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CDNI                                                            K. Leung
Internet-Draft                                            F. Le Faucheur
Intended status: Standards Track                           Cisco Systems
Expires: February 13, 2016                            R. van Brandenburg
                                                                     TNO
                                                               B. Downey
                                                            Verizon Labs
                                                               M. Fisher
                                                      Limelight Networks
                                                         August 12, 2015


               URI Signing for CDN Interconnection (CDNI)
                     draft-ietf-cdni-uri-signing-05

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.  Some
   of the information may be accessed by the CDN via configuration or
   CDNI metadata.

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 February 13, 2016.








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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Background on URI Signing . . . . . . . . . . . . . . . .   4
     1.3.  CDNI URI Signing Overview . . . . . . . . . . . . . . . .   6
     1.4.  URI Signing in a non-CDNI context . . . . . . . . . . . .   8
   2.  Signed URI Information Elements . . . . . . . . . . . . . . .   8
     2.1.  Enforcement Information Elements  . . . . . . . . . . . .  10
     2.2.  Signature Computation Information Elements  . . . . . . .  11
     2.3.  URI Signature Information Elements  . . . . . . . . . . .  12
     2.4.  URI Signing Package Attribute . . . . . . . . . . . . . .  13
     2.5.  User Agent Attributes . . . . . . . . . . . . . . . . . .  14
   3.  Creating the Signed URI . . . . . . . . . . . . . . . . . . .  14
     3.1.  Calculating the URI Signature . . . . . . . . . . . . . .  15
     3.2.  Packaging the URI Signature . . . . . . . . . . . . . . .  18
   4.  Validating a URI Signature  . . . . . . . . . . . . . . . . .  19
     4.1.  Information Element Extraction  . . . . . . . . . . . . .  19
     4.2.  Signature Validation  . . . . . . . . . . . . . . . . . .  20
     4.3.  Distribution Policy Enforcement . . . . . . . . . . . . .  22
   5.  Relationship with CDNI Interfaces . . . . . . . . . . . . . .  23
     5.1.  CDNI Control Interface  . . . . . . . . . . . . . . . . .  23
     5.2.  CDNI Footprint & Capabilities Advertisement Interface . .  23
     5.3.  CDNI Request Routing Redirection Interface  . . . . . . .  24
     5.4.  CDNI Metadata Interface . . . . . . . . . . . . . . . . .  24
     5.5.  CDNI Logging Interface  . . . . . . . . . . . . . . . . .  27
   6.  URI Signing Message Flow  . . . . . . . . . . . . . . . . . .  28
     6.1.  HTTP Redirection  . . . . . . . . . . . . . . . . . . . .  28
     6.2.  DNS Redirection . . . . . . . . . . . . . . . . . . . . .  31
   7.  HTTP Adaptive Streaming . . . . . . . . . . . . . . . . . . .  34
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  35
   10. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . . .  36



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   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  37
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  37
     12.2.  Informative References . . . . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  38

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]:

   "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.



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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.

   o  Original URI: The URI before URI Signing is applied.

   o  Signed URI: Any URI that contains a URI Signature.

   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.





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   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
   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).




















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                   --------
                  /        \
                  |   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
   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 6).















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                                      +-------------------------+
                                      |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

   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




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   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.

2.  Signed URI Information Elements

   The concept behind URI Signing is based on embedding in the Target
   CDN URI/Redirection URI 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.




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   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.

   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 URI.  A typical Signed URI will only contain one embedded
      URI Signature Information Element.

   In addition, the this document specifies the following URI attribute:

   o  URI Signing Package Attribute: The URI attribute that encapsulates
      all the URI Signing information elements in an encoded format.
      Only this attribute is exposed in the Signed URI as a URI query
      parameter.

   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.




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   Note that all the URI Signing information elements and the URI query
   attribute are mandatory to implement, but not mandatory to use.

2.1.  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 the URI
   Signature of a given request 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 expires.
      This is represented as an integer denoting the number of seconds
      since midnight 1/1/1970 UTC (i.e.  UNIX epoch).  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 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 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.

   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.

   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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2.2.  Signature Computation Information Elements

   This section identifies the set of information elements that may be
   needed to verify the URI (signature).  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.

   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 URI Signing Package
      Attribute, 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 URI Signing Package Attribute.

   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 URI Signing
      Package Attribute.

   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 URI Signing Package
      Attribute, 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 URI Signing Package Attribute, the default is
      EC-DSA.

   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, 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.  The



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   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 2.3.

   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 2.3.

2.3.  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.

   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 when symmetric keys are used.

   The Digital Signature attribute contains the digital signature used
   to verify the Signed URI 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



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   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.

2.4.  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.  URI Signing Package Attribute is appended
   to the Original URI to create the Signed URI.

   The primary advantage of the URI Signing Package Attribute is that it
   avoids having to expose the URI Signing Information Elements directly
   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 attributes in the Signed URI.

   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 attribute exposed in the Signed URI.  The
   attribute MUST be the last parameter in the query string 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



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   as the URI Signing Package Attribute to avoid namespace collision and
   potential failure of the URI Signing validation.

   The 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).

2.5.  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
   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.

3.  Creating the Signed URI

   The following procedure for signing a URI defines the algorithms in
   this version of URI Signing.  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.  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




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   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 and appending it to the Original 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.

3.1.  Calculating the URI Signature

   Calculate the URI Signature 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 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, step 4 can be skipped.

       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, step 5 can be skipped.

       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.




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       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
           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=120942
               2976&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.




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           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").

       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=12094
               22976&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



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               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.  "://example.com/content.mov?ET=1209422976&CIP=192.
               0.2.1&KID=http://example.com/public/keys/123&DS=r:CFB03ED
               B33810AB6C79EE3C47FBD86D227D702F25F66C01CF03F59F1E005668D
               :s:57ED0E8DF7E786C87E39177DD3398A7FB010E6A4C0DC8AA71331A9
               29A29EA24E" )

3.2.  Packaging the URI Signature

   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




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       Y2IxNDQ2YTY0MzEzNTJhYWIwZmI2ZTBkY2EzMGUzMDM1NjU5M2E5N2FjYjk3MjIwM
       jEyMGRjNDgyYmRkYWY=").  Note: this is the completed Signed URI.

4.  Validating a URI Signature

   The process of validating a Signed URI 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, and finally, validation of the information elements to
   ensure proper enforcement of the distribution policy.  The integrity
   of the Signed URI 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.

4.1.  Information Element Extraction

   Extract the information elements embedded in the URI.  Note that some
   steps are to be skipped if the corresponding URI Signing information
   elements are not embedded in the Signed URI.

   1.   Extract the value from 'URISigningPackage' attribute.  This
        value is the encoded URI Signing Package 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 'URISigningPackage' attribute.

   2.   Decode the string using Base-64 Data Encoding [RFC4648] to
        obtain all the URI Signing information elements (e.g.  "ET=12094
        22976&CIP=192.0.2.1&KID=example:keys:123&MD=1ecb1446a6431352aab0
        fb6e0dca30e30356593a97acb972202120dc482bddaf").

   3.   Extract the value from "VER" if the information element exists
        in the query string.  Determine the version of the URI Signing
        algorithm used to process the Signed URI.  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
        URI, 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 in
        the query string.  The existence of this information element
        indicates a symmetric key is used.





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   5.   Extract the value from "DS" if the information element exists in
        the query string.  The existence of this information element
        indicates an asymmetric key is used.

   6.   If neither "MD" or "DS" attribute is in the URI, 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
        in the query string.  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 in
        the query string.  The existence of this information element
        indicates content delivery is enforced based on time.

   9.   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.

   10.  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.

   11.  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.

4.2.  Signature Validation

   Validate the URI Signature for the Signed URI.

   1.  Copy the Original URI, excluding the "scheme name" part, into a
       buffer to hold the message for performing the operations below.

   2.  Remove the "URISigningPackage" attribute from the message.
       Remove any subsequent part of the query string after the
       "URISigningPackage" attribute.

   3.  Append the decoded value from "URISigningPackage" attribute
       (which contains all the URI Signing Information Elements).





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   4.  Depending on the type of key used to sign the URI, validate the
       message digest or digital signature for symmetric key or
       asymmetric keys, respectively.

       A.  For symmetric key, HMAC algorithm 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 Signed
               URI, obtain the shared key via CDNI metadata or
               configuration.

           b.  If "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.  Otherwise,
               the "HF" information element is not in the Signed URI.
               In this case, 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.

       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



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               "KID_NUM" information element is present in the Signed
               URI, obtain the public key via CDNI metadata or
               configuration.

           b.  If "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.
               Otherwise, the "DSA" information element is not in the
               Signed URI.  In this case, the default DSA is EC-DSA.

           c.  Extract the value from the "DS" information element.
               This is the 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.

4.3.  Distribution Policy Enforcement

   Note the steps are to be skipped if the corresponding URI Signing
   information elements are not in the Signed URI.  The absence of a
   given Enforcement Information Element indicates enforcement of its
   purpose is not necessary in the CSP's distribution policy.

   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, then 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, then the request is denied.





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5.  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?).

5.1.  CDNI Control Interface

   URI Signing has no impact on this interface.

5.2.  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.

5.3.  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

5.4.  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.

5.5.  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.

6.  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.

6.1.  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.

6.2.  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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7.  HTTP Adaptive Streaming

   The authors note that in order to perform URI signing for individual
   content segments of HTTP Adaptive Bitrate content, specific URI
   signing mechanisms are needed.  Such mechanisms are currently out-of-
   scope of this document.  More details on this topic is covered in
   Models for HTTP-Adaptive-Streaming-Aware CDNI [RFC6983].  [Editor
   note: DASH draft discussion]

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)





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   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

   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.




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      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
      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.







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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.

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-19 (work in progress), July 2015.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC6707]  Niven-Jenkins, B., Le Faucheur, F., and N. Bitar, "Content
              Distribution Network Interconnection (CDNI) Problem
              Statement", RFC 6707, DOI 10.17487/RFC6707, September
              2012, <http://www.rfc-editor.org/info/rfc6707>.

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-11 (work in progress), July 2015.

   [I-D.ietf-cdni-redirection]
              Niven-Jenkins, B. and R. Brandenburg, "Request Routing
              Redirection Interface for CDN Interconnection", draft-
              ietf-cdni-redirection-11 (work in progress), July 2015.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [RFC3174]  Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
              <http://www.rfc-editor.org/info/rfc3174>.



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   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952,
              DOI 10.17487/RFC5952, August 2010,
              <http://www.rfc-editor.org/info/rfc5952>.

   [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, DOI 10.17487/RFC6983, July 2013,
              <http://www.rfc-editor.org/info/rfc6983>.

   [RFC7336]  Peterson, L., Davie, B., and R. van Brandenburg, Ed.,
              "Framework for Content Distribution Network
              Interconnection (CDNI)", RFC 7336, DOI 10.17487/RFC7336,
              August 2014, <http://www.rfc-editor.org/info/rfc7336>.

   [RFC7337]  Leung, K., Ed. and Y. Lee, Ed., "Content Distribution
              Network Interconnection (CDNI) Requirements", RFC 7337,
              DOI 10.17487/RFC7337, August 2014,
              <http://www.rfc-editor.org/info/rfc7337>.

Authors' Addresses

   Kent Leung
   Cisco Systems
   3625 Cisco Way
   San Jose  95134
   USA

   Phone: +1 408 526 5030
   Email: kleung@cisco.com











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   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


   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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