URI Signing for CDN Interconnection (CDNI)

This document uses the terminology defined in CDNI Problem Statement . This document also uses the terminology of Keyed-Hashing for Message Authentication (HMAC) including the following terms (reproduced here for convenience): MAC: message authentication code. 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. HMAC-SHA1: HMAC instantiation using SHA1 as the cryptographic hash function. HMAC-MD5: HMAC instantiation using MD5 as the cryptographic hash function. In addition, the following terms are used throughout this document: URI Signature: Message digest or digital signature that is computed with an algorithm for protecting the URI. Original URI: The URI before URI Signing is applied. Signed URI: Any URI that contains a URI signature. 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. 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.

The following section provides an informative overview of how URI Signing works in CDNI scenarios. In order to do so, URI Signing is first explained in terms of a single CDN delivering content on behalf of a CSP. A CSP and CDN are assumed to have a trust relationship that enables the CSP to authorize access to a content item by including a set of attributes in the URI before redirecting a UA to the CDN. Using these attributes, it is possible for a CDN to check an incoming content request to see whether it was authorized by the CSP (e.g. based on the UA's IP address or a time window). Of course, the attributes need to be added to the URI in a way that prevents a UA from changing the attributes, thereby leaving the CDN to think that the request was authorized by the CSP when in fact it wasn't. For this reason, a URI Signing mechanism includes in the URI a message digest or digital signature that allows a CDN to check the authenticity of the URI. The message digest or digital signature can be calculated based on a shared secret between the CSP and CDN or using asymetric public/private keys, respectively. Figure 1, shown below, presents an overview of the URI Signing mechanism in the case of a CSP with a single CDN. In this particular example, the CSP and CDN have exchanged a (symmetric) shared secret key. Once the UA sends a content request to the CSP (#1), the CSP responds by directing the UA towards the CDN using an embedded Target CDN URI (#2). The CSP may include in this URI the IP address of the UA and/or a time window. Finally, it signs the URI using the shared secret. Once the UA receives the response with the embedded URI, it sends a new request using the embedded URI to the CDN (#3). Upon receiving the request, the CDN checks to see if the URI is authentic by verifying the URI signature. In addition, it checks whether the IP address of the UA matches that in the URI and if the time window is still valid. After these values are confirmed to be valid, the CDN starts the content delivery process (#4).

-------- / \ | CSP |< * * * * * * * * * * * \ / Trust * -------- relationship * ^ | (symmetric key) * | | * 1. Request | | 2. Signed * for | | URI * content | | * | v v +------+ 3. Signed URI -------- | User |----------------->/ \ | Agent| | CDN | | |<-----------------\ / +------+ 4. Content -------- Delivery In CDNI scenarios, URI Signing operates the same way in the initial steps (#1-#3) 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. Depending on whether HTTP-based or DNS-based request routing is in use, the Upstream CDN responds by directing the UA towards the Downstream CDN using either a Redirection URI 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.

-------- / \ | CSP |< * * * * * * * * * * * \ / Trust * -------- relationship * ^ | (symmetric key) * | | * 1. Request | | 2. Signed * for | | URI * content | | * | v 3. Signed URI v +------+ or DNS request -------- | User |----------------->/ \ | Agent| | uCDN | | |<-----------------\ / +------+ 4. Redirection URI-------- ^ | or DNS Reply ^ | | * Trust relationship | | * (symmetric key) | | 5. Redirection URI v | | or Signed URI -------- | +------------------->/ \ [May be | | dCDN | cascaded +-----------------------\ / CDNs] 6. Content -------- delivery The trust relationships between CSP, Upstream CDN, and Downstream CDN have direct implications for URI Signing. In the case shown in Figure 2, the CDN that the CSP has a trust relationship with is the Upstream CDN. The delivery of the content may be delegated to the Downstream CDN, which has a relationship with the Upstream CDN but may have no relationship with the CSP. In CDNI, there are two methods for request routing: DNS-based and HTTP-based. For DNS-based request routing, the Signed URI (i.e. Target CDN URI) provided by the CSP reaches the Downstream CDN directly. In the case where the Downstream CDN does not have a trust relationship with the CSP, this means that only an asymetric 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, 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 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.

The concept behind URI Signing is based on embedding in the Target CDN URI/Redirection URI some attributes that can be validated to ensure the UA has legitimate access to the content. In the URI signing mechanism that is described in this section, four types of attributes may be embedded in the URI: Enforcement Attributes: Attributes 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. Signature Computation Attributes: Attributes 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 attributes that describe how the URI signature was generated. Examples of Signature Computation Attributes include the used HMAC's hash function and/or the key identifier. URI Signature Attributes: The attribute that contains the actual message digest or digital signature representing the URI signature and conveying the integrity and authenticity of the URI. URI Signing Token Attribute: The attribute that obfuscates all the other URI Signing attributes in the Signed URI. 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 CDN (or CSP) signing the URI and a public key for the verification of the Signed URI. Regardless of the type of key used, the entity that validates the URI has to obtain the key. There are very different requirements for key distribution 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.

This section identifies the set of attributes that may be needed to enforce the CSP distribution policy. These attributes are protected by the URI signature. New attributes 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 attributes used for URI signature in a given request is a deployment decision. The defined keyword for each query string attribute is specified in parenthesis below. The following attributes are used to enforce the distribution policy: Expiry Time (ET) [optional] - Time when the Signed URI expires. This is represented in 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). 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 . The request is rejected if sourced from a client with a different IP address. The Expiry Time attribute 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 attribute is used to restrict content access to a particular User Agent, based on its IP address for whom the content access was authorized.

This section identifies the set of attributes that may be needed to verify the URI (signature). New attributes may be introduced in the future if new URI signing algorithms are developed. The defined keyword for each query string attribute is specified in parenthesis below. The following attributes are used to verify the URI (signature). Version (VER) [optional] - An integer used for identifying the version of URI signing method. Key ID (KID) [optiona] - A string used for obtaining the key (e.g. database lookup, URI reference) which is needed to validate the URI signature. Hash Function (HF) [optional] - A string used for identifying the hash function to compute the URI signature (e.g. "MD5", "SHA1") with HMAC. The Version attribute 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, the version is considered to have been set to 1. More versions may be defined in the future. The Key ID attribute is used to retrieved the key which is needed as input to the algorithm for validating the Signed URI. The Hash Function attribute indicates the hash function to be used for HMAC-based message digest computation.

The following attributes are used to convey the actual URI signature. Message Digest (MD) [mandatory for symmetric key] - A string used for the message digest generated by the URI signer. Digital Signature (DS) [mandatory for asymmetric keys] - A string used for the digital signature provided by the URI signer. The Message Digest attribute contains the message digest used to validate the Signed URI when symmetric key is used. In the case of symmetric key, HMAC algorithm is used for the following reasons: 1) Ability to use hash functions (i.e. no changes needed) with well understood cryptographic properties that perform well and for which code is freely and widely available, 2) Easy to replace the embedded hash function in case faster or more secure hash functions are found or required, 3) Original performance of the hash function is maintained without incurring a significant degradation, and 4) Simple way to use and handle keys. The Digital Signature attribute contains the digital signature used to verify the Signed URI when asymmetric keys are used. 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.

As an option to avoid exposing all the URI Signing attributes in the URI, the attributes can be obfuscated by including only the URI Signing token in the Signed URI. This also reduces the number of attributes that are appended to the Original URI to just one. The intent is to hide the information (e.g. IP address) from view for the common user who is not aware of the encoding scheme. It is not a security method since anyone who knows the encoding scheme is able to obtain the clear text. The following attribute is used to convey the tokenized set of URI Signing attributes in the Signed URI. URI Signing Token (UST) [optional] - The encoded token containing the URI Signing attributes. The URI Signing Token attribute contains the URI Signing attributes in Base-64 Data Encoding format. When this attribute is used, it is the only URI Signing attribute exposed in the Signed URI. The attribute MUST be the last attribute in the query string of the URI. The CDNI Metadata Interface may override the encoding format used in the "UST" attribute.

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 URI Signing attribute is not needed to enforce the distribution policy. A URI (as defined in URI Generic Syntax) 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 referenced by an 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. Note: The following URI signing steps are specified to generate a Signed URI. However, 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. The URI Signing attributes are appended to the protected portion of the URI to compute the URI signature. Copy the Original URI, excluding the "scheme name" part, into a buffer to hold the message for performing the operations below. Check if the URI already contains a query string. If not, append a "?" character. If yes, append an "&" character. If the version needs to be specified, then append the string "VER=1". This represents the version of URI Signing specified by this document. If time window enforcement is needed, then perform the this step and the next two steps. Append the string "&ET=". Get the current time in seconds since epoch (as an integer). Add the validity time in seconds as an integer. Convert this integer to a string and append to the message. If client IP address enforcement is needed, then perform this step and the next step. Append the string "&CIP=". Convert the client's IP address in dotted decimal notation format (i.e. for IPv4 address) or canonical text representation (for IPv6 address) to a string and append to the message. 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. For symmetric key, HMAC is used. Obtain the shared key to be used for signing the URI. If the key identifier needs to be specified, then perform this step and the next step. Append the string "&KID=". Append the key identifier (e.g. "example:keys:123") needed by the entity to locate the shared key for validating the URI signature. If the hash function for HMAC needs to be specified, then perform this step and the next step. Append the string "&HF=". Append the string for the type of hash function (e.g. "MD5", "SHA-1") Append the string "&MD=". The message contains the complete section of the URI that is protected. (e.g. "://example.com/content.mov?VER=1&ET=1209422976&CIP=10.0.0.1&KID=example:keys:123&HF=SHA-1&MD="). Compute the message digest (note: this is the URI signature) using the HMAC algorithm with the shared key and message as the two inputs to the hash function which is specified by the "HF" attribute. Convert the message digest to its equivalent human readable value. Append the string for the message digest (e.g. "://example.com/content.mov?VER=1&ET=1209422976&CIP=10.0.0.1&KID=example:keys:123&HF=SHA-1&MD=4fb1c1adf1588fbe11cc6a04c6e69f35"). For asymmetric keys, EC DSA is used. 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. If the key identifier needs to be specified, then perform this step and the next step. Append the string "&KID=". 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 the Key ID URI contains only the "scheme name", "authority", and "path" parts. Append the string "&DS=". The message contains the complete section of the URI that is protected. (e.g. "://example.com/content.mov?VER=1&ET=1209422976&CIP=10.0.0.1&KID=http://example.com/public/keys/123&DS="). Compute the message digest using SHA-1 (without a key) for the message. Compute the digital signature (note: this is the URI signature) using the EC DSA algorithm with the private EC key and message digest (obtained in previous step) as inputs. Convert the digital signature to its equivalent human readable value. Append the string for the digital signature which contains the values for the 'r' and 's' parameters. The (r,s) pair is denoted by ':' (e.g. "://example.com/content.mov?VER=1&ET=1209422976&CIP=10.0.0.1&KID=http://example.com/public/keys/123&DS=r:CFB03EDB33810AB6C79EE3C47FBD86D227D702F25F66C01CF03F59F1E005668D:s:57ED0E8DF7E786C87E39177DD3398A7FB010E6A4C0DC8AA71331A929A29EA24E" ) Generate the Signed URI (i.e. when tokenizing the URI Signing attributes is not necessary) by prepending the "scheme name" part to the message (e.g. http://example.com/content.mov?VER=1&ET=1209422976&CIP=10.0.0.1&KID=http://example.com/public/keys/123&DS=r:CFB03EDB33810AB6C79EE3C47FBD86D227D702F25F66C01CF03F59F1E005668D:s:57ED0E8DF7E786C87E39177DD3398A7FB010E6A4C0DC8AA71331A929A29EA24E" ). Note: this is the completed Signed URI. When tokenizing the URI Signing attributes is desired, follow the procedure below. Generate the URI Signing token in this step and the next step. Remove the Original URI portion from the message to obtain all the URI Signing attributes, including the URI signature ("VER=1&ET=1209422976&CIP=10.0.0.1&KID=example:keys:123&HF=SHA-1&MD=4fb1c1adf1588fbe11cc6a04c6e69f35"). Compute the URI Signing token using Base-64 Data Encoding on the message (e.g. "VkVSPTEmRVQ9MTIwOTQyMjk3NiZDSVA9MTAuMC4wLjEmS0lEPWZvb2JhcjprZXlzOjEyMyZIRj1TSEEtMSZNRD00ZmIxYzFhZGYxNTg4ZmJlMTFjYzZhMDRjNmU2OWYzNQ==") Note: This is the value for the URI Signing token. Append the URI Signing token to the Original URI in this step and the next three steps. Copy the entire Original URI into a buffer to hold the message. Check if the URI already contains a query string. If not, append a "?" character. If yes, append an "&" character. Append the string "UST=" to the message. Append the URI Signing token to the message (e.g. "http://example.com/content.mov?UST=VkVSPTEmRVQ9MTIwOTQyMjk3NiZDSVA9MTAuMC4wLjEmS0lEPWZvb2JhcjprZXlzOjEyMyZIRj1TSEEtMSZNRD00ZmIxYzFhZGYxNTg4ZmJlMTFjYzZhMDRjNmU2OWYzNQ=="). Note: This is the complete Signed URI.

The following steps are specified to validate a Signed URI. However, it is possible to use some other algorithm and implementation as long as the same result is achieved. Note that some steps are to be skipped if the corresponding URI Signing attribute is not embedded in the Signed URI. The absence of a given attribute indicates enforcement of its purpose is not necessary in the distribution policy. Extract the value from "UST" attribute if the attribute exists. This value is the encoded URI Signing token. If there is no token in the URI, then skip the next step. Decode the string using Base-64 Data Encoding (or another encoding method specified by configuration or CDNI metada) to obtain all the URI Signing attributes (e.g. "VER=1&ET=1209422976&CIP=10.0.0.1&KID=example:keys:123&HF=SHA-1&MD=4fb1c1adf1588fbe11cc6a04c6e69f35"). Extract the value from "VER" attribute if the attribute exists. Determine the version of the URI Signing algorithm used to process the Signed URI. If the attribute is not in the URI, then obtain the version number in another manner (e.g. configuration or CDNI metadata). Extract the value from "CIP" attribute if the attribute exists. Validate that the request came from the same IP address as indicated in the "CIP" attribute. If the IP address is incorrect, then the request is denied. Extract the value from "ET" attribute if the attribute exists. Validate that the request arrived before expiration time based on the "ET" attribute. If the time expired, then the request is denied. Extract the value from "MD" attribute if the attribute exists. The attribute indicates symmetric key is used. Extract the value from "DS" attribute if the attribute exists. The attribute indicates asymmetric key is used. If neither "MD" or "DS" attribute is in the URI, then no URI signature exists and the request is denied. Validate the URI signature for the Signed URI. Copy the Original URI, excluding the "scheme name" part, into a buffer to hold the message for performing the operations below. Remove the "UST" attribute from the message. Append the decoded value from "UST" attribute (which contains all the URI Signing attributes). 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. For symmetric key, HMAC algorithm is used. Extract the value from the "KID" attribute if the attribute exists. Use the key identifier (e.g. "example:keys:123") to locate the shared key, which may be one of the keys available to use (i.e. set by configuration or CDNI metadata). If the attribute is not in the URI, then obtain the key in another manner (e.g. configuration or CDNI metadata). Extract the value from the "HF" attribute if the attribute exists. Determine the type of hash function (e.g. "MD5", "SHA-1") to use for HMAC. If the attribute is not in the URI, then obtain the hash function type in another manner (e.g. configuration or CDNI metadata). Extract the value from the "MD" attribute. This is the received message digest. Convert the message digest to binary format. This will be used to compare with the computed value later. Remove the value part of the "MD" attribute (but not the '=' character) from the message. The message is ready for validation of the message digest (e.g. "://example.com/content.mov?VER=1&ET=1209422976&CIP=10.0.0.1&KID=example:keys:123&HF=SHA-1&MD="). Compute the message digest using the HMAC algorithm with the shared key and message as the two inputs to the hash function which is specified by the "HF" attribute. Compare the result with the received message digest to validate the Signed URI. For asymmetric keys, EC DSA is used. Extract the value from the "KID" attribute. Use the key identifier (e.g. "http://example.com/public/keys/123") to obtain the EC public key, which may be one of the keys available to use (i.e. set by configuration or CDNI metadata). If the attribute is not in the URI, then obtain the key in another manner (e.g. configuration or CDNI metadata). Extract the value from the "DS" attribute. This is the digital signature. Convert the digital signature to binary format. This will be used for verification later. Remove the value part of the "DS" attribute (but not the '=' character) from the message. The message is ready for validation of the digital signature (e.g. "://example.com/content.mov?VER=1&ET=1209422976&CIP=10.0.0.1&KID=http://example.com/public/keys/123&DS="). Compute the message digest using SHA-1 (without a key) for the message. Verify the digital signature using the EC DSA algorithm with the public EC key, received digital signature, and message digest (obtained in previous step) as inputs. This validates the Signed URI.

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?).

The Downstream CDN advertises its capability to support URI Signing via the CDNI Request Routing/Footprint & Capabilities Advertisement Interface (CDNI FCI). The supported version of URI Signing needs to be included to allow for future extendebility. TBD: To be taken into account by Footprint & Capabilities design team working on this area. URI Signing version

Editor's Note: Check if there is an impact on CDNI RI? TBD: CDNI Redirection Interface is work in progress.

The following CDNI Metadata objects are specified for URI Signing. Note that the Key ID information 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 a 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 MUST provide the public key for the content or information to authorize the received Key ID attribute. TBD: CDNI Metadata Interface is work in progress. Content access control indication. Type of access control. Specifically, access to content is subject to URI Signing. URI Signing required indication means Downstream CDN ensures URI must be signed and validated before content delivery. Otherwise, Downstream CDN does not perform validation regardless if URI is signed or not. Version of URI Signing to use for validating the Signed URI Key value along with its key index (i.e. Key ID) and type (asymmetric or symmetric) used for validating URI signature. There may be one or more keys available to use for validation. Authorization to distribute the key(s) to Downstream CDNs Hash function for HMAC to be used for validation (i.e. enforce a specific hash function for security level) Encoding format to override the "UST" attribute. (Editor Note: Is this needed in CDNI Metadata or defined in a new CDNI attribute?)

The Downstream CDN reports that enforcement of the access control was applied to the request for content delivery. TBD: CDNI Logging interface is work in progress. URI signature validation events (e.g. invalid client IP address, expired signed URI, incorrect URI signature, successful validation) Delivery log with confirmation of access control enforcement (i.e Delivery CDN enforced URI Signing before content delivery)

URI Signing supports both HTTP-based and DNS-based request routing. HMAC 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.

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 RR 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.Authorisation 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 | | |------------------->| | Using the CDNI Request Routing/Footprint & Capabilities Advertisement interface, the Downstream CDN advertises its capabilities including URI Signing support to the Authoritative CDN. CSP provides to the Authoritative CDN the information needed to validate URI signatures from that CSP. For example, this information may include a hashing function, algorithm, and a key value. 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 a hashing algorithm and private key corresponding to the trust relationship between the Authoritative CDN and the Downstream CDN. 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 If the authorization decision is negative, the CSP rejects the request. 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. On receipt of the corresponding content request, the authoritative CDN validates the URI Signature in the URI using the information provided by the CSP. If the validation is negative, the authoritative CDN rejects the request 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 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 If the validation is negative, the Downstream CDN rejects the request and sends an error code (e.g. 403) in the HTTP response. If the validation is positive, the Downstream CDN serves the request and delivers the content. 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.

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 RR 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.authorisation 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 | | |------------------->| | Using the CDNI Request Routing interface, the Downstream CDN advertises its capabilities including URI Signing support to the Authoritative CDN. 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. Using the CDNI Metadata interface, the Authoritative CDN communicates to a Downstream CDN the information needed to validate cryptographic signatures from the CSP. In the case of symmetric key, the Authoritative CDN checks if the Downstream CDN is allowed by CSP to obtain the shared secret key. 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. If the authorization decision is negative, the CSP rejects the request 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. End user sends DNS request to the authoritative CDN. On receipt of the DNS request, the authoritative CDN redirects the request to the Downstream CDN. End user sends DNS request to the Downstream CDN. On receipt of the DNS request, the Downstream CDN responds with IP address of one of its Surrogates. 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 If the validation is negative, the Downstream CDN rejects the request and sends an error code (e.g. 403) in the HTTP response. If the validation is positive, the Downstream CDN serves the request and delivers the content. 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 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.

TBD - HTTP ABR calls for specific support by URI Signing ("flexible URI signing") as discussed in . This will be added in a future version of this document.

[Editor note: (Is there a need to/How to) register official query string attribute keywords to be used for URI Signing? Need anything from IANA?] This document requests IANA to create three new registries for the attributes (a.k.a. keywords) and their defined values in the URI Signing token. This document highlights the use of the following query string attribute in the URI to support URI Signing. There is no intention to claim any query string attribute for URI beyond the CDNI URI Signing context. That means the entities that sign the URI or validate the URI signature comply to the keyword specified in the query string for the URI Signing function only when URI Signing is used and only in the context of CDNI. The following Enforcement Attributes names are allocated: ET (Expiry time) CIP (Client IP address) The following Signature Computation Attributes names are allocated: VER (Version): 1(Base) KID (Key ID) HF (Hash Function): "MD5", "SHA1" The following URI Signature Attributes names are allocated: MD (Message Digest) DS (Digital Signature) The following URI Signing Token Attributes names are allocated: UST (URI Signing Token)

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 approriate. In general it holds that the level of protection against illegitimate access can be increased by including more Enforcement Attributes in the URI. The current version of this document includes attributes for enforcing Client IP Address and Expiration Time, however this list can be extended with other, more complex, attributes that are able to provide some form of protection against some of the vulnerabilities highlighted below. That said, there are a number of aspects that limit the level of security offered by URI signing and that anybody implementing URI signing should be aware of. Replay attacks: Any (valid) Signed URI can be used to perform replay attacks. The vulnerability to replay attacks can be reduced by picking a relatively short window for the Expiration Time attribute, although this is limited by the fact that any HTTP-based request needs a window of at least a couple of seconds to prevent any sudden network issues from preventing legitimate UAs access to the content. One way to reduce exposure to replay attacks is to include in the URI a unique one-time access ID. Whenever the Downstream CDN receives a request with a given unique 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. TBD: ... 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.

The authors would like to thank the following people for their contributions in reviewing this document and providing feedback: Kevin Ma, Ben Niven-Jenkins, Thierry Magnien, Dan York, Bhaskar Bhupalam, and Matt Caulfield.