--------
               /        \
               |   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 uCDN and the dCDN. In step #3, UA may send an HTTP request or a DNS request. Depending on whether HTTP-based or DNS-based request routing is used, the uCDN responds by directing the UA towards the dCDN using either a Redirection URI (which is a Signed URI generated by the uCDN) or a DNS reply, respectively (#4). Once the UA receives the response, it sends the Redirection URI/Target CDN URI to the dCDN (#5). The received URI is validated by the dCDN before delivering the content (#6). This is depicted in the figure below. Note: The CDNI call flows are covered in Detailed URI Signing Operation [operation].

                                   +-------------------------+
                                   |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, uCDN, and dCDN 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 uCDN. The delivery of the content may be delegated to the dCDN, which has a relationship with the uCDN 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 dCDN directly. In the case where the dCDN does not have a trust relationship with the CSP, this means that either an asymmetric public/private key method needs to be used for computing the signed JWT (because the CSP and dCDN are not able to exchange symmetric shared secret keys), or the CSP needs to allow the uCDN to redistribute shared keys to a subset of their dCDNs .

For HTTP-based request routing, the Signed URI (i.e., Target CDN URI) provided by the CSP reaches the uCDN. After this URI has been verified to be correct by the uCDN, the uCDN creates and signs a new Redirection URI to redirect the UA to the dCDN. Since this new URI also has a new signed JWT, this new signature can be based around the trust relationship between the uCDN and dCDN, and the relationship between the dCDN and CSP is not relevant. Given the fact that such a relationship between uCDN and dCDN always exists, both asymmetric public/private keys and symmetric shared secret keys can be used for URI Signing with HTTP-based request routing. Note that the signed Redirection URI MUST maintain the same, or higher, level of security as the original Signed URI.

1.4. URI Signing in a non-CDNI context

While the URI signing method 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 method 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. JWT Format and Processing Requirements

The concept behind URI Signing is based on embedding a signed JSON Web Token (JWT) [RFC7519] in the Target CDN URI/Redirection URI. The signed JWT contains a number of claims that can be validated to ensure the UA has legitimate access to the content.

In addition, this document specifies the following URI attribute:

• URI Signing Package (URISigningPackage): The URI attribute that encapsulates all the URI Signing claims in a signed JWT encoded format. This attribute is exposed in the Signed URI as a URI query parameter or as a URL path parameter.

The parameter name of the URI Signing Package Attribute is defined in the CDNI Metadata. If the CDNI Metadata interface is not used, or does not include a parameter name for the URI Signing Package Attribute, the parameter name can be set by configuration (out of scope of this document).

2.1. JWT Claims

This section identifies the set of claims that can be used to enforce the CSP distribution policy. New claims can be introduced in the future to extend the distribution policy capabilities.

In order to provide distribution policy flexibility, the exact subset of claims used in a given signed JWT is a runtime decision. Claim requirements are defined in the CDNI Metadata. If the CDNI Metadata interface is not used, or does not include claim requirements, the claim requirements can be set by configuration (out of scope of this document).

The following claims (where the "JSON Web Token Claims" registry claim name is specified in parenthesis below) are used to enforce the distribution policies. All of the listed claims are mandatory to implement in a URI Signing implementation, but are not mandatory to use in a given signed JWT. (The "optional" and "mandatory" identifiers in square brackets refer to whether or not a given claim MUST be present in a URI Signing JWT.) A CDN MUST be able to parse and process all of the claims listed below. If the signed JWT contains any claims which the CDN does not understand (i.e., is unable to parse and process), the CDN MUST reject the request.

• Issuer (iss) [optional] - The semantics in [RFC7519] Section 4.1.1 MUST be followed. This claim MAY be used to validate authorization of the issuer of a signed JWT and also MAY be used to confirm that the indicated key was provided by said issuer. If the CDN validating the signed JWT does not support Issuer validation, or if the Issuer in the signed JWT does not match the list of known acceptable Issuers, the CDN MUST reject the request. If the received signed JWT contains an Issuer claim, then any JWT subsequently generated for CDNI redirection MUST also contain an Issuer claim, and the Issuer value MUST be updated to identify the redirecting CDN. If the received signed JWT does not contain an Issuer claim, an Issuer claim MAY be added to a signed JWT generated for CDNI redirection.
• URI Container (sub) [mandatory] - The semantics in [RFC7519] Section 4.1.2 MUST be followed. Container for holding the URI representation before the URI Signing Package is added. This representation can take one of several forms detailed in Section 2.1.1. If the URI pattern/regex in the signed JWT does not match the URI of the content request, the CDN validating the signed JWT MUST reject the request. When redirecting a URI, the CDN generating the new signed JWT MAY change the URI Container to comport with the URI being used in the redirection.
• Client IP (aud) [optional] - The semantics in [RFC7519] Section 4.1.3 MUST be followed. IP address, or IP prefix, for which the Signed URI is valid. This is represented in CIDR notation, with dotted decimal format for IPv4 or canonical text representation for IPv6 addresses [RFC5952]. The request is rejected if sourced from a client outside of the specified IP range. Since the client IP is considered personally identifiable information this field MUST be a JSON Web Encryption (JWE [RFC7516]) Object in compact serialization form. If the CDN validating the signed JWT does not support Client IP validation, or if the Client IP in the signed JWT does not match the source IP address in the content request request, the CDN MUST reject the request. If the received signed JWT contains a Client IP claim, then any JWT subsequently generated for CDNI redirection MUST also contain a Client IP claim, and the Client IP value MUST be the same as in the received signed JWT. A signed JWT generated for CDNI redirection MUST NOT add a Client IP claim if no Client IP claim existed in the received signed JWT.
• Expiry Time (exp) [optional] - The semantics in [RFC7519] Section 4.1.4 MUST be followed, though URI Signing implementations MUST not allow for any time synchronization "leeway". Note: The time on the entities that generate and validate the signed URI SHOULD be in sync. In the CDNI case, this means that CSP, uCDN and dCDN servers need to be time-synchronized. It is RECOMMENDED to use NTP for time synchronization. If the CDN validating the signed JWT does not support Expiry Time validation, or if the Expiry Time in the signed JWT corresponds to a time earlier than the time of the content request request, the CDN MUST reject the request. If the received signed JWT contains a Expiry Time claim, then any JWT subsequently generated for CDNI redirection MUST also contain an Expiry Time claim, and the Expiry Time value MUST be the same as in the received signed JWT. A signed JWT generated for CDNI redirection MUST NOT add an Expiry Time claim if no Expiry Time claim existed in the received signed JWT.
• Not Before (nbf) [optional] - The semantics in [RFC7519] Section 4.1.5 MUST be followed, though URI Signing implementations MUST not allow for any time synchronization "leeway". Note: The time on the entities that generate and validate the signed URI SHOULD be in sync. In the CDNI case, this means that the CSP, uCDN, and dCDN servers need to be time-synchronized. It is RECOMMENDED to use NTP for time synchronization. If the CDN validating the signed JWT does not support Not Before time validation, or if the Not Before time in the signed JWT corresponds to a time later than the time of the content request request, the CDN MUST reject the request. If the received signed JWT contains a Not Before time claim, then any JWT subsequently generated for CDNI redirection MUST also contain a Not Before time claim, and the Not Before time value MUST be the same as in the received signed JWT. A signed JWT generated for CDNI redirection MUST NOT add a Not Before time claim if no Not Before time claim existed in the received signed JWT.
• Issued At (iat) [optional] - The semantics in [RFC7519] Section 4.1.6 MUST be followed. Note: The time on the entities that generate and validate the signed URI SHOULD be in sync. In the CDNI case, this means that CSP, uCDN, and dCDN servers need to be time-synchronized. It is RECOMMENDED to use NTP for time synchronization. If the received signed JWT contains an Issued At claim, then any JWT subsequently generated for CDNI redirection MUST also contain an Issued At claim, and the Issuer value MUST be updated to identify the time the new JWT was generated. If the received signed JWT does not contain an Issued At claim, an Issued At claim MAY be added to a signed JWT generated for CDNI redirection.
• Nonce (jti) [optional] - The semantics in [RFC7519] Section 4.1.7 MUST be followed. Can be used to prevent replay attacks if the CDN stores a list of all previously used Nonce values, and validates that the Nonce in the current JWT has never been used before. If the signed JWT contains a Nonce claim and the CDN validating the signed JWT does not support Nonce storage, then the CDN MUST reject the request. If the received signed JWT contains a Nonce claim, then any JWT subsequently generated for CDNI redirection MUST also contain a Nonce claim, and the Nonce value MUST be the same as in the received signed JWT. If the received signed JWT does not contain a Nonce claim, a Nonce claim MAY be added to a signed JWT generated for CDNI redirection.

Note: See the Security Considerations [security] section on the limitations of using an expiration time and client IP address for distribution policy enforcement.

2.1.1. URI Container Forms

The URI Container (sub) claim takes one of the following forms. More forms may be added in the future to extend the capabilities.

2.1.1.1. URI Simple Container (uri:)

When prefixed with 'uri:', the string following 'uri:' is the URI that MUST be matched with a simple string match to the requested URI.

2.1.1.2. URI Pattern Container (uri-pattern:)

Prefixed with 'uri-pattern:', this string contains one or more URI Patterns that describes for which content the Signed URI is valid. Each URI Pattern contains an expression to match against the requested URI, to check whether the requested content is allowed to be served. Multiple URI Patterns may be concatenated in a single URI Pattern by separating them with a semi-colon (';') character. Each URI Pattern follows the [RFC3986] URI format, including the '://' that delimits the URI scheme from the hierarchy part. The pattern may include the special literals:

• ';' - separates individual patterns when the string contains multiple URI patterns.
• '*' - matches any sequence of characters, including the empty string.
• '?' - matches exactly one character.
• '$' - used to escape the special literals; MUST be followed by exactly one of ';', '*', '?', or '$'.

The following is an example of a valid URI Pattern:

*://*/folder/content-83112371/quality_*/segment????.mp4
                  

An example of two concatenated URI Patterns is the following (whitespace is inserted after the ';' for readability and should not be present in the actual representation):

http://*/folder/content-83112371/manifest/*.xml;
http://*/folder/content-83112371/quality_*/segment????.mp4
                  

In order to increase the performance of string parsing the URI Pattern, implementations can check often-used URI Pattern prefixes to quickly check whether certain URI components can be ignored. For example, URI Pattern prefixes '*://*/' or '*://*:*' will be used in case the scheme and authority components of the URI are ignored for purposes of pattern enforcement.

2.1.1.3. URI Regular Expression Container (uri-regex:)

Prefixed with 'uri-regex:', this string is any PCRE [PCRE839] compatible regular expression used to match against the requested URI.

Note: Because '\' has special meaning in JSON [RFC7159] as the escape character within JSON strings, the regular expression character '\' MUST be escaped as '\\'.

An example of a 'uri-regex:' is the following:

.*\\://.*/folder/content-83112371/quality_.*/segment.{3}\\.mp4
                  

Note: Due to computational complexity of executing arbitrary regular expressions, it is RECOMMENDED to only execute after validating the JWT to ensure its authenticity.

3. Relationship with CDNI Interfaces

Some of the CDNI Interfaces need enhancements to support URI Signing. As an example: A dCDN that supports URI Signing needs to be able to advertise this capability to the uCDN. The uCDN needs to select a dCDN based on such capability when the CSP requires access control to enforce its distribution policy via URI Signing. Also, the uCDN needs to be able to distribute via the CDNI Metadata interface the information necessary to allow the dCDN 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?).

3.1. CDNI Control Interface

URI Signing has no impact on this interface.

3.2. CDNI Footprint & Capabilities Advertisement Interface

The CDNI Request Routing: Footprint and Capabilities Semantics document [I-D.ietf-cdni-footprint-capabilities-semantics] defines support for advertising CDNI Metadata capabilities, via CDNI Payload Type. The CDNI Payload Type registered in Section 6.1 can be used for capability advertisement.

3.3. 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 uCDN signs the URI provided by the dCDN. URI Signing therefore has has no impact on this interface.

3.4. 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, a new CDNI metadata object is specified.

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 dCDN to ensure that the URI must be signed and validated before delivering content. Otherwise, the dCDN does not perform validation, regardless of whether or not the URI is signed.
  • Type: Boolean
  • Mandatory-to-Specify: No. The default is true.
• Property: issuers
  • Description: A list of valid Issuers against which the Issuer claim in the signed JWT may be validated.
  • Type: Array of Strings
  • Mandatory-to-Specify: No. The default is an empty list. An empty list means that any Issuer is acceptable.
• Property: package-attribute
  • Description: The name to use for the URI Signing Package.
  • Type: String
  • Mandatory-to-Specify: No. Default is "URISigningPackage".

The following is an example of a URI Signing metadata payload with all default values:

{
  "generic-metadata-type": "MI.UriSigning"
  "generic-metadata-value": {}
}

          

The following is an example of a URI Signing metadata payload with explicit values:

{
  "generic-metadata-type": "MI.UriSigning"
  "generic-metadata-value":
    {
      "enforce": true,
      "issuers": ["csp", "ucdn1", "ucdn2"],
      "package-attribute": "usp"
    }
}

          

3.5. CDNI Logging Interface

For URI Signing, the dCDN 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], using the new "cdni_http_request_v2" record-type registered in Section 6.2.1.

• 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:
    • "000" : no signed JWT validation performed
    • "200" : signed JWT validation performed and validated
    • "400" : signed JWT validation performed and rejected because of incorrect signature
    • "401" : signed JWT validation performed and rejected because of Expiration Time enforcement
    • "402" : signed JWT validation performed and rejected because of Client IP enforcement
    • "403" : signed JWT validation performed and rejected because of URI Pattern enforcement
    • "404" : signed JWT validation performed and rejected because of Issuer enforcement
    • "405" : signed JWT validation performed and rejected because of Not Before enforcement
    • "500" : unable to perform signed JWT validation because of malformed URI
  • occurrence: there MUST be zero or exactly one instance of this field.
• s-uri-signing-deny-reason (optional):
  • format: QSTRING
  • field value: a string for providing further information in case the signed JWT was rejected, e.g., for debugging purposes.
  • occurrence: there MUST be zero or exactly one instance of this field.

4. URI Signing Message Flow

URI Signing supports both HTTP-based and DNS-based request routing. JSON Web Token (JWT) [RFC7519] defines a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a signed JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted.

4.1. HTTP Redirection

For HTTP-based request routing, a set of information that is unique to a given end user content request is included in a signed JWT, using key information that is specific to a pair of adjacent CDNI hops (e.g. between the CSP and the uCDN, between the uCDN and a dCDN). This allows a CDNI hop to ascertain the authenticity of a given request received from a previous CDNI hop.

The URI signing method described below is based on the following steps (assuming HTTP redirection, iterative request routing, and a CDN path with two CDNs). Note that uCDN and uCDN are used exchangeably.

     End-User           dCDN                 uCDN                 CSP
     |                    |                    |                    |
     |            1.CDNI FCI interface used to |                    |
     |         advertise URI Signing capability|                    |
     |                    |------------------->|                    |
     |                    |                    |                    |
     |              2.Provides information to validate signed JWT   |
     |                    |                    |<-------------------|
     |                    |                    |                    |
     |        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.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 dCDN advertises its capabilities including URI Signing support to the uCDN.
2. CSP provides to the uCDN the information needed to validate signed JWTs from that CSP. For example, this information may include a key value.
3. Using the CDNI Metadata interface, the uCDN communicates to a dCDN the information needed to validate signed JWTs from the uCDN for the given CSP. For example, this information may include the URI query string parameter name for the URI Signing Package Attribute.
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 uCDN validates the signed JWT in the URI using the information provided by the CSP.
8. If the validation is negative, the uCDN rejects the request.
9. If the validation is positive, the uCDN 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 dCDN.
10. On receipt of the corresponding content request, the dCDN validates the signed JWT in the Signed URI using the information provided by the uCDN in the CDNI Metadata.
11. If the validation is negative, the dCDN rejects the request and sends an error code (e.g., 403 Forbidden) in the HTTP response.
12. If the validation is positive, the dCDN serves the request and delivers the content.
13. At a later time, dCDN 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 and asymmetric keys because the key information only needs to be specific to a pair of adjacent CDNI hops.

4.2. DNS Redirection

For DNS-based request routing, the CSP and uCDN 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 dCDNs. 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 dCDNs. Authorized Delivery CDNs need to obtain the key information to validate the Signed URI.

The URI signing method described below is based on the following steps (assuming iterative DNS request routing and a CDN path with two CDNs).

     End-User            dCDN                 uCDN                CSP
     |                    |                    |                    |
     |            1.CDNI FCI interface used to |                    |
     |         advertise URI Signing capability|                    |
     |                    |------------------->|                    |
     |                    |                    |                    |
     |              2.Provides information to validate signed JWT   |
     |                    |                    |<-------------------|
     |        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>    |
     |<-------------------------------------------------------------|
     |                    |                    |                    |
     |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 dCDN advertises its capabilities including URI Signing support to the uCDN.
2. CSP provides to the uCDN the information needed to validate cryptographic signatures from that CSP. For example, this information may include a key.
3. Using the CDNI Metadata interface, the uCDN communicates to a dCDN 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 uCDN checks if the dCDN is allowed by CSP to obtain the shared secret key.
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 uCDN.
8. On receipt of the DNS request, the uCDN redirects the request to the dCDN.
9. End user sends DNS request to the dCDN.
10. On receipt of the DNS request, the dCDN responds with IP address of one of its Surrogates.
11. On receipt of the corresponding content request, the dCDN validates the cryptographic signature in the URI using the information provided by the uCDN in the CDNI Metadata.
12. If the validation is negative, the dCDN rejects the request and sends an error code (e.g., 403) in the HTTP response.
13. If the validation is positive, the dCDN serves the request and delivers the content.
14. At a later time, dCDN 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 needs to be distributed across multiple, possibly non-adjacent, CDNI hops is the public key, which 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.

5. 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]. In addition, [I-D.brandenburg-cdni-uri-signing-for-has] provides an extension to the algorithm defined in this document that deals specifically with URI signing of segmented content.

6. IANA Considerations

6.1. CDNI Payload Type

This document requests the registration of the following CDNI Payload Type under the IANA "CDNI Payload Type" registry:

Payload Type	Specification
MI.UriSigning	RFCthis

[RFC Editor: Please replace RFCthis with the published RFC number for this document.]

6.1.1. CDNI UriSigning Payload Type

Purpose: The purpose of this payload type is to distinguish UriSigning MI objects (and any associated capability advertisement).

Interface: MI/FCI

Encoding: see Section 3.4

6.2. CDNI Logging Record Type

This document requests the registration of the following CDNI Logging record-type under the IANA "CDNI Logging record-types" registry:

record-types	Reference	Description
cdni_http_request_v2	RFCthis	Extension to CDNI Logging Record version 1 for content delivery using HTTP, to include URI Signing logging fields

[RFC Editor: Please replace RFCthis with the published RFC number for this document.]

6.2.1. CDNI Logging Record Version 2 for HTTP

The "cdni_http_request_v2" record-type supports all of the fields supported by the "cdni_http_request_v1" record-type [I-D.ietf-cdni-logging] plus the two additional fields "s-uri-signing" and "s-uri-signing-deny-reason", registered by this document in Section 6.3. The name, format, field value, and occurence information for the two new fields can be found in Section 3.5 of this document.

6.3. CDNI Logging Field Names

This document requests the registration of the following CDNI Logging fields under the IANA "CDNI Logging Field Names" registry:

Field Name	Reference
s-uri-signing	RFCthis
s-uri-signing-deny-reason	RFCthis

[RFC Editor: Please replace RFCthis with the published RFC number for this document.]

7. Security Considerations

This document describes the concept of URI Signing and how it can be used to provide access authorization in the case of CDNI. The primary goal of URI Signing is to make sure that only authorized UAs are able to access the content, with a 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 claims in the signed JWT. The current version of this document includes claims for enforcing Issuer, Client IP Address, Not Before time, 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: A (valid) Signed URI may be used to perform replay attacks. The vulnerability to replay attacks can be reduced by picking a relatively short window between the Not Before time and Expiration Time attributes, although this is limited by the fact that any HTTP-based request needs a window of at least a couple of seconds to prevent a sudden network issues from preventing legitimate UAs access to the content. One may also reduce exposure to replay attacks by including a unique one-time access ID via the Nonce attribute (jti claim). Whenever the dCDN receives a request with a given unique ID, it adds that ID to the list of 'used' IDs. In the case an illegitimate UA tries to use the same URI through a replay attack, the dCDN can deny the request based on the already-used access ID.
• Illegitimate clients 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 dCDN. This results in the dCDN 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, e.g., attributes that can be found in HTTP headers.

The shared key between CSP and uCDN may be distributed to dCDNs - including cascaded CDNs. Since this key can be used to legitimately sign a URL for content access authorization, it is 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.

8. Privacy

The privacy protection concerns described in CDNI Logging Interface [I-D.ietf-cdni-logging] apply when the client's IP address (aud) is embedded in the Signed URI. For this reason, the mechanism described in Section 2 encrypts the Client IP before including it in the URI Signing Package (and thus the URL itself).

9. 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, Leif Hedstrom and Gancho Tenev. In addition, Matt Caulfield provided content for the CDNI Metadata Interface section.

10. References

10.1. Normative References

[I-D.ietf-cdni-logging]	Faucheur, F., Bertrand, G., Oprescu, I. and R. Peterkofsky, "CDNI Logging Interface", Internet-Draft draft-ietf-cdni-logging-27, June 2016.
[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[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.
[RFC7159]	Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014.
[RFC7516]	Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", RFC 7516, DOI 10.17487/RFC7516, May 2015.
[RFC7519]	Jones, M., Bradley, J. and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015.

10.2. Informative References

[I-D.brandenburg-cdni-uri-signing-for-has]	Brandenburg, R., "URI Signing for HTTP Adaptive Streaming (HAS)", Internet-Draft draft-brandenburg-cdni-uri-signing-for-has-03, June 2016.
[I-D.ietf-cdni-footprint-capabilities-semantics]	Seedorf, J., Peterson, J., Previdi, S., Brandenburg, R. and K. Ma, "CDNI Request Routing: Footprint and Capabilities Semantics", Internet-Draft draft-ietf-cdni-footprint-capabilities-semantics-20, May 2016.
[I-D.ietf-cdni-metadata]	Niven-Jenkins, B., Murray, R., Caulfield, M. and K. Ma, "CDN Interconnection Metadata", Internet-Draft draft-ietf-cdni-metadata-21, August 2016.
[I-D.ietf-cdni-redirection]	Niven-Jenkins, B. and R. Brandenburg, "Request Routing Redirection interface for CDN Interconnection", Internet-Draft draft-ietf-cdni-redirection-20, August 2016.
[PCRE839]	Hazel, P., "Perl Compatible Regular Expressions", Version 8.39, June 2016.
[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.
[RFC5952]	Kawamura, S. and M. Kawashima, "A Recommendation for IPv6 Address Text Representation", RFC 5952, DOI 10.17487/RFC5952, August 2010.
[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.
[RFC7336]	Peterson, L., Davie, B. and R. van Brandenburg, "Framework for Content Distribution Network Interconnection (CDNI)", RFC 7336, DOI 10.17487/RFC7336, August 2014.
[RFC7337]	Leung, K. and Y. Lee, "Content Distribution Network Interconnection (CDNI) Requirements", RFC 7337, DOI 10.17487/RFC7337, August 2014.

Appendix A. Signed URI Package Example

This section contains two examples of token usage: a simple example with only the required claims present, and a complex example which demonstrates the full JWT claims set, including an encrypted Client IP (aud).

Note: All of the examples have whitespace added to improve formatting and readability, but are not present in the generated content.

Both examples use the following signing key to generate the Signed URI Package:

{
  "kty": "EC",
  "kid": "P5UpOv0eMq1wcxLf7WxIg09JdSYGYFDOWkldueaImf0",
  "use": "sig",
  "crv": "P-256",
  "x": "be807S4O7dzB6I4hTiCUvmxCI6FuxWba1xYBlLSSsZ8",
  "y": "rOGC4vI69g-WF9AGEVI37sNNwbjIzBxSjLvIL7f3RBA",
  "d": "yaowezrCLTU6yIwUL5RQw67cHgvZeMTLVZXjUGb1A1M"
}

A.1. Simple Example

This example is the simplest possible example containing the only required field (sub).

The JWT Claim Set before signing:

{
  "sub": "uri:http://cdni.example/foo/bar/baz"
}

The Signed JWT:

eyJhbGciOiJFUzI1NiIsImtpZCI6IlA1VXBPdjBlTXExd2N4TGY3V3hJZzA5SmRTWU
dZRkRPV2tsZHVlYUltZjAifQ.eyJzdWIiOiJ1cmk6aHR0cDovL2NkbmkuZXhhbXBsZ
S9mb28vYmFyL2JheiJ9.oC4yKwUchowx4KhMsI8MQ-Sq_1s3fC8NCi-IWcmNEE9MQz
EEQfurJ1su2Op_dtYuc_fG8NixSVubz3HWKM4Rsw

A.2. Complex Example

This example uses all optional fields, including Client IP (aud) which is encrpyted. This significantly increases the size of the signed JWT token.

Shared key used for encrpyting the Client IP (aud):

{
  "kty": "oct",
  "kid": "f-WbjxBC3dPuI3d24kP2hfvos7Qz688UTi6aB0hN998",
  "use": "enc",
  "alg": "A128GCM",
  "k": "4uFxxV7fhNmrtiah2d1fFg"
}

JWE for client IP (aud) of [2001:db8::1/32]:

eyJhbGciOiJkaXIiLCJraWQiOiJmLVdianhCQzNkUHVJM2QyNGtQMmhmdm9zN1F6Nj
g4VVRpNmFCMGhOOTk4IiwiZW5jIjoiQTEyOEdDTSJ9..Ewl05cq3jmUe1Bv1.CHif9
OMPmsMPgJ8tZgvD0A.R3I2C8nfppY2wBfc4xEPPQ

The JWT Claim Set before signing:

{
  "aud": "eyJhbGciOiJkaXIiLCJraWQiOiJmLVdianhCQzNkUHVJM2QyNGtQMmhm
dm9zN1F6Njg4VVRpNmFCMGhOOTk4IiwiZW5jIjoiQTEyOEdDTSJ9..Ewl05cq3jmUe
1Bv1.CHif9OMPmsMPgJ8tZgvD0A.R3I2C8nfppY2wBfc4xEPPQ",
  "exp": 1474243500,
  "iat": 1474243200,
  "iss": "Upstream CDN Inc",
  "jti": "5DAafLhZAfhsbe",
  "nbf": 1474243200,
  "sub": "uri-regex:http://cdni\\.example/foo/bar/baz/[0-9]{3}\\.png"
}

The Signed JWT:

eyJhbGciOiJFUzI1NiIsImtpZCI6IlA1VXBPdjBlTXExd2N4TGY3V3hJZzA5SmRTWU
dZRkRPV2tsZHVlYUltZjAifQ.eyJhdWQiOiJleUpoYkdjaU9pSmthWElpTENKcmFXU
WlPaUptTFZkaWFuaENRek5rVUhWSk0yUXlOR3RRTW1obWRtOXpOMUY2TmpnNFZWUnB
ObUZDTUdoT09UazRJaXdpWlc1aklqb2lRVEV5T0VkRFRTSjkuLkV3bDA1Y3Ezam1VZ
TFCdjEuQ0hpZjlPTVBtc01QZ0o4dFpndkQwQS5SM0kyQzhuZnBwWTJ3QmZjNHhFUFB
RIiwiZXhwIjoxNDc0MjQzNTAwLCJpYXQiOjE0NzQyNDMyMDAsImlzcyI6IlVwc3RyZ
WFtIENETiBJbmMiLCJqdGkiOiI1REFhZkxoWkFmaHNiZSIsIm5iZiI6MTQ3NDI0MzI
wMCwic3ViIjoidXJpLXJlZ2V4Omh0dHA6Ly9jZG5pXFwuZXhhbXBsZS9mb28vYmFyL
2Jhei9bMC05XXszfVxcLnBuZyJ9.AtDNW7mwFIJPqsWAn9ojzj4imE-vTowR-FRzil
vnSQuQMz_u4sIspxe6RoXo_Ti8rVMgJ0jOdSvVnQUJZdfRUQ

Authors' Addresses