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4. JSON Web Key (JWK) Format

A JWK is a JSON object that represents a cryptographic key. The members of the object represent properties of the key, including its value. This JSON object MAY contain whitespace and/or line breaks before or after any JSON values or structural characters, in accordance with Section 2 of RFC 7159 [RFC7159]. This document defines the key parameters that are not algorithm specific and, thus, common to many keys.

In addition to the common parameters, each JWK will have members that are key type specific. These members represent the parameters of the key. Section 6 of the JSON Web Algorithms (JWA) [JWA] specification defines multiple kinds of cryptographic keys and their associated members.

The member names within a JWK MUST be unique; JWK parsers MUST either reject JWKs with duplicate member 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].

Additional members can be present in the JWK; if not understood by implementations encountering them, they MUST be ignored. Member names used for representing key parameters for different keys types need not be distinct. Any new member name should either be registered in the IANA "JSON Web Key Parameters" registry established by Section 8.1 or be a value that contains a Collision-Resistant Name.

4.1. "kty" (Key Type) Parameter​

The "kty" (key type) parameter identifies the cryptographic algorithm family used with the key, such as "RSA" or "EC". "kty" values should either be registered in the IANA "JSON Web Key Types" registry established by [JWA] or be a value that contains a Collision-Resistant Name. The "kty" value is a case-sensitive string. This member MUST be present in a JWK.

A list of defined "kty" values can be found in the IANA "JSON Web Key Types" registry established by [JWA]; the initial contents of this registry are the values defined in Section 6.1 of [JWA].

The key type definitions include specification of the members to be used for those key types. Members used with specific "kty" values can be found in the IANA "JSON Web Key Parameters" registry established by Section 8.1.

4.2. "use" (Public Key Use) Parameter​

The "use" (public key use) parameter identifies the intended use of the public key. The "use" parameter is employed to indicate whether a public key is used for encrypting data or verifying the signature on data.

Values defined by this specification are:

  • "sig" (signature)
  • "enc" (encryption)

Other values MAY be used. The "use" value is a case-sensitive string. Use of the "use" member is OPTIONAL, unless the application requires its presence.

When a key is used to wrap another key and a public key use designation for the first key is desired, the "enc" (encryption) key use value is used, since key wrapping is a kind of encryption. The "enc" value is also to be used for public keys used for key agreement operations.

Additional "use" (public key use) values can be registered in the IANA "JSON Web Key Use" registry established by Section 8.2. Registering any extension values used is highly recommended when this specification is used in open environments, in which multiple organizations need to have a common understanding of any extensions used. However, unregistered extension values can be used in closed environments, in which the producing and consuming organization will always be the same.

4.3. "key_ops" (Key Operations) Parameter​

The "key_ops" (key operations) parameter identifies the operation(s) for which the key is intended to be used. The "key_ops" parameter is intended for use cases in which public, private, or symmetric keys may be present.

Its value is an array of key operation values. Values defined by this specification are:

  • "sign" (compute digital signature or MAC)
  • "verify" (verify digital signature or MAC)
  • "encrypt" (encrypt content)
  • "decrypt" (decrypt content and validate decryption, if applicable)
  • "wrapKey" (encrypt key)
  • "unwrapKey" (decrypt key and validate decryption, if applicable)
  • "deriveKey" (derive key)
  • "deriveBits" (derive bits not to be used as a key)

(Note that the "key_ops" values intentionally match the "KeyUsage" values defined in the Web Cryptography API [W3C.CR-WebCryptoAPI-20141211] specification.)

Other values MAY be used. The key operation values are case-sensitive strings. Duplicate key operation values MUST NOT be present in the array. Use of the "key_ops" member is OPTIONAL, unless the application requires its presence.

Multiple unrelated key operations SHOULD NOT be specified for a key because of the potential vulnerabilities associated with using the same key with multiple algorithms. Thus, the combinations "sign" with "verify", "encrypt" with "decrypt", and "wrapKey" with "unwrapKey" are permitted, but other combinations SHOULD NOT be used.

Additional "key_ops" (key operations) values can be registered in the IANA "JSON Web Key Operations" registry established by Section 8.3. The same considerations about registering extension values apply to the "key_ops" member as do for the "use" member.

The "use" and "key_ops" JWK members SHOULD NOT be used together; however, if both are used, the information they convey MUST be consistent. Applications should specify which of these members they use, if either is to be used by the application.

4.4. "alg" (Algorithm) Parameter​

The "alg" (algorithm) parameter identifies the algorithm intended for use with the key. The values used should either be registered in the IANA "JSON Web Signature and Encryption Algorithms" registry established by [JWA] or be a value that contains a Collision-Resistant Name. The "alg" value is a case-sensitive ASCII string. Use of this member is OPTIONAL.

4.5. "kid" (Key ID) Parameter​

The "kid" (key ID) parameter is used to match a specific key. This is used, for instance, to choose among a set of keys within a JWK Set during key rollover. The structure of the "kid" value is unspecified. When "kid" values are used within a JWK Set, different keys within the JWK Set SHOULD use distinct "kid" values. (One example in which different keys might use the same "kid" value is if they have different "kty" (key type) values but are considered to be equivalent alternatives by the application using them.) The "kid" value is a case-sensitive string. Use of this member is OPTIONAL. When used with JWS or JWE, the "kid" value is used to match a JWS or JWE "kid" Header Parameter value.

4.6. "x5u" (X.509 URL) Parameter​

The "x5u" (X.509 URL) parameter is a URI [RFC3986] that refers to a resource for an X.509 public key certificate or certificate chain [RFC5280]. The identified resource MUST provide a representation of the certificate or certificate chain that conforms to RFC 5280 [RFC5280] in PEM-encoded form, with each certificate delimited as specified in Section 6.1 of RFC 4945 [RFC4945]. The key in the first certificate MUST match the public key represented by other members of the JWK. The protocol used to acquire the resource MUST provide integrity protection; an HTTP GET request to retrieve the certificate MUST use TLS [RFC2818] [RFC5246]; the identity of the server MUST be validated, as per Section 6 of RFC 6125 [RFC6125]. Use of this member is OPTIONAL.

While there is no requirement that optional JWK members providing key usage, algorithm, or other information be present when the "x5u" member is used, doing so may improve interoperability for applications that do not handle PKIX certificates [RFC5280]. If other members are present, the contents of those members MUST be semantically consistent with the related fields in the first certificate. For example, if the "use" member is present, then it MUST correspond to the usage that is specified in the certificate, when the certificate includes this information. Similarly, if the "alg" member is present, it MUST correspond to the algorithm specified in the certificate.

4.7. "x5c" (X.509 Certificate Chain) Parameter​

The "x5c" (X.509 certificate chain) parameter contains a chain of one or more PKIX certificates [RFC5280]. The certificate chain is represented as a JSON array of certificate value strings. Each string in the array is a base64-encoded (Section 4 of [RFC4648] -- not base64url-encoded) DER [ITU.X690.1994] PKIX certificate value. The PKIX certificate containing the key value MUST be the first certificate. This MAY be followed by additional certificates, with each subsequent certificate being the one used to certify the previous one. The key in the first certificate MUST match the public key represented by other members of the JWK. Use of this member is OPTIONAL.

As with the "x5u" member, optional JWK members providing key usage, algorithm, or other information MAY also be present when the "x5c" member is used. If other members are present, the contents of those members MUST be semantically consistent with the related fields in the first certificate. See the last paragraph of Section 4.6 for additional guidance on this.

4.8. "x5t" (X.509 Certificate SHA-1 Thumbprint) Parameter​

The "x5t" (X.509 certificate SHA-1 thumbprint) parameter is a base64url-encoded SHA-1 thumbprint (a.k.a. digest) of the DER encoding of an X.509 certificate [RFC5280]. Note that certificate thumbprints are also sometimes known as certificate fingerprints. The key in the certificate MUST match the public key represented by other members of the JWK. Use of this member is OPTIONAL.

As with the "x5u" member, optional JWK members providing key usage, algorithm, or other information MAY also be present when the "x5t" member is used. If other members are present, the contents of those members MUST be semantically consistent with the related fields in the referenced certificate. See the last paragraph of Section 4.6 for additional guidance on this.

4.9. "x5t#S256" (X.509 Certificate SHA-256 Thumbprint) Parameter​

The "x5t#S256" (X.509 certificate SHA-256 thumbprint) parameter is a base64url-encoded SHA-256 thumbprint (a.k.a. digest) of the DER encoding of an X.509 certificate [RFC5280]. Note that certificate thumbprints are also sometimes known as certificate fingerprints. The key in the certificate MUST match the public key represented by other members of the JWK. Use of this member is OPTIONAL.

As with the "x5u" member, optional JWK members providing key usage, algorithm, or other information MAY also be present when the "x5t#S256" member is used. If other members are present, the contents of those members MUST be semantically consistent with the related fields in the referenced certificate. See the last paragraph of Section 4.6 for additional guidance on this.

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