Internet DRAFT - draft-barnes-atoca-escape
draft-barnes-atoca-escape
ATOCA R. Barnes
Internet-Draft A. Chi
Intended status: Informational BBN Technologies
Expires: April 12, 2013 October 9, 2012
Encoding of Secure Common Alert Protocol Entities (ESCAPE)
draft-barnes-atoca-escape-02.txt
Abstract
Recipients of emergency alerts need to be able to verify that an
alert they receive was issued by an authorized source. The Common
Alerting Protocol (CAP) provides a standard way of encoding alert
information. This document describes a security wrapper for Common
Alerting Protocol objects to allow alerts to be signed by alert
originators.
Please send feedback to the atoca@ietf.org mailing list.
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 April 12, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Provisioning Requirements . . . . . . . . . . . . . . . . 4
1.2. Open Questions . . . . . . . . . . . . . . . . . . . . . . 5
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. ESCAPE Encoding . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Basic MIME encoding . . . . . . . . . . . . . . . . . . . 6
3.2. Alert Tokens . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. S/MIME Encapsulation . . . . . . . . . . . . . . . . . . . 7
3.4. Validity . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Processing Rules . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Alert Originator (Signer) . . . . . . . . . . . . . . . . 9
4.2. Alert Relay (Re-signer) . . . . . . . . . . . . . . . . . 10
4.3. Alert Recipient (Verifier) . . . . . . . . . . . . . . . . 10
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
Emergency alerting is an increasingly important function of
telecommunications networks, allowing authorities to distribute
warnings of impending danger to large numbers of end users in a short
period of time. However, because emergency alerts are such important
messages to users, there is much more potential for abuse of alerting
than other messaging systems. If an attacker can introduce a false
emergency alert, he may be able to cause mass action, such as the
evacuation of a building or city.
Traditionally, the security of alerting systems has been based mainly
on the security of the system by which authorities provide alerts to
broadcast points, and on the link-layer security of broadcast media
that deliver alerts to end users. For example, alerting systems for
cellular networks typically rely on sending alerts over the cellular
control plane, so that only the local carrier can deliver alerts to
an end device. Alerting via broadcast media such as television or
radio is protected by legal limitations on the ability to transmit
above certain power thresholds in portions of spectrum used for
broadcast media (e.g., television stations).
In the context of the Internet, it is impossible to rely on link-
layer security because IP runs over many types of link that have no
analogous access controls. Indeed, alerts may transit multiple
different types of network en route to end devices. For example, if
an alert is delivered to a device that routes between cellular,
Ethernet, and 802.11 interfaces, the router may need to translate an
alert delivered by the cellular control plane into an IP datagram
that can be delivered over Ethernet or 802.11. There is thus a need
for an end-to-end security mechanism for alerts, so that an endpoint
can verify that an alert is authentic even if the alert arrives over
an untrusted interface.
This document describes ESCAPE, a secure container format for the
Common Alerting Protocol (CAP) [CAP]. CAP documents provide
information about an emergency alert; ESCAPE-wrapped CAP documents
also provide security information that can authenticate the
originator of the alert. Using this additional information, end
alert recipients can verify that ESCAPE-wrapped alerts were
originated by entities they trust, and reject false alerts from
untrusted entities.
Note that ESCAPE validation is not a complete security solution for
alerting. ESCAPE validation will authenticate the originator of an
alert; it is up to the device to determine whether the originator is
trusted. In general, this will require that devices be provisioned
with credentials for trusted entities, either via manual provisioning
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(e.g., static keys in device firmware) or by some dynamic
provisioning protocol.
Likewise, ESCAPE validation only proves that an alert came from an
authorized originator, not whether the alert is relevant to the
recipient in a more general sense. For example, the recipient may be
outside the target area of the alert (as described by the "<area>"
element of the CAP document). Alert applicability involves more than
authenticity: it includes location, jurisdiction, and possibly other
locale-specific factors. Other specifications may make additional
requirements on the contents of CAP documents, or require endpoints
to make checks on encoded CAP document beyond the basic validity
check required by this document.
1.1. Provisioning Requirements
For purposes of this document, we assume that potential recipients of
ESCAPE objects can be provisioned with two types of credentials:
public keys and "alert token hashes". A public key for an authority
is used to validate digital signatures by that authority. Alert
tokens provide a faster rough authentication.
An alert token is a single-use binary string that is used as a fast
authentication check, according to the process described in
[RFC2289]. Clients are provisioned with a cryptographic hash that
was produced by multiple iterations of a hash function against a
secret binary string. Although the final hash is published to
recipients through some provisioning process, the sequence of
preimages, including the original secret string, are known only to an
authorized server. These preimages are used in reverse order as the
alert tokens. On receiving an alert token, a client hashes it one or
more times to verify that it is indeed a preimage of the publicly
provisioned hash, or optimally, the immediate preimage of the latest
alert token. (Thus, alongside alert tokens, the provisioning system
must specify the hash function used to verify the alert token as well
as an iteration limit.) The detailed steps for verifying an ESCAPE
object using a collection of provisioned public keys and alert tokens
is described in Section 4.3.
The high-level operation of an ESCAPE-based alerting system can be
summarized as follows:
1. Endpoints are provisioned with public keys and/or alert token
hashes for authorities.
2. When an authority wishes to generate an alert, it includes an
alert token and signs it using its private key (Section 4.1)
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3. The alert is distributed to one or more endpoints.
4. Along the way, an intermediary may add a signature to the object
(Section 4.2).
5. When the alert arrives at an endpoint, the endpoint validates the
signature and alert token (Section 4.3). If both are valid, it
renders the alert to the user.
This document defines procedures for generating, re-signing, and
verifying alerts (steps 2, 4, and 5 above). The mechanisms used for
provisioning trusted credentials (step 1) and for the actual delivery
of alerts (step 3) are beyond the scope of this document.
1.2. Open Questions
Should we always apply GZIP to the entire encoded message? Pro:
Slightly smaller message size. Con: Will need to require GZIP for
all messages or add content transfer encoding indication.
Should we allow DER-encoded CAP as well as XML-encoded CAP? Pro:
Smaller message size. Con: Clients need to support two encodings,
although MIME content indication allows simple switching.
Should we constrain crypto algorithms? Pro: Marginally simpler
implementation. Con: Need to maintain a list of supported
algorithms. Could also do this in another document.
Should we require a public-key signature, or allow reliance on token
checks alone? Pro: Enables cases with no public-key operations.
Con: Risk of replay attacks using old tokens.
Should we move the Alert-Token field from the inner (signed) MIME
entity to the outer (unsigned) MIME entity? Pro: Allows relays to
add tokens. Con: Allows relays and attackers to remove or change
tokens.
Should we use JOSE instead of S/MIME? Pro: Simpler to implement;
might make it easier to have multiple, detached signatures with their
own alert tokens. Con: Less mature, fewer libraries.
2. Definitions
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 RFC 2119 [RFC2119].
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3. ESCAPE Encoding
The ESCAPE format encapsulates a CAP document as an S/MIME object
[RFC5751]. First, the CAP document is encoded as a MIME entity
[RFC2045], then the MIME entity is signed using S/MIME.
3.1. Basic MIME encoding
CAP XML documents have MIME type "application/cap+xml". An alert
originator may choose to apply the gzip compression scheme to the
alert before sending it. If the alert is compressed, the originator
must encode the compressed alert using the base64 encoding scheme
before transmitting it [RFC1952][RFC4648]. A CAP MIME body thus has
the following properties:
o A "Content-Type" header field MUST be present, with the single
value "application/cap+xml".
o A "Content-Encoding" header field MAY be present. If present, it
MUST have the value "gzip", and there MUST be a "Content-Transfer-
Encoding" header with the value "base64".
o The body of the MIME entity MUST be a CAP document, either
directly, or processed through the gzip and base64 encodings.
3.2. Alert Tokens
ESCAPE introduces a new MIME header, Alert-Token, which provides a
rough form of authentication. If alert recipients are configured
with an algorithm for checking the validity of a token, along with
the appropriate authentication material, the recipient of an alert
message can check the validity of the value in the Alert-Token field
before performing full S/MIME validation on the ESCAPE object. The
Alert-Token field contains a single opaque binary string, encoded in
base64. The ABNF syntax ([RFC5234]) for the field is as follows,
where "base64" is as defined in RFC 4566 [RFC4566]. (Here we also
follow the usual conventions with regard to whitespace in MIME
headers.)
Alert-Token = "Alert-Token" ":" base64
An ESCAPE MIME entity MAY contain one or more Alert-Token header
fields. Any header fields other than "Content-Type", "Content-
Encoding", "Content-Transfer-Encoding", and "Alert-Token" MAY be
ignored by alert recipients.
The Alert-Token authentication system implements an iterative hash-
preimage scheme based on the same technique used by Lamport's One-
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Time Password system [RFC2289]. Such a scheme can provide rough
authentication when the communication channel is adequately
integrity-protected (e.g., by the signature on the ESCAPE object).
The Alert Originator, such as a government authority, first generates
a sequence of one-time-use alert tokens, which are initially kept
secret, known only to the Alert Originator. To do so, the Alert
Originator chooses an iteration limit (n), and computes a hash chain
starting from a secret binary string (s), and iteratively applies a
cryptographic hash function (h) up to n times:
s, h(s), h^2(s), ... , h^(n-1)(s), h^n(s)
The final hash, h^n(s), is publicly provisioned (out of band) to the
clients, along with the iteration limit, n. All previous elements of
the sequence are initially kept secret by the authority for use as
alert tokens. Note that once a token in the above sequence is
public, so are all subsequent tokens. Each independent authority
computes its own sequence of tokens, so each authority provisions one
ordered pair (final_hash, iteration_limit) to the recipients.
When sending an emergency alert, the Alert Originator inserts an
alert token in an Alert-Token header in the ESCAPE message and
transmits the alert. The value of this token will generally be the
immediate hash-preimage of the previous alert token. An Alert
Recipient device receiving the message verifies the Alert-Token by
iteratively computing the hash value of the alert token until it
matches the most recent public hash, or until the iteration limit is
reached. If a match is found, then the alert token is considered
valid. Otherwise, the alert token is considered invalid.
To diminish the chances of replay, when the Alert Recipient
encounters a valid Alert-Token, it MUST update its record of the
"most recent public hash" and decrement the iteration limit. Replays
of the same token MUST be rejected.
3.3. S/MIME Encapsulation
After a CAP message has been encoded into a MIME entity, an S/MIME
signature is applied, following the S/MIME procedures for
constructing a signed message of type "multipart/signed" (Section 3.4
of RFC 5751 [RFC5751]). The following constraints apply to the
S/MIME encoding used in ESCAPE messages.
o The ESCAPE message MUST have type "multipart/signed".
o If the ESCAPE message contains header fields other than "Content-
Type", then they MAY be ignored.
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o The S/MIME body part SHOULD NOT include certificates for signers,
in order to minimize message size.
o The S/MIME body part SHOULD identify signers by subject key
identifier (SKI) rather than by subject name, in order to allow
for both certified and uncertified public keys [RFC5652]. If the
SKI is used to identify signers, then it MUST be equal to the 160-
bit SHA-1 hash of the value of the BIT STRING subjectPublicKey as
specified in Section 4.2.1.2 of [RFC5280].
Except the constraints above, software to verify ESCAPE alerts MUST
include full S/MIME support, including all defined cryptographic
algorithms [RFC3370][RFC5754]. Implementations MAY include
additional algorithms, but alert signers SHOULD NOT sign alerts with
non-standard algorithms, since some recipients may not be able to
process them.
3.4. Validity
An ESCAPE object is valid if and only if all of the following
conditions are true:
o If the verifying entity is configured with a valid alert token
hash for the cognizant Alert Originator, then there must be at
least one Alert-Token header field containing a valid token.
o If the verifying entity is configured with trusted public keys,
then the S/MIME signature on the object must be valid, and must be
verified using a trusted public key.
An entity verifying an ESCAPE object MUST verify both of these
criteria, but MAY check them in either order and omit further checks
after the object fails one check. In particular, performing the
token check before decoding and verifying the CMS signature may avoid
the work of signature verification.
Note that the alert token mechanism is a much weaker form of
authentication than a public-key signature. For example, a malicious
actor that receives an alert token could use it to send bogus alerts
to entities that had not yet received it. An alert recipient that is
not provisioned with trusted public keys should thus treat alerts
verified only by an alert token as suspect (e.g., by providing
warnings in a user interface). A verifying entity SHOULD NOT accept
ESCAPE objects if it is configured with neither trusted public keys
nor valid tokens.
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4. Processing Rules
There are three main phases in the life-cycle of an ESCAPE object.
First, it is created and signed by an alert originator. Second, it
may pass through an alert relay that adds a signature under its key.
Finally, it is received and verified by an end recipient. This
section describes the steps that each type of entity follows to sign,
re-sign or verify an ESCAPE object.
4.1. Alert Originator (Signer)
Inputs:
o CAP document
o Private key for signing, and corresponding subject key identifier
o gzip flag
o Token hash sequence (possibly empty), with the index of the most
recently used token hash
Processing steps:
1. Encode the CAP document as a MIME entity.
A. Add a "Content-Type" header field with value "application/
cap+xml".
B. If the gzip flag is set, add a "Content-Encoding" header
field with value "gzip" and a "Content-Transfer-Encoding"
header field with value "base64".
C. Add an "Alert-Token" header field and insert a previously
unused alert token, from earlier in the hash sequence than
the earliest used token. Update the "earliest used token"
index.
D. If the gzip flag is set, gzip the CAP document, then gzip and
base64-encode the CAP document and set it as the message
body.
E. If the gzip flag is not set, set the CAP document as the
message body.
2. Compute the signature over the MIME entity using the signing key
and create a CMS SignedData structure that identifies the signer
using the corresponding subject key ID.
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3. Combine the CAP MIME entity and the computed CMS SignedData
structure into a "multipart/signed" S/MIME object.
Output: ESCAPE message
4.2. Alert Relay (Re-signer)
Inputs:
o ESCAPE object
o Private key for signing and corresponding subject key ID
Processing steps:
1. Extract the CAP MIME entity and CMS SignedData object from the
ESCAPE message.
2. Compute the signature over the CAP MIME entity using the signing
key.
3. Append the signature value and subject key ID to the CMS
SignedData object as a new SignerInfo.
4. Combine the CAP MIME entity and the computed CMS SignedData
structure into a "multipart/signed" S/MIME object.
Output: ESCAPE message
4.3. Alert Recipient (Verifier)
Inputs:
o ESCAPE object
o Set of trusted public keys (possibly empty)
o Set of trusted alert token hashes, with corresponding iteration
limits (possibly empty)
Processing steps:
1. Extract the CAP MIME entity and the CMS SignedData object from
the ESCAPE message.
2. Check that the MIME headers for the CAP MIME entity have the
correct values.
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* Content-Type must be "application/cap+xml"
* Content-Encoding must be "gzip" or absent
* Content-Transfer-Encoding must be present and equal to
"base64" if Content-Encoding is present, otherwise absent.
If any of these headers are invalid, then return the CAP message
is malformed. Return FALSE.
3. Extract the CAP entity body. If the Content-Encoding is "gzip",
then base64-decode and un-gzip the CAP entity body.
4. If the set of trusted alert token hashes includes a token hash
for the cognizant Alert Originator, then verify that the Alert-
Token values in the CAP MIME entity is a valid token, using the
algorithm described in Section 3.2. If no valid alert token is
provided, then return FALSE.
5. If the set of trusted public keys is non-empty, then verify that
at least one of the SignerInfos within the CMS SignedData object
contains a valid signature under a trusted key. If no valid,
trusted signature is found, then return FALSE.
6. Return TRUE.
Output: Verification status
5. Examples
Consider the following CAP message:
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<?xml version = "1.0" encoding = "UTF-8"?>
<alert xmlns = "urn:oasis:names:tc:emergency:cap:1.1">
<identifier>43b080713727</identifier>
<sender>hsas@dhs.gov</sender>
<sent>2003-04-02T14:39:01-05:00</sent>
<status>Actual</status>
<msgType>Alert</msgType>
<scope>Public</scope>
<info>
<category>Security</category>
<event>Homeland Security Advisory System Update</event>
<urgency>Immediate</urgency>
<severity>Severe</severity>
<certainty>Likely</certainty>
<senderName>U.S. Government,
Department of Homeland Security</senderName>
<headline>Homeland Security Sets Code ORANGE</headline>
<description>The Department of Homeland Security has
elevated the Homeland Security Advisory System threat level
to ORANGE / High in response to intelligence which may
indicate a heightened threat of terrorism.</description>
<instruction> A High Condition is declared when there is a
high risk of terrorist attacks. In addition to the
Protective Measures taken in the previous Threat Conditions,
Federal departments and agencies should consider agency-
specific Protective Measures in accordance with their
existing plans.</instruction>
<web>http://www.dhs.gov/dhspublic/display?theme=29</web>
<parameter>
<valueName>HSAS</valueName>
<value>ORANGE</value>
</parameter>
<resource>
<resourceDesc>Image file (GIF)</resourceDesc>
<uri>http://www.dhs.gov/dhspublic/getAdvisoryImage</uri>
</resource>
<area>
<areaDesc>U.S. nationwide and interests worldwide</areaDesc>
</area>
</info>
</alert>
Suppose an alert signer has the following RSA key pair, encoded as a
PEM-encoded private key and self-signed certificate [RFC1421]:
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-----BEGIN PRIVATE KEY-----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-----END PRIVATE KEY-----
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
Then if the signer signs the alert with the above private key and the
token "foobar", he will create the following ESCAPE message:
Content-Type: multipart/signed;
protocol="application/pkcs7-signature";
micalg="sha1";
boundary="----C16CFF6F1CB606631B8BBD4B5B43051F"
------C16CFF6F1CB606631B8BBD4B5B43051F
Alert-Token: asdfasdfasdf
Content-Type: application/cap+xml
<?xml version = "1.0" encoding = "UTF-8"?>
<alert xmlns = "urn:oasis:names:tc:emergency:cap:1.1">
<identifier>43b080713727</identifier>
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<sender>hsas@dhs.gov</sender>
<sent>2003-04-02T14:39:01-05:00</sent>
<status>Actual</status>
<msgType>Alert</msgType>
<scope>Public</scope>
<info>
<category>Security</category>
<event>Homeland Security Advisory System Update</event>
<urgency>Immediate</urgency>
<severity>Severe</severity>
<certainty>Likely</certainty>
<senderName>U.S. Government,
Department of Homeland Security</senderName>
<headline>Homeland Security Sets Code ORANGE</headline>
<description>The Department of Homeland Security has
elevated the Homeland Security Advisory System threat level
to ORANGE / High in response to intelligence which may
indicate a heightened threat of terrorism.</description>
<instruction> A High Condition is declared when there is a
high risk of terrorist attacks. In addition to the
Protective Measures taken in the previous Threat Conditions,
Federal departments and agencies should consider agency-
specific Protective Measures in accordance with their
existing plans.</instruction>
<web>http://www.dhs.gov/dhspublic/display?theme=29</web>
<parameter>
<valueName>HSAS</valueName>
<value>ORANGE</value>
</parameter>
<resource>
<resourceDesc>Image file (GIF)</resourceDesc>
<uri>http://www.dhs.gov/dhspublic/getAdvisoryImage</uri>
</resource>
<area>
<areaDesc>U.S. nationwide and interests worldwide</areaDesc>
</area>
</info>
</alert>
------C16CFF6F1CB606631B8BBD4B5B43051F
Content-Type: application/pkcs7-signature; name="smime.p7s"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="smime.p7s"
MIIBxQYJKoZIhvcNAQcCoIIBtjCCAbICAQMxCTAHBgUrDgMCGjALBgkqhkiG9w0B
BwExggGTMIIBjwIBA4AUKcxEepHj4Yr7+WDTS28DWxXcIvMwBwYFKw4DAhqggdgw
GAYJKoZIhvcNAQkDMQsGCSqGSIb3DQEHATAcBgkqhkiG9w0BCQUxDxcNMTExMDA0
MTUzMzM4WjAjBgkqhkiG9w0BCQQxFgQUG0dU/Z+LJg/29/4nvzkou4Bion4weQYJ
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Internet-Draft ESCAPE October 2012
KoZIhvcNAQkPMWwwajALBglghkgBZQMEASowCwYJYIZIAWUDBAEWMAsGCWCGSAFl
AwQBAjAKBggqhkiG9w0DBzAOBggqhkiG9w0DAgICAIAwDQYIKoZIhvcNAwICAUAw
BwYFKw4DAgcwDQYIKoZIhvcNAwICASgwDQYJKoZIhvcNAQEBBQAEgYBDIjpmJ2uP
nbFJqb35p7dGKdoWyh0Q0LUKr9SxOWkmvK9K6AB/Bodzlo1U5hGVqX10p7HqUWW9
SMt3DXB8sxSbEOrD0HUsdsQvmoulfWNAX5ZphS7jvy1LeR9qrYp8zyzUd1bWSOZA
kQKwpg6PRyVYArqG8uAD00CW0elL34WKLQ==
------C16CFF6F1CB606631B8BBD4B5B43051F--
If the signer also applies the GZIP encoding and attaches the token,
he will create the following ESCAPE message:
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Content-Type: multipart/signed;
protocol="application/pkcs7-signature";
micalg="sha1";
boundary="----C6A0932DF53B0609D38DC49A7E492DB3"
------C6A0932DF53B0609D38DC49A7E492DB3
Alert-Token: foobar
Content-Type: application/cap+xml
Content-Transfer-Encoding: base64
Content-Encoding: gzip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------C6A0932DF53B0609D38DC49A7E492DB3
Content-Type: application/pkcs7-signature; name="smime.p7s"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="smime.p7s"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------C6A0932DF53B0609D38DC49A7E492DB3--
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6. IANA Considerations
This document requires no action by IANA.
7. Security Considerations
This document defines a secure alert format that allows alert
originators to apply S/MIME digital signatures to a CAP alerts
[RFC5751], and to enclose an additional rough authenticator based on
a one-time password scheme [RFC2289]. The security considerations
discussed in the specifications for those security mechanisms apply
here as well.
This document does not address the question of which signers or alert
tokens should be accepted as authorized alert originators. There is
a need for some out of band process for provisioning public keys and
alert token hashes to potential alert recipients. Obviously, if that
process can be exploited to cause alert recipients to trust an
unauthorized public key, then affected recipients will be at risk of
accepting inappropriate alerts under that public key (assuming the
attacker can deliver the alert to the recipient). The risk is lower
with regard to alert token hashes, because they are only used as a
rough check to avoid signature verification on obviously bogus
alerts. If an attacker can cause only unauthorized alert hashes to
be provisioned as trusted, and not unauthorized public keys, then he
will only be able to waste resources on recipient devices by forcing
them to verify bogus signatures.
Finally, a note on the choice of security technology. The CAP
specification does provide for alert signing, using XML-DSig. In
this document, we use S/MIME as a simpler mechanism for signing.
Because S/MIME signs over a serialization of an XML document rather
than the logical structure of the document, it does not require XML
canonicalization (as XML-DSig does). Using S/MIME also means that
ESCAPE can accommodate alerts that are not encoded in XML, such as
DER-encoded CAP alerts, both because the signature computation is
agnostic to the format of the signed content and because MIME
provides content type indication.
8. Acknowledgements
[TODO]
9. References
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9.1. Normative References
[CAP] Botterell, A. and E. Jones, "Common Alerting Protocol
v1.1", October 2005.
[RFC1421] Linn, J., "Privacy Enhancement for Internet Electronic
Mail: Part I: Message Encryption and Authentication
Procedures", RFC 1421, February 1993.
[RFC1952] Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G.
Randers-Pehrson, "GZIP file format specification version
4.3", RFC 1952, May 1996.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2289] Haller, N., Metz, C., Nesser, P., and M. Straw, "A One-
Time Password System", RFC 2289, February 1998.
[RFC3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
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Internet-Draft ESCAPE October 2012
Message Syntax", RFC 5754, January 2010.
9.2. Informative References
[I-D.ietf-atoca-requirements]
Schulzrinne, H., Norreys, S., Rosen, B., and H.
Tschofenig, "Requirements, Terminology and Framework for
Exigent Communications", draft-ietf-atoca-requirements-03
(work in progress), March 2012.
Authors' Addresses
Richard Barnes
BBN Technologies
9861 Broken Land Parkway
Columbia, MD 21046
US
Phone: +1 410 290 6169
Andrew Chi
BBN Technologies
10 Moulton St
Cambridge, MA 02138
US
Phone: +1 617 873 2574
Barnes & Chi Expires April 12, 2013 [Page 19]
