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JOSE – JSON Object Signing and Encryption

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Federated Identity Management has become very widespread in past years – in addition to enterprise deployments a lot of popular web services allow users to carry their identity over multiple sites. Social networking sites especially are in a good position to drive the federated identity management, as they have both critical mass of users and the incentive to become an identity provider. As the users move away from a single device to using multiple portable devices, there is a constant pressure to make the federated identity protocols simpler (with respect to complexity), more user friendly (especially for developers) and easier to implement (on wide range of devices and platforms).

Unfortunately older technologies are deeply rooted in enterprise environments and are unsuitable for Internet (Kerberos), or are based on more or less complicated data serialization (e.g. OpenID 2.0 or SAML). Canonicalization, whitespace handling and representation of binary data are among the challenges that various serialization formats face.

OpenID Connect

Another approach to the problem of serialization of structured data in context of identity management is to use already widespread and simple JSON in combination with base64url encoding of data. The advantage in context of federated identity management is obvious – while being sufficiently universal, they have almost native support in web clients. Getting this level of simplicity and interoperability is very compelling, despite these having some shortcomings in e.g. bandwidth-efficiency.

This approach is taken by an upcoming standard OpenID Connect, a third revision of OpenID protocol. The protocol describes a method of providing identity-based claims from identity provider to relying party, with end user being authenticated by Identity Provider and authorizing the request. The communication between client, relying party and identity provider follows OAUTH 2.0 protocol, just like in previous version of the protocol, OpenID 2.0. Claims have a format of JSON key-value hash and the task of protection of integrity, and possibly confidentiality, is addressed by JSON Object Signing and Encryption (JOSE) standard.

JOSE

The standard provides a general approach to signing and encryption of any content, not necessarily in JSON. However, it is deliberately built on JSON and base64url to be easily usable in web applications. Also, while being used in OpenID Connect, it can be used as a building block in other protocols.

JOSE is still an upcoming standard, but final revisions should be available shortly. It consists of several upcoming RFCs:

  • JWA – JSON Web Algorithms, describes cryptographic algorithms used in JOSE
  • JWK – JSON Web Key, describes format and handling of cryptographic keys in JOSE
  • JWS – JSON Web Signature, describes producing and handling signed messages
  • JWE – JSON Web Encryption, describes producting and handling encrypted messages
  • JWT – JSON Web Token, describes representation of claims encoded in JSON and protected by JWS or JWE

JWK

JSON Web Key is a data structure representing a cryptographic key with both the cryptographic data and other attributes, such as key usage.

{ 
  "kty":"EC",
  "crv":"P-256",
  "x":"MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4",
  "y":"4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM",
  "use":"enc",
  "kid":"1"
}

Mandatory “kty” key type parameter describes the cryptographic algorithm associated with the key. Depending on the key type, other parameters might be used – as shown in the example elliptic curve key would contain “crv” parameter identifying the curve, “x” and “y” coordinates of point, optional “use” to denote intended usage of the key and “kid” as key ID. The specification now describes three key types: “EC” for Elliptic Curve, “RSA” for, well, RSA, and “oct” for octet sequence denoting the shared symmetric key.

JWS

JSON Web Signature standard describes process of creation and validation of datastructure representing signed payload. As example take following string as a payload:

'{"iss":"joe",
 "exp":1300819380,
 "http://example.com/is_root":true}'

Incidentally, this string contains JSON data, but this is not relevant for the signing procedure and it might as well be any data. Before signing, the payload is always converted to base64url encoding:

eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFtcGxlLm
NvbS9pc19yb290Ijp0cnVlfQ

Additional parameters are associated with each payload. Required parameter is “alg”, which denotes the algorithm used for generating a signature (one of the possible values is “none” for unprotected messages). The parameters are included in final JWS in either protected or unprotected header. The data in protected header is integrity protected and base64url encoded, whereas unprotected header human readable associated data.

As example, the protected header will contain following data:

{"alg":"ES256"}

which in base64url encoding look like this:

eyJhbGciOiJFUzI1NiJ9

The “ES356″ here is identifier for ECDSA signature algorithm using P-256 curve and SHA-256 digest algorithm.

Unprotected header can contain a key id parameter:

{"kid":"e9bc097a-ce51-4036-9562-d2ade882db0d"}

The base64url encoded payload and protected header are concatenated with ‘.’ to form a raw data, which is fed to the signature algorithm to produce the final signature.

Finally, the JWS output is serialized using one of JSON or Compact serializations. Compact serialization is simple concatenation of comma separated base64url encoded protected header, payload and signature. JSON serialization is a human readable JSON object, which for the example above would look like this:

{
  "payload": "eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6
              Ly9leGFtcGxlLmNvbS9pc19yb290Ijp0cnVlfQ",
  "protected":"eyJhbGciOiJFUzI1NiJ9",
  "header":
    {"kid":"e9bc097a-ce51-4036-9562-d2ade882db0d"},
     "signature":
     "DtEhU3ljbEg8L38VWAfUAqOyKAM6-Xx-F4GawxaepmXFCgfTjDxw5djxLa8IS
      lSApmWQxfKTUJqPP3-Kg6NU1Q"
}

Such process for generating signature is pretty straightforward, yet still supports some advanced use-cases, such as multiple signatures with separate headers.

JWE

JSON Web Encryption follows the same logic as JWS with a few differences:

  • by default, for each message new content encryption key (CEK) should be generated. This key is used to encrypt the plaintext and is attached to the final message. Public key of recipient or a shared key is used only to encrypt the CEK (unless direct encryption is used, see below).
  • only AEAD (Authenticated Encryption with Associated Data) algorithms are defined in the standard, so users do not have to think about how to combine JWE with JWS.

Just like with JWS, header data of JWE object can be transmitted in either integrity protected, unprotected or per-recipient unprotected header. The final JSON serialized output then has the following structure:

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

The CEK is encrypted for each recipient separately, using different algorithms. This gives us ability to encrypt a message to recipients with different keys, e.g. RSA, shared symmetric and EC key.

The two used algorithms need to be specified as a header parameters. “alg” parameter specified the algorithm used to protect the CEK, while “enc” parameter specifies the algorithm used to encrypt the plaintext using CEK as key. Needless to say, “alg” can have a value of “dir”, which marks direct usage of the key, instead of using CEK.

As example, assume we have RSA public key of the first recipient and share a symmetric key with second recipient. The “alg” parameter for the first recipient will have value “RSA1_5″ denoting RSAES-PKCS1-V1_5 algorithm and “A128KW” denoting AES 128 Keywrap for the second recipient, along with key IDs:

{"alg":"RSA1_5","kid":"2011-04-29"}

and

{"alg":"A128KW","kid":"7"}

These algorithms will be used to encrypt content encryption key (CEK) to each of the recipients. After CEK is generated, we use it to encrypt the plaintext with AES 128 in CBC mode with HMAC SHA 256 for integrity:

{"enc":"A128CBC-HS256"}

We can protect this information by putting it into a protected header, which, when base64url encoded, will look like this:

eyJlbmMiOiJBMTI4Q0JDLUhTMjU2In0

This data will be fed as associated data to AEAD encryption algorithm and therefore be protected by the final signature tag.

Putting this all together, the resulting JWE object will looks like this:

{
  "protected": "eyJlbmMiOiJBMTI4Q0JDLUhTMjU2In0",
  "recipients":[
    {"header": {"alg":"RSA1_5","kid":"2011-04-29"},
     "encrypted_key":
       "UGhIOguC7IuEvf_NPVaXsGMoLOmwvc1GyqlIKOK1nN94nHPoltGRhWhw7Zx0-
        kFm1NJn8LE9XShH59_i8J0PH5ZZyNfGy2xGdULU7sHNF6Gp2vPLgNZ__deLKx
        GHZ7PcHALUzoOegEI-8E66jX2E4zyJKx-YxzZIItRzC5hlRirb6Y5Cl_p-ko3
        YvkkysZIFNPccxRU7qve1WYPxqbb2Yw8kZqa2rMWI5ng8OtvzlV7elprCbuPh
        cCdZ6XDP0_F8rkXds2vE4X-ncOIM8hAYHHi29NX0mcKiRaD0-D-ljQTP-cFPg
        wCp6X-nZZd9OHBv-B3oWh2TbqmScqXMR4gp_A"},
    {"header": {"alg":"A128KW","kid":"7"},
     "encrypted_key":
        "6KB707dM9YTIgHtLvtgWQ8mKwboJW3of9locizkDTHzBC2IlrT1oOQ"}],
  "iv": "AxY8DCtDaGlsbGljb3RoZQ",
  "ciphertext": "KDlTtXchhZTGufMYmOYGS4HffxPSUrfmqCHXaI9wOGY",
  "tag": "Mz-VPPyU4RlcuYv1IwIvzw"
}

JWA

JSON Web Algorithms defines algorithms and their identifiers to be used in JWS and JWE. The three parameters that specify algorithms are “alg” for JWS, “alg” and “enc” for JWE.

"enc": 
    A128CBC-HS256, A192CBC-HS384, A256CBC-HS512 (AES in CBC with HMAC), 
    A128GCM, A192GCM, A256GCM

"alg" for JWS: 
    HS256, HS384, HS512 (HMAC with SHA), 
    RS256, RS384, RS512 (RSASSA-PKCS-v1_5 with SHA), 
    ES256, ES384, ES512 (ECDSA with SHA), 
    PS256, PS384, PS512 (RSASSA-PSS with SHA for digest and MGF1)

"alg" for JWE: 
    RSA1_5, RSA-OAEP, RSA-OAEP-256, 
    A128KW, A192KW, A256KW (AES Keywrap), 
    dir (direct encryption), 
    ECDH-ES (EC Diffie Hellman Ephemeral+Static key agreement), 
    ECDH-ES+A128KW, ECDH-ES+A192KW, ECDH-ES+A256KW (with AES Keywrap), 
    A128GCMKW, A192GCMKW, A256GCMKW (AES in GCM Keywrap), 
    PBES2-HS256+A128KW, PBES2-HS384+A192KW, PBES2-HS512+A256KW 
    (PBES2 with HMAC SHA and AES keywrap)

On the first look the wealth of choice for “alg” in JWE is balanced by just two options for “enc”. Thanks to “enc” and “alg” being separate, algorithms suitable for encrypting cryptographic key and content can be separately defined. AES Keywrap scheme defined in RFC 3394 is a preferred way to protect cryptographic key. The scheme uses fixed value of IV, which is checked after decryption and provides integrity protection without making the encrypted key longer (by adding IV and authentication tag). But here`s a catch – while A128KW refers to AES Keywrap algorithm as defined in RFC 3394, word “keywrap” in A128GCMKW is used in a more general sense as synonym to encryption, so it denotes simple encryption of key with AES in GCM mode.

JWT

While previous parts of JOSE provide a general purpose cryptographic primitives for arbitrary data, JSON Web Token standard is more tied to the OpenID Connect. JWT object is simply JSON hash with claims, that is either signed with JWS or encrypted with JWE and serialized using compact serialization. Beware of a terminological quirk – when JWT is used as plaintext in JWE or JWS, it is referred to as nested JWT (rather than signed, or encrypted).

JWT standard defines claims – name/value pair asserting information about subject. The claims include

  • “iss” to identify issuer of the claim
  • “sub” identifying subject of JWT
  • “aud” (audience) identifying intended recipients
  • “exp” to mark expiration time of JWT
  • “nbf” (not before) to mark time before which JWT must be rejected
  • “iat” (issued at) to mark time when JWT was created
  • “jti” (JWT ID) as unique identifier for JWT

While standard mandates what are mandatory values of the claims, all of them are optional to use in a valid JWT. This means applications can use any structure for JWT if it`s not intended to use publicly, and for public JWT set of claims is defined and collisions in names are prevented.

The good

The fact that JOSE combines JSON and base64url encoding making it simple and web friendly is a clear win. Although we will definitely see JOSE adopted in web environment first, it does have ambition to become more general purpose standard.

The design promotes secure choices, e.g. use of unique CEK per message, which makes users default to secure configurations while still giving option to use less secure methods (“dir” for encryption, “none” in JWS). Being a new standard authors did seize the opportunity to define only secure algorithms. This is certainly good, but as the advances in cryptography weaken the algorithms, ability to deprecate algorithms (with backwards compatibility always being an issue) will be more important in the future.

The bad

Despite the effort to keep the standard simple, some complexities inevitably slipped through. One obvious comes from a different serializations. JOSE standards support two serializations – JSON and Compact. JSON serialization is human readable and gives the users more freedom as it supports some advances features like multiple recipients. However, since single recipient is a much more common case, standard also defines a variant called flattened JSON serialization. In this type nested parameters from nested fields (like “recipients”) is moved directly to top level JSON object, making multiple recipients with flattened serialization impossible.

The compact serialization is created, according to the standard, for “space constrained environments”. The result of the serialization is comma separated concatenation of base64url encoded segments of the original JSON object. For example for JWS the serialization is constructed as follows:

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

Astute readers notice that compact serialization further restricts the available features of JWS, e.g. it is not possible to include unprotected header anymore. The space saving comes from dropping the keys that denote the parts of JSON object (“payload”, “signature” etc.). On the other hand, base64url encoding expands the length of the cleartext data (i.e. unprotected header), so in extreme example compact serialized JWS might actually be longer that JSON serialized one if the header contains enough data (to compensate inefficiency of specifying keys in the object) and is stored as unprotected header. Of importance is also the number of dot separated sections, since their number is the only method of differentiating between compact serialized JWE and JWS. Other proposed extensions, such as Key Managed JSON Web Signature (KMJWS), must take this into account.

The standard also still contains several ambiguities, e.g. JWK defines a JWK Set and states that “The member names within a JWK Set MUST be unique;”  without specifying what member name actually is.

The ugly

The JOSE standard is already incorporated in OpenID Connect standard. As the standards evolved side by side, OpenID Connect standard is based on older revisions of JOSE. More importantly, 15.6.1. Pre-Final IETF Specifications section of OpenID Connect states:

“Implementers should be aware that this specification uses several IETF specifications that are not yet final specifications … While every effort will be made to prevent breaking changes to these specifications, should they occur, OpenID Connect implementations should continue to use the specifically referenced draft versions above in preference to the final versions …”

The compatibility issues are always bane of cryptographic standard and this decision to prefer pre-final revisions of JOSE standard might force implementations to make some hard decisions.

Future

The JOSE standard seems to be quickly approaching the final revisions and we will most probably see more of it on the web. Implementations for most of the popular languages are in place and we will see whether the decision to award Special European Identity Award for Best Innovation for Security in the API Economy to JOSE will also stand the test of time.

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