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RFC 7515

 
 
 

JSON Web Signature (JWS)

Part 2 of 3, p. 15 to 36
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5.  Producing and Consuming JWSs

5.1.  Message Signature or MAC Computation

   To create a JWS, the following steps are performed.  The order of the
   steps is not significant in cases where there are no dependencies
   between the inputs and outputs of the steps.

   1.  Create the content to be used as the JWS Payload.

   2.  Compute the encoded payload value BASE64URL(JWS Payload).

   3.  Create the JSON object(s) containing the desired set of Header
       Parameters, which together comprise the JOSE Header (the JWS
       Protected Header and/or the JWS Unprotected Header).

   4.  Compute the encoded header value BASE64URL(UTF8(JWS Protected
       Header)).  If the JWS Protected Header is not present (which can
       only happen when using the JWS JSON Serialization and no
       "protected" member is present), let this value be the empty
       string.

   5.  Compute the JWS Signature in the manner defined for the
       particular algorithm being used over the JWS Signing Input
       ASCII(BASE64URL(UTF8(JWS Protected Header)) || '.' ||
       BASE64URL(JWS Payload)).  The "alg" (algorithm) Header Parameter
       MUST be present in the JOSE Header, with the algorithm value
       accurately representing the algorithm used to construct the JWS
       Signature.

   6.  Compute the encoded signature value BASE64URL(JWS Signature).

   7.  If the JWS JSON Serialization is being used, repeat this process
       (steps 3-6) for each digital signature or MAC operation being
       performed.

   8.  Create the desired serialized output.  The JWS Compact
       Serialization of this result is BASE64URL(UTF8(JWS Protected
       Header)) || '.' || BASE64URL(JWS Payload) || '.' || BASE64URL(JWS
       Signature).  The JWS JSON Serialization is described in
       Section 7.2.

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5.2.  Message Signature or MAC Validation

   When validating a JWS, the following steps are performed.  The order
   of the steps is not significant in cases where there are no
   dependencies between the inputs and outputs of the steps.  If any of
   the listed steps fails, then the signature or MAC cannot be
   validated.

   When there are multiple JWS Signature values, it is an application
   decision which of the JWS Signature values must successfully validate
   for the JWS to be accepted.  In some cases, all must successfully
   validate, or the JWS will be considered invalid.  In other cases,
   only a specific JWS Signature value needs to be successfully
   validated.  However, in all cases, at least one JWS Signature value
   MUST successfully validate, or the JWS MUST be considered invalid.

   1.  Parse the JWS representation to extract the serialized values for
       the components of the JWS.  When using the JWS Compact
       Serialization, these components are the base64url-encoded
       representations of the JWS Protected Header, the JWS Payload, and
       the JWS Signature, and when using the JWS JSON Serialization,
       these components also include the unencoded JWS Unprotected
       Header value.  When using the JWS Compact Serialization, the JWS
       Protected Header, the JWS Payload, and the JWS Signature are
       represented as base64url-encoded values in that order, with each
       value being separated from the next by a single period ('.')
       character, resulting in exactly two delimiting period characters
       being used.  The JWS JSON Serialization is described in
       Section 7.2.

   2.  Base64url-decode the encoded representation of the JWS Protected
       Header, following the restriction that no line breaks,
       whitespace, or other additional characters have been used.

   3.  Verify that the resulting octet sequence is a UTF-8-encoded
       representation of a completely valid JSON object conforming to
       RFC 7159 [RFC7159]; let the JWS Protected Header be this JSON
       object.

   4.  If using the JWS Compact Serialization, let the JOSE Header be
       the JWS Protected Header.  Otherwise, when using the JWS JSON
       Serialization, let the JOSE Header be the union of the members of
       the corresponding JWS Protected Header and JWS Unprotected
       Header, all of which must be completely valid JSON objects.
       During this step, verify that the resulting JOSE Header does not
       contain duplicate Header Parameter names.  When using the JWS

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       JSON Serialization, this restriction includes that the same
       Header Parameter name also MUST NOT occur in distinct JSON object
       values that together comprise the JOSE Header.

   5.  Verify that the implementation understands and can process all
       fields that it is required to support, whether required by this
       specification, by the algorithm being used, or by the "crit"
       Header Parameter value, and that the values of those parameters
       are also understood and supported.

   6.  Base64url-decode the encoded representation of the JWS Payload,
       following the restriction that no line breaks, whitespace, or
       other additional characters have been used.

   7.  Base64url-decode the encoded representation of the JWS Signature,
       following the restriction that no line breaks, whitespace, or
       other additional characters have been used.

   8.  Validate the JWS Signature against the JWS Signing Input
       ASCII(BASE64URL(UTF8(JWS Protected Header)) || '.' ||
       BASE64URL(JWS Payload)) in the manner defined for the algorithm
       being used, which MUST be accurately represented by the value of
       the "alg" (algorithm) Header Parameter, which MUST be present.
       See Section 10.6 for security considerations on algorithm
       validation.  Record whether the validation succeeded or not.

   9.  If the JWS JSON Serialization is being used, repeat this process
       (steps 4-8) for each digital signature or MAC value contained in
       the representation.

   10. If none of the validations in step 9 succeeded, then the JWS MUST
       be considered invalid.  Otherwise, in the JWS JSON Serialization
       case, return a result to the application indicating which of the
       validations succeeded and failed.  In the JWS Compact
       Serialization case, the result can simply indicate whether or not
       the JWS was successfully validated.

   Finally, note that it is an application decision which algorithms may
   be used in a given context.  Even if a JWS can be successfully
   validated, unless the algorithm(s) used in the JWS are acceptable to
   the application, it SHOULD consider the JWS to be invalid.

5.3.  String Comparison Rules

   Processing a JWS inevitably requires comparing known strings to
   members and values in JSON objects.  For example, in checking what
   the algorithm is, the Unicode string "alg" will be checked against
   the member names in the JOSE Header to see if there is a matching

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   Header Parameter name.  The same process is then used to determine if
   the value of the "alg" Header Parameter represents a supported
   algorithm.

   The JSON rules for doing member name comparison are described in
   Section 8.3 of RFC 7159 [RFC7159].  Since the only string comparison
   operations that are performed are equality and inequality, the same
   rules can be used for comparing both member names and member values
   against known strings.

   These comparison rules MUST be used for all JSON string comparisons
   except in cases where the definition of the member explicitly calls
   out that a different comparison rule is to be used for that member
   value.  Only the "typ" and "cty" member values defined in this
   specification do not use these comparison rules.

   Some applications may include case-insensitive information in a case-
   sensitive value, such as including a DNS name as part of a "kid" (key
   ID) value.  In those cases, the application may need to define a
   convention for the canonical case to use for representing the case-
   insensitive portions, such as lowercasing them, if more than one
   party might need to produce the same value so that they can be
   compared.  (However, if all other parties consume whatever value the
   producing party emitted verbatim without attempting to compare it to
   an independently produced value, then the case used by the producer
   will not matter.)

   Also, see the JSON security considerations in Section 10.12 and the
   Unicode security considerations in Section 10.13.

6.  Key Identification

   It is necessary for the recipient of a JWS to be able to determine
   the key that was employed for the digital signature or MAC operation.
   The key employed can be identified using the Header Parameter methods
   described in Section 4.1 or can be identified using methods that are
   outside the scope of this specification.  Specifically, the Header
   Parameters "jku", "jwk", "kid", "x5u", "x5c", "x5t", and "x5t#S256"
   can be used to identify the key used.  These Header Parameters MUST
   be integrity protected if the information that they convey is to be
   utilized in a trust decision; however, if the only information used
   in the trust decision is a key, these parameters need not be
   integrity protected, since changing them in a way that causes a
   different key to be used will cause the validation to fail.

   The producer SHOULD include sufficient information in the Header
   Parameters to identify the key used, unless the application uses
   another means or convention to determine the key used.  Validation of

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   the signature or MAC fails when the algorithm used requires a key
   (which is true of all algorithms except for "none") and the key used
   cannot be determined.

   The means of exchanging any shared symmetric keys used is outside the
   scope of this specification.

   Also, see Appendix D for notes on possible key selection algorithms.

7.  Serializations

   JWSs use one of two serializations: the JWS Compact Serialization or
   the JWS JSON Serialization.  Applications using this specification
   need to specify what serialization and serialization features are
   used for that application.  For instance, applications might specify
   that only the JWS JSON Serialization is used, that only JWS JSON
   Serialization support for a single signature or MAC value is used, or
   that support for multiple signatures and/or MAC values is used.  JWS
   implementations only need to implement the features needed for the
   applications they are designed to support.

7.1.  JWS Compact Serialization

   The JWS Compact Serialization represents digitally signed or MACed
   content as a compact, URL-safe string.  This string is:

      BASE64URL(UTF8(JWS Protected Header)) || '.' ||
      BASE64URL(JWS Payload) || '.' ||
      BASE64URL(JWS Signature)

   Only one signature/MAC is supported by the JWS Compact Serialization
   and it provides no syntax to represent a JWS Unprotected Header
   value.

7.2.  JWS JSON Serialization

   The JWS JSON Serialization represents digitally signed or MACed
   content as a JSON object.  This representation is neither optimized
   for compactness nor URL-safe.

   Two closely related syntaxes are defined for the JWS JSON
   Serialization: a fully general syntax, with which content can be
   secured with more than one digital signature and/or MAC operation,
   and a flattened syntax, which is optimized for the single digital
   signature or MAC case.

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7.2.1.  General JWS JSON Serialization Syntax

   The following members are defined for use in top-level JSON objects
   used for the fully general JWS JSON Serialization syntax:

   payload
      The "payload" member MUST be present and contain the value
      BASE64URL(JWS Payload).

   signatures
      The "signatures" member value MUST be an array of JSON objects.
      Each object represents a signature or MAC over the JWS Payload and
      the JWS Protected Header.

   The following members are defined for use in the JSON objects that
   are elements of the "signatures" array:

   protected
      The "protected" member MUST be present and contain the value
      BASE64URL(UTF8(JWS Protected Header)) when the JWS Protected
      Header value is non-empty; otherwise, it MUST be absent.  These
      Header Parameter values are integrity protected.

   header
      The "header" member MUST be present and contain the value JWS
      Unprotected Header when the JWS Unprotected Header value is non-
      empty; otherwise, it MUST be absent.  This value is represented as
      an unencoded JSON object, rather than as a string.  These Header
      Parameter values are not integrity protected.

   signature
      The "signature" member MUST be present and contain the value
      BASE64URL(JWS Signature).

   At least one of the "protected" and "header" members MUST be present
   for each signature/MAC computation so that an "alg" Header Parameter
   value is conveyed.

   Additional members can be present in both the JSON objects defined
   above; if not understood by implementations encountering them, they
   MUST be ignored.

   The Header Parameter values used when creating or validating
   individual signature or MAC values are the union of the two sets of
   Header Parameter values that may be present: (1) the JWS Protected
   Header represented in the "protected" member of the signature/MAC's
   array element, and (2) the JWS Unprotected Header in the "header"

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   member of the signature/MAC's array element.  The union of these sets
   of Header Parameters comprises the JOSE Header.  The Header Parameter
   names in the two locations MUST be disjoint.

   Each JWS Signature value is computed using the parameters of the
   corresponding JOSE Header value in the same manner as for the JWS
   Compact Serialization.  This has the desirable property that each JWS
   Signature value represented in the "signatures" array is identical to
   the value that would have been computed for the same parameter in the
   JWS Compact Serialization, provided that the JWS Protected Header
   value for that signature/MAC computation (which represents the
   integrity-protected Header Parameter values) matches that used in the
   JWS Compact Serialization.

   In summary, the syntax of a JWS using the general JWS JSON
   Serialization is as follows:

     {
      "payload":"<payload contents>",
      "signatures":[
       {"protected":"<integrity-protected header 1 contents>",
        "header":<non-integrity-protected header 1 contents>,
        "signature":"<signature 1 contents>"},
       ...
       {"protected":"<integrity-protected header N contents>",
        "header":<non-integrity-protected header N contents>,
        "signature":"<signature N contents>"}]
     }

   See Appendix A.6 for an example JWS using the general JWS JSON
   Serialization syntax.

7.2.2.  Flattened JWS JSON Serialization Syntax

   The flattened JWS JSON Serialization syntax is based upon the general
   syntax but flattens it, optimizing it for the single digital
   signature/MAC case.  It flattens it by removing the "signatures"
   member and instead placing those members defined for use in the
   "signatures" array (the "protected", "header", and "signature"
   members) in the top-level JSON object (at the same level as the
   "payload" member).

   The "signatures" member MUST NOT be present when using this syntax.
   Other than this syntax difference, JWS JSON Serialization objects
   using the flattened syntax are processed identically to those using
   the general syntax.

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   In summary, the syntax of a JWS using the flattened JWS JSON
   Serialization is as follows:

     {
      "payload":"<payload contents>",
      "protected":"<integrity-protected header contents>",
      "header":<non-integrity-protected header contents>,
      "signature":"<signature contents>"
     }

   See Appendix A.7 for an example JWS using the flattened JWS JSON
   Serialization syntax.

8.  TLS Requirements

   Implementations supporting the "jku" and/or "x5u" Header Parameters
   MUST support TLS.  Which TLS version(s) ought to be implemented will
   vary over time and depend on the widespread deployment and known
   security vulnerabilities at the time of implementation.  At the time
   of this writing, TLS version 1.2 [RFC5246] is the most recent
   version.

   To protect against information disclosure and tampering,
   confidentiality protection MUST be applied using TLS with a
   ciphersuite that provides confidentiality and integrity protection.
   See current publications by the IETF TLS working group, including RFC
   6176 [RFC6176], for guidance on the ciphersuites currently considered
   to be appropriate for use.  Also, see "Recommendations for Secure Use
   of Transport Layer Security (TLS) and Datagram Transport Layer
   Security (DTLS)" [RFC7525] for recommendations on improving the
   security of software and services using TLS.

   Whenever TLS is used, the identity of the service provider encoded in
   the TLS server certificate MUST be verified using the procedures
   described in Section 6 of RFC 6125 [RFC6125].

9.  IANA Considerations

   The following registration procedure is used for all the registries
   established by this specification.

   Values are registered on a Specification Required [RFC5226] basis
   after a three-week review period on the jose-reg-review@ietf.org
   mailing list, on the advice of one or more Designated Experts.
   However, to allow for the allocation of values prior to publication,
   the Designated Experts may approve registration once they are
   satisfied that such a specification will be published.

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   Registration requests sent to the mailing list for review should use
   an appropriate subject (e.g., "Request to register header parameter:
   example").

   Within the review period, the Designated Experts will either approve
   or deny the registration request, communicating this decision to the
   review list and IANA.  Denials should include an explanation and, if
   applicable, suggestions as to how to make the request successful.
   Registration requests that are undetermined for a period longer than
   21 days can be brought to the IESG's attention (using the
   iesg@ietf.org mailing list) for resolution.

   Criteria that should be applied by the Designated Experts includes
   determining whether the proposed registration duplicates existing
   functionality, whether it is likely to be of general applicability or
   useful only for a single application, and whether the registration
   description is clear.

   IANA must only accept registry updates from the Designated Experts
   and should direct all requests for registration to the review mailing
   list.

   It is suggested that multiple Designated Experts be appointed who are
   able to represent the perspectives of different applications using
   this specification, in order to enable broadly informed review of
   registration decisions.  In cases where a registration decision could
   be perceived as creating a conflict of interest for a particular
   Expert, that Expert should defer to the judgment of the other
   Experts.

9.1.  JSON Web Signature and Encryption Header Parameters Registry

   This specification establishes the IANA "JSON Web Signature and
   Encryption Header Parameters" registry for Header Parameter names.
   The registry records the Header Parameter name and a reference to the
   specification that defines it.  The same Header Parameter name can be
   registered multiple times, provided that the parameter usage is
   compatible between the specifications.  Different registrations of
   the same Header Parameter name will typically use different Header
   Parameter Usage Locations values.

9.1.1.  Registration Template

   Header Parameter Name:
      The name requested (e.g., "kid").  Because a core goal of this
      specification is for the resulting representations to be compact,
      it is RECOMMENDED that the name be short -- not to exceed 8
      characters without a compelling reason to do so.  This name is

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      case sensitive.  Names may not match other registered names in a
      case-insensitive manner unless the Designated Experts state that
      there is a compelling reason to allow an exception.

   Header Parameter Description:
      Brief description of the Header Parameter (e.g., "Key ID").

   Header Parameter Usage Location(s):
      The Header Parameter usage locations, which should be one or more
      of the values "JWS" or "JWE".

   Change Controller:
      For Standards Track RFCs, list the "IESG".  For others, give the
      name of the responsible party.  Other details (e.g., postal
      address, email address, home page URI) may also be included.

   Specification Document(s):
      Reference to the document or documents that specify the parameter,
      preferably including URIs that can be used to retrieve copies of
      the documents.  An indication of the relevant sections may also be
      included but is not required.

9.1.2.  Initial Registry Contents

   This section registers the Header Parameter names defined in
   Section 4.1 in this registry.

   o  Header Parameter Name: "alg"
   o  Header Parameter Description: Algorithm
   o  Header Parameter Usage Location(s): JWS
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.1 of RFC 7515

   o  Header Parameter Name: "jku"
   o  Header Parameter Description: JWK Set URL
   o  Header Parameter Usage Location(s): JWS
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.2 of RFC 7515

   o  Header Parameter Name: "jwk"
   o  Header Parameter Description: JSON Web Key
   o  Header Parameter Usage Location(s): JWS
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.3 of RFC 7515

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   o  Header Parameter Name: "kid"
   o  Header Parameter Description: Key ID
   o  Header Parameter Usage Location(s): JWS
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.4 of RFC 7515

   o  Header Parameter Name: "x5u"
   o  Header Parameter Description: X.509 URL
   o  Header Parameter Usage Location(s): JWS
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.5 of RFC 7515

   o  Header Parameter Name: "x5c"
   o  Header Parameter Description: X.509 Certificate Chain
   o  Header Parameter Usage Location(s): JWS
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.6 of RFC 7515

   o  Header Parameter Name: "x5t"
   o  Header Parameter Description: X.509 Certificate SHA-1 Thumbprint
   o  Header Parameter Usage Location(s): JWS
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.7 of RFC 7515

   o  Header Parameter Name: "x5t#S256"
   o  Header Parameter Description: X.509 Certificate SHA-256 Thumbprint
   o  Header Parameter Usage Location(s): JWS
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.8 of RFC 7515

   o  Header Parameter Name: "typ"
   o  Header Parameter Description: Type
   o  Header Parameter Usage Location(s): JWS
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.9 of RFC 7515

   o  Header Parameter Name: "cty"
   o  Header Parameter Description: Content Type
   o  Header Parameter Usage Location(s): JWS
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.10 of RFC 7515

   o  Header Parameter Name: "crit"
   o  Header Parameter Description: Critical
   o  Header Parameter Usage Location(s): JWS
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.11 of RFC 7515

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9.2.  Media Type Registration

9.2.1.  Registry Contents

   This section registers the "application/jose" media type [RFC2046] in
   the "Media Types" registry [IANA.MediaTypes] in the manner described
   in RFC 6838 [RFC6838], which can be used to indicate that the content
   is a JWS or JWE using the JWS Compact Serialization or the JWE
   Compact Serialization.  This section also registers the "application/
   jose+json" media type in the "Media Types" registry, which can be
   used to indicate that the content is a JWS or JWE using the JWS JSON
   Serialization or the JWE JSON Serialization.

   o  Type name: application
   o  Subtype name: jose
   o  Required parameters: n/a
   o  Optional parameters: n/a
   o  Encoding considerations: 8bit; application/jose values are encoded
      as a series of base64url-encoded values (some of which may be the
      empty string), each separated from the next by a single period
      ('.') character.
   o  Security considerations: See the Security Considerations section
      of RFC 7515.
   o  Interoperability considerations: n/a
   o  Published specification: RFC 7515
   o  Applications that use this media type: OpenID Connect, Mozilla
      Persona, Salesforce, Google, Android, Windows Azure, Xbox One,
      Amazon Web Services, and numerous others that use JWTs
   o  Fragment identifier considerations: n/a
   o  Additional information:

         Magic number(s): n/a
         File extension(s): n/a
         Macintosh file type code(s): n/a

   o  Person & email address to contact for further information:
      Michael B. Jones, mbj@microsoft.com
   o  Intended usage: COMMON
   o  Restrictions on usage: none
   o  Author: Michael B. Jones, mbj@microsoft.com
   o  Change Controller: IESG
   o  Provisional registration?  No

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   o  Type name: application
   o  Subtype name: jose+json
   o  Required parameters: n/a
   o  Optional parameters: n/a
   o  Encoding considerations: 8bit; application/jose+json values are
      represented as a JSON Object; UTF-8 encoding SHOULD be employed
      for the JSON object.
   o  Security considerations: See the Security Considerations section
      of RFC 7515
   o  Interoperability considerations: n/a
   o  Published specification: RFC 7515
   o  Applications that use this media type: Nimbus JOSE + JWT library
   o  Fragment identifier considerations: n/a
   o  Additional information:

         Magic number(s): n/a
         File extension(s): n/a
         Macintosh file type code(s): n/a

   o  Person & email address to contact for further information:
      Michael B. Jones, mbj@microsoft.com
   o  Intended usage: COMMON
   o  Restrictions on usage: none
   o  Author: Michael B. Jones, mbj@microsoft.com
   o  Change Controller: IESG
   o  Provisional registration?  No

10.  Security Considerations

   All of the security issues that are pertinent to any cryptographic
   application must be addressed by JWS/JWE/JWK agents.  Among these
   issues are protecting the user's asymmetric private and symmetric
   secret keys and employing countermeasures to various attacks.

   All the security considerations in "XML Signature Syntax and
   Processing Version 2.0" [W3C.NOTE-xmldsig-core2-20130411], also apply
   to this specification, other than those that are XML specific.
   Likewise, many of the best practices documented in "XML Signature
   Best Practices" [W3C.NOTE-xmldsig-bestpractices-20130411] also apply
   to this specification, other than those that are XML specific.

10.1.  Key Entropy and Random Values

   Keys are only as strong as the amount of entropy used to generate
   them.  A minimum of 128 bits of entropy should be used for all keys,
   and depending upon the application context, more may be required.

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   Implementations must randomly generate public/private key pairs, MAC
   keys, and padding values.  The use of inadequate pseudorandom number
   generators (PRNGs) to generate cryptographic keys can result in
   little or no security.  An attacker may find it much easier to
   reproduce the PRNG environment that produced the keys, searching the
   resulting small set of possibilities rather than brute-force
   searching the whole key space.  The generation of quality random
   numbers is difficult.  RFC 4086 [RFC4086] offers important guidance
   in this area.

10.2.  Key Protection

   Implementations must protect the signer's private key.  Compromise of
   the signer's private key permits an attacker to masquerade as the
   signer.

   Implementations must protect the MAC key.  Compromise of the MAC key
   may result in undetectable modification of the authenticated content.

10.3.  Key Origin Authentication

   The key management technique employed to obtain public keys must
   authenticate the origin of the key; otherwise, it is unknown what
   party signed the message.

   Likewise, the key management technique employed to distribute MAC
   keys must provide data origin authentication; otherwise, the contents
   are delivered with integrity from an unknown source.

10.4.  Cryptographic Agility

   See Section 8.1 of [JWA] for security considerations on cryptographic
   agility.

10.5.  Differences between Digital Signatures and MACs

   While MACs and digital signatures can both be used for integrity
   checking, there are some significant differences between the security
   properties that each of them provides.  These need to be taken into
   consideration when designing protocols and selecting the algorithms
   to be used in protocols.

   Both signatures and MACs provide for integrity checking -- verifying
   that the message has not been modified since the integrity value was
   computed.  However, MACs provide for origination identification only
   under specific circumstances.  It can normally be assumed that a
   private key used for a signature is only in the hands of a single
   entity (although perhaps a distributed entity, in the case of

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   replicated servers); however, a MAC key needs to be in the hands of
   all the entities that use it for integrity computation and checking.
   Validation of a MAC only provides corroboration that the message was
   generated by one of the parties that knows the symmetric MAC key.
   This means that origination can only be determined if a MAC key is
   known only to two entities and the recipient knows that it did not
   create the message.  MAC validation cannot be used to prove
   origination to a third party.

10.6.  Algorithm Validation

   The digital signature representations for some algorithms include
   information about the algorithm used inside the signature value.  For
   instance, signatures produced with RSASSA-PKCS1-v1_5 [RFC3447] encode
   the hash function used, and many libraries actually use the hash
   algorithm specified inside the signature when validating the
   signature.  When using such libraries, as part of the algorithm
   validation performed, implementations MUST ensure that the algorithm
   information encoded in the signature corresponds to that specified
   with the "alg" Header Parameter.  If this is not done, an attacker
   could claim to have used a strong hash algorithm while actually using
   a weak one represented in the signature value.

10.7.  Algorithm Protection

   In some usages of JWS, there is a risk of algorithm substitution
   attacks, in which an attacker can use an existing digital signature
   value with a different signature algorithm to make it appear that a
   signer has signed something that it has not.  These attacks have been
   discussed in detail in the context of Cryptographic Message Syntax
   (CMS) [RFC6211].  This risk arises when all of the following are
   true:

   o  Verifiers of a signature support multiple algorithms.

   o  Given an existing signature, an attacker can find another payload
      that produces the same signature value with a different algorithm.

   o  The payload crafted by the attacker is valid in the application
      context.

   There are several ways for an application to mitigate algorithm
   substitution attacks:

   o  Use only digital signature algorithms that are not vulnerable to
      substitution attacks.  Substitution attacks are only feasible if
      an attacker can compute pre-images for a hash function accepted by

Top      Up      ToC       Page 30 
      the recipient.  All JWA-defined signature algorithms use SHA-2
      hashes, for which there are no known pre-image attacks, as of the
      time of this writing.

   o  Require that the "alg" Header Parameter be carried in the JWS
      Protected Header.  (This is always the case when using the JWS
      Compact Serialization and is the approach taken by CMS [RFC6211].)

   o  Include a field containing the algorithm in the application
      payload, and require that it be matched with the "alg" Header
      Parameter during verification.  (This is the approach taken by
      PKIX [RFC5280].)

10.8.  Chosen Plaintext Attacks

   Creators of JWSs should not allow third parties to insert arbitrary
   content into the message without adding entropy not controlled by the
   third party.

10.9.  Timing Attacks

   When cryptographic algorithms are implemented in such a way that
   successful operations take a different amount of time than
   unsuccessful operations, attackers may be able to use the time
   difference to obtain information about the keys employed.  Therefore,
   such timing differences must be avoided.

10.10.  Replay Protection

   While not directly in scope for this specification, note that
   applications using JWS (or JWE) objects can thwart replay attacks by
   including a unique message identifier as integrity-protected content
   in the JWS (or JWE) message and having the recipient verify that the
   message has not been previously received or acted upon.

10.11.  SHA-1 Certificate Thumbprints

   A SHA-1 hash is used when computing "x5t" (X.509 certificate SHA-1
   thumbprint) values, for compatibility reasons.  Should an effective
   means of producing SHA-1 hash collisions be developed and should an
   attacker wish to interfere with the use of a known certificate on a
   given system, this could be accomplished by creating another
   certificate whose SHA-1 hash value is the same and adding it to the
   certificate store used by the intended victim.  A prerequisite to
   this attack succeeding is the attacker having write access to the
   intended victim's certificate store.

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   Alternatively, the "x5t#S256" (X.509 certificate SHA-256 thumbprint)
   Header Parameter could be used instead of "x5t".  However, at the
   time of this writing, no development platform is known to support
   SHA-256 certificate thumbprints.

10.12.  JSON Security Considerations

   Strict JSON [RFC7159] validation is a security requirement.  If
   malformed JSON is received, then the intent of the producer is
   impossible to reliably discern.  Ambiguous and potentially
   exploitable situations could arise if the JSON parser used does not
   reject malformed JSON syntax.  In particular, any JSON inputs not
   conforming to the JSON-text syntax defined in RFC 7159 MUST be
   rejected in their entirety by JSON parsers.

   Section 4 of "The JavaScript Object Notation (JSON) Data Interchange
   Format" [RFC7159] states, "The names within an object SHOULD be
   unique", whereas this specification states that

      The Header Parameter names within the JOSE Header MUST be unique;
      JWS parsers MUST either reject JWSs with duplicate Header
      Parameter names or use a JSON parser that returns only the
      lexically last duplicate member name, as specified in
      Section 15.12 ("The JSON Object") of ECMAScript 5.1 [ECMAScript].

   Thus, this specification requires that the "SHOULD" in Section 4 of
   [RFC7159] be treated as a "MUST" by producers and that it be either
   treated as a "MUST" or treated in the manner specified in ECMAScript
   5.1 by consumers.  Ambiguous and potentially exploitable situations
   could arise if the JSON parser used does not enforce the uniqueness
   of member names or returns an unpredictable value for duplicate
   member names.

   Some JSON parsers might not reject input that contains extra
   significant characters after a valid input.  For instance, the input
   "{"tag":"value"}ABCD" contains a valid JSON-text object followed by
   the extra characters "ABCD".  Implementations MUST consider JWSs
   containing such input to be invalid.

10.13.  Unicode Comparison Security Considerations

   Header Parameter names and algorithm names are Unicode strings.  For
   security reasons, the representations of these names must be compared
   verbatim after performing any escape processing (as per Section 8.3
   of RFC 7159 [RFC7159]).  This means, for instance, that these JSON
   strings must compare as being equal ("sig", "\u0073ig"), whereas
   these must all compare as being not equal to the first set or to each
   other ("SIG", "Sig", "si\u0047").

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   JSON strings can contain characters outside the Unicode Basic
   Multilingual Plane.  For instance, the G clef character (U+1D11E) may
   be represented in a JSON string as "\uD834\uDD1E".  Ideally, JWS
   implementations SHOULD ensure that characters outside the Basic
   Multilingual Plane are preserved and compared correctly;
   alternatively, if this is not possible due to these characters
   exercising limitations present in the underlying JSON implementation,
   then input containing them MUST be rejected.

11.  References

11.1.  Normative References

   [ECMAScript] Ecma International, "ECMAScript Language Specification,
                5.1 Edition", ECMA 262, June 2011,
                <http://www.ecma-international.org/ecma-262/5.1/
                ECMA-262.pdf>.

   [IANA.MediaTypes]
                IANA, "Media Types",
                <http://www.iana.org/assignments/media-types>.

   [ITU.X690.2008]
                International Telecommunications Union, "Information
                Technology - ASN.1 encoding rules: Specification of
                Basic Encoding Rules (BER), Canonical Encoding Rules
                (CER) and Distinguished Encoding Rules (DER)", ITU-T
                Recommendation X.690, 2008.

   [JWA]        Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
                DOI 10.17487/RFC7518, May 2015,
                <http://www.rfc-editor.org/info/rfc7518>.

   [JWK]        Jones, M., "JSON Web Key (JWK)", RFC 7517,
                DOI 10.17487/RFC7517, May 2015,
                <http://www.rfc-editor.org/info/rfc7517>.

   [RFC20]      Cerf, V., "ASCII format for Network Interchange",
                STD 80, RFC 20, DOI 10.17487/RFC0020, October 1969,
                <http://www.rfc-editor.org/info/rfc20>.

   [RFC2045]    Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                Extensions (MIME) Part One: Format of Internet Message
                Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
                <http://www.rfc-editor.org/info/rfc2045>.

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   [RFC2046]    Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                Extensions (MIME) Part Two: Media Types", RFC 2046,
                DOI 10.17487/RFC2046, November 1996,
                <http://www.rfc-editor.org/info/rfc2046>.

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119,
                DOI 10.17487/RFC2119, March 1997,
                <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2818]    Rescorla, E., "HTTP Over TLS", RFC 2818,
                DOI 10.17487/RFC2818, May 2000,
                <http://www.rfc-editor.org/info/rfc2818>.

   [RFC3629]    Yergeau, F., "UTF-8, a transformation format of ISO
                10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
                2003, <http://www.rfc-editor.org/info/rfc3629>.

   [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>.

   [RFC4945]    Korver, B., "The Internet IP Security PKI Profile of
                IKEv1/ISAKMP, IKEv2, and PKIX", RFC 4945,
                DOI 10.17487/RFC4945, August 2007,
                <http://www.rfc-editor.org/info/rfc4945>.

   [RFC4949]    Shirey, R., "Internet Security Glossary, Version 2",
                FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
                <http://www.rfc-editor.org/info/rfc4949>.

   [RFC5246]    Dierks, T. and E. Rescorla, "The Transport Layer
                Security (TLS) Protocol Version 1.2", RFC 5246,
                DOI 10.17487/RFC5246, August 2008,
                <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5280]    Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
                Housley, R., and W. Polk, "Internet X.509 Public Key
                Infrastructure Certificate and Certificate Revocation
                List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May
                2008, <http://www.rfc-editor.org/info/rfc5280>.

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   [RFC6125]    Saint-Andre, P. and J. Hodges, "Representation and
                Verification of Domain-Based Application Service
                Identity within Internet Public Key Infrastructure Using
                X.509 (PKIX) Certificates in the Context of Transport
                Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125,
                March 2011, <http://www.rfc-editor.org/info/rfc6125>.

   [RFC6176]    Turner, S. and T. Polk, "Prohibiting Secure Sockets
                Layer (SSL) Version 2.0", RFC 6176,
                DOI 10.17487/RFC6176, March 2011,
                <http://www.rfc-editor.org/info/rfc6176>.

   [RFC7159]    Bray, T., Ed., "The JavaScript Object Notation (JSON)
                Data Interchange Format", RFC 7159,
                DOI 10.17487/RFC7159, March 2014,
                <http://www.rfc-editor.org/info/rfc7159>.

   [UNICODE]    The Unicode Consortium, "The Unicode Standard",
                <http://www.unicode.org/versions/latest/>.

11.2.  Informative References

   [CanvasApp]  Facebook, "Canvas Applications",
                <http://developers.facebook.com/docs/authentication/
                canvas>.

   [JSS]        Bradley, J. and N. Sakimura, Ed., "JSON Simple Sign",
                September 2010, <http://jsonenc.info/jss/1.0/>.

   [JWE]        Jones, M. and J. Hildebrand, "JSON Web Encryption
                (JWE)", RFC 7516, DOI 10.17487/RFC7516, May 2015,
                <http://www.rfc-editor.org/info/rfc7516>.

   [JWT]        Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
                (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
                <http://www.rfc-editor.org/info/rfc7519>.

   [MagicSignatures]
                Panzer, J., Ed., Laurie, B., and D. Balfanz, "Magic
                Signatures", January 2011,
                <http://salmon-protocol.googlecode.com/svn/trunk/
                draft-panzer-magicsig-01.html>.

   [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>.

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   [RFC3447]    Jonsson, J. and B. Kaliski, "Public-Key Cryptography
                Standards (PKCS) #1: RSA Cryptography Specifications
                Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
                2003, <http://www.rfc-editor.org/info/rfc3447>.

   [RFC4086]    Eastlake 3rd, D., Schiller, J., and S. Crocker,
                "Randomness Requirements for Security", BCP 106,
                RFC 4086, DOI 10.17487/RFC4086, June 2005,
                <http://www.rfc-editor.org/info/rfc4086>.

   [RFC4122]    Leach, P., Mealling, M., and R. Salz, "A Universally
                Unique IDentifier (UUID) URN Namespace", RFC 4122,
                DOI 10.17487/RFC4122, July 2005,
                <http://www.rfc-editor.org/info/rfc4122>.

   [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>.

   [RFC6211]    Schaad, J., "Cryptographic Message Syntax (CMS)
                Algorithm Identifier Protection Attribute", RFC 6211,
                DOI 10.17487/RFC6211, April 2011,
                <http://www.rfc-editor.org/info/rfc6211>.

   [RFC6838]    Freed, N., Klensin, J., and T. Hansen, "Media Type
                Specifications and Registration Procedures", BCP 13,
                RFC 6838, DOI 10.17487/RFC6838, January 2013,
                <http://www.rfc-editor.org/info/rfc6838>.

   [RFC7525]    Sheffer, Y., Holz, R., and P. Saint-Andre,
                "Recommendations for Secure Use of Transport Layer
                Security (TLS) and Datagram Transport Layer Security
                (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
                2015, <http://www.rfc-editor.org/info/rfc7525>.

   [SHS]        National Institute of Standards and Technology, "Secure
                Hash Standard (SHS)", FIPS PUB 180-4, March 2012,
                <http://csrc.nist.gov/publications/fips/fips180-4/
                fips-180-4.pdf>.

   [W3C.NOTE-xmldsig-bestpractices-20130411]
                Hirsch, F. and P. Datta, "XML Signature Best Practices",
                World Wide Web Consortium Note
                NOTE-xmldsig-bestpractices-20130411, April 2013,
                <http://www.w3.org/TR/2013/
                NOTE-xmldsig-bestpractices-20130411/>.

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   [W3C.NOTE-xmldsig-core2-20130411]
                Eastlake, D., Reagle, J., Solo, D., Hirsch, F.,
                Roessler, T., Yiu, K., Datta, P., and S. Cantor, "XML
                Signature Syntax and Processing Version 2.0", World Wide
                Web Consortium Note NOTE-xmldsig-core2-20130411, April
                2013,
                <http://www.w3.org/TR/2013/NOTE-xmldsig-core2-20130411/>.


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