CDNI K. Leung
Internet-Draft F. Le Faucheur
Intended status: Standards Track Cisco Systems
Expires: December 3, 2015 R. van Brandenburg
TNO
B. Downey
Verizon Labs
M. Fisher
Limelight Networks
June 1, 2015
URI Signing for CDN Interconnection (CDNI)
draft-ietf-cdni-uri-signing-04
Abstract
This document describes how the concept of URI signing supports the
content access control requirements of CDNI and proposes a URI
signing scheme.
The proposed URI signing method specifies the information needed to
be included in the URI and the algorithm used to authorize and to
validate access requests for the content referenced by the URI. The
algorithm includes specific provisions for allowing access control of
HTTP Adaptive Streaming content that is characterized by independent
chunks referenced by a manifest file.
The proposed mechanism is also applicable in a non-CDNI context.
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-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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 December 3, 2015.
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Background on URI Signing . . . . . . . . . . . . . . . . 5
1.3. CDNI URI Signing Overview . . . . . . . . . . . . . . . . 6
1.4. URI Signing in a non-CDNI context . . . . . . . . . . . . 8
2. URI Signing and HTTP Adaptive Streaming . . . . . . . . . . . 9
3. Signed URI Information Elements . . . . . . . . . . . . . . . 10
3.1. Enforcement Information Elements . . . . . . . . . . . . 11
3.2. Signature Computation Information Elements . . . . . . . 13
3.3. URI Signature Information Elements . . . . . . . . . . . 14
3.4. URI Signing Package Attribute . . . . . . . . . . . . . . 15
3.5. User Agent Attributes . . . . . . . . . . . . . . . . . . 16
4. Generating a URI Signature . . . . . . . . . . . . . . . . . 17
4.1. Creating a Signed URI . . . . . . . . . . . . . . . . . . 17
4.1.1. Calculating the URI Signature (Signed URI) . . . . . 18
4.1.2. Packaging the URI Signature (Signed URI) . . . . . . 21
4.2. Creating an initial Signed Token . . . . . . . . . . . . 22
4.2.1. Calculating the URI Signature (initial Signed Token) 22
4.2.2. Packaging the URI Signature (initial Signed Token) . 26
4.2.2.1. Communicating the Signed Token in a HTTP 3xx
Redirection message . . . . . . . . . . . . . . . 26
4.2.2.2. Communicating the Signed Token in a HTTP 2xx
Successful message . . . . . . . . . . . . . . . 27
5. Validating a URI Signature . . . . . . . . . . . . . . . . . 28
5.1. Information Element Extraction . . . . . . . . . . . . . 28
5.2. Signature Validation . . . . . . . . . . . . . . . . . . 30
5.3. Distribution Policy Enforcement . . . . . . . . . . . . . 32
5.4. Subsequent Signed Token Generation . . . . . . . . . . . 33
5.4.1. Calculating the URI Signature (subsequent Signed
Token) . . . . . . . . . . . . . . . . . . . . . . . 33
5.4.2. Packaging the URI Signature (subsequent Signed Token) 37
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5.4.2.1. Communicating the Signed Token in a HTTP 3xx
Redirection message . . . . . . . . . . . . . . . 37
5.4.2.2. Communicating the Signed Token in a HTTP 2xx
Successful message . . . . . . . . . . . . . . . 38
6. Relationship with CDNI Interfaces . . . . . . . . . . . . . . 39
6.1. CDNI Control Interface . . . . . . . . . . . . . . . . . 39
6.2. CDNI Footprint & Capabilities Advertisement Interface . . 39
6.3. CDNI Request Routing Redirection Interface . . . . . . . 40
6.4. CDNI Metadata Interface . . . . . . . . . . . . . . . . . 40
6.5. CDNI Logging Interface . . . . . . . . . . . . . . . . . 43
7. URI Signing Message Flow . . . . . . . . . . . . . . . . . . 44
7.1. HTTP Redirection . . . . . . . . . . . . . . . . . . . . 44
7.2. DNS Redirection . . . . . . . . . . . . . . . . . . . . . 47
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 50
9. Security Considerations . . . . . . . . . . . . . . . . . . . 51
10. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 52
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 53
12.1. Normative References . . . . . . . . . . . . . . . . . . 53
12.2. Informative References . . . . . . . . . . . . . . . . . 53
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 54
1. Introduction
This document describes the concept of URI Signing and how it can be
used to provide access authorization in the case of interconnected
CDNs (CDNI). The primary goal of URI Signing is to make sure that
only authorized User Agents (UAs) are able to access the content,
with a Content Service Provider (CSP) being able to authorize every
individual request. It should be noted that URI Signing is not a
content protection scheme; if a CSP wants to protect the content
itself, other mechanisms, such as DRM, are more appropriate.
The overall problem space for CDN Interconnection (CDNI) is described
in CDNI Problem Statement [RFC6707]. In this document, along with
the CDNI Requirements [RFC7337] document and the CDNI Framework
[RFC7336] the need for interconnected CDNs to be able to implement an
access control mechanism that enforces the CSP's distribution policy
is described.
Specifically, CDNI Framework [RFC7336] states:
"The CSP may also trust the CDN operator to perform actions such as
..., and to enforce per-request authorization performed by the CSP
using techniques such as URI signing."
In particular, the following requirement is listed in CDNI
Requirements [RFC7337]:
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"MI-16 [HIGH] The CDNI Metadata Distribution interface shall allow
signaling of authorization checks and validation that are to be
performed by the surrogate before delivery. For example, this could
potentially include:
* need to validate URI signed information (e.g. Expiry time, Client
IP address)."
This document proposes a URI Signing scheme that allows Surrogates in
interconnected CDNs to enforce a per-request authorization performed
by the CSP. Splitting the role of performing per-request
authorization by CSP and the role of validation of this authorization
by the CDN allows any arbitrary distribution policy to be enforced
across CDNs without the need of CDNs to have any awareness of the
actual CSP distribution policy.
1.1. Terminology
This document uses the terminology defined in CDNI Problem Statement
[RFC6707].
This document also uses the terminology of Keyed-Hashing for Message
Authentication (HMAC) [RFC2104] including the following terms
(reproduced here for convenience):
o MAC: message authentication code.
o HMAC: Hash-based message authentication code (HMAC) is a specific
construction for calculating a MAC involving a cryptographic hash
function in combination with a secret key.
o HMAC-SHA256: HMAC instantiation using SHA-256 as the cryptographic
hash function.
o SHA-1: Secure Hash Algorithm 1 (SHA-1) [RFC3174] is the
cryptographic hash function.
In addition, the following terms are used throughout this document:
o URI Signature: Message digest or digital signature that is
computed with an algorithm for protecting the URI and/or a set of
URI Signing Information Elements.
o URI Signing Information Element: An element containing information
used in the URI Signature validation process that is signalled
along with the URI Signature in either the Signed URI or in a
Signed Token. It is protected by the URI Signature.
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o Original URI: The URI before URI Signing is applied.
o Signed URI: Any URI that contains a URI Signature. It can be used
to retrieve one particular resource, indicated by the URI.
o Signed Token: A set of URI Signing Information Elements protected
by a URI Signature that can be used to retrieve a pre-determined
set of resources. It can be communicated via a URL, an HTTP
Header or a Cookie. A Signed Token differs from a Signed URI in
the sense that the token is valid for multiple resources and it's
embedded URI Signature thus does not protect a particular URI. It
can be used to form a Signed Token Chain in the case of HTTP
Adaptive Streaming content.
o Target CDN URI: Embedded URI created by the CSP to direct UA
towards the Upstream CDN. The Target CDN URI can be signed by the
CSP and verified by the Upstream CDN.
o Redirection URI: URI created by the Upstream CDN to redirect UA
towards the Downstream CDN. The Redirection URI can be signed by
the Upstream CDN and verified by the Downstream CDN. In a
cascaded CDNI scenario, there can be more than one Redirection
URI.
1.2. Background on URI Signing
The next section provides an overview of how URI Signing works in a
CDNI environment. As background information, URI Signing is first
explained in terms of a single CDN delivering content on behalf of a
CSP.
A CSP and CDN are assumed to have a trust relationship that enables
the CSP to authorize access to a content item by including a set of
attributes in the URI before redirecting a UA to the CDN. Using
these attributes, it is possible for a CDN to check an incoming
content request to see whether it was authorized by the CSP (e.g.
based on the UA's IP address or a time window). Of course, the
attributes need to be added to the URI in a way that prevents a UA
from changing the attributes, thereby leaving the CDN to think that
the request was authorized by the CSP when in fact it wasn't. For
this reason, a URI Signing mechanism includes in the URI a message
digest or digital signature that allows a CDN to check the
authenticity of the URI. The message digest or digital signature can
be calculated based on a shared secret between the CSP and CDN or
using CSP's asymmetric public/private key pair, respectively.
Figure 1, shown below, presents an overview of the URI Signing
mechanism in the case of a CSP with a single CDN. When the UA
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browses for content on CSP's website (#1), it receives HTML web pages
with embedded content URIs. Upon requesting these URIs, the CSP
redirects to a CDN, creating a Target CDN URI (#2) (alternatively,
the Target CDN URI itself is embedded in the HTML). The Target CDN
URI is the Signed URI which may include the IP address of the UA and/
or a time window and always contains the URI Signature which is
generated by the CSP using the shared secret or a private key. Once
the UA receives the response with the embedded URI, it sends a new
HTTP request using the embedded URI to the CDN (#3). Upon receiving
the request, the CDN checks to see if the Signed URI is authentic by
verifying the URI signature. In addition, it checks whether the IP
address of the HTTP request matches that in the Signed URI and if the
time window is still valid. After these values are confirmed to be
valid, the CDN delivers the content (#4).
--------
/ \
| CSP |< * * * * * * * * * * *
\ / Trust *
-------- relationship *
^ | *
| | *
1. Browse | | 2. Signed *
for | | URI *
content | | *
| v v
+------+ 3. Signed URI --------
| User |----------------->/ \
| Agent| | CDN |
| |<-----------------\ /
+------+ 4. Content --------
Delivery
Figure 1: Figure 1: URI Signing in a CDN Environment
1.3. CDNI URI Signing Overview
In a CDNI environment, URI Signing operates the same way in the
initial steps #1 and #2 but the later steps involve multiple CDNs in
the process of delivering the content. The main difference from the
single CDN case is a redirection step between the Upstream CDN and
the Downstream CDN. In step #3, UA may send HTTP request or DNS
request. Depending on whether HTTP-based or DNS-based request
routing is used, the Upstream CDN responds by directing the UA
towards the Downstream CDN using either a Redirection URI (which is a
Signed URI generated by the Upstream CDN) or a DNS reply,
respectively (#4). Once the UA receives the response, it sends the
Redirection URI/Target CDN URI to the Downstream CDN (#5). The
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received URI is validated by the Downstream CDN before delivering the
content (#6). This is depicted in the figure below. Note: The CDNI
call flows are covered in Detailed URI Signing Operation (Section 7).
+-------------------------+
|Request Redirection Modes|
+-------------------------+
| a) HTTP |
| b) DNS |
+-------------------------+
--------
/ \< * * * * * * * * * * * * * *
| CSP |< * * * * * * * * * * * *
\ / Trust * *
-------- relationship * *
^ | * *
| | 2. Signed * *
1. Browse | | URI in * *
for | | HTML * *
content | | * *
| v 3.a)Signed URI v *
+------+ b)DNS request -------- * Trust
| User |----------------->/ \ * relationship
| Agent| | uCDN | * (optional)
| |<-----------------\ / *
+------+ 4.a)Redirection URI------- *
^ | b)DNS Reply ^ *
| | * *
| | Trust relationship * *
| | * *
6. Content | | 5.a)Redirection URI * *
delivery | | b)Signed URI(after v v
| | DNS exchange) --------
| +---------------------->/ \ [May be
| | dCDN | cascaded
+--------------------------\ / CDNs]
--------
+-----------------------------------------+
| Key | Asymmetric | Symmetric |
+-----------------------------------------+
|HTTP |Public key (uCDN)|Shared key (uCDN)|
|DNS |Public key (CSP) |Shared key (CSP) |
+-----------------------------------------+
Figure 2: URI Signing in a CDNI Environment
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The trust relationships between CSP, Upstream CDN, and Downstream CDN
have direct implications for URI Signing. In the case shown in
Figure 2, the CDN that the CSP has a trust relationship with is the
Upstream CDN. The delivery of the content may be delegated to the
Downstream CDN, which has a relationship with the Upstream CDN but
may have no relationship with the CSP.
In CDNI, there are two methods for request routing: DNS-based and
HTTP-based. For DNS-based request routing, the Signed URI (i.e.
Target CDN URI) provided by the CSP reaches the Downstream CDN
directly. In the case where the Downstream CDN does not have a trust
relationship with the CSP, this means that only an asymmetric public/
private key method can be used for computing the URI Signature
because the CSP and Downstream CDN are not able to exchange symmetric
shared secret keys. Since the CSP is unlikely to have relationships
with all the Downstream CDNs that are delegated to by the Upstream
CDN, the CSP may choose to allow the Authoritative CDN to
redistribute the shared key to a subset of their Downstream CDNs .
For HTTP-based request routing, the Signed URI (i.e. Target CDN URI)
provided by the CSP reaches the Upstream CDN. After this URI has
been verified to be correct by the Upstream CDN, the Upstream CDN
creates and signs a new Redirection URI to redirect the UA to the
Downstream CDN. Since this new URI also has a new URI Signature,
this new signature can be based around the trust relationship between
the Upstream CDN and Downstream CDN, and the relationship between the
Downstream CDN and CSP is not relevant. Given the fact that such a
relationship between Upstream CDN and Downstream CDN always exists,
both asymmetric public/private keys and symmetric shared secret keys
can be used for URI Signing. Note that the signed Redirection URI
SHOULD maintain the same level of security as the original Signed
URI.
1.4. URI Signing in a non-CDNI context
While the URI signing scheme defined in this document was primarily
created for the purpose of allowing URI Signing in CDNI scenarios,
e.g. between a uCDN and a dCDN or between a CSP and a dCDN, there is
nothing in the defined URI Signing scheme that precludes it from
being used in a non-CDNI context. As such, the described mechanism
could be used in a single-CDN scenario such as shown in Figure 1 in
Section 1.2, for example to allow a CSP that uses different CDNs to
only have to implement a single URI Signing mechanism.
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2. URI Signing and HTTP Adaptive Streaming
For content that is delivered via an HTTP Adaptive Streaming (HAS)
protocol, such as MPEG DASH [Editor's Note: Include reference],
special provisions need to be made in order to ensure URI Signing can
be applied. In general, HAS protocols work by breaking large objects
(e.g. videos), into a sequence of small, independent chunks. Such
chunks are then referenced by a separate manifest file, which either
includes a list of URLs to the chunks or specifies an algorithm
through which a User Agent can construct the URLs to the chunks.
Requests for chunks therefore originate from the manifest file and,
unless the URLs in the manifest file point to the CSP, are not
subjected to redirection and URI Signing. This opens up the
vulnerability of malicious User Agents sharing the manifest file and
deep-linking to the chunks.
One method for dealing with this vulnerability would be to include in
the manifest itself Signed URIs that point to the individual chunks.
There exist a number of issues with that approach. First, it
requires the CDN delivering the manifest to rewrite the manifest file
for each User Agent, which would require the CDN to be aware of the
exact HAS protocol and version used. Secondly, it would require the
expiration time of the Signed URIs to be valid for at least the full
duration of the content described by the manifest. Since it is not
uncommon for a manifest file to contain a video item of more than 30
minutes in length, this would require the Signed URIs to be valid for
a long time, thereby reducing their effectiveness and that of the URI
Signing mechanism in general. For a more detailed analysis of how
HAS protocols affect CDNI, see Models for HTTP-Adaptive-Streaming-
Aware CDNI [RFC6983].
In order to allow for effective access control of HAS content, the
URI signing scheme defined in this document instead supports a
mechanism through which subsequent chunk requests can be chained
together. As part of the URI validation procedure, the CDN can
generate a Signed Token that the UA can use to do a subsequent
request. More specifically, whenever a UA successfully retrieves a
chunk, it receives, in the HTTP 2xx Successful message, a Signed
Token that it can use whenever it requests the next chunk. As long
as each Signed Token in the chain is correctly validated before a new
one is generated, the chain is not broken and the User Agent can
successfully retrieve additional chunks. Given the fact that with
HAS protocols, it is usually not possible to determine a priori which
chunk will be requested next (i.e. to allow for seeking within the
content and for switching to a different quality level), the Signed
Token includes a scoping mechanism that allows it to be valid for
more than one URL.
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In order for this chaining of Signed Tokens to work, it is necessary
for a UA to extract the Signed Token from the HTTP 2xx Successful
message of an earlier request and use it to retrieve the next chunk.
The exact mechanism by which the client does this depends on the
exact HAS protocol and since this document is only concerned with the
generation and validation of incoming request, this process is
outside the scope of this document. However, in order to also
support legacy UAs that do not include any specific provisions for
the handling of Signed Tokens, this document does define a legacy
mechanism using HTTP Cookies that allows such UAs to support the
concept of chained Signed Tokens without requiring any support on the
UA side.
3. Signed URI Information Elements
The concept behind URI Signing is based on embedding in either the
Signed URI or Signed Token a number of information elements that can
be validated to ensure the UA has legitimate access to the content.
These information elements are appended, in an encapsulated form, to
the Original URI to form the Signed URI. Alternatively, the
encapsulated elements are placed in the Signed Token.
For the purposes of the URI signing mechanism described in this
document, three types of information elements may be embedded in the
URI:
o Enforcement Information Elements: Information Elements that are
used to enforce a distribution policy defined by the CSP.
Examples of enforcement attributes are IP address of the UA and
time window. Another example is the Path Pattern information
element that is used to define the scope of a Signed Token.
o Signature Computation Information Elements: Information Elements
that are used by the CDN to verify the URI signature embedded in
the received URI. In order to verify a URI Signature, the CDN
requires some information elements that describe how the URI
Signature was generated. Examples of Signature Computation
Elements include the used HMACs hash function and/or the key
identifier.
o URI Signature Information Elements: The information elements that
carry the actual message digest or digital signature representing
the URI signature used for checking the integrity and authenticity
of the Signed URI or Signed Token. A typical Signed URI or Token
will only contain one embedded URI Signature Information Element.
In addition, the this document specifies the following URI attribute:
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o URI Signing Package Attribute: The URI attribute is a container
that encapsulates all the URI Signing Information Elements in an
encoded format. In the case of a Signed URI, only this attribute
is exposed as a URI query string parameter. In the case of a
Signed Token, this attribute constitutes the Signed Token and is
communicated in either the URI, a dedicated header, or as an HTTP
Cookie.
Two types of keys can be used for URI Signing: asymmetric keys and
symmetric keys. Asymmetric keys are based on a public/private key
pair mechanism and always contain a private key only known to the
entity signing the URI (either CSP or uCDN) and a public key for the
verification of the Signed URI. With symmetric keys, the same key is
used by both the signing entity for signing the URI as well as by the
validating entity for validating the Signed URI. Regardless of the
type of keys used, the validating entity has to obtain the key
(either the public or the symmetric key). There are very different
requirements for key distribution (out of scope of this document)
with asymmetric keys and with symmetric keys. Key distribution for
symmetric keys requires confidentiality to prevent another party from
getting access to the key, since it could then generate valid Signed
URIs for unauthorized requests. Key distribution for asymmetric keys
does not require confidentiality since public keys can typically be
distributed openly (because they cannot be used for URI signing) and
private keys are kept by the URI signing function.
Note that all the URI Signing Information Elements and the URI
Signing Package Attribute query string attribute are mandatory to
implement, but not mandatory to use.
3.1. Enforcement Information Elements
This section identifies the set of information elements that may be
needed to enforce the CSP distribution policy. New information
elements may be introduced in the future to extend the capabilities
of the distribution policy.
In order to provide flexibility in distribution policies to be
enforced, the exact subset of information elements used in a Signed
URI or Signed Token is a deployment decision. The defined keyword
for each information element is specified in parenthesis below.
The following information elements are used to enforce the
distribution policy:
o Expiry Time (ET) [optional] - Time when the Signed URI or Signed
Token expires. This is represented as an integer denoting the
number of seconds since midnight 1/1/1970 UTC (i.e. UNIX epoch).
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The request is rejected if the received time is later than this
timestamp. Note: The time, including time zone, on the entities
that generate and validate the Signed URI or Signed Token need to
be in sync (e.g. NTP is used).
o Client IP (CIP) [optional] - IP address of the client for which
this Signed URI or Signed Token is generated. This is represented
in dotted decimal format for IPv4 or canonical text representation
for IPv6 address [RFC5952] . The request is rejected if sourced
from a client with a different IP address.
o Path Pattern (PP) [mandatory for Signed Token] - Path Pattern that
describes for which content the Signed Token is valid. The Path
Pattern contains an expression to match against the requested URI
path to check whether the requested content is allowed to be
requested with the Signed Token. A Path Pattern contains a
sequence of one or more path segments seperated by a slash ('/')
character. The pattern may include the wildcards '*' and '?',
where '*' matches any sequence of characters (including the empty
string) and '?' matches exactly one character. The three literals
'\', '*' and '?' should be escaped as '\\', '\*' and '\?'. All
other characters are treated as literals. The following is an
example of a valid Path Pattern: '*/folder/content-83112371/
quality_*/segment????.mp4'. The Path Pattern Information Element
MUST NOT be used in a Signed URI.
The Expiry Time Information Element ensures that the content
authorization expires after a predetermined time. This limits the
time window for content access and prevents replay of the request
beyond the authorized time window.
The Client IP Information Element is used to restrict content access
to a particular User Agent, based on its IP address for whom the
content access was authorized.
The Path Pattern Information Element is used to restrict content
access to a particular set of URLs, based on the path component of
the URI on which it is available. This is primarily useful for
content delivered via an HTTP Adaptive Streaming protocol using a
manifest file, where it is often not known a priori which segment
will be requested next.
Note: See the Security Considerations (Section 9) section on the
limitations of using an expiration time and client IP address for
distribution policy enforcement.
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3.2. Signature Computation Information Elements
This section identifies the set of information elements that may be
needed to verify the URI Signature and/or calculate a new Signed
Token. New information elements may be introduced in the future if
new URI signing algorithms are developed.
The defined keyword for each information element is specified in
parenthesis below.
The following information elements are used to validate the URI by
recreating the URI Signature and/or calculate a new Signed Token.
o Version (VER) [optional] - An 8-bit unsigned integer used for
identifying the version of URI signing method. If this
Information Element is not present in the Signed URI or Signed
Token, the default version is 1.
o Key ID (KID) [optional] - A string used for obtaining the key
(e.g. database lookup, URI reference) which is needed to validate
the URI Signature. The KID and KID_NUM information elements MUST
NOT be present in the same Signed URI or Signed Token.
o Numerical Key ID (KID_NUM) [optional] - A 64-bit unsigned integer
used as an optional alternative for KID. The KID and KID_NUM
information elements MUST NOT be present in the same Signed URI or
Signed Token.
o Hash Function (HF) [optional] - A string used for identifying the
hash function to compute the URI signature with HMAC. If this
Information Element is not present in the Signed URI or Signed
Token, the default hash function is SHA-256.
o Digital Signature Algorithm (DSA) [optional] - Algorithm used to
calculate the Digital Signature. If this Information Element is
not present in the Signed URI or Signed Token, the default is EC-
DSA.
o URI Signing Cookie Flag (USCF) [optional] - The presence of this
flag indicates the URI Signing Information Elements contents of
the URI Signing Package Attribute are communicated via the Cookie
header of the HTTP request instead of via the query string
component of the URI.
o Expiration Time Setting (ETS) [optional] - An 16-bit unsigned
integer (in seconds) used for setting the value of the Expiry Time
information element in newly generated Signed Tokens in case a
chain of Signed Tokens is used.
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The Version Information Element indicates which version of URI
signing scheme is used (including which attributes and algorithms are
supported). The present document specifies Version 1. If the
Version attribute is not present in the Signed URI or Signed Token,
then the version is obtained from the CDNI metadata, else it is
considered to have been set to the default value of 1. More versions
may be defined in the future.
The Key ID Information Element is used to retrieved the key which is
needed as input to the algorithm for validating the Signed URI or
Signed Token. The method used for obtaining the actual key from the
reference included in the Key ID Information Element is outside the
scope of this document. Instead of using the KID element, which is a
string, it is possible to use the KID_NUM element for numerical Key
identifiers instead. The KID_NUM element is a 64-bit unsigned
integer. In cases where numerical KEY IDs are used, it is
RECOMMENDED to use KID_NUM instead of KID.
The Hash Function Information Element indicates the hash function to
be used for HMAC-based message digest computation. The Hash Function
Information Element is used in combination with the Message Digest
Information Element defined in section Section 3.3.
The Digital Signature Algorithm Information Element indicates the
digital signature function to be in the case asymmetric keys are
used. The Digital Signature Algorithm Information Element is used in
combination with the Digital Signature Information Element defined in
section Section 3.3.
The URI Signing Cookie Flag Information Element is used to indicate
the contents of the URI Signing Package attribute is communicated via
the Cookie header of the HTTP request instead of via the query string
component of the URI. The primary use case for this is the case
where the chained Signed Token mechanism described in Section 2 is
used in combination with legacy UAs.
The Expiration Time Setting Information Element is used to
communicate to the CDN to which duration the Expiry Time information
element should be set whenever a new Signed Token is generated. This
information element is only used in combination with the chained
Signed Token mechanism for HTTP Adaptive Streaming content as
described in Section 2.
3.3. URI Signature Information Elements
This section identifies the set of information elements that carry
the URI Signature that is used for checking the integrity and
authenticity of the URI.
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The defined keyword for each information element is specified in
parenthesis below.
The following information elements are used to carry the actual URI
Signature.
o Message Digest (MD) [mandatory for symmetric key] - A string used
for the message digest generated by the URI signing entity.
o Digital Signature (DS) [mandatory for asymmetric keys] - A string
used for the digital signature provided by the URI signing entity.
The Message Digest attribute contains the message digest used to
validate the Signed URI or Signed Token when symmetric keys are used.
The Digital Signature attribute contains the digital signature used
to verify the Signed URI or Signed Token when asymmetric keys are
used.
In the case of symmetric key, HMAC algorithm is used for the
following reasons: 1) Ability to use hash functions (i.e. no changes
needed) with well understood cryptographic properties that perform
well and for which code is freely and widely available, 2) Easy to
replace the embedded hash function in case faster or more secure hash
functions are found or required, 3) Original performance of the hash
function is maintained without incurring a significant degradation,
and 4) Simple way to use and handle keys. The default HMAC algorithm
used is SHA-256.
In the case of asymmetric keys, Elliptic Curve Digital Signature
Algorithm (EC DSA) - a variant of DSA - is used because of the
following reasons: 1) Key size is small while still offering good
security, 2) Key is easy to store, and 3) Computation is faster than
DSA or RSA.
3.4. URI Signing Package Attribute
The URI Signing Package Attribute is an encapsulation container for
the URI Signing Information Elements defined in the previous
sections. The URI Signing Information Elements are encoded and
stored in this attribute. In the case of a Signed URI, the URI
Signing Package Attribute is appended to the Original URI to create
the Signed URI. In the case of a Signed Token, the URI Signing
Package Attribute can be communicated via the query string component
of a URI, via a dedicated HTTP header, or as an HTTP Cookie.
The primary advantage of the URI Signing Package Attribute is that it
avoids having to expose the URI Signing Information Elements directly
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in the query string of the URI, thereby reducing the potential for a
namespace collision space within the URI query string. A side-
benefit of the attribute is the obfuscation performed by the URI
Signing Package Attribute hides the information (e.g. client IP
address) from view of the common user, who is not aware of the
encoding scheme. Obviously, this is not a security method since
anyone who knows the encoding scheme is able to obtain the clear
text. Note that any parameters appended to the query string after
the URI Signing Package Attribute are not validated and hence do not
affect URI Signing.
The following attribute is used to carry the encoded set of URI
Signing Information Elementsin the Signed URI or the Signed Token.
o URI Signing Package (URISigningPackage) - The encoded attribute
containing all the CDNI URI Signing Information Elements used for
URI Signing.
The URI Signing Package Attribute contains the URI Signing
Information Elements in the Base-64 encoding with URL and Filename
Safe Alphabet (a.k.a. "base64url") as specified in the Base-64 Data
Encoding [RFC4648] document. The URI Signing Package Attribute is
the only URI Signing Information Element exposed in a Signed URI.
The attribute MUST be the last parameter in the query string
component of the URI when the Signed URI is generated. However, a
client or CDN may append other query parameters unrelated to URI
Signing to the Signed URI. Such additional query parameters SHOULD
NOT use the same name as the URI Signing Package Attribute to avoid
namespace collision and potential failure of the URI Signing
validation. In the case of a Signed Token, the URI Signing Package
Attribute fully constitutes the Signed Token.
The query string parameter name of the URI Signing Package Attribute
shall be defined in the CDNI Metadata interface. If the CDNI
Metadata interface is not used, or does not include a parameter name
for the URI Signing Package Attribute, the parameter name is set by
configuration (out of scope of this document).
3.5. User Agent Attributes
For some use cases, such as logging, it might be useful to allow the
UA, or another entity, add one or more attributes to the Signed URI
for purposes other than URI Signing without causing URI Signing to
fail. In order to do so, such attributes MUST be appended after the
URI Signing Packacke Attribute. Any attributes appended in such way
after the URI Signature has been calculated are not validated for the
purpose of content access authorization. Adding any such attributes
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to the Signed URI before the URI Signing Packacke Attribute will
cause the URI Signing validation to fail.
Note that a malicious UA might potentially use the ability to append
attributes to the Signed URI in order to try to influence the content
that is delivered. For example, the UA might append '&quality=HD' to
try to make the dCDN deliver an HD version of the requested content.
Since such an additional attribute is appended after the URI Signing
Package Attribute it is not validated and will not affect the outcome
of the URI validation. In order to deal with this vulnerability, a
dCDN is RECOMMENDED to ignore any query strings appended after the
URI Signing Package Attribute for the purpose of content selection.
4. Generating a URI Signature
This section describes the process of generating a URI Signature.
The nature of this process depends on whether a Signed URI or Signed
Token is being generated. A Signed Token will typically be used as
part of a chain of Signed Tokens for the delivery of HTTP Adaptive
Streaming content (see Section 2). Section 4.1 defines creating a
regular Signed URI. Section 4.2 defines creating the initial Signed
Token for HAS content.
4.1. Creating a Signed URI
The following procedure defines the URI Signing algorithm for
creating a Signed URI for regular (i.e. non-HTTP Adaptive Streaming)
content in which the Path Pattern information element is not used.
Note that some steps may be skipped if the CSP does not enforce a
distribution policy and the Enforcement Information Elements are
therefore not necessary. A URI (as defined in URI Generic Syntax
[RFC3986]) contains the following parts: scheme name, authority,
path, query, and fragment. In a Signed URI, the entire URI except
the "scheme name" part is protected by the URI signature. This
allows the URI signature to be validated correctly in the case when a
client performs a fallback to another scheme (e.g. HTTP) for a
content item referenced by a URI with a specific scheme (e.g. RTSP).
The benefit is that the content access is protected regardless of the
type of transport used for delivery. If the CSP wants to ensure a
specific protocol is used for content delivery, that information is
passed by CDNI metadata. Note: Support for changing of the URL
scheme requires that the default port is used, or that the protocols
must both run on the same non-standard port.
The process of generating a Signed URI can be divided into two sets
of steps: first, calculating the URI Signature and then, packaging
the URI Signature along with the URI Signing Information Elements
into a URI Signing Package Attribute and appending it to the Original
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URI. Note it is possible to use some other algorithm and
implementation as long as the same result is achieved. An example
for the Original URI, "http://example.com/content.mov", is used to
clarify the steps.
4.1.1. Calculating the URI Signature (Signed URI)
Calculate the URI Signature for use with a Signed URI by following
the procedure below.
1. Copy the Original URI, excluding the "scheme name" part, into a
buffer to hold the message for performing the operations below.
2. Check if the Original URI already contains a query string. If
not, append a "?" character. If yes, append an "&" character.
3. If the version is the default value (i.e. "1"), skip this step.
Otherwise, specify the version by appending the string "VER=#",
where '#' represents the new version number. The following steps
in the procedure is based on the initial version of URI Signing
specified by this document. For other versions, reference the
associated RFC for the URI signing procedure.
4. If time window enforcement is not needed, skip this step.
A. If an information element was added to the message, append an
"&" character. Append the string "ET=". Note in the case of
re-signing a URI, the information element is carried over
from the received Signed URI.
B. Get the current time in seconds since epoch (as an integer).
Add the validity time in seconds as an integer. Note in the
case of re-signing a URI, the value MUST remain the same as
the received Signed URI.
C. Convert this integer to a string and append to the message.
5. If client IP enforcement is not needed, skip this step.
A. If an information element was added to the message, append an
"&" character. Append the string "CIP=". Note in the case
of re-signing a URI, the attribute is carried over from the
received Signed URI.
B. Convert the client's IP address in dotted decimal notation
format (i.e. for IPv4 address) or canonical text
representation (for IPv6 address [RFC5952]) to a string and
append to the message. Note in the case of re-signing an
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URI, the value MUST remain the same as the received Signed
URI.
6. Depending on the type of key used to sign the URI, compute the
message digest or digital signature for symmetric key or
asymmetric keys, respectively.
A. For symmetric key, HMAC is used.
1. Obtain the shared key to be used for signing the URI.
2. If the key identifier is not needed, skip this step. If
an information element was added to the message, append
an "&" character. Append the string "KID=" in case a
string-based Key ID is used, or "KID_NUM=" in case a
numerical Key ID is used. Append the key identifier
(e.g. "example:keys:123" or "56128239") needed by the
entity to locate the shared key for validating the URI
signature.
3. Optional: If the hash function for the HMAC uses the
default value ("SHA-256"), skip this step. If an
information element was added to the message, append an
"&" character. append the string "HF=". Append the
string for the new type of hash function to be used.
Note that re-signing a URI MUST use the same hash
function as the received Signed URI or one of the
allowable hash functions designated by the CDNI metadata.
4. If an information element was added to the message,
append an "&" character. Append the string "MD=". The
message now contains the complete section of the URI that
is protected (e.g. "://example.com/content.mov?ET=1209422
976&CIP=192.0.2.1&KID=example:keys:123&MD=").
5. Compute the message digest using the HMAC algorithm and
the default SHA-256 hash function, or another hash
function if specified by the HF Information Element, with
the shared key and message as the two inputs to the hash
function.
6. Convert the message digest to its equivalent hexadecimal
format.
7. Append the string for the message digest (e.g.
"://example.com/content.mov?ET=1209422976&CIP=192.0.2.1&K
ID=example:keys:123&MD=1ecb1446a6431352aab0fb6e0dca30e303
56593a97acb972202120dc482bddaf").
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B. For asymmetric keys, EC DSA is used.
1. Generate the EC private and public key pair. Store the
EC public key in a location that's reachable for any
entity that needs to validate the URI signature.
2. If the key identifier is not needed, skip this step. If
an information element was added to the message, append
an "&" character. Append the string "KID=" in case a
string-based Key ID is used, or "KID_NUM=" in case a
numerical Key ID is used. Append the key identifier
(e.g. "http://example.com/public/keys/123") needed by the
entity to locate the shared key for validating the URI
signature. Note that in the case the Key ID URI is a URL
to a public key, the Key ID URI SHOULD only contain the
"scheme name", "authority", and "path" parts (i.e. query
string is not allowed).
3. Optional: If the digital signature algorithm uses the
default value ("EC-DSA"), skip this step. If an
information element was added to the message, append an
"&" character. Append the string "DSA=". Append the
string denoting the new digital signature function.
4. If an information element was added to the message,
append an "&" character. Append the string "DS=". The
message now contains the complete section of the URI that
is protected. (e.g. "://example.com/content.mov?ET=120942
2976&CIP=192.0.2.1&KID=http://example.com/public/
keys/123&DS=").
5. Compute the message digest using SHA-1 (without a key)
for the message. Note: The digital signature generated
in the next step is calculated over the SHA-1 message
digest, instead of over the cleartype message, to reduce
the length of the digital signature, and thereby the
length of the URI Signing Package Attribute and the
resulting Signed URI. Since SHA-1 is not used for
cryptographic purposes here, the security concerns around
SHA-1 do not apply.
6. Compute the digital signature, using the EC-DSA algorithm
by default or another algorithm if specified by the DSA
Information Element, with the private EC key and message
digest (obtained in previous step) as inputs.
7. Convert the digital signature to its equivalent
hexadecimal format.
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8. Append the string for the digital signature. In the case
where EC-DSA algorithm is used, this string contains the
values for the 'r' and 's' parameters, delimited by ':'
(e.g. "://example.com/content.mov?ET=1209422976&CIP=192.0
.2.1&KID=http://example.com/public/keys/123&DS=r:CFB03EDB
33810AB6C79EE3C47FBD86D227D702F25F66C01CF03F59F1E005668D:
s:57ED0E8DF7E786C87E39177DD3398A7FB010E6A4C0DC8AA71331A92
9A29EA24E" )
4.1.2. Packaging the URI Signature (Signed URI)
Apply the URI Signing Package Attribute by following the procedure
below to generate the Signed URI.
1. Remove the Original URI portion from the message to obtain all
the URI Signing Information Elements, including the URI signature
(e.g. "ET=1209422976&CIP=192.0.2.1&KID=example:keys:123&MD=1ecb1
446a6431352aab0fb6e0dca30e30356593a97acb972202120dc482bddaf").
2. Compute the URI Signing Package Attribute using Base-64 Data
Encoding [RFC4648] on the message (e.g. "VkVSPTEmRVQ9MTIwOTQyMjk
3NiZDSVA9MTkyLjAuMi4xJktJRD1leGFtcGxlOmtleXM6MTIzJk1EPTFlY2IxNDQ2
YTY0MzEzNTJhYWIwZmI2ZTBkY2EzMGUzMDM1NjU5M2E5N2FjYjk3MjIwMjEyMGRjN
DgyYmRkYWY="). Note: This is the value for the URI Signing
Package Attribute.
3. Copy the entire Original URI into a buffer to hold the message.
4. Check if the Original URI already contains a query string. If
not, append a "?" character. If yes, append an "&" character.
5. Append the parameter name used to indicate the URI Signing
Package Attribute, as communicated via the CDNI Metadata
interface, followed by an "=". If none is communicated by the
CDNI Metadata interface, it defaults to "URISigningPackage". For
example, if the CDNI Metadata interface specifies "SIG", append
the string "SIG=" to the message.
6. Append the URI Signing token to the message (e.g.
"http://example.com/content.mov?URISigningPackage=VkVSPTEmRVQ9MTI
wOTQyMjk3NiZDSVA9MTkyLjAuMi4xJktJRD1leGFtcGxlOmtleXM6MTIzJk1EPTFl
Y2IxNDQ2YTY0MzEzNTJhYWIwZmI2ZTBkY2EzMGUzMDM1NjU5M2E5N2FjYjk3MjIwM
jEyMGRjNDgyYmRkYWY="). Note: this is the completed Signed URI.
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4.2. Creating an initial Signed Token
The following procedure defines the algorithm for creating the
initial Signed Token of a Signed Token chain as for example used with
HTTP Adaptive Streaming content. Note that the process described in
this section is only performed for creating the initial Signed Token
of a particular chain. Subsequent Signed Tokens forming the same
chain are generated as part of the URI Signature Validation process
described in Section 5. The creation of the initial Signed Token
will typically be done by the CSP the first time a particular UA
requests the manifest file. Choosing appropriate values of the
Enforcement Information Elements in the initial Signed Token requires
some knowledge of the structure of the HTTP Adapative Streaming
content that is being requested.
In contrast with a Signed URI, where the URI Signature is calculated
over the Original URI, a Signed Token does not protect the Original
URI. Instead, the Path Pattern Information Element is used to convey
the set of resources for which the particular Signed Token is valid.
The process of generating a initial Signed Token can be divided into
two sets of steps: first, calculating the URI Signature and then,
packaging the URI Signature along with the URI Signing Information
Elements into a URI Signing Package to construct a Signed Token and
appending the Signed Token to the message. Note it is possible to
use some other algorithm and implementation as long as the same
result is achieved. An example for the Original URI,
"http://example.com/folder/content-83112371/manifest.xml", is used to
clarify the steps.
4.2.1. Calculating the URI Signature (initial Signed Token)
Calculate the URI Signature for use with a Signed Token by following
the procedure below.
1. Create a new buffer for constructing the Signed Token in the
steps below.
2. If the URI Signing version used is the default one (i.e. "1"),
skip this step. Otherwise, specify the version by appending the
string "VER=#", where '#' represents the new version number. The
following steps in the procedure is based on the initial version
of URI Signing specified by this document. For other versions,
reference the associated RFC for the URI signing procedure.
3. If time window enforcement is not needed, this step can be
skipped.
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A. If an information element was added to the message, append an
"&" character. Append the string "ET=".
B. Get the current time in seconds since epoch (as an integer).
Add the validity time of the initial Signed Token in seconds
as an integer.
C. Convert this integer to a string and append to the message.
D. Append an "&" character. Append the string "ETS=".
E. Append the Expiration Time Setting (in seconds) in the form
of a string to the message. Note: the length of the
Expiration Time Setting should be appropriate given the
segment duration of the HTTP Adaptive Streaming content in
question. As an example, if the segment duration is 10
seconds, the Expiration Time Setting should be at minimum 10
seconds, and preferably a bit more.
4. If client IP enforcement is not needed, this step can be skipped.
A. If an information element was added to the message, append an
"&" character. Append the string "CIP=".
B. Convert the client's IP address in dotted decimal notation
format (i.e. for IPv4 address) or canonical text
representation (for IPv6 address [RFC5952]) to a string and
append to the message.
5. If an information element was added to the message, append an "&"
character. Append the string "PP=".
6. Append the value of the Path Pattern in the form of a string to
the message. Note: the value of the Path Pattern element should
be appropriate given the file and folder structure of the HTTP
Adaptive Streaming content in question. As an example, if the
Original URI for the manifest file is 'http://example.com/folder/
content-83112371/manifest.xml', a suitable Path Pattern might be
'*/content-83112371/*/segment????.mp4'.
7. Depending on the type of key used to sign the URI, compute the
message digest or digital signature for symmetric key or
asymmetric keys, respectively.
A. For symmetric key, HMAC is used.
1. Obtain the shared key to be used for signing the URI.
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2. If the key identifier is not needed, skip this step. If
an information element was added to the message, append
an "&" character. Append the string "KID=" in case a
string-based Key ID is used, or "KID_NUM=" in case a
numerical Key ID is used. Append the key identifier
(e.g. "example:keys:123" or "56128239") needed by the
entity to locate the shared key for validating the URI
signature.
3. Optional: If the hash function for the HMAC uses the
default value ("SHA-256"), skip this step. If an
information element was added to the message, append an
"&" character. append the string "HF=". Append the
string for the new type of hash function to be used.
Note one MUST use the same hash function as communicated
in the original concatenated information element string
or one of the allowable hash functions designated by the
CDNI metadata.
4. If an information element was added to the message,
append an "&" character. Append the string "MD=". The
message now contains the complete set of URI Signing
Information Elements over which the URI Signature is
computed (e.g.
"ET=1209422976&ETS=15&CIP=192.0.2.1&PP=*/content-
83112371/*/segment????.mp4&KID=example:keys:123&MD=").
5. Compute the message digest using the HMAC algorithm and
the default SHA-256 hash function, or another hash
function if specified by the HF Information Element, with
the shared key and message as the two inputs to the hash
function.
6. Convert the message digest to its equivalent hexadecimal
format.
7. Append the string for the message digest (e.g.
"ET=1209422976&ETS=15&CIP=192.0.2.1&PP=*/content-83112371
/*/segment????.mp4&KID=example:keys:123&MD=d6117d7db8a68b
d59f6e7e3343484831acd8f23bbaa7f44b285a2f3bb6f02cfd").
B. For asymmetric keys, EC DSA is used.
1. Generate the EC private and public key pair. Store the
EC public key in a location that's reachable for any
entity that needs to validate the URI signature.
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2. If the key identifier is not needed, skip this step. If
an information element was added to the message, append
an "&" character. Append the string "KID=" in case a
string-based Key ID is used, or "KID_NUM=" in case a
numerical Key ID is used. Append the key identifier
(e.g. "http://example.com/public/keys/123") needed by the
entity to locate the shared key for validating the URI
signature. Note that in the case the Key ID URI is a URL
to a public key, the Key ID URI SHOULD only contain the
"scheme name", "authority", and "path" parts (i.e. query
string is not allowed).
3. Optional: If the digital signature algorithm uses the
default value ("EC-DSA"), skip this step. If an
information element was added to the message, append an
"&" character. Append the string "DSA=". Append the
string denoting the new digital signature function.
4. If an information element was added to the message,
append an "&" character. Append the string "DS=". The
message now contains the complete set of URI Signing
Information Elements over which the URI Signature is
computed (e.g.
"ET=1209422976&ETS=15&CIP=192.0.2.1&PP=*/content-
83112371/*/segment????.mp4&KID=example:keys:123&DS=").
5. Compute the message digest using SHA-1 (without a key)
for the message. Note: The digital signature generated
in the next step is calculated over the SHA-1 message
digest, instead of over the cleartype message, to reduce
the length of the digital signature, and thereby the
length of the Signed Token. Since SHA-1 is not used for
cryptographic purposes here, the security concerns around
SHA-1 do not apply.
6. Compute the digital signature, using the EC-DSA algorithm
by default or another algorithm if specified by the DSA
Information Element, with the private EC key and message
digest (obtained in previous step) as inputs.
7. Convert the digital signature to its equivalent
hexadecimal format.
8. Append the string for the digital signature. In the case
where EC-DSA algorithm is used, this string contains the
values for the 'r' and 's' parameters, delimited by ':'
(e.g. "ET=1209422976&ETS=15&CIP=192.0.2.1&PP=*/content-8
3112371/*/segment????.mp4&KID=example:keys:123&DS=r:CFB03
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EDB33810AB6C79EE3C47FBD86D227D702F25F66C01CF03F59F1E00566
8D:s:57ED0E8DF7E786C87E39177DD3398A7FB010E6A4C0DC8AA71331
A929A29EA24E" )
4.2.2. Packaging the URI Signature (initial Signed Token)
The following steps depend on whether the Signed Token is
communicated to the UA via an HTTP 3xx Redirection message or via an
HTTP 2xx Successful message. In the case of a redirection response,
the Signed Token can be communicated as part of the query string
component of the URL signalled via the Location header of the
Redirection message. In the case of a 2xx Successful message, the
Signed Token can be communicated via either a dedicated header or,
for legacy UAs, via the Set-Cookie header.
4.2.2.1. Communicating the Signed Token in a HTTP 3xx Redirection
message
The following steps describe how the Signed Token can be communicated
to the UA via an HTTP 3xx Redirection message.
1. Copy the entire Original URI into a buffer to hold the message.
2. Check if the Original URI already contains a query string. If
not, append a "?" character. If yes, append an "&" character.
3. Append the parameter name used to indicate the URI Signing
Package Attribute, as communicated via the CDNI Metadata
interface, followed by an "=". If none is communicated by the
CDNI Metadata interface, it defaults to "URISigningPackage". For
example, if the CDNI Metadata interface specifies "SIG", append
the string "SIG=" to the message.
4. Encode the Signed Token by applying Base-64 Data Encoding
[RFC4648] on the value of the Signed Token (e.g. "RVQ9MTIwOTQyMj
k3NiZhbXA7RVRTPTE1JmFtcDtDSVA9MTkyLjAuMi4xJmFtcDtQUD0qL2NvbnRlbnQ
tODMxMTIzNzEvKi9zZWdtZW50Pz8/Py5tcDQmYW1wO0tJRD1leGFtcGxlOmtleXM6
MTIzJmFtcDtNRD1kNjExN2Q3ZGI4YTY4YmQ1OWY2ZTdlMzM0MzQ4NDgzMWFjZDhmM
jNiYmFhN2Y0NGIyODVhMmYzYmI2ZjAyY2Zk").
5. Append the URI Signing token to the message (e.g.
"http://example.com/folder/content-83112371/manifest.xml?URISigni
ngPackage=RVQ9MTIwOTQyMjk3NiZhbXA7RVRTPTE1JmFtcDtDSVA9MTkyLjAuMi4
xJmFtcDtQUD0qL2NvbnRlbnQtODMxMTIzNzEvKi9zZWdtZW50Pz8/Py5tcDQmYW1w
O0tJRD1leGFtcGxlOmtleXM6MTIzJmFtcDtNRD1kNjExN2Q3ZGI4YTY4YmQ1OWY2Z
TdlMzM0MzQ4NDgzMWFjZDhmMjNiYmFhN2Y0NGIyODVhMmYzYmI2ZjAyY2Zk").
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6. Place the message in the Location header of the HTTP 3xx
Redirection message returned to the UA.
4.2.2.2. Communicating the Signed Token in a HTTP 2xx Successful
message
The following steps describe how the Signed Token can be communicated
to the UA via an HTTP 2xx Successful message.
4.2.2.2.1. Header-based
The following steps are used in case the UA is expected to implement
a mechanisms for extracting the Signed Token from the dedicated URI
Signing HTTP Header and place in the query string component of the
next request.
1. Encode the Signed Token by applying Base-64 Data Encoding
[RFC4648] on the value of the Signed Token (e.g. "RVQ9MTIwOTQyMj
k3NiZhbXA7RVRTPTE1JmFtcDtDSVA9MTkyLjAuMi4xJmFtcDtQUD0qL2NvbnRlbnQ
tODMxMTIzNzEvKi9zZWdtZW50Pz8/Py5tcDQmYW1wO0tJRD1leGFtcGxlOmtleXM6
MTIzJmFtcDtNRD1kNjExN2Q3ZGI4YTY4YmQ1OWY2ZTdlMzM0MzQ4NDgzMWFjZDhmM
jNiYmFhN2Y0NGIyODVhMmYzYmI2ZjAyY2Zk").
2. Place the value of the encoded Signed Token in the URI Signing
Header of the HTTP 2xx Successful message of the content being
returned to the UA. Note: The HTTP Header name to use is
communicated via the CDNI Metadata Interface or set via
configuration. Otherwise, it defaults to 'URISigningPackage'.
4.2.2.2.2. Cookie-based
The following steps are used in combination with legacy UAs that do
not support a dedicated URI Signing HTTP header.
1. Encode the Signed Token by applying Base-64 Data Encoding
[RFC4648] on the value of the Signed Token (e.g. "RVQ9MTIwOTQyMj
k3NiZhbXA7RVRTPTE1JmFtcDtDSVA9MTkyLjAuMi4xJmFtcDtQUD0qL2NvbnRlbnQ
tODMxMTIzNzEvKi9zZWdtZW50Pz8/Py5tcDQmYW1wO0tJRD1leGFtcGxlOmtleXM6
MTIzJmFtcDtNRD1kNjExN2Q3ZGI4YTY4YmQ1OWY2ZTdlMzM0MzQ4NDgzMWFjZDhmM
jNiYmFhN2Y0NGIyODVhMmYzYmI2ZjAyY2Zk").
2. Add a 'URISigningPackage' cookie to the HTTP 2xx Successful
message of the content being returned to the UA, with the value
set to the encoded Signed Token.
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5. Validating a URI Signature
The process of validating a Signed URI or Signed Token can be divided
into three sets of steps: first, extraction of the URI Signing
information elements, then validation of the URI signature to ensure
the integrity of the Signed URI or Signed Token, and finally,
validation of the information elements to ensure proper enforcement
of the distribution policy. In case the chained Signed Token
mechanism for HTTP Adaptive Streaming, as defined in Section 2, is
used, a fourth step for constructing and communicating the next
Signed Token, is added. Note the first three steps in the algorithm
described below apply irrespective of whether a Signed URI or Signed
Token is received.
In the algorithm below, the integrity of the Signed URI or Signed
Token is confirmed before distribution policy enforcement because
validation procedure would detect the right event when the URI is
tampered with. Note it is possible to use some other algorithm and
implementation as long as the same result is achieved.
5.1. Information Element Extraction
Extract the information elements embedded in the Signed URI or Signed
Token. Note that some steps are to be skipped if the corresponding
URI Signing Information Elements are not embedded in the Signed URI
or Signed Token.
1. Check if the query string component of the received URI contains
the 'URISigningPackage' attribute. If there are multiple
instances of this attribute, the first one is used and the
remaining ones are ignored. This ensures that the Signed URI
can be validated despite a client appending another instance of
the URI Signing Package attribute. If the query string
component of the received URI does not contain the URI Signing
Package attribute, check if the HTTP request contains a
'URISigningPackage' cookie and use that as the URI Signing
Package in the following steps. If the request does not contain
the URI Signing Package query string attribute and does not
contain a URISigningPackage cookie, the request is denied.
2. Decode the URI Signing Package using Base-64 Data Encoding
[RFC4648] to obtain all the URI Signing Information Elements in
the form of a concatenated string (e.g. "ET=1209422976&CIP=192.
0.2.1&KID=example:keys:123&MD=1ecb1446a6431352aab0fb6e0dca30e303
56593a97acb972202120dc482bddaf").
3. Extract the value from "VER", if the information element exists.
Determine the version of the URI Signing algorithm used to
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process the Signed URI or Signed Token. If the CDNI Metadata
interface is used, check to see if the used version of the URI
Signing algorithm is among the allowed set of URI Signing
versions specified by the metadata. If this is not the case,
the request is denied. If the information element is not in the
information elements string, then obtain the version number in
another manner (e.g. configuration, CDNI metadata or default
value).
4. Extract the value from "MD", if the information element exists.
The existence of this information element indicates a symmetric
key is used.
5. Extract the value from "DS", if the information element exists.
The existence of this information element indicates an
asymmetric key is used.
6. If neither the "MD" or "DS" attribute exists, then no URI
Signature exists and the request is denied. If both the "MD"
and the "DS" information elements are present, the Signed URI is
considered to be malformed and the request is denied.
7. Extract the value from "CIP", if the information element exists.
The existence of this information element indicates content
delivery is enforced based on client IP address.
8. Extract the value from "ET", if the information element exists.
The existence of this information element indicates content
delivery is enforced based on time.
9. Extract the value from "PP", if the information element exists.
The existence of this information element indicates content
delivery is enforced based on whether there is a match between
the path of the requested resource and the Path Pattern
information element. The existence of this element further
indicates that a chain of Signed Tokens is used, and a new
Signed Token should be generated and communicated upon
successful validation.
10. Extract the value from the "KID" or "KID_NUM" information
element, if they exist. The existence of either of these
information elements indicates a key can be referenced. If both
the "KID" and the "KID_NUM" information elements are present,
the Signed URI is considered to be malformed and the request is
denied.
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11. Extract the value from the "HF" information element, if it
exists. The existence of this information element indicates a
different hash function than the default.
12. Extract the value from the "DSA" information element, if it
exists. The existence of this information element indicates a
different digital signature algorithm than the default.
13. Extract the value from "USCF", if the information element
exists. The existence of this information element indicates
cookie-based communication for legacy UAs should be used for
signalling the next Signed Token in case a HTTP 2xx Succcessful
message is sent to the user.
14. Extract the value from "ETS", if the information element exists.
This information element indicates the validity time of the next
Signed Token in the chain.
5.2. Signature Validation
Validate the URI Signature for the Signed URI or Signed Token.
1. Create a new buffer for validating the Signed URI or Signed Token
in the steps below.
2. If the received URI Signing Package contains the Path Pattern
Information Element, this step can be skipped.
A. Copy the received URI, excluding the "scheme name" part, into
the buffer.
B. Remove the "URISigningPackage" attribute from the message, if
it exists. Remove any subsequent part of the query string
after the "URISigningPackage" attribute.
C. Append the decoded value from the "URISigningPackage"
attribute (which contains all the URI Signing Information
Elements).
3. If the received URI Signing Package does NOT contain the Path
Pattern Information Element, this step can be skipped. Copy the
decoded contents of the Signed Token in the buffer.
4. Depending on the type of key used to create the Signed URI or
Signed Token, validate the message digest or digital signature
for symmetric key or asymmetric keys, respectively.
A. For symmetric key, HMAC algorithm is used.
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a. If either the "KID" or "KID_NUM" information element
exists, validate that the key identifier is in the
allowable KID set as listed in the CDNI metadata or
configuration. The request is denied when the key
identifier is not allowed. If neither the "KID" or
"KID_NUM" information element is present in the received
URI Signing Package, obtain the shared key via CDNI
metadata or configuration.
b. If the "HF" information element exists, validate that the
hash function is in the allowable "HF" set as listed in
the CDNI metadata or configuration. The request is
denied when the hash function is not allowed. If the
"HF" information element is not in the received URI
Signing Package, the default hash function is SHA-256.
c. Extract the value from the "MD" information element.
This is the received message digest.
d. Convert the message digest to binary format. This will
be used to compare with the computed value later.
e. Remove the value part of the "MD" information element
(but not the '=' character) from the message. The
message is ready for validation of the message digest
(e.g. "://example.com/content.mov?ET=1209422976&CIP=192.0
.2.1&KID=example:keys:123&MD=").
f. Compute the message digest using the HMAC algorithm with
the shared key and message as the two inputs to the hash
function.
g. Compare the result with the received message digest to
validate the Signed URI or Signed Token. If there is no
match, the request is denied.
B. For asymmetric keys, a digital signature function is used.
a. If either the "KID" or "KID_NUM" information element
exists, validate that the key identifier is in the
allowable KID set as listed in the CDNI metadata or
configuration. The request is denied when the key
identifier is not allowed. If neither the "KID" or
"KID_NUM" information element is present in the received
URI Signing Package, obtain the public key via CDNI
metadata or configuration.
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b. If the "DSA" information element exists, validate that
the digital signature algorithm is in the allowable "DSA"
set as listed in the CDNI metadata or configuration. The
request is denied when the DSA is not allowed. If the
"DSA" information element is not in the received URI
Signing Package, the default DSA is EC-DSA.
c. Extract the value from the "DS" information element.
This is the received digital signature.
d. Convert the digital signature to binary format. This
will be used for verification later.
e. Remove the value part of the "DS" information element
(but not the '=' character) from the message. The
message is ready for validation of the digital signature
(e.g. "://example.com/content.mov?ET=1209422976&CIP=192.0
.2.1&KID=http://example.com/public/keys/123&DS=").
f. Compute the message digest using SHA-1 (without a key)
for the message.
g. Verify the digital signature using the digital signature
function (e.g. EC-DSA) with the public key, received
digital signature, and message digest (obtained in
previous step) as inputs. This validates the Signed URI
or Signed Token. If signature is determined to be
invalid, the request is denied.
5.3. Distribution Policy Enforcement
Note the associated steps are to be skipped if the corresponding URI
Signing Information Elements are not in the received URI Signing
Package. The absence of a given Enforcement Information Element
indicates enforcement of its purpose is not necessary in the CSP's
distribution policy. The exception is the Path Pattern Information
Element, which is mandatory for Signed Tokens.
1. If the "CIP" information element exists, validate that the
request came from the same IP address as indicated in the "CIP"
information element. If the IP address is incorrect, the request
is denied.
2. If the "ET" information element exists, validate that the request
arrived before expiration time based on the "ET" information
element. If the time expired, the request is denied.
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3. If the "PP" information element exists, validate that the
requested resource is in the allowed set by matching the received
URI against the Path Pattern information element. If there is no
match, the request is denied.
5.4. Subsequent Signed Token Generation
The following steps describe how to generate a subsequent Signed
Token in a chain of Signed Tokens and are to be skipped in case the
chained Signed Token mechanism for HTTP Adaptive Streaming content is
not used. Note that the process for generating an initial Signed
Token is described in Section 4.2 and the process below is used for
generating all subsequent tokens after the initial one.
The process of generating a subsequent Signed Token can be divided
into two sets of steps: first, calculating the URI Signature and
then, packaging the URI Signature along with the URI Signing
Information Elements into a URI Signing Package to construct a new
Signed Token and appending the Signed Token to the message. Note it
is possible to use some other algorithm and implementation as long as
the same result is achieved.
5.4.1. Calculating the URI Signature (subsequent Signed Token)
Calculate the URI Signature for use with the new Signed Token by
following the procedure below.
1. Create a new buffer for constructing the new Signed Token in the
steps below.
2. If the URI Signing version used in the received URI Signing
Package is the default one (i.e. "1"), skip this step.
Otherwise, specify the version by appending the string "VER=#",
where '#' represents the version number. The following steps in
the procedure is based on the initial version of URI Signing
specified by this document. For other versions, reference the
associated RFC for the URI signing procedure.
3. If the received URI Signing Package does not contain the "ET"
information element, skip this step.
A. If an information element was added to the message, append an
"&" character. Append the string "ET=".
B. If the received URI Signing Package contains the "ETS"
information element, perform this step.
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1. Get the value of the "ETS" information element and
convert it to an integer.
2. Get the current time in seconds sinds epoch (as an
integer) and add the value of the "ETS" information
element as seconds.
3. Convert the result to a string and append it to the
message.
4. Append the "&" character and the "ETS=" string.
5. Append the value of the "ETS" information element in the
received URI Signing Package.
C. If the received URI Signing Package does not contain the
"ETS" information element, perform this step. Get the value
of the "ET" information element from the original
concatenated information element string and append it to the
message.
4. If the received URI Signing Package does not contain the "CIP"
information element, skip this step.
A. If an information element was added to the message, append an
"&" character. Append the string "CIP=".
B. Append the value of the "CIP" information element in the
received URI Signing Package.
5. If an information element was added to the message, append an "&"
character. Append the string "PP=". Append the value of the
"PP" information element in the received URI Signing Package.
6. Depending on the type of key used to sign the received Signed
Token, compute the message digest or digital signature for
symmetric key or asymmetric keys, respectively.
A. For symmetric key, HMAC is used.
1. Obtain the shared key to be used for signing the Signed
Token.
2. If the key identifier is not needed, skip this step. If
an information element was added to the message, append
an "&" character. Append the string "KID=" in case a
string-based Key ID is used, or "KID_NUM=" in case a
numerical Key ID is used. Append the key identifier
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(e.g. "example:keys:123" or "56128239") needed by the
entity to locate the shared key for validating the URI
signature.
3. Optional: If the hash function for the HMAC uses the
default value ("SHA-256"), skip this step. If an
information element was added to the message, append an
"&" character. append the string "HF=". Append the
string for the new type of hash function to be used.
Note one MUST use the same hash function as communicated
in the received URI Signing Package or one of the
allowable hash functions designated by the CDNI metadata.
4. If an information element was added to the message,
append an "&" character. Append the string "MD=". The
message now contains the complete set of URI Signing
Information Elements over which the URI Signature is
computed (e.g.
"ET=1209422976&ETS=15&CIP=192.0.2.1&PP=*/content-
83112371/*/segment????.mp4&KID=example:keys:123&MD=").
5. Compute the message digest using the HMAC algorithm and
the default SHA-256 hash function, or another hash
function if specified by the HF Information Element, with
the shared key and message as the two inputs to the hash
function.
6. Convert the message digest to its equivalent hexadecimal
format.
7. Append the string for the message digest (e.g.
"ET=1209422976&ETS=15&CIP=192.0.2.1&PP=*/content-83112371
/*/segment????.mp4&KID=example:keys:123&MD=d6117d7db8a68b
d59f6e7e3343484831acd8f23bbaa7f44b285a2f3bb6f02cfd").
B. For asymmetric keys, EC DSA is used.
1. Generate the EC private and public key pair. Store the
EC public key in a location that's reachable for any
entity that needs to validate the URI signature.
2. If the key identifier is not needed, skip this step. If
an information element was added to the message, append
an "&" character. Append the string "KID=" in case a
string-based Key ID is used, or "KID_NUM=" in case a
numerical Key ID is used. Append the key identifier
(e.g. "http://example.com/public/keys/123") needed by the
entity to locate the shared key for validating the URI
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signature. Note that in the case the Key ID URI is a URL
to a public key, the Key ID URI SHOULD only contain the
"scheme name", "authority", and "path" parts (i.e. query
string is not allowed).
3. Optional: If the digital signature algorithm uses the
default value ("EC-DSA"), skip this step. If an
information element was added to the message, append an
"&" character. Append the string "DSA=". Append the
string denoting the new digital signature function.
4. If an information element was added to the message,
append an "&" character. Append the string "DS=". The
message now contains the complete set of URI Signing
Information Elements over which the URI Signature is
computed (e.g.
"ET=1209422976&ETS=15&CIP=192.0.2.1&PP=*/content-
83112371/*/segment????.mp4&KID=example:keys:123&DS=").
5. Compute the message digest using SHA-1 (without a key)
for the message. Note: The digital signature generated
in the next step is calculated over the SHA-1 message
digest, instead of over the cleartype message, to reduce
the length of the digital signature, and thereby the
length of the URI Signing Package Attribute and the
resulting Signed URI. Since SHA-1 is not used for
cryptographic purposes here, the security concerns around
SHA-1 do not apply.
6. Compute the digital signature, using the EC-DSA algorithm
by default or another algorithm if specified by the DSA
Information Element, with the private EC key and message
digest (obtained in previous step) as inputs.
7. Convert the digital signature to its equivalent
hexadecimal format.
8. Append the string for the digital signature. In the case
where EC-DSA algorithm is used, this string contains the
values for the 'r' and 's' parameters, delimited by ':'
(e.g. "ET=1209422976&ETS=15&CIP=192.0.2.1&PP=*/content-8
3112371/*/segment????.mp4&KID=example:keys:123&DS=r:CFB03
EDB33810AB6C79EE3C47FBD86D227D702F25F66C01CF03F59F1E00566
8D:s:57ED0E8DF7E786C87E39177DD3398A7FB010E6A4C0DC8AA71331
A929A29EA24E" )
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5.4.2. Packaging the URI Signature (subsequent Signed Token)
The following steps depend on whether the Signed Token is
communicated to the UA via an HTTP 3xx Redirection message or via an
HTTP 2xx Successful message. In the case of a redirection response,
the Signed Token can be communicated as part of the query string
component of the URL signalled via the Location header of the
Redirection message. In the case of a 2xx Successful message, the
Signed Token can be communicated via either a dedicated HTTP header
or, for legacy UAs, via the Set-Cookie header. If the received URI
Signing Package contains the 'USCF' Information Element, the new
Signed Token MUST be communicated via the Cookie method. If the
received URI Signing Package does NOT contain the 'USCF' Information
Element, the new Signed Token SHALL be communicated via the dedicated
HTTP header.
5.4.2.1. Communicating the Signed Token in a HTTP 3xx Redirection
message
The following steps describe how the new Signed Token can be
communicated to the UA via an HTTP 3xx Redirection message.
1. Copy the target URI of the HTTP 3xx Redirection message into a
buffer to hold the message.
2. Check if the URI already contains a query string. If not, append
a "?" character. If yes, append an "&" character.
3. Append the parameter name used to indicate the URI Signing
Package Attribute, as communicated via the CDNI Metadata
interface, followed by an "=". If none is communicated by the
CDNI Metadata interface, it defaults to "URISigningPackage". For
example, if the CDNI Metadata interface specifies "SIG", append
the string "SIG=" to the message.
4. Encode the Signed Token by applying Base-64 Data Encoding
[RFC4648] on the value of the Signed Token (e.g. "RVQ9MTIwOTQyMj
k3NiZhbXA7RVRTPTE1JmFtcDtDSVA9MTkyLjAuMi4xJmFtcDtQUD0qL2NvbnRlbnQ
tODMxMTIzNzEvKi9zZWdtZW50Pz8/Py5tcDQmYW1wO0tJRD1leGFtcGxlOmtleXM6
MTIzJmFtcDtNRD1kNjExN2Q3ZGI4YTY4YmQ1OWY2ZTdlMzM0MzQ4NDgzMWFjZDhmM
jNiYmFhN2Y0NGIyODVhMmYzYmI2ZjAyY2Zk").
5. Append the URI Signing token to the message (e.g.
"http://example.com/folder/content-83112371/manifest.xml?URISigni
ngPackage=RVQ9MTIwOTQyMjk3NiZhbXA7RVRTPTE1JmFtcDtDSVA9MTkyLjAuMi4
xJmFtcDtQUD0qL2NvbnRlbnQtODMxMTIzNzEvKi9zZWdtZW50Pz8/Py5tcDQmYW1w
O0tJRD1leGFtcGxlOmtleXM6MTIzJmFtcDtNRD1kNjExN2Q3ZGI4YTY4YmQ1OWY2Z
TdlMzM0MzQ4NDgzMWFjZDhmMjNiYmFhN2Y0NGIyODVhMmYzYmI2ZjAyY2Zk").
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6. Place the message in the Location header of the HTTP 3xx
Redirection message returned to the UA.
5.4.2.2. Communicating the Signed Token in a HTTP 2xx Successful
message
The following steps describe how the new Signed Token can be
communicated to the UA via an HTTP 2xx Successful message.
5.4.2.2.1. Header-based
If the received URI Signing Package does NOT contain the 'USCF'
Information Element, the new Signed Token SHALL be communicated via
the following method.
1. Encode the Signed Token by applying Base-64 Data Encoding
[RFC4648] on the value of the Signed Token (e.g. "RVQ9MTIwOTQyMj
k3NiZhbXA7RVRTPTE1JmFtcDtDSVA9MTkyLjAuMi4xJmFtcDtQUD0qL2NvbnRlbnQ
tODMxMTIzNzEvKi9zZWdtZW50Pz8/Py5tcDQmYW1wO0tJRD1leGFtcGxlOmtleXM6
MTIzJmFtcDtNRD1kNjExN2Q3ZGI4YTY4YmQ1OWY2ZTdlMzM0MzQ4NDgzMWFjZDhmM
jNiYmFhN2Y0NGIyODVhMmYzYmI2ZjAyY2Zk").
2. Place the value of the encoded Signed Token in the URI Signing
Header of the HTTP 2xx Successful message of the content being
returned to the UA. Note: The HTTP Header name to use is
communicated via the CDNI Metadata Interface or set via
configuration. Otherwise, it defaults to 'URISigningPackage'.
5.4.2.2.2. Cookie-based
If the received URI Signing Package contains the 'USCF' Information
Element, the new Signed Token MUST be communicated via the following
method.
1. Encode the Signed Token by applying Base-64 Data Encoding
[RFC4648] on the value of the Signed Token (e.g. "RVQ9MTIwOTQyMj
k3NiZhbXA7RVRTPTE1JmFtcDtDSVA9MTkyLjAuMi4xJmFtcDtQUD0qL2NvbnRlbnQ
tODMxMTIzNzEvKi9zZWdtZW50Pz8/Py5tcDQmYW1wO0tJRD1leGFtcGxlOmtleXM6
MTIzJmFtcDtNRD1kNjExN2Q3ZGI4YTY4YmQ1OWY2ZTdlMzM0MzQ4NDgzMWFjZDhmM
jNiYmFhN2Y0NGIyODVhMmYzYmI2ZjAyY2Zk").
2. Add a 'URISigningPackage' cookie to the HTTP 2xx Successful
message of the content being returned to the UA, with the value
set to the encoded Signed Token.
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6. Relationship with CDNI Interfaces
Some of the CDNI Interfaces need enhancements to support URI Signing.
As an example: A Downstream CDN that supports URI Signing needs to be
able to advertise this capability to the Upstream CDN. The Upstream
CDN needs to select a Downstream CDN based on such capability when
the CSP requires access control to enforce its distribution policy
via URI Signing. Also, the Upstream CDN needs to be able to
distribute via the CDNI Metadata interface the information necessary
to allow the Downstream CDN to validate a Signed URI . Events that
pertain to URI Signing (e.g. request denial or delivery after access
authorization) need to be included in the logs communicated through
the CDNI Logging interface (Editor's Note: Is this within the scope
of the CDNI Logging interface?).
6.1. CDNI Control Interface
URI Signing has no impact on this interface.
6.2. CDNI Footprint & Capabilities Advertisement Interface
The Downstream CDN advertises its capability to support URI Signing
via the CDNI Footprint & Capabilities Advertisement interface (FCI).
The supported version of URI Signing needs to be included to allow
for future extensibility.
In general, new information elements introduced to enhance URI
Signing requires a draft and a new version. ForInformation Elements,
For Enforcement Information Elements, there is no need to
advertise the based information elements such as "CIP" and "ET".
For Signature Computation Information Elements:
No need to advertise "VER" Information Element unless it's not
"1". In this case, a draft is needed to describe the new
version.
Advertise value of the "HF" Information Element (i.e. SHA-256)
to indicate support for the hash function; Need IANA assignment
for new hash function.
Advertise value of the "DSA" Information Element (i.e. EC-DSA)
to indicate support for the DSA; Need IANA assignment for new
digital signature algorithm.
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Advertise "MD" Information Element (i.e. EC-DSA) to indicate
support for symmetric key method; A new draft is needed for an
alternative method.
Advertise "DS" Information Element (i.e. EC-DSA) to indicate
support for asymmetric key method; A new draft is needed for an
alternative method.
For URI Signing Package Attribute, there is no need to advertise
the base attribute.
6.3. CDNI Request Routing Redirection Interface
The CDNI Request Routing Redirection Interface
[I-D.ietf-cdni-redirection] describes the recursive request
redirection method. For URI Signing, the Upstream CDN signs the URI
provided by the Downstream CDN. This approach has the following
benefits:
Consistency with interative request routing method
URI Signing is fully operational even when Downstream CDN does not
have the signing function (which may be the case when the
Downstream CDN operates only as a delivering CDN)
Upstream CDN can act as a conversion gateway for the requesting
routing interface between Upstream CDN and CSP and request routing
interface between Upstream CDN and Downstream CDN since these two
interfaces may not be the same
6.4. CDNI Metadata Interface
The CDNI Metadata Interface [I-D.ietf-cdni-metadata] describes the
CDNI metadata distribution in order to enable content acquisition and
delivery. For URI Signing, additional CDNI metadata objects are
specified. In general, an Empty set means "all". These are the CDNI
metadata objects used for URI Signing.
The UriSigning Metadata object contains information to enable URI
signing and validation by a dCDN. The UriSigning properties are
defined below.
Property: enforce
Description: URI Signing enforcement flag. Specifically, this
flag indicates if the access to content is subject to URI
Signing. URI Signing requires the Downstream CDN to ensure
that the URI must be signed and validated before content
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delivery. Otherwise, Downstream CDN does not perform
validation regardless if URI is signed or not.
Type: Boolean
Mandatory-to-Specify: No. If a UriSigning object is present in
the metadata for a piece of content (even if the object is
empty), then URI signing should be enforced. If no UriSigning
object is present in the metadata for a piece of content, then
the URI signature should not be validated.
Property: key-id
Description: Designated key identifier used for URI Signing
computation when the Signed URI does not contain the Key ID
information element.
Type: String
Mandatory-to-Specify: No. A Key ID is not essential for all
implementations of URI signing.
Property: key-id-set
Description: Allowable Key ID set that the Signed URI's Key ID
information element can reference.
Type: List of Strings
Mandatory-to-Specify: No. Default is to allow any Key ID.
Property: hash-function
Description: Designated hash function used for URI Signing
computation when the Signed URI does not contain the Hash
Function information element.
Type: String (limited to the hash function strings in the
registry defined by the IANA Considerations (Section 8)
section)
Mandatory-to-Specify: No. Default is SHA-256.
Property: hash-function-set
Description: Allowable Hash Function set that the Signed URI's
Hash Function information element can reference.
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Type: List of Strings
Mandatory-to-Specify: No. Default is to allow any hash
function.
Property: digital-signature-algorithm
Description: Designated digital signature function used for URI
Signing computation when the Signed URI does not contain the
Digital Signature Algorithm information element.
Type: String (limited to the digital signature algorithm
strings in the registry defined by the IANA Considerations
(Section 8) section).
Mandatory-to-Specify: No. Default is EC-DSA.
Property: digital-signature-algorithm-set
Description: Allowable digital signature function set that the
Signed URI's Digital Signature Algorithm information element
can reference.
Type: List of Strings
Mandatory-to-Specify: No. Default is to allow any DSA.
Property: version
Description: Designated version used for URI Signing
computation when the Signed URI does not contain the VER
attribute.
Type: Integer
Mandatory-to-Specify: No. Default is 1.
Property: version-set
Description: Allowable version set that the Signed URI's VER
attribute can reference.
Type: List of Integers
Mandatory-to-Specify: No. Default is to allow any version.
Property: package-attribute
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Description: Overwrite the default name for the URL Signing
Package Attribute.
Type: String
Mandatory-to-Specify: No. Default is "URISigningPackage".
Note that the Key ID information element is not needed if only one
key is provided by the CSP or the Upstream CDN for the content item
or set of content items covered by the CDNI Metadata object. In the
case of asymmetric keys, it's easy for any entity to sign the URI for
content with a private key and provide the public key in the Signed
URI. This just confirms that the URI Signer authorized the delivery.
But it's necessary for the URI Signer to be the content owner. So,
the CDNI Metadata interface or configuration MUST provide the
allowable Key ID set to authorize the Key ID information element
embedded in the Signed URI.
6.5. CDNI Logging Interface
For URI Signing, the Downstream CDN reports that enforcement of the
access control was applied to the request for content delivery. When
the request is denied due to enforcement of URI Signing, the reason
is logged.
The following CDNI Logging field for URI Signing SHOULD be supported
in the HTTP Request Logging Record as specified in CDNI Logging
Interface [I-D.ietf-cdni-logging].
o s-uri-signing (mandatory):
* format: 3DIGIT
* field value: this characterises the uri signing validation
performed by the Surrogate on the request. The allowed values
are:
+ "0" : no uri signature validation performed
+ "1" : uri signature validation performed and validated
+ "2" : uri signature validation performed and rejected
* occurrence: there MUST be zero or exactly one instance of this
field.
o s-uri-signing-deny-reason (optional):
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* format: QSTRING
* field value: the rejection reason when uri signature performed
by the Surrogate on the request. Examples:
+ "invalid client IP address"
+ "expired signed URI"
+ "incorrect URI signature"
* occurrence: there MUST be zero or exactly one instance of this
field.
7. URI Signing Message Flow
URI Signing supports both HTTP-based and DNS-based request routing.
HMAC [RFC2104] defines a hash-based message authentication code
allowing two parties that share a symmetric key or asymmetric keys to
establish the integrity and authenticity of a set of information
(e.g. a message) through a cryptographic hash function.
7.1. HTTP Redirection
For HTTP-based request routing, HMAC is applied to a set of
information that is unique to a given end user content request using
key information that is specific to a pair of adjacent CDNI hops
(e.g. between the CSP and the Authoritative CDN, between the
Authoritative CDN and a Downstream CDN). This allows a CDNI hop to
ascertain the authenticity of a given request received from a
previous CDNI hop.
The URI signing scheme described below is based on the following
steps (assuming HTTP redirection, iterative request routing and a CDN
path with two CDNs). Note that Authoritative CDN and Upstream CDN
are used exchangeably.
End-User dCDN uCDN CSP
| | | |
| 1.CDNI FCI interface used to | |
| advertise URI Signing capability| |
| |------------------->| |
| | | |
| 2.Provides information to validate URI signature|
| | |<-------------------|
| | | |
| 3.CDNI Metadata interface used to| |
| provide URI Signing attributes| |
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| |<-------------------| |
|4.Authorization request | |
|------------------------------------------------------------->|
| | | [Apply distribution
| | | policy] |
| | | |
| | (ALT: Authorization decision)
|5.Request is denied | | <Negative> |
|<-------------------------------------------------------------|
| | | |
|6.CSP provides signed URI | <Positive> |
|<-------------------------------------------------------------|
| | | |
|7.Content request | | |
|---------------------------------------->| [Validate URI |
| | | signature] |
| | | |
| | (ALT: Validation result) |
|8.Request is denied | <Negative>| |
|<----------------------------------------| |
| | | |
|9.Re-sign URI and redirect to <Positive>| |
| dCDN (newly signed URI) | |
|<----------------------------------------| |
| | | |
|10.Content request | | |
|------------------->| [Validate URI | |
| | signature] | |
| | | |
| (ALT: Validation result) | |
|11.Request is denied| <Negative> | |
|<-------------------| | |
| | | |
|12.Content delivery | <Positive> | |
|<-------------------| | |
: : : :
: (Later in time) : : :
|13.CDNI Logging interface to include URI Signing information |
| |------------------->| |
Figure 3: HTTP-based Request Routing with URI Signing
1. Using the CDNI Footprint & Capabilities Advertisement interface,
the Downstream CDN advertises its capabilities including URI
Signing support to the Authoritative CDN.
2. CSP provides to the Authoritative CDN the information needed to
validate URI signatures from that CSP. For example, this
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information may include a hashing function, algorithm, and a key
value.
3. Using the CDNI Metadata interface, the Authoritative CDN
communicates to a Downstream CDN the information needed to
validate URI signatures from the Authoritative CDN for the given
CSP. For example, this information may include the URI query
string parameter name for the URI Signing Package Attribute, a
hashing algorithm and/or a key corresponding to the trust
relationship between the Authoritative CDN and the Downstream
CDN.
4. When a UA requests a piece of protected content from the CSP,
the CSP makes a specific authorization decision for this unique
request based on its arbitrary distribution policy
5. If the authorization decision is negative, the CSP rejects the
request.
6. If the authorization decision is positive, the CSP computes a
Signed URI that is based on unique parameters of that request
and conveys it to the end user as the URI to use to request the
content.
7. On receipt of the corresponding content request, the
authoritative CDN validates the URI Signature in the URI using
the information provided by the CSP.
8. If the validation is negative, the authoritative CDN rejects the
request
9. If the validation is positive, the authoritative CDN computes a
Signed URI that is based on unique parameters of that request
and provides to the end user as the URI to use to further
request the content from the Downstream CDN
10. On receipt of the corresponding content request, the Downstream
CDN validates the URI Signature in the Signed URI using the
information provided by the Authoritative CDN in the CDNI
Metadata
11. If the validation is negative, the Downstream CDN rejects the
request and sends an error code (e.g. 403) in the HTTP response.
12. If the validation is positive, the Downstream CDN serves the
request and delivers the content.
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13. At a later time, Downstream CDN reports logging events that
includes URI signing information.
With HTTP-based request routing, URI Signing matches well the general
chain of trust model of CDNI both with symmetric key and asymmetric
keys because the key information only need to be specific to a pair
of adjacent CDNI hops.
7.2. DNS Redirection
For DNS-based request routing, the CSP and Authoritative CDN must
agree on a trust model appropriate to the security requirements of
the CSP's particular content. Use of asymmetric public/private keys
allows for unlimited distribution of the public key to Downstream
CDNs. However, if a shared secret key is preferred, then the CSP may
want to restrict the distribution of the key to a (possibly empty)
subset of trusted Downstream CDNs. Authorized Delivery CDNs need to
obtain the key information to validate the Signed UR, which is
computed by the CSP based on its distribution policy.
The URI signing scheme described below is based on the following
steps (assuming iterative DNS request routing and a CDN path with two
CDNs). Note that Authoritative CDN and Upstream CDN are used
exchangeably.
End-User dCDN uCDN CSP
| | | |
| 1.CDNI FCI interface used to | |
| advertise URI Signing capability| |
| |------------------->| |
| | | |
| 2.Provides information to validate URI signature|
| | |<-------------------|
| 3.CDNI Metadata interface used to| |
| provide URI Signing attributes| |
| |<-------------------| |
|4.Authorization request | |
|------------------------------------------------------------->|
| | | [Apply distribution
| | | policy] |
| | | |
| | (ALT: Authorization decision)
|5.Request is denied | | <Negative> |
|<-------------------------------------------------------------|
| | | |
|6.Provides signed URI | <Positive> |
|<-------------------------------------------------------------|
| | | |
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|7.DNS request | | |
|---------------------------------------->| |
| | | |
|8.Redirect DNS to dCDN | |
|<----------------------------------------| |
| | | |
|9.DNS request | | |
|------------------->| | |
| | | |
|10.IP address of Surrogate | |
|<-------------------| | |
| | | |
|11.Content request | | |
|------------------->| [Validate URI | |
| | signature] | |
| | | |
| (ALT: Validation result) | |
|12.Request is denied| <Negative> | |
|<-------------------| | |
| | | |
|13.Content delivery | <Positive> | |
|<-------------------| | |
: : : :
: (Later in time) : : :
|14.CDNI Logging interface to report URI Signing information |
| |------------------->| |
Figure 4: DNS-based Request Routing with URI Signing
1. Using the CDNI Footprint & Capabilities Advertisement interface,
the Downstream CDN advertises its capabilities including URI
Signing support to the Authoritative CDN.
2. CSP provides to the Authoritative CDN the information needed to
validate cryptographic signatures from that CSP. For example,
this information may include a hash function, algorithm, and a
key.
3. Using the CDNI Metadata interface, the Authoritative CDN
communicates to a Downstream CDN the information needed to
validate cryptographic signatures from the CSP (e.g. the URI
query string parameter name for the URI Signing Package
Attribute). In the case of symmetric key, the Authoritative CDN
checks if the Downstream CDN is allowed by CSP to obtain the
shared secret key.
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4. When a UA requests a piece of protected content from the CSP,
the CSP makes a specific authorization decision for this unique
request based on its arbitrary distribution policy.
5. If the authorization decision is negative, the CSP rejects the
request
6. If the authorization decision is positive, the CSP computes a
cryptographic signature that is based on unique parameters of
that request and includes it in the URI provided to the end user
to request the content.
7. End user sends DNS request to the authoritative CDN.
8. On receipt of the DNS request, the authoritative CDN redirects
the request to the Downstream CDN.
9. End user sends DNS request to the Downstream CDN.
10. On receipt of the DNS request, the Downstream CDN responds with
IP address of one of its Surrogates.
11. On receipt of the corresponding content request, the Downstream
CDN validates the cryptographic signature in the URI using the
information provided by the Authoritative CDN in the CDNI
Metadata
12. If the validation is negative, the Downstream CDN rejects the
request and sends an error code (e.g. 403) in the HTTP response.
13. If the validation is positive, the Downstream CDN serves the
request and delivers the content.
14. At a later time, Downstream CDN reports logging events that
includes URI signing information.
With DNS-based request routing, URI Signing matches well the general
chain of trust model of CDNI when used with asymmetric keys because
the only key information that need to be distributed across multiple
CDNI hops including non-adjacent hops is the public key, that is
generally not confidential.
With DNS-based request routing, URI Signing does not match well the
general chain of trust model of CDNI when used with symmetric keys
because the symmetric key information needs to be distributed across
multiple CDNI hops including non-adjacent hops. This raises a
security concern for applicability of URI Signing with symmetric keys
in case of DNS-based inter-CDN request routing.
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8. IANA Considerations
[Editor's note: (Is there a need to) register default value for URI
Signing Package Attribute URI query string parameter name (i.e.
URISigningPackage) to be used for URI Signing? Need anything from
IANA?]
[Editor's note: To do: Convert to proper IANA Registry format]
This document requests IANA to create three new URI Signing
registries for the Information Elements and their defined values to
be used for URI Signing.
The following Enforcement Information Element names are allocated:
o ET (Expiry time)
o CIP (Client IP address)
The following Signature Computation Information Element names are
allocated:
o VER (Version): 1 (Base)
o KID (Key ID)
o KID_NUM (Numerical Key ID)
o HF (Hash Function): "SHA-256"
o DSA (Digital Signature Algorithm): "EC-DSA"
The following URI Signature Information Element names are allocated:
o MD (Message Digest for Symmetric Key)
o DS (Digital Signature for Asymmetric Keys)
The IANA is requested to allocate a new entry to the CDNI Logging
Field Names Registry as specified in CDNI Logging Interface
[I-D.ietf-cdni-logging] in accordance to the "Specification Required"
policy [RFC5226]
o s-url-signing
o s-url-signing-deny-reason
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The IANA is requested to allocate a new entry to the "CDNI
GenericMetadata Types" Registry as specified in CDNI Metadata
Interface [I-D.ietf-cdni-metadata] in accordance to the
"Specification Required" policy [RFC5226]:
+------------+---------------+---------+------+------+
| Type name | Specification | Version | MTE | STR |
+------------+---------------+---------+------+------+
| UriSigning | RFCthis | 1 | true | true |
+------------+---------------+---------+------+------+
The IANA is also requested to allocate a new MIME type under the IANA
MIME Media Type registry for the UriSigning metadata object:
application/cdni.UriSigning.v1
9. Security Considerations
This document describes the concept of URI Signing and how it can be
used to provide access authorization in the case of interconnected
CDNs (CDNI). The primary goal of URI Signing is to make sure that
only authorized UAs are able to access the content, with a Content
Service Provider (CSP) being able to authorize every individual
request. It should be noted that URI Signing is not a content
protection scheme; if a CSP wants to protect the content itself,
other mechanisms, such as DRM, are more appropriate.
In general, it holds that the level of protection against
illegitimate access can be increased by including more Enforcement
Information Elements in the URI. The current version of this
document includes elements for enforcing Client IP Address and
Expiration Time, however this list can be extended with other, more
complex, attributes that are able to provide some form of protection
against some of the vulnerabilities highlighted below.
That said, there are a number of aspects that limit the level of
security offered by URI signing and that anybody implementing URI
signing should be aware of.
Replay attacks: Any (valid) Signed URI can be used to perform
replay attacks. The vulnerability to replay attacks can be
reduced by picking a relatively short window for the Expiration
Time attribute, although this is limited by the fact that any
HTTP-based request needs a window of at least a couple of seconds
to prevent any sudden network issues from preventing legitimate
UAs access to the content. One way to reduce exposure to replay
attacks is to include in the URI a unique one-time access ID.
Whenever the Downstream CDN receives a request with a given unique
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access ID, it adds that access ID to the list of 'used' IDs. In
the case an illegitimate UA tries to use the same URI through a
replay attack, the Downstream CDN can deny the request based on
the already-used access ID.
Illegitimate client behind a NAT: In cases where there are
multiple users behind the same NAT, all users will have the same
IP address from the point of view of the Downstream CDN. This
results in the Downstream CDN not being able to distinguish
between the different users based on Client IP Address and
illegitimate users being able to access the content. One way to
reduce exposure to this kind of attack is to not only check for
Client IP but also for other attributes that can be found in the
HTTP headers.
The shared key between CSP and Authoritative CDN may be distributed
to Downstream CDNs - including cascaded CDNs. Since this key can be
used to legitimately sign a URL for content access authorization,
it's important to know the implications of a compromised shared key.
In the case where asymmetric keys are used, the KID information
element might contain the URL to the public key. To prevent
malicious clients from signing their own URIs and inserting the
associated public key URL in the KID field, thereby passing URI
validation, it is important that CDNs check whether the URI conveyed
in the KID field is in the allowable set of KIDs as listed in the
CDNI metadata or set via configuration.
10. Privacy
The privacy protection concerns described in CDNI Logging Interface
[I-D.ietf-cdni-logging] apply when the client's IP address (CIP
attribute) is embedded in the Signed URI. This means that, when
anonymization is enabled, the value of the URI Signing Package
Attribute MUST be removed from the logging record.
11. Acknowledgements
The authors would like to thank the following people for their
contributions in reviewing this document and providing feedback:
Scott Leibrand, Kevin Ma, Ben Niven-Jenkins, Thierry Magnien, Dan
York, Bhaskar Bhupalam, Matt Caulfield, Samuel Rajakumar, Iuniana
Oprescu and Leif Hedstrom. In addition, Matt Caulfield provided
content for the CDNI Metadata Interface section.
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12. References
12.1. Normative References
[I-D.ietf-cdni-logging]
Faucheur, F., Bertrand, G., Oprescu, I., and R.
Peterkofsky, "CDNI Logging Interface", draft-ietf-cdni-
logging-18 (work in progress), March 2015.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6707] Niven-Jenkins, B., Le Faucheur, F., and N. Bitar, "Content
Distribution Network Interconnection (CDNI) Problem
Statement", RFC 6707, September 2012.
12.2. Informative References
[I-D.ietf-cdni-metadata]
Niven-Jenkins, B., Murray, R., Caulfield, M., and K. Ma,
"CDN Interconnection Metadata", draft-ietf-cdni-
metadata-09 (work in progress), March 2015.
[I-D.ietf-cdni-redirection]
Niven-Jenkins, B. and R. Brandenburg, "Request Routing
Redirection Interface for CDN Interconnection", draft-
ietf-cdni-redirection-09 (work in progress), April 2015.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, September 2001.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, August 2010.
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[RFC6983] van Brandenburg, R., van Deventer, O., Le Faucheur, F.,
and K. Leung, "Models for HTTP-Adaptive-Streaming-Aware
Content Distribution Network Interconnection (CDNI)", RFC
6983, July 2013.
[RFC7336] Peterson, L., Davie, B., and R. van Brandenburg,
"Framework for Content Distribution Network
Interconnection (CDNI)", RFC 7336, August 2014.
[RFC7337] Leung, K. and Y. Lee, "Content Distribution Network
Interconnection (CDNI) Requirements", RFC 7337, August
2014.
Authors' Addresses
Kent Leung
Cisco Systems
3625 Cisco Way
San Jose 95134
USA
Phone: +1 408 526 5030
Email: kleung@cisco.com
Francois Le Faucheur
Cisco Systems
Greenside, 400 Avenue de Roumanille
Sophia Antipolis 06410
France
Phone: +33 4 97 23 26 19
Email: flefauch@cisco.com
Ray van Brandenburg
TNO
Anna van Buerenplein 1
Den Haag 2595DC
the Netherlands
Phone: +31 88 866 7000
Email: ray.vanbrandenburg@tno.nl
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Bill Downey
Verizon Labs
60 Sylvan Road
Waltham, Massachusetts 02451
USA
Phone: +1 781 466 2475
Email: william.s.downey@verizon.com
Michel Fisher
Limelight Networks
222 S Mill Ave
Tempe, AZ 85281
USA
Phone: +1 360 419 5185
Email: mfisher@llnw.com
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