Network Working Group C. Jennings
Internet-Draft P. Jones
Intended status: Standards Track Cisco Systems
Expires: October 30, 2017 A. Roach
Mozilla
April 28, 2017
SRTP Double Encryption Procedures
draft-ietf-perc-double-04
Abstract
In some conferencing scenarios, it is desirable for an intermediary
to be able to manipulate some RTP parameters, while still providing
strong end-to-end security guarantees. This document defines SRTP
procedures that use two separate but related cryptographic contexts
to provide "hop-by-hop" and "end-to-end" security guarantees. Both
the end-to-end and hop-by-hop cryptographic transforms can utilize an
authenticated encryption with associated data scheme 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
working documents as Internet-Drafts. The list of current Internet-
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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 October 30, 2017.
Copyright Notice
Copyright (c) 2017 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
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Cryptographic Contexts . . . . . . . . . . . . . . . . . . . 3
4. Original Header Block . . . . . . . . . . . . . . . . . . . . 4
5. RTP Operations . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Encrypting a Packet . . . . . . . . . . . . . . . . . . . 6
5.2. Relaying a Packet . . . . . . . . . . . . . . . . . . . . 6
5.3. Decrypting a Packet . . . . . . . . . . . . . . . . . . . 8
6. RTCP Operations . . . . . . . . . . . . . . . . . . . . . . . 9
7. Recommended Inner and Outer Cryptographic Transforms . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9.1. RTP Header Extension . . . . . . . . . . . . . . . . . . 11
9.2. DTLS-SRTP . . . . . . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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 context from the sending endpoint
to the receiving endpoint that can encrypt and authenticate the media
end-to-end while still allowing certain RTP header information to be
changed by the Media Distributor. At the same time, a separate
cryptographic context provides integrity and optional confidentiality
for the media flowing between the Media Distributor and the
endpoints. See the framework document that describes this concept in
more detail in more detail in
[I-D.ietf-perc-private-media-framework].
This specification defines an SRTP transform that uses the AES-GCM
transform [RFC7714] to encrypt an RTP packet for the end-to-end
cryptographic context. The output of this is treated as an RTP
packet and again encrypted with an SRTP transform used in the hop-by-
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hop cryptographic context 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. 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 transform before it is sent. The receiving endpoint
decrypts and checks integrity using the hop-by-hop cryptographic
transform 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 as 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 of if
including the media payload.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Terms used throughout this document include:
o Media Distributor: media distribution device that routes media
from one endpoint to other endpoints
o E2E: end-to-end, meaning the link from one endpoint through one or
more Media Distributors to the endpoint at the other end.
o HBH: hop-by-hop, meaning the link from the endpoint to or from the
Media Distributor.
o OHB: Original Header Block is an RTP header extension that
contains the original values from the RTP header that might have
been changed by a Media Distributor.
3. Cryptographic Contexts
This specification uses two cryptographic contexts: an inner ("end-
to-end") context 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") context 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 encrypted using the hop-by-hop cryptographic context.
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The RECOMMENDED cipher for the hop-by-hop and end-to-end contexts 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 contexts 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) transforms.
o Assign the key and salt values generated for the inner (end-to-
end) transform to the first half of the key and salt for the
double transform.
o Assign the key and salt values for the outer (hop-by-hop)
transform to the second half of the key and salt for the double
transform.
Obviously, if the Media Distributor is to be able to modify header
fields but not decrypt the payload, then it must have cryptographic
context for the outer transform, but not the inner transform. 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") transform is to use
[I-D.ietf-perc-dtls-tunnel].
4. Original Header Block
Any SRTP packet processed following these procedures MAY contain an
Original Header Block (OHB) RTP header extension.
The OHB contains the original values of any modified header fields
and MUST be placed after any already-existing RTP header extensions.
Placement of the OHB after any original header extensions is
important so that the receiving endpoint can properly authenticate
the original packet and any originally included RTP header
extensions. The receiving endpoint will authenticate the original
packet by restoring the modified RTP header field values and header
extensions. It does this by copying the original values from the OHB
and then removing the OHB extension and any other RTP header
extensions that appear after the OHB extension.
The Media Distributor is only permitted to modify the extension (X)
bit, payload type (PT) field, and the RTP sequence number field.
The OHB extension is either one octet in length, two octets in
length, or three octets in length. The length of the OHB indicates
what data is contained in the extension.
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If the OHB is one octet in length, it contains the original PT field
value. In this case, the OHB has this form:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+---------------+
| ID | len=0 |R| PT |
+-+-+-+-+-+-+-+-+---------------+
Note that "R" indicates a reserved bit that MUST be set to zero when
sending a packet and ignored upon receipt. ID is the RTP Header
Extension identifier negotiated in the SDP.
If the OHB is two octets in length, it contains the original RTP
packet sequence number. In this case, the OHB has this form:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-------------------------------+
| ID | len=1 | Sequence Number |
+-+-+-+-+-+-+-+-+-------------------------------+
If the OHB is three octets in length, it contains the original PT
field value and RTP packet sequence number. In this case, the OHB
has this form:
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 6 4 5 6 7 8 9 1
+-+-+-+-+-+-+-+-+---------------+-------------------------------+
| ID | len=2 |R| PT | Sequence Number |
+-+-+-+-+-+-+-+-+---------------+-------------------------------+
If a Media Distributor modifies an original RTP header value, the
Media Distributor MUST include the OHB extension to reflect the
changed value, setting the X bit in the RTP header to 1 if no header
extensions were originally present. If another Media Distributor
along the media path makes additional changes to the RTP header and
any original value is already present in the OHB, the Media
Distributor must extend the OHB by adding the changed value to the
OHB. To properly preserve original RTP header values, a Media
Distributor MUST NOT change a value already present in the OHB
extension.
5. RTP Operations
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5.1. Encrypting a Packet
To encrypt a packet, the endpoint encrypts the packet using the inner
cryptographic context and then encrypts using the outer cryptographic
context. The processes is as follows:
o Form an RTP packet. If there are any header extensions, they MUST
use [RFC5285].
o Apply the inner cryptographic transform to the RTP packet. If
encrypting RTP header extensions end-to-end, then [RFC6904] MUST
be used when encrypting the RTP packet using the inner
cryptographic context.
o If the endpoint wishes to insert header extensions that can be
modified by an Media Distributor, it MUST insert an OHB header
extension at the end of any header extensions protected end-to-end
(if any), then add any Media Distributor-modifiable header
extensions. In other cases, the endpoint SHOULD still insert an
OHB header extension. The OHB MUST replicate the information
found in the RTP header following the application of the inner
cryptographic transform. If not already set, the endpoint MUST
set the X bit in the RTP header to 1 when introducing the OHB
extension.
o Apply the outer cryptographic transform 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 context.
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 does not have a notion of outer or inner
cryptographic contexts. Rather, the Media Distributor has a single
cryptographic context. The cryptographic transform 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 cryptographic context.
In order to modify a packet, the Media Distributor decrypts the
packet, modifies the packet, updates the OHB with any modifications
not already present in the OHB, and re-encrypts the packet using the
cryptographic context used for next hop.
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o Apply the cryptographic transform to the packet. If decrypting
RTP header extensions hop-by-hop, then [RFC6904] MUST be used.
o Change any required parameters
o If a changed RTP header field is not already in the OHB, add it
with its original value to the OHB. A Media Distributor can add
information to the OHB, but MUST NOT change existing information
in the OHB.
o If the Media Distributor resets a parameter to its original value,
it MAY drop it from the OHB as long as there are no other header
extensions following the OHB. Note that this might result in a
decrease in the size of the OHB. It is also possible for the
Media Distributor to remove the OHB entirely if all parameters in
the RTP header are reset to their original values and no other
header extensions follow the OHB. If the OHB is removed and no
other extension is present, the X bit in the RTP header MUST be
set to 0.
o The Media Distributor MUST NOT delete any header extensions before
the OHB, but MAY add, delete, or modify any that follow the OHB.
* If the Media Distributor adds any header extensions, it must
append them and it must maintain the order of the original
header extensions in the [RFC5285] block.
* If the Media Distributor appends header extensions, then it
MUST add the OHB header extension (if not present), even if the
OHB merely replicates the original header field values, and
append the new extensions following the OHB. The OHB serves as
a demarcation point between original RTP header extensions
introduced by the endpoint and those introduced by a Media
Distributor.
o The Media Distributor MAY modify any header extension appearing
after the OHB, but MUST NOT modify header extensions that are
present before the OHB.
o Apply the cryptographic transform to the packet. If the RTP
Sequence Number has been modified, SRTP processing happens as
defined in SRTP and will end up using the new Sequence Number. If
encrypting RTP header extensions hop-by-hop, then [RFC6904] MUST
be used.
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5.3. Decrypting a Packet
To decrypt a packet, the endpoint first decrypts and verifies using
the outer cryptographic context, then uses the OHB to reconstruct the
original packet, which it decrypts and verifies with the inner
cryptographic context.
o Apply the outer cryptographic transform 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 context.
o Form a new synthetic SRTP packet with:
* Header = Received header, with header fields replaced with
values from OHB (if present).
* Insert all header extensions up to the OHB extension, but
exclude the OHB and any header extensions that follow the OHB.
If there are no extensions remaining, then the X bit MUST bet
set to 0. If there are extensions remaining, then the
remaining extensions MUST be padded to the first 32-bit
boundary and the overall length of the header extensions
adjusted accordingly.
* Payload is the encrypted payload from the outer SRTP packet.
o Apply the inner cryptographic transform 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.
If decrypting RTP header extensions end-to-end, then [RFC6904]
MUST be used when decrypting the RTP packet using the inner
cryptographic context.
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.
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The PT and sequence number from the inner SRTP packet can be used for
collection of various statistics.
If any of the following RTP headers extensions are found in the outer
SRTP packet, they MAY be used:
o Mixer-to-client audio level indicators (See [RFC6465])
6. RTCP Operations
Unlike RTP, which is encrypted both hop-by-hop and end-to-end using
two separate cryptographic contexts, RTCP is encrypted using only the
outer (HBH) cryptographic context. The procedures for RTCP
encryption are specified in [RFC3711] and this document introduces no
additional steps.
7. Recommended Inner and Outer Cryptographic Transforms
This specification recommends and defines AES-GCM as both the inner
and outer cryptographic transforms, identified as
DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM and
DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM. These transforms provide
for authenticated encryption and will consume additional processing
time double-encrypting for HBH and E2E. 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 transform)_(outer transform).
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 transforms 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
option of using that for the outer transform. Similarly, if a new
transform was defined that provided only integrity, that would also
be reasonable to use for the HBH as the payload data is already
encrypted by the E2E.
The AES-GCM cryptographic transform introduces an additional 16
octets to the length of the packet. When using AES-GCM for both the
inner and outer cryptographic transforms, the total additional length
is 32 octets. If no other header extensions are present in the
packet and the OHB is introduced, that will consume an additional 8
octets. If other extensions are already present, the OHB will
consume up to 4 additional octets.
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8. Security Considerations
To summarize what is encrypted and authenticated, we will refer to
all the RTP fields and headers created by the sender and before the
pay load as the initial envelope and the RTP payload information with
the media as the payload. Any additional headers added by the Media
Distributor are referred to as the extra envelope. The sender uses
the E2E key to encrypts the payload and authenticate the payload +
initial envelope which using an AEAD cipher results in a slight
longer new payload. Then the sender uses the HBH key to encrypt the
new payload and authenticate the initial envelope and new payload.
The Media Distributor has the HBH key so it can check the
authentication of the received packet across the initial envelope and
payload data but it can't decrypt the payload as it does not have the
E2E key. It can add extra envelope information. It then
authenticates the initial plus extra envelope information plus
payload with a HBH key. This HBH for the outgoing packet is
typically different than the HBH key for the incoming packet.
The receiver can check the authentication of the initial and extra
envelope information. This, along with the OHB, is used to construct
a synthetic packet that is should be identical to one the sender
created and the receiver can check that it is identical and then
decrypt the original payload.
The end result is that if the authentications succeed, the receiver
knows exactly what the original sender sent, as well as exactly which
modifications were made by the Media Distributor.
It is obviously critical that the intermediary has only the outer
transform parameters and not the inner transform parameters. We rely
on an external key management protocol to assure this property.
Modifications by the intermediary result in the recipient getting two
values for changed parameters (original and modified). The recipient
will have to choose which to use; there is risk in using either that
depends on the session setup.
The security properties for both the inner and outer key holders are
the same as the security properties of classic SRTP.
9. IANA Considerations
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9.1. RTP Header Extension
This document defines a new extension URI in the RTP Compact Header
Extensions part of the Real-Time Transport Protocol (RTP) Parameters
registry, according to the following data:
Extension URI: urn:ietf:params:rtp-hdrext:ohb
Description: Original Header Block
Contact: Cullen Jennings <mailto:fluffy@iii.ca>
Reference: RFCXXXX
Note to RFC Editor: Replace RFCXXXX with the RFC number of this
specification.
9.2. DTLS-SRTP
We request IANA to add the following values to defines a DTLS-SRTP
"SRTP Protection Profile" defined in [RFC5764].
+-------+------------------------------------------+-----------+
| Value | Profile | Reference |
+-------+------------------------------------------+-----------+
| {TBD} | DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM | RFCXXXX |
| {TBD} | DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM | RFCXXXX |
+-------+------------------------------------------+-----------+
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:
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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: 32 octets
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: 32 octets
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 (E2E)
transform and the second half is used for the outer (HBH) transform.
10. Acknowledgments
Many thanks to Richard Barnes for sending significant text for this
specification. Thank you for reviews and improvements from David
Benham, Paul Jones, Suhas Nandakumar, Nils Ohlmeier, and Magnus
Westerlund.
11. References
11.1. 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,
<http://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,
<http://www.rfc-editor.org/info/rfc3711>.
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[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
2008, <http://www.rfc-editor.org/info/rfc5285>.
[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,
<http://www.rfc-editor.org/info/rfc5764>.
[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904,
DOI 10.17487/RFC6904, April 2013,
<http://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,
<http://www.rfc-editor.org/info/rfc7714>.
11.2. Informative References
[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-00
(work in progress), March 2017.
[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", draft-ietf-perc-private-media-framework-03
(work in progress), March 2017.
[I-D.ietf-perc-srtp-ekt-diet]
Jennings, C., Mattsson, J., McGrew, D., and D. Wing,
"Encrypted Key Transport for Secure RTP", draft-ietf-perc-
srtp-ekt-diet-03 (work in progress), March 2017.
[RFC6465] Ivov, E., Ed., Marocco, E., Ed., and J. Lennox, "A Real-
time Transport Protocol (RTP) Header Extension for Mixer-
to-Client Audio Level Indication", RFC 6465,
DOI 10.17487/RFC6465, December 2011,
<http://www.rfc-editor.org/info/rfc6465>.
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Authors' Addresses
Cullen Jennings
Cisco Systems
Email: fluffy@iii.ca
Paul E. Jones
Cisco Systems
Email: paulej@packetizer.com
Adam Roach
Mozilla
Email: adam@nostrum.com
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