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RFC 7516

 
 
 

JSON Web Encryption (JWE)

Part 2 of 3, p. 15 to 28
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5.  Producing and Consuming JWEs

5.1.  Message Encryption

   The message encryption process is as follows.  The order of the steps
   is not significant in cases where there are no dependencies between
   the inputs and outputs of the steps.

   1.   Determine the Key Management Mode employed by the algorithm used
        to determine the Content Encryption Key value.  (This is the
        algorithm recorded in the "alg" (algorithm) Header Parameter of
        the resulting JWE.)

   2.   When Key Wrapping, Key Encryption, or Key Agreement with Key
        Wrapping are employed, generate a random CEK value.  See RFC
        4086 [RFC4086] for considerations on generating random values.
        The CEK MUST have a length equal to that required for the
        content encryption algorithm.

   3.   When Direct Key Agreement or Key Agreement with Key Wrapping are
        employed, use the key agreement algorithm to compute the value
        of the agreed upon key.  When Direct Key Agreement is employed,
        let the CEK be the agreed upon key.  When Key Agreement with Key
        Wrapping is employed, the agreed upon key will be used to wrap
        the CEK.

   4.   When Key Wrapping, Key Encryption, or Key Agreement with Key
        Wrapping are employed, encrypt the CEK to the recipient and let
        the result be the JWE Encrypted Key.

   5.   When Direct Key Agreement or Direct Encryption are employed, let
        the JWE Encrypted Key be the empty octet sequence.

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   6.   When Direct Encryption is employed, let the CEK be the shared
        symmetric key.

   7.   Compute the encoded key value BASE64URL(JWE Encrypted Key).

   8.   If the JWE JSON Serialization is being used, repeat this process
        (steps 1-7) for each recipient.

   9.   Generate a random JWE Initialization Vector of the correct size
        for the content encryption algorithm (if required for the
        algorithm); otherwise, let the JWE Initialization Vector be the
        empty octet sequence.

   10.  Compute the encoded Initialization Vector value BASE64URL(JWE
        Initialization Vector).

   11.  If a "zip" parameter was included, compress the plaintext using
        the specified compression algorithm and let M be the octet
        sequence representing the compressed plaintext; otherwise, let M
        be the octet sequence representing the plaintext.

   12.  Create the JSON object(s) containing the desired set of Header
        Parameters, which together comprise the JOSE Header: one or more
        of the JWE Protected Header, the JWE Shared Unprotected Header,
        and the JWE Per-Recipient Unprotected Header.

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

   14.  Let the Additional Authenticated Data encryption parameter be
        ASCII(Encoded Protected Header).  However, if a JWE AAD value is
        present (which can only be the case when using the JWE JSON
        Serialization), instead let the Additional Authenticated Data
        encryption parameter be ASCII(Encoded Protected Header || '.' ||
        BASE64URL(JWE AAD)).

   15.  Encrypt M using the CEK, the JWE Initialization Vector, and the
        Additional Authenticated Data value using the specified content
        encryption algorithm to create the JWE Ciphertext value and the
        JWE Authentication Tag (which is the Authentication Tag output
        from the encryption operation).

   16.  Compute the encoded ciphertext value BASE64URL(JWE Ciphertext).

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   17.  Compute the encoded Authentication Tag value BASE64URL(JWE
        Authentication Tag).

   18.  If a JWE AAD value is present, compute the encoded AAD value
        BASE64URL(JWE AAD).

   19.  Create the desired serialized output.  The Compact Serialization
        of this result is the string BASE64URL(UTF8(JWE Protected
        Header)) || '.' || BASE64URL(JWE Encrypted Key) || '.' ||
        BASE64URL(JWE Initialization Vector) || '.' || BASE64URL(JWE
        Ciphertext) || '.' || BASE64URL(JWE Authentication Tag).  The
        JWE JSON Serialization is described in Section 7.2.

5.2.  Message Decryption

   The message decryption process is the reverse of the encryption
   process.  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 these steps fail, the encrypted content cannot be
   validated.

   When there are multiple recipients, it is an application decision
   which of the recipients' encrypted content must successfully validate
   for the JWE to be accepted.  In some cases, encrypted content for all
   recipients must successfully validate or the JWE will be considered
   invalid.  In other cases, only the encrypted content for a single
   recipient needs to be successfully validated.  However, in all cases,
   the encrypted content for at least one recipient MUST successfully
   validate or the JWE MUST be considered invalid.

   1.   Parse the JWE representation to extract the serialized values
        for the components of the JWE.  When using the JWE Compact
        Serialization, these components are the base64url-encoded
        representations of the JWE Protected Header, the JWE Encrypted
        Key, the JWE Initialization Vector, the JWE Ciphertext, and the
        JWE Authentication Tag, and when using the JWE JSON
        Serialization, these components also include the base64url-
        encoded representation of the JWE AAD and the unencoded JWE
        Shared Unprotected Header and JWE Per-Recipient Unprotected
        Header values.  When using the JWE Compact Serialization, the
        JWE Protected Header, the JWE Encrypted Key, the JWE
        Initialization Vector, the JWE Ciphertext, and the JWE
        Authentication Tag 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 four
        delimiting period characters being used.  The JWE JSON
        Serialization is described in Section 7.2.

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   2.   Base64url decode the encoded representations of the JWE
        Protected Header, the JWE Encrypted Key, the JWE Initialization
        Vector, the JWE Ciphertext, the JWE Authentication Tag, and the
        JWE AAD, following the restriction that no line breaks,
        whitespace, or other additional characters have been used.

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

   4.   If using the JWE Compact Serialization, let the JOSE Header be
        the JWE Protected Header.  Otherwise, when using the JWE JSON
        Serialization, let the JOSE Header be the union of the members
        of the JWE Protected Header, the JWE Shared Unprotected Header
        and the corresponding JWE Per-Recipient 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 JWE 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 algorithms being used, or by the "crit"
        Header Parameter value, and that the values of those parameters
        are also understood and supported.

   6.   Determine the Key Management Mode employed by the algorithm
        specified by the "alg" (algorithm) Header Parameter.

   7.   Verify that the JWE uses a key known to the recipient.

   8.   When Direct Key Agreement or Key Agreement with Key Wrapping are
        employed, use the key agreement algorithm to compute the value
        of the agreed upon key.  When Direct Key Agreement is employed,
        let the CEK be the agreed upon key.  When Key Agreement with Key
        Wrapping is employed, the agreed upon key will be used to
        decrypt the JWE Encrypted Key.

   9.   When Key Wrapping, Key Encryption, or Key Agreement with Key
        Wrapping are employed, decrypt the JWE Encrypted Key to produce
        the CEK.  The CEK MUST have a length equal to that required for
        the content encryption algorithm.  Note that when there are
        multiple recipients, each recipient will only be able to decrypt
        JWE Encrypted Key values that were encrypted to a key in that
        recipient's possession.  It is therefore normal to only be able

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        to decrypt one of the per-recipient JWE Encrypted Key values to
        obtain the CEK value.  Also, see Section 11.5 for security
        considerations on mitigating timing attacks.

   10.  When Direct Key Agreement or Direct Encryption are employed,
        verify that the JWE Encrypted Key value is an empty octet
        sequence.

   11.  When Direct Encryption is employed, let the CEK be the shared
        symmetric key.

   12.  Record whether the CEK could be successfully determined for this
        recipient or not.

   13.  If the JWE JSON Serialization is being used, repeat this process
        (steps 4-12) for each recipient contained in the representation.

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

   15.  Let the Additional Authenticated Data encryption parameter be
        ASCII(Encoded Protected Header).  However, if a JWE AAD value is
        present (which can only be the case when using the JWE JSON
        Serialization), instead let the Additional Authenticated Data
        encryption parameter be ASCII(Encoded Protected Header || '.' ||
        BASE64URL(JWE AAD)).

   16.  Decrypt the JWE Ciphertext using the CEK, the JWE Initialization
        Vector, the Additional Authenticated Data value, and the JWE
        Authentication Tag (which is the Authentication Tag input to the
        calculation) using the specified content encryption algorithm,
        returning the decrypted plaintext and validating the JWE
        Authentication Tag in the manner specified for the algorithm,
        rejecting the input without emitting any decrypted output if the
        JWE Authentication Tag is incorrect.

   17.  If a "zip" parameter was included, uncompress the decrypted
        plaintext using the specified compression algorithm.

   18.  If there was no recipient for which all of the decryption steps
        succeeded, then the JWE MUST be considered invalid.  Otherwise,
        output the plaintext.  In the JWE JSON Serialization case, also
        return a result to the application indicating for which of the
        recipients the decryption succeeded and failed.

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   Finally, note that it is an application decision which algorithms may
   be used in a given context.  Even if a JWE can be successfully
   decrypted, unless the algorithms used in the JWE are acceptable to
   the application, it SHOULD consider the JWE to be invalid.

5.3.  String Comparison Rules

   The string comparison rules for this specification are the same as
   those defined in Section 5.3 of [JWS].

6.  Key Identification

   The key identification methods for this specification are the same as
   those defined in Section 6 of [JWS], except that the key being
   identified is the public key to which the JWE was encrypted.

7.  Serializations

   JWEs use one of two serializations: the JWE Compact Serialization or
   the JWE 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 JWE JSON Serialization is used, that only JWE JSON
   Serialization support for a single recipient is used, or that support
   for multiple recipients is used.  JWE implementations only need to
   implement the features needed for the applications they are designed
   to support.

7.1.  JWE Compact Serialization

   The JWE Compact Serialization represents encrypted content as a
   compact, URL-safe string.  This string is:

      BASE64URL(UTF8(JWE Protected Header)) || '.' ||
      BASE64URL(JWE Encrypted Key) || '.' ||
      BASE64URL(JWE Initialization Vector) || '.' ||
      BASE64URL(JWE Ciphertext) || '.' ||
      BASE64URL(JWE Authentication Tag)

   Only one recipient is supported by the JWE Compact Serialization and
   it provides no syntax to represent JWE Shared Unprotected Header, JWE
   Per-Recipient Unprotected Header, or JWE AAD values.

7.2.  JWE JSON Serialization

   The JWE JSON Serialization represents encrypted content as a JSON
   object.  This representation is neither optimized for compactness nor
   URL safe.

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   Two closely related syntaxes are defined for the JWE JSON
   Serialization: a fully general syntax, with which content can be
   encrypted to more than one recipient, and a flattened syntax, which
   is optimized for the single-recipient case.

7.2.1.  General JWE JSON Serialization Syntax

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

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

   unprotected
      The "unprotected" member MUST be present and contain the value JWE
      Shared Unprotected Header when the JWE Shared 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.

   iv
      The "iv" member MUST be present and contain the value
      BASE64URL(JWE Initialization Vector) when the JWE Initialization
      Vector value is non-empty; otherwise, it MUST be absent.

   aad
      The "aad" member MUST be present and contain the value
      BASE64URL(JWE AAD)) when the JWE AAD value is non-empty;
      otherwise, it MUST be absent.  A JWE AAD value can be included to
      supply a base64url-encoded value to be integrity protected but not
      encrypted.

   ciphertext
      The "ciphertext" member MUST be present and contain the value
      BASE64URL(JWE Ciphertext).

   tag
      The "tag" member MUST be present and contain the value
      BASE64URL(JWE Authentication Tag) when the JWE Authentication Tag
      value is non-empty; otherwise, it MUST be absent.

   recipients
      The "recipients" member value MUST be an array of JSON objects.
      Each object contains information specific to a single recipient.
      This member MUST be present with exactly one array element per

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      recipient, even if some or all of the array element values are the
      empty JSON object "{}" (which can happen when all Header Parameter
      values are shared between all recipients and when no encrypted key
      is used, such as when doing Direct Encryption).

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

   header
      The "header" member MUST be present and contain the value JWE Per-
      Recipient Unprotected Header when the JWE Per-Recipient
      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.

   encrypted_key
      The "encrypted_key" member MUST be present and contain the value
      BASE64URL(JWE Encrypted Key) when the JWE Encrypted Key value is
      non-empty; otherwise, it MUST be absent.

   At least one of the "header", "protected", and "unprotected" members
   MUST be present so that "alg" and "enc" Header Parameter values are
   conveyed for each recipient computation.

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

   Some Header Parameters, including the "alg" parameter, can be shared
   among all recipient computations.  Header Parameters in the JWE
   Protected Header and JWE Shared Unprotected Header values are shared
   among all recipients.

   The Header Parameter values used when creating or validating per-
   recipient ciphertext and Authentication Tag values are the union of
   the three sets of Header Parameter values that may be present: (1)
   the JWE Protected Header represented in the "protected" member, (2)
   the JWE Shared Unprotected Header represented in the "unprotected"
   member, and (3) the JWE Per-Recipient Unprotected Header represented
   in the "header" member of the recipient's array element.  The union
   of these sets of Header Parameters comprises the JOSE Header.  The
   Header Parameter names in the three locations MUST be disjoint.

   Each JWE Encrypted Key value is computed using the parameters of the
   corresponding JOSE Header value in the same manner as for the JWE
   Compact Serialization.  This has the desirable property that each JWE
   Encrypted Key value in the "recipients" array is identical to the

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   value that would have been computed for the same parameter in the JWE
   Compact Serialization.  Likewise, the JWE Ciphertext and JWE
   Authentication Tag values match those produced for the JWE Compact
   Serialization, provided that the JWE Protected Header value (which
   represents the integrity-protected Header Parameter values) matches
   that used in the JWE Compact Serialization.

   All recipients use the same JWE Protected Header, JWE Initialization
   Vector, JWE Ciphertext, and JWE Authentication Tag values, when
   present, resulting in potentially significant space savings if the
   message is large.  Therefore, all Header Parameters that specify the
   treatment of the plaintext value MUST be the same for all recipients.
   This primarily means that the "enc" (encryption algorithm) Header
   Parameter value in the JOSE Header for each recipient and any
   parameters of that algorithm MUST be the same.

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

     {
      "protected":"<integrity-protected shared header contents>",
      "unprotected":<non-integrity-protected shared header contents>,
      "recipients":[
       {"header":<per-recipient unprotected header 1 contents>,
        "encrypted_key":"<encrypted key 1 contents>"},
       ...
       {"header":<per-recipient unprotected header N contents>,
        "encrypted_key":"<encrypted key N contents>"}],
      "aad":"<additional authenticated data contents>",
      "iv":"<initialization vector contents>",
      "ciphertext":"<ciphertext contents>",
      "tag":"<authentication tag contents>"
     }

   See Appendix A.4 for an example JWE using the general JWE JSON
   Serialization syntax.

7.2.2.  Flattened JWE JSON Serialization Syntax

   The flattened JWE JSON Serialization syntax is based upon the general
   syntax, but flattens it, optimizing it for the single-recipient case.
   It flattens it by removing the "recipients" member and instead
   placing those members defined for use in the "recipients" array (the
   "header" and "encrypted_key" members) in the top-level JSON object
   (at the same level as the "ciphertext" member).

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

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

     {
      "protected":"<integrity-protected header contents>",
      "unprotected":<non-integrity-protected header contents>,
      "header":<more non-integrity-protected header contents>,
      "encrypted_key":"<encrypted key contents>",
      "aad":"<additional authenticated data contents>",
      "iv":"<initialization vector contents>",
      "ciphertext":"<ciphertext contents>",
      "tag":"<authentication tag contents>"
     }

   Note that when using the flattened syntax, just as when using the
   general syntax, any unprotected Header Parameter values can reside in
   either the "unprotected" member or the "header" member, or in both.

   See Appendix A.5 for an example JWE using the flattened JWE JSON
   Serialization syntax.

8.  TLS Requirements

   The Transport Layer Security (TLS) requirements for this
   specification are the same as those defined in Section 8 of [JWS].

9.  Distinguishing between JWS and JWE Objects

   There are several ways of distinguishing whether an object is a JWS
   or JWE.  All these methods will yield the same result for all legal
   input values; they may yield different results for malformed inputs.

   o  If the object is using the JWS Compact Serialization or the JWE
      Compact Serialization, the number of base64url-encoded segments
      separated by period ('.') characters differs for JWSs and JWEs.
      JWSs have three segments separated by two period ('.') characters.
      JWEs have five segments separated by four period ('.') characters.

   o  If the object is using the JWS JSON Serialization or the JWE JSON
      Serialization, the members used will be different.  JWSs have a
      "payload" member and JWEs do not.  JWEs have a "ciphertext" member
      and JWSs do not.

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   o  The JOSE Header for a JWS can be distinguished from the JOSE
      Header for a JWE by examining the "alg" (algorithm) Header
      Parameter value.  If the value represents a digital signature or
      MAC algorithm, or is the value "none", it is for a JWS; if it
      represents a Key Encryption, Key Wrapping, Direct Key Agreement,
      Key Agreement with Key Wrapping, or Direct Encryption algorithm,
      it is for a JWE.  (Extracting the "alg" value to examine is
      straightforward when using the JWS Compact Serialization or the
      JWE Compact Serialization and may be more difficult when using the
      JWS JSON Serialization or the JWE JSON Serialization.)

   o  The JOSE Header for a JWS can also be distinguished from the JOSE
      Header for a JWE by determining whether an "enc" (encryption
      algorithm) member exists.  If the "enc" member exists, it is a
      JWE; otherwise, it is a JWS.

10.  IANA Considerations

10.1.  JSON Web Signature and Encryption Header Parameters Registration

   This section registers the Header Parameter names defined in
   Section 4.1 in the IANA "JSON Web Signature and Encryption Header
   Parameters" registry established by [JWS].

10.1.1.  Registry Contents

   o  Header Parameter Name: "alg"
   o  Header Parameter Description: Algorithm
   o  Header Parameter Usage Location(s): JWE
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.1 of RFC 7516

   o  Header Parameter Name: "enc"
   o  Header Parameter Description: Encryption Algorithm
   o  Header Parameter Usage Location(s): JWE
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.2 of RFC 7516

   o  Header Parameter Name: "zip"
   o  Header Parameter Description: Compression Algorithm
   o  Header Parameter Usage Location(s): JWE
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.3 of RFC 7516

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   o  Header Parameter Name: "jku"
   o  Header Parameter Description: JWK Set URL
   o  Header Parameter Usage Location(s): JWE
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.4 of RFC 7516

   o  Header Parameter Name: "jwk"
   o  Header Parameter Description: JSON Web Key
   o  Header Parameter Usage Location(s): JWE

   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.5 of RFC 7516

   o  Header Parameter Name: "kid"
   o  Header Parameter Description: Key ID
   o  Header Parameter Usage Location(s): JWE
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.6 of RFC 7516

   o  Header Parameter Name: "x5u"
   o  Header Parameter Description: X.509 URL
   o  Header Parameter Usage Location(s): JWE
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.7 of RFC 7516

   o  Header Parameter Name: "x5c"
   o  Header Parameter Description: X.509 Certificate Chain
   o  Header Parameter Usage Location(s): JWE
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.8 of RFC 7516

   o  Header Parameter Name: "x5t"
   o  Header Parameter Description: X.509 Certificate SHA-1 Thumbprint
   o  Header Parameter Usage Location(s): JWE
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.9 of RFC 7516

   o  Header Parameter Name: "x5t#S256"
   o  Header Parameter Description: X.509 Certificate SHA-256 Thumbprint
   o  Header Parameter Usage Location(s): JWE
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.10 of RFC 7516

   o  Header Parameter Name: "typ"
   o  Header Parameter Description: Type
   o  Header Parameter Usage Location(s): JWE
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.11 of RFC 7516

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   o  Header Parameter Name: "cty"
   o  Header Parameter Description: Content Type
   o  Header Parameter Usage Location(s): JWE
   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.12 of RFC 7516

   o  Header Parameter Name: "crit"
   o  Header Parameter Description: Critical
   o  Header Parameter Usage Location(s): JWE

   o  Change Controller: IESG
   o  Specification Document(s): Section 4.1.13 of RFC 7516

11.  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 the JWS specification also apply
   to this specification.  Likewise, all the security considerations in
   XML Encryption 1.1 [W3C.REC-xmlenc-core1-20130411] also apply, other
   than those that are XML specific.

11.1.  Key Entropy and Random Values

   See Section 10.1 of [JWS] for security considerations on key entropy
   and random values.  In addition to the uses of random values listed
   there, note that random values are also used for Content Encryption
   Keys (CEKs) and Initialization Vectors (IVs) when performing
   encryption.

11.2.  Key Protection

   See Section 10.2 of [JWS] for security considerations on key
   protection.  In addition to the keys listed there that must be
   protected, implementations performing encryption must protect the key
   encryption key and the Content Encryption Key.  Compromise of the key
   encryption key may result in the disclosure of all contents protected
   with that key.  Similarly, compromise of the Content Encryption Key
   may result in disclosure of the associated encrypted content.

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11.3.  Using Matching Algorithm Strengths

   Algorithms of matching strengths should be used together whenever
   possible.  For instance, when AES Key Wrap is used with a given key
   size, using the same key size is recommended when AES GCM is also
   used.  If the key encryption and content encryption algorithms are
   different, the effective security is determined by the weaker of the
   two algorithms.

   Also, see RFC 3766 [RFC3766] for information on determining strengths
   for public keys used for exchanging symmetric keys.

11.4.  Adaptive Chosen-Ciphertext Attacks

   When decrypting, particular care must be taken not to allow the JWE
   recipient to be used as an oracle for decrypting messages.  RFC 3218
   [RFC3218] should be consulted for specific countermeasures to attacks
   on RSAES-PKCS1-v1_5.  An attacker might modify the contents of the
   "alg" Header Parameter from "RSA-OAEP" to "RSA1_5" in order to
   generate a formatting error that can be detected and used to recover
   the CEK even if RSAES-OAEP was used to encrypt the CEK.  It is
   therefore particularly important to report all formatting errors to
   the CEK, Additional Authenticated Data, or ciphertext as a single
   error when the encrypted content is rejected.

   Additionally, this type of attack can be prevented by restricting the
   use of a key to a limited set of algorithms -- usually one.  This
   means, for instance, that if the key is marked as being for
   "RSA-OAEP" only, any attempt to decrypt a message using the "RSA1_5"
   algorithm with that key should fail immediately due to invalid use of
   the key.

11.5.  Timing Attacks

   To mitigate the attacks described in RFC 3218 [RFC3218], the
   recipient MUST NOT distinguish between format, padding, and length
   errors of encrypted keys.  It is strongly recommended, in the event
   of receiving an improperly formatted key, that the recipient
   substitute a randomly generated CEK and proceed to the next step, to
   mitigate timing attacks.


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