCM. Other combinations of SRTP ciphers that support the
procedures in this document can be added to the IANA registry.
The keys and salt for these algorithms are generated with the
following steps:
o Generate key and salt values of the length required for the
combined inner (end-to-end) and outer (hop-by-hop) algorithms.
o Assign the key and salt values generated for the inner (end-to-
end) algorithm to the first half of the key and the first half of
the salt for the double algorithm.
o Assign the key and salt values for the outer (hop-by-hop)
algorithm to the second half of the key and second half of the
salt for the double algorithm. The first half of the key is
referred to as the inner key while the second half is referred to
as the outer key. When a key is used by a cryptographic
algorithm, the salt used is the part of the salt generated with
that key.
o the SSRC is the same for both the inner and out outer algorithms
as it can not be changed.
o The SEQ and ROC are tracked independently for the inner and outer
algorithms.
If the Media Distributor is to be able to modify header fields but
not decrypt the payload, then it must have cryptographic key for the
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outer algorithm, but not the inner (end-to-end) algorithm. This
document does not define how the Media Distributor should be
provisioned with this information. One possible way to provide
keying material for the outer (hop-by-hop) algorithm is to use
[I-D.ietf-perc-dtls-tunnel].
3.1. Key Derivation
Although SRTP uses a single master key to derive keys for an SRTP
session, this transform requires separate inner and outer keys. In
order to allow the inner and outer keys to be managed independently
via the master key, the transforms defined in this document MUST be
used with the following pseudo-random function (PRF), which preserves
the separation between the two halves of the key. Given a positive
integer "n" representing the desired output length, a master key
"k_master", and an input "x":
PRF\_double\_n(k\_master,x) = PRF\_(n/2)(inner(k\_master),x) ||
PRF\_(n/2)(outer(k\_master),x)
Here "PRF_n(k, x)" represents the AES_CM PRF KDF (see Section 4.3.3
of [RFC3711]) for DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM algorithm
and AES_256_CM_PRF KDF [RFC6188] for
DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM algorithm. "inner(key)"
represents the first half of the key, and "outer(key)" represents the
second half of the key.
4. Original Header Block
The Original Header Block (OHB) contains the original values of any
modified RTP header fields. In the encryption process, the OHB is
included in an SRTP packet as described in Section 5. 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.
The OHB can reflect modifications to the following fields in an RTP
header: the payload type, the sequence number, and the marker bit.
All other fields in the RTP header MUST remain unmodified; since the
OHB cannot reflect their original values, the receiver will be unable
to verify the E2E integrity of the packet.
The OHB has the following syntax (in ABNF [RFC5234]):
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OCTET = %x00-FF
PT = OCTET
SEQ = 2OCTET
Config = OCTET
OHB = [ PT ] [ SEQ ] Config
If present, the PT and SEQ parts of the OHB contain the original
payload type and sequence number fields, respectively. The final
"config" octet of the OHB specifies whether these fields are present,
and the original value of the marker bit (if necessary):
+-+-+-+-+-+-+-+-+
|R R R R B M P Q|
+-+-+-+-+-+-+-+-+
o P: PT is present
o Q: SEQ is present
o M: Marker bit is present
o B: Value of marker bit
o R: Reserved, MUST be set to 0
In particular, an all-zero OHB config octet (0x00) indicates that
there have been no modifications from the original header.
If the marker bit is not present (M=0), then B MUST be set to zero.
That is, if "C" represents the value of the config octet, then the
masked value "C & 0x0C" MUST NOT have the value "0x80".
5. RTP Operations
As implied by the use of the word "double" above, this transform
applies AES-GCM to the SRTP packet twice. This allows media
distributors to be able to modify some header fields while allowing
endpoints to verify the end-to-end integrity of a packet.
The first, "inner" application of AES-GCM encrypts the SRTP payload
and integrity-protects a version of the SRTP header with extensions
truncated. Omitting extensions from the inner integrity check means
that they can be modified by a media distributor holding only the
"outer" key.
The second, "outer" application of AES-GCM encrypts the ciphertext
produced by the inner encryption (i.e., the encrypted payload and
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authentication tag), plus an OHB that expresses any changes made
between the inner and outer transforms.
A media distributor that has the outer key but not the inner key may
modify the header fields that can be included in the OHB by
decrypting, modifying, and re-encrypting the packet.
5.1. Encrypting a Packet
To encrypt a packet, the endpoint encrypts the packet using the inner
(end-to-end) cryptographic key and then encrypts using the outer
(hop-by-hop) cryptographic key. The encryption also supports a mode
for repair packets that only does the outer (hop-by-hop) encryption.
The processes is as follows:
1. Form an RTP packet. If there are any header extensions, they
MUST use [RFC8285].
2. If the packet is for repair mode data, skip to step 6.
3. Form a synthetic RTP packet with the following contents:
* Header: The RTP header of the original packet with the
following modifications:
* The X bit is set to zero
* The header is truncated to remove any extensions (i.e., keep
only the first 12 + 4 * CC bytes of the header)
* Payload: The RTP payload of the original packet (including
padding when present)
4. Apply the inner cryptographic algorithm to the synthetic RTP
packet from the previous step.
5. Replace the header of the protected RTP packet with the header of
the original packet (to restore any header extensions and reset
the X bit), and append an empty OHB (0x00) to the encrypted
payload (with the authentication tag) obtained from the step 4.
6. Apply the outer cryptographic algorithm to the RTP packet. If
encrypting RTP header extensions hop-by-hop, then [RFC6904] MUST
be used when encrypting the RTP packet using the outer
cryptographic key.
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When using EKT [I-D.ietf-perc-srtp-ekt-diet], the EKT Field comes
after the SRTP packet exactly like using EKT with any other SRTP
transform.
5.2. Relaying a Packet
The Media Distributor has the part of the key for the outer (hop-by-
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.
In order to modify a packet, the Media Distributor decrypts the
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.
1. Apply the outer (hop-by-hop) cryptographic algorithm to decrypt
the packet. If decrypting RTP header extensions hop-by-hop, then
[RFC6904] MUST be used. Note that the RTP payload produced by
this decryption operation contains the original encrypted payload
with the tag from the inner transform and the OHB appended.
2. Make any desired changes to the fields are allowed to be changed,
i.e., PT, SEQ, and M. The Media Distributor MAY also make
modifications to header extensions, without the need to reflect
these changes in the OHB.
3. Reflect any changes to header fields in the OHB:
* If Media Distributor changed a field that is not already in
the OHB, then it MUST add the original value of the field to
the OHB. Note that this might result in an increase in the
size of the OHB.
* If the Media Distributor took a field that had previously been
modified and reset to its original value, then it SHOULD drop
the corresponding information from the OHB. Note that this
might result in a decrease in the size of the OHB.
* Otherwise, the Media Distributor MUST NOT modify the OHB.
4. Apply the outer (hop-by-hop) cryptographic algorithm to the
packet. If the RTP Sequence Number has been modified, SRTP
processing happens as defined in SRTP and will end up using the
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new Sequence Number. If encrypting RTP header extensions hop-by-
hop, then [RFC6904] MUST be used.
In order to avoid nonce reuse, the cryptographic contexts used in
step 1 and step 5 MUST use different, independent master keys. Note
that this means that the key used for decryption by the MD MUST be
different from the key used for re-encryption to the end recipient.
Note that if multiple MDs modify the same packet, then the first MD
to alter a given header field is the one that adds it to the OHB. If
a subsequent MD changes the value of a header field that has already
been changed, then the original value will already be in the OHB, so
no update to the OHB is required.
A Media Distributor that decrypts, modifies, and re-encrypts packets
in this way MUST use an independent key for each recipient, and MUST
NOT re-encrypt the packet using the sender's keys. If the Media
Distributor decrypts and re-encrypts with the same key and salt, it
will result in the reuse of a (key, nonce) pair, undermining the
security of AES-GCM.
5.3. Decrypting a Packet
To decrypt a packet, the endpoint first decrypts and verifies using
the outer (hop-by-hop) cryptographic key, then uses the OHB to
reconstruct the original packet, which it decrypts and verifies with
the inner (end-to-end) cryptographic key.
1. Apply the outer cryptographic algorithm to the packet. If the
integrity check does not pass, discard the packet. The result of
this is referred to as the outer SRTP packet. If decrypting RTP
header extensions hop-by-hop, then [RFC6904] MUST be used when
decrypting the RTP packet using the outer cryptographic key.
2. If the packet is for repair mode data, skip the rest of the
steps. Note that the packet that results from the repair
algorithm will still have encrypted data that needs to be
decrypted as specified by the repair algorithm sections.
3. Remove the inner authentication tag and the OHB from the end of
the payload of the outer SRTP packet.
4. Form a new synthetic SRTP packet with:
* Header = Received header, with the following modifications:
* Header fields replaced with values from OHB (if any)
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* The X bit is set to zero
* The header is truncated to remove any extensions (i.e., keep
only the first 12 + 4 * CC bytes of the header)
* Payload is the encrypted payload from the outer SRTP packet
(after the inner tag and OHB have been stripped).
* Authentication tag is the inner authentication tag from the
outer SRTP packet.
5. Apply the inner cryptographic algorithm to this synthetic SRTP
packet. Note if the RTP Sequence Number was changed by the Media
Distributor, the synthetic packet has the original Sequence
Number. If the integrity check does not pass, discard the
packet.
Once the packet has been successfully decrypted, the application
needs to be careful about which information it uses to get the
correct behavior. The application MUST use only the information
found in the synthetic SRTP packet and MUST NOT use the other data
that was in the outer SRTP packet with the following exceptions:
o The PT from the outer SRTP packet is used for normal matching to
SDP and codec selection.
o The sequence number from the outer SRTP packet is used for normal
RTP ordering.
The PT and sequence number from the inner SRTP packet can be used for
collection of various statistics.
If the RTP header of the outer packet contains extensions, they MAY
be used. However, because extensions are not protected end-to-end,
implementations SHOULD reject an RTP packet containing headers that
would require end-to-end protection.
6. RTCP Operations
Unlike RTP, which is encrypted both hop-by-hop and end-to-end using
two separate cryptographic keys, RTCP is encrypted using only the
outer (hop-by-hop) cryptographic key. The procedures for RTCP
encryption are specified in [RFC3711] and this document introduces no
additional steps.
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7. Use with Other RTP Mechanisms
Media Distributors sometimes interact with RTP media packets sent by
endpoints, e.g., to provide recovery or receive commands via DTMF.
When media packets are encrypted end-to-end, these procedures require
modification. (End-to-end interactions, including end-to-end
recovery, are not affected by end-to-end encryption.)
Repair mechanisms, in general, will need to perform recovery on
encrypted packets (double-encrypted when using this transform), since
the Media Distributor does not have access to the plaintext of the
packet, only an intermediate, E2E-encrypted form.
When the recovery mechanism calls for the recovery packet itself to
be encrypted, it is encrypted with only the outer, hop-by-hop key.
This allows a media distributor to generate recovery packets without
having access to the inner, end-to-end keys. However, it also
results in recovery packets being triple-encrypted, twice for the
base transform, and once for the recovery protection.
7.1. RTP Retransmission (RTX)
When using RTX [RFC4588] with double, the cached payloads MUST be the
double-encrypted packets, i.e., the bits that are sent over the wire
to the other side. When encrypting a retransmission packet, it MUST
be encrypted the packet in repair mode (i.e., with only the hop-by-
hop key).
If the Media Distributor were to cache the inner, E2E-encrypted
payload and retransmit that with an RTX OSN field prepended, then the
modifications to the payload would cause the inner integrity check to
fail at the receiver.
A typical RTX receiver would decrypt the packet, undo the RTX
transformation, then process the resulting packet normally by using
the steps in Section 5.3.
7.2. Redundant Audio Data (RED)
When using RED [RFC2198] with double, the processing at the sender
and receiver is the same as when using RED with any other SRTP
transform.
The main difference between double and any other transform is that in
an intermediated environment, usage of RED must be end-to-end. A
Media Distributor cannot synthesize RED packets, because it lacks
access to the plaintext media payloads that are combined to form a
RED payload.
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Note that FlexFEC may often provide similar or better repair
capabilities compared to RED. For most applications, FlexFEC is a
better choice than RED; in particular, FlexFEC has modes in which the
Media Distributor can synthesize recovery packets.
7.3. Forward Error Correction (FEC)
When using Flex FEC [I-D.ietf-payload-flexible-fec-scheme] with
double, repair packets MUST be constructed by first double-encrypting
the packet, then performing FEC. Processing of repair packets
proceeds in the opposite order, performing FEC recovery and then
decrypting. This ensures that the original media is not revealed to
the Media Distributor but at the same time allows the Media
Distributor to repair media. When encrypting a packet that contains
the Flex FEC data, which is already encrypted, it MUST be encrypted
with only the outer, hop-by-hop transform.
The algorithm recommended in [I-D.ietf-rtcweb-fec] for repair of
video is Flex FEC [I-D.ietf-payload-flexible-fec-scheme]. Note that
for interoperability with WebRTC, [I-D.ietf-rtcweb-fec] recommends
not using additional FEC only m-line in SDP for the repair packets.
7.4. DTMF
When DTMF is sent using the mechanism in [RFC4733], it is end-to-end
encrypted and the relay can not read it, so it cannot be used to
control the relay. Other out of band methods to control the relay
need to be used instead.
8. Recommended Inner and Outer Cryptographic Algorithms
This specification recommends and defines AES-GCM as both the inner
and outer cryptographic algorithms, identified as
DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM and
DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM. These algorithm provide
for authenticated encryption and will consume additional processing
time double-encrypting for hop-by-hop and end-to-end. However, the
approach is secure and simple, and is thus viewed as an acceptable
trade-off in processing efficiency.
Note that names for the cryptographic transforms are of the form
DOUBLE_(inner algorithm)_(outer algorithm).
While this document only defines a profile based on AES-GCM, it is
possible for future documents to define further profiles with
different inner and outer algorithms in this same framework. For
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
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option of using that for the outer algorithm. Similarly, if a new
transform was defined that provided only integrity, that would also
be reasonable to use for the outer transform as the payload data is
already encrypted by the inner transform.
The AES-GCM cryptographic algorithm introduces an additional 16
octets to the length of the packet. When using AES-GCM for both the
inner and outer cryptographic algorithms, the total additional length
is 32 octets. The OHB will consume an additional 1-4 octets.
Packets in repair mode will carry additional repair data, further
increasing their size.
9. Security Considerations
This SRTP transform provides protection against two classes of
attacker: An network attacker that knows neither the inner nor outer
keys, and a malicious MD that knows the outer key. Obviously, it
provides no protections against an attacker that holds both the inner
and outer keys.
The protections with regard to the network are the same as with the
normal SRTP AES-GCM transforms. The major difference is that the
double transforms are designed to work better in a group context. In
such contexts, it is important to note that because these transforms
are symmetric, they do not protect against attacks within the group.
Any member of the group can generate valid SRTP packets for any SSRC
in use by the group.
With regard to a malicious MD, the recipient can verify the integrity
of the base header fields and confidentiality and integrity of the
payload. The recipient has no assurance, however, of the integrity
of the header extensions in the packet.
The main innovation of this transform relative to other SRTP
transforms is that it allows a partly-trusted MD to decrypt, modify,
and re-encrypt a packet. When this is done, the cryptographic
contexts used for decryption and re-encryption MUST use different,
independent master keys. If the same context is used, the nonce
formation rules for SRTP will cause the same key and nonce to be used
with two different plaintexts, which substantially degrades the
security of AES-GCM.
In other words, from the perspective of the MD, re-encrypting packets
using this protocol will involve the same cryptographic operations as
if it had established independent AES-GCM crypto contexts with the
sender and the receiver. If the MD doesn't modify any header fields,
then an MD that supports AES-GCM could be unused unmodified.
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10. IANA Considerations
10.1. DTLS-SRTP
We request IANA to add the following values to defines a DTLS-SRTP
"SRTP Protection Profile" defined in [RFC5764].
+------------+------------------------------------------+-----------+
| Value | Profile | Reference |
+------------+------------------------------------------+-----------+
| {0x00, | DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM | RFCXXXX |
| 0x09} | | |
| {0x00, | DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM | RFCXXXX |
| 0x0A} | | |
+------------+------------------------------------------+-----------+
Note to IANA: Please assign value RFCXXXX and update table to point
at this RFC for these values.
The SRTP transform parameters for each of these protection are:
DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM
cipher: AES_128_GCM then AES_128_GCM
cipher_key_length: 256 bits
cipher_salt_length: 192 bits
aead_auth_tag_length: 256 bits
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM
cipher: AES_256_GCM then AES_256_GCM
cipher_key_length: 512 bits
cipher_salt_length: 192 bits
aead_auth_tag_length: 256 bits
auth_function: NULL
auth_key_length: N/A
auth_tag_length: N/A
maximum lifetime: at most 2^31 SRTCP packets and
at most 2^48 SRTP packets
The first half of the key and salt is used for the inner (end-to-end)
algorithm and the second half is used for the outer (hop-by-hop)
algorithm.
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11. Acknowledgments
Thank you for reviews and improvements to this specification from
Alex Gouaillard, David Benham, Magnus Westerlund, Nils Ohlmeier, Paul
Jones, Roni Even, and Suhas Nandakumar. In addition, thank you to
Sergio Garcia Murillo proposed the change of transporting the OHB
information in the RTP payload instead of the RTP header.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764,
DOI 10.17487/RFC5764, May 2010,
<https://www.rfc-editor.org/info/rfc5764>.
[RFC6188] McGrew, D., "The Use of AES-192 and AES-256 in Secure
RTP", RFC 6188, DOI 10.17487/RFC6188, March 2011,
<https://www.rfc-editor.org/info/rfc6188>.
[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904,
DOI 10.17487/RFC6904, April 2013,
<https://www.rfc-editor.org/info/rfc6904>.
[RFC7714] McGrew, D. and K. Igoe, "AES-GCM Authenticated Encryption
in the Secure Real-time Transport Protocol (SRTP)",
RFC 7714, DOI 10.17487/RFC7714, December 2015,
<https://www.rfc-editor.org/info/rfc7714>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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[RFC8285] Singer, D., Desineni, H., and R. Even, Ed., "A General
Mechanism for RTP Header Extensions", RFC 8285,
DOI 10.17487/RFC8285, October 2017,
<https://www.rfc-editor.org/info/rfc8285>.
12.2. Informative References
[I-D.ietf-payload-flexible-fec-scheme]
Zanaty, M., Singh, V., Begen, A., and G. Mandyam, "RTP
Payload Format for Flexible Forward Error Correction
(FEC)", draft-ietf-payload-flexible-fec-scheme-20 (work in
progress), May 2019.
[I-D.ietf-perc-dtls-tunnel]
Jones, P., Ellenbogen, P., and N. Ohlmeier, "DTLS Tunnel
between a Media Distributor and Key Distributor to
Facilitate Key Exchange", draft-ietf-perc-dtls-tunnel-05
(work in progress), April 2019.
[I-D.ietf-perc-private-media-framework]
Jones, P., Benham, D., and C. Groves, "A Solution
Framework for Private Media in Privacy Enhanced RTP
Conferencing (PERC)", draft-ietf-perc-private-media-
framework-12 (work in progress), June 2019.
[I-D.ietf-perc-srtp-ekt-diet]
Jennings, C., Mattsson, J., McGrew, D., Wing, D., and F.
Andreasen, "Encrypted Key Transport for DTLS and Secure
RTP", draft-ietf-perc-srtp-ekt-diet-10 (work in progress),
July 2019.
[I-D.ietf-rtcweb-fec]
Uberti, J., "WebRTC Forward Error Correction
Requirements", draft-ietf-rtcweb-fec-10 (work in
progress), July 2019.
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
DOI 10.17487/RFC2198, September 1997,
<https://www.rfc-editor.org/info/rfc2198>.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
DOI 10.17487/RFC4588, July 2006,
<https://www.rfc-editor.org/info/rfc4588>.
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[RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
Digits, Telephony Tones, and Telephony Signals", RFC 4733,
DOI 10.17487/RFC4733, December 2006,
<https://www.rfc-editor.org/info/rfc4733>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
Appendix A. Encryption Overview
The following figure shows a double encrypted SRTP packet. The sides
indicate the parts of the packet that are encrypted and authenticated
by the hop-by-hop and end-to-end operations.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<++
|V=2|P|X| CC |M| PT | sequence number | IO
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IO
| timestamp | IO
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IO
| synchronization source (SSRC) identifier | IO
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ IO
| contributing source (CSRC) identifiers | IO
| .... | IO
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
| RTP extension (OPTIONAL) ... | |O
+>+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
O I | payload ... | IO
O I | +-------------------------------+ IO
O I | | RTP padding | RTP pad count | IO
O +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
O | | E2E authentication tag | |O
O | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |O
O | | OHB ... | |O
+>| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |+
| | | HBH authentication tag | ||
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ||
| | ||
| +- E2E Encrypted Portion E2E Authenticated Portion ---+|
| |
+--- HBH Encrypted Portion HBH Authenticated Portion ----+
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Authors' Addresses
Cullen Jennings
Cisco Systems
Email: fluffy@iii.ca
Paul E. Jones
Cisco Systems
Email: paulej@packetizer.com
Richard Barnes
Cisco Systems
Email: rlb@ipv.sx
Adam Roach
Mozilla
Email: adam@nostrum.com
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