Internet DRAFT - draft-jennings-perc-double
draft-jennings-perc-double
Network Working Group C. Jennings
Internet-Draft P. Jones
Intended status: Standards Track Cisco Systems
Expires: September 22, 2016 A. Roach
Mozilla
March 21, 2016
SRTP Double Encryption Procedures
draft-jennings-perc-double-01
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 22, 2016.
Copyright Notice
Copyright (c) 2016 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 . . . . . . . . . . . . . . . . . . . 5
5.2. Modifying a Packet . . . . . . . . . . . . . . . . . . . 6
5.3. Decrypting a Packet . . . . . . . . . . . . . . . . . . . 7
6. RTCP Operations . . . . . . . . . . . . . . . . . . . . . . . 8
7. Recommended Inner and Outer Cryptographic Transforms . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9.1. RTP Header Extension . . . . . . . . . . . . . . . . . . 10
9.2. DTLS-SRTP . . . . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
Cloud conferencing systems that are based on switched conferencing
have a central media distribution device (MDD) 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 MDD. At the same time, a separate
cryptographic context provides integrity and optional confidentiality
for the media flowing between the MDD and the endpoints. See the
framework document that describes this concept in more detail in more
detail in [I-D.jones-perc-private-media-framework].
This specification RECOMMENDS the SRTP 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-hop cryptographic context
between the endpoint and the MDD. The MDD decrypts and checks
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integrity of the hop-by-hop security. The MDD 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 MDD changed using the
information in the Original Header Block before decrypting and
checking the end-to-end integrity.
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 MDD: 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 MDDs to the endpoint at the other end.
o HBH: hop-by-hop, meaning the link from the endpoint to or from the
MDD.
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 an MDD.
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 MDDs to
ensure the integrity of media over a single hop and to enable an MDD
to modify certain RTP header fields. RTCP is also encrypted using
the hop-by-hop cryptographic context. 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:
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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.
o Assign the key and salt values for the outer (hop-by-hop)
transform.
As a special case, the outer cryptographic transform MAY be the NULL
cipher (see [RFC3711]) if a secure transport such as [RFC6347] is
used over a hop (i.e., between an endpoint and MDD or between two
MDDs). In that case, the key and salt values generated would be the
length required only for the inner cryptographic transform.
Obviously, if the MDD 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 MDD 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.jones-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 MDD 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.
If the OHB is one octet in length, it contains both the original X
bit and PT field value. In this case, the OHB has this form:
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0
0 1 2 3 4 5 6 7
+---------------+
|X| PT |
+---------------+
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
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-------------------------------+
| Sequence Number |
+-------------------------------+
If the OHB is three octets in length, it contains the original X bit,
PT field value, and 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
+---------------+-------------------------------+
|X| PT | Sequence Number |
+---------------+-------------------------------+
If an MDD modifies an original RTP header value, the MDD MUST include
the OHB extension to reflect the changed value. If another MDD along
the media path makes additional changes to the RTP header and any
original value is not already present in the OHB, the MDD must extend
the OHB by adding the changed value to the OHB. To properly preserve
original RTP header values, an MDD MUST NOT change a value already
present in the OHB extension.
5. RTP Operations
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.
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o If the endpoint wishes to insert header extensions that can be
modified by an MDD, it MUST insert an OHB header extension at the
end of any header extensions protected end-to-end, then add any
MDD-modifiable header extensions. The OHB MUST replicate the
information found in the RTP header following the application of
the inner cryptographic transform. For example, if the packet had
no header extensions when the inner cryptographic transform was
applied, the X bit would be 0. If the endpoint introduces an OHB
and then adds MDD-modifiable header extensions, the X bit in the
OHB would be 0. After introducing the OHB and MDD-modifiable
header extensions, of course, the X bit in the RTP header would be
set to 1.
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.
5.2. Modifying a Packet
The MDD does not have a notion of outer or inner cryptographic
contexts. Rather, the MDD 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 MDD 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.
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. An MDD MAY add information to
the OHB, but MUST NOT change existing information in the OHB.
o If the MDD 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.
o The MDD MUST NOT delete any header extensions before the OHB, but
MAY add, delete, or modify any that follow the OHB.
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* If the MDD 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 MDD 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 an MDD.
o The MDD 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 encrypting
RTP header extensions hop-by-hop, then [RFC6904] MUST be used.
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 the original X bit is 1, then the remaining extensions MUST
be padded to the first 32-bit boundary and the overall length
of the header extensions adjusted accordingly. If the original
X bit is 0, then the header extensions would be removed
entirely.
* Payload is the original encrypted payload.
o Apply the inner cryptographic transform to this synthetic SRTP
packet. If the integrity check does not pass, discard the packet.
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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 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.
If any of the following RTP headers extensions are found in the outer
SRTP packet, they MAY be used:
o TBD
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).
This specification also allows for the NULL cipher to be used as the
outer cryptographic transform in cases where a secure transport is
used over the hop, with those transforms identified as
DOUBLE_AEAD_AES_128_GCM_NULL_NULL and
DOUBLE_AEAD_AES_256_GCM_NULL_NULL.
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Open Issue: It is not clear if the NULL ciphers are needed or not.
The authors plan to remove them from the next version of the draft
unless there is a reasonable support and reasons to keep them in.
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.
8. Security Considerations
It is obviously critical that the intermediary have 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.
The NULL cipher MUST be used in conjunction with an encrypted
transport for both RTP and RTCP. Use of the NULL cipher for the
outer cryptographic context without the use of an encrypted transport
exposes the RTCP traffic to undetectable modification as it is
transmitted over the network. Likewise, RTP traffic under the same
conditions would be subject to modification that would not be
detectable by the MDD. While the endpoint could detect modification
of the end-to-end information, reliance on information like payload
type value in the packet received from the MDD could present problems
such as attempting to decode media with the wrong codec.
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9. IANA Considerations
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, TBD} | DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM | RFCXXXX |
| {TBD, TBD} | DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM | RFCXXXX |
| {TBD, TBD} | DOUBLE_AEAD_AES_128_GCM_NULL_NULL | RFCXXXX |
| {TBD, TBD} | DOUBLE_AEAD_AES_256_GCM_NULL_NULL | 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
DOUBLE_AEAD_AES_128_GCM_NULL_NULL
cipher: AES_128_GCM then identity transform
cipher_key_length: 128 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 16 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_NULL_NULL
cipher: AES_256_GCM then identity transform
cipher_key_length: 256 bits
cipher_salt_length: 96 bits
aead_auth_tag_length: 16 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
Except when the NULL cipher is used for the outer (HBH) transform,
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.
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For those that use the NULL cipher for the outer transform, the the
key and salt is applied only to the inner transform.
10. Acknowledgments
Many thanks to review from GET YOUR NAME HERE. Please, send
comments.
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>.
[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
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[I-D.jones-perc-dtls-tunnel]
Jones, P., "DTLS Tunnel between Media Distribution Device
and Key Management Function to Facilitate Key Exchange",
draft-jones-perc-dtls-tunnel-02 (work in progress),
March 2016.
[I-D.jones-perc-private-media-framework]
Jones, P., Ismail, N., and D. Benham, "A Solution
Framework for Private Media in Privacy Enhanced RTP
Conferencing", draft-jones-perc-private-media-framework-02
(work in progress), March 2016.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
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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