Network Working Group M. Jones
Internet-Draft Microsoft
Intended status: Standards Track D. Balfanz
Expires: September 29, 2011 Google
J. Bradley
independent
Y. Goland
Microsoft
J. Panzer
Google
N. Sakimura
Nomura Research Institute
P. Tarjan
Facebook
March 28, 2011
JSON Web Signature (JWS)
draft-jones-json-web-signature-00
Abstract
JSON Web Signature (JWS) is a means of representing signed content
using JSON data structures. Related encryption capabilities are
described in the separate JSON Web Encryption (JWE) specification.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 29, 2011.
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Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. JSON Web Signature (JWS) Overview . . . . . . . . . . . . . . 5
3.1. Example JWS . . . . . . . . . . . . . . . . . . . . . . . 5
4. JWS Header . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Reserved Header Parameter Names . . . . . . . . . . . . . 6
4.2. Public Header Parameter Names . . . . . . . . . . . . . . 9
4.3. Private Header Parameter Names . . . . . . . . . . . . . . 9
5. Rules for Creating and Validating a JWS . . . . . . . . . . . 9
6. Base64url encoding as used by JWSs . . . . . . . . . . . . . . 11
7. Signing JWSs with Cryptographic Algorithms . . . . . . . . . . 11
7.1. Creating a JWS with HMAC SHA-256 . . . . . . . . . . . . . 12
7.2. Creating a JWS with RSA SHA-256 . . . . . . . . . . . . . 13
7.3. Creating a JWS with ECDSA P-256 SHA-256 . . . . . . . . . 14
7.4. Additional Algorithms . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9.1. Unicode Comparison Security Issues . . . . . . . . . . . . 16
10. Open Issues and Things To Be Done (TBD) . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.1. Normative References . . . . . . . . . . . . . . . . . . . 19
11.2. Informative References . . . . . . . . . . . . . . . . . . 20
Appendix A. JWS Examples . . . . . . . . . . . . . . . . . . . . 20
A.1. JWS using HMAC SHA-256 . . . . . . . . . . . . . . . . . . 20
A.1.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . 20
A.1.2. Decoding . . . . . . . . . . . . . . . . . . . . . . . 22
A.1.3. Validating . . . . . . . . . . . . . . . . . . . . . . 22
A.2. JWS using RSA SHA-256 . . . . . . . . . . . . . . . . . . 23
A.2.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . 23
A.2.2. Decoding . . . . . . . . . . . . . . . . . . . . . . . 26
A.2.3. Validating . . . . . . . . . . . . . . . . . . . . . . 26
A.3. JWS using ECDSA P-256 SHA-256 . . . . . . . . . . . . . . 27
A.3.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . 27
A.3.2. Decoding . . . . . . . . . . . . . . . . . . . . . . . 28
A.3.3. Validating . . . . . . . . . . . . . . . . . . . . . . 29
Appendix B. Notes on implementing base64url encoding without
padding . . . . . . . . . . . . . . . . . . . . . . . 29
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 30
Appendix D. Document History . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
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1. Introduction
JSON Web Signature (JWS) is a compact signature format intended for
space constrained environments such as HTTP Authorization headers and
URI query parameters. The JWS signature mechanisms are independent
of the type of content being signed, allowing arbitrary content to be
signed. A related encryption capability is described in a separate
JSON Web Encryption (JWE) [JWE] specification.
2. Terminology
JSON Web Signature (JWS) A data structure cryptographically securing
a JWS Header Input and a JWS Payload Input with a JWS Crypto
Output.
JWS Header Input A string containing a base64url encoded JSON object
that describes the cryptographic operations applied to the JWS
Header Input and the JWS Payload Input.
JWS Payload Input A string containing base64url encoded content.
JWS Crypto Output A string containing base64url encoded
cryptographic material that secures the contents of the JWS Header
Input and the JWS Payload Input.
Decoded JWS Header Input JWS Header Input that has been base64url
decoded back into a JSON object.
Decoded JWS Payload Input JWS Payload Input that has been base64url
decoded.
Decoded JWS Crypto Output JWS Crypto Output that has been base64url
decoded back into cryptographic material.
JWS Signing Input The concatenation of the JWS Header Input, a
period ('.') character, and the JWS Payload Input.
Header Parameter Names The names of the members within the JSON
object represented in a JWS Header Input.
Header Parameter Values The values of the members within the JSON
object represented in a JWS Header Input.
Digital Signature For the purposes of this specification, we use
this term to encompass both Hash-based Message Authentication
Codes (HMACs), which can provide authenticity but not non-
repudiation, and digital signatures using public key algorithms,
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which can provide both. Readers should be aware of this
distinction, despite the decision to use a single term for both
concepts to improve readability of the specification.
Base64url Encoding For the purposes of this specification, this term
always refers to the he URL- and filename-safe Base64 encoding
described in RFC 4648 [RFC4648], Section 5, with the '=' padding
characters omitted, as permitted by Section 3.2.
3. JSON Web Signature (JWS) Overview
JWSs represent content that is base64url encoded and digitally
signed, and optionally encrypted, using JSON data structures. A
portion of the base64url encoded content that is signed is the JWS
Payload Input.
An accompanying base64url encoded JSON object - the JWS Header Input
- describes the signature method used.
The names within the header JSON object MUST be unique. These names
are referred to as Header Parameter Names. The corresponding values
are referred to as Header Parameter Values.
JWSs contain a signature that ensures the integrity of the contents
of the JWS Header Input and the JWS Payload Input. This signature
value is the JWS Crypto Output. The JSON Header object MUST contain
an "alg" parameter, the value of which is a string that unambiguously
identifies the algorithm used to sign the JWS Header Input and the
JWS Payload Input to produce the JWS Crypto Output.
3.1. Example JWS
The following example JSON header object declares that the encoded
object is a JSON Web Token (JWT) [JWT] and the JWS Header Input and
the JWS Payload Input are signed using the HMAC SHA-256 algorithm:
{"typ":"JWT",
"alg":"HS256"}
Base64url encoding the UTF-8 representation of the JSON header object
yields this JWS Header Input value:
eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9
The following is an example of a JSON object that can be encoded to
produce a JWS Payload Input. (Note that the payload can be any
base64url encoded content, and need not be a base64url encoded JSON
object.)
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{"iss":"joe",
"exp":1300819380,
"http://example.com/is_root":true}
Base64url encoding the UTF-8 representation of the JSON object yields
the following JWS Payload Input.
eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFtcGxlLmNvbS9pc19yb290Ijp0cnVlfQ
Signing the UTF-8 representation of the JWS Signing Input (the
concatenation of the JWS Header Input, a period ('.') character, and
the JWS Payload Input) with the HMAC SHA-256 algorithm and base64url
encoding the result, as per Section 7.1, yields this JWS Crypto
Output value:
dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk
This computation is illustrated in more detail in Appendix A.1.
4. JWS Header
The members of the JSON object represented by the Decoded JWS Header
Input describe the signature applied to the JWS Header Input and the
JWS Payload Input and optionally additional properties of the JWS.
Implementations MUST understand the entire contents of the header;
otherwise, the JWS MUST be rejected for processing.
4.1. Reserved Header Parameter Names
The following header parameter names are reserved. All the names are
short because a core goal of JWSs is for the representations to be
compact.
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+-----------+--------+--------------+-------------------------------+
| Header | JSON | Header | Header Parameter Semantics |
| Parameter | Value | Parameter | |
| Name | Type | Syntax | |
+-----------+--------+--------------+-------------------------------+
| alg | string | StringAndURI | The "alg" (algorithm) header |
| | | | parameter identifies the |
| | | | cryptographic algorithm used |
| | | | to secure the JWS. A list of |
| | | | reserved alg values is in |
| | | | Table 3. The processing of |
| | | | the "alg" (algorithm) header |
| | | | parameter, if present, |
| | | | requires that the value of |
| | | | the "alg" header parameter |
| | | | MUST be one that is both |
| | | | supported and for which there |
| | | | exists a key for use with |
| | | | that algorithm associated |
| | | | with the signer of the |
| | | | content. This header |
| | | | parameter is REQUIRED. |
| typ | string | String | The "typ" (type) header |
| | | | parameter is used to declare |
| | | | the type of the signed |
| | | | content. This header |
| | | | parameter is OPTIONAL. |
| jku | string | URL | The "jku" (JSON Key URL) |
| | | | header parameter is a URL |
| | | | that points to JSON-encoded |
| | | | public key certificates that |
| | | | can be used to validate the |
| | | | signature. The specification |
| | | | for this encoding is TBD. |
| | | | This header parameter is |
| | | | OPTIONAL. |
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| kid | string | String | The "kid" (key ID) header |
| | | | parameter is a hint |
| | | | indicating which specific key |
| | | | owned by the signer should be |
| | | | used to validate the |
| | | | signature. This allows |
| | | | signers to explicitly signal |
| | | | a change of key to |
| | | | recipients. Omitting this |
| | | | parameter is equivalent to |
| | | | setting it to an empty |
| | | | string. The interpretation |
| | | | of the contents of the "kid" |
| | | | parameter is unspecified. |
| | | | This header parameter is |
| | | | OPTIONAL. |
| x5u | string | URL | The "x5u" (X.509 URL) header |
| | | | parameter is a URL that |
| | | | points to an X.509 public key |
| | | | certificate that can be used |
| | | | to validate the signature. |
| | | | This certificate MUST conform |
| | | | to RFC 5280 [RFC5280]. This |
| | | | header parameter is OPTIONAL. |
| x5t | string | String | The "x5t" (x.509 certificate |
| | | | thumbprint) header parameter |
| | | | provides a base64url encoded |
| | | | SHA-256 thumbprint (a.k.a. |
| | | | digest) of the DER encoding |
| | | | of an X.509 certificate that |
| | | | can be used to match a |
| | | | certificate. This header |
| | | | parameter is OPTIONAL. |
+-----------+--------+--------------+-------------------------------+
Table 1: Reserved Header Parameter Definitions
Additional reserved header parameter names MAY be defined via the
IANA JSON Web Signature Header Parameters registry, as per Section 8.
The syntax values used above are defined as follows:
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+--------------+----------------------------------------------------+
| Syntax Name | Syntax Definition |
+--------------+----------------------------------------------------+
| IntDate | The number of seconds from 1970-01-01T0:0:0Z as |
| | measured in UTC until the desired date/time. See |
| | RFC 3339 [RFC3339] for details regarding |
| | date/times in general and UTC in particular. |
| String | Any string value MAY be used. |
| StringAndURI | Any string value MAY be used but a value |
| | containing a ":" character MUST be a URI as |
| | defined in RFC 3986 [RFC3986]. |
| URL | A URL as defined in RFC 1738 [RFC1738]. |
+--------------+----------------------------------------------------+
Table 2: Header Parameter Syntax Definitions
4.2. Public Header Parameter Names
Additional header parameter names can be defined by those using JWSs.
However, in order to prevent collisions, any new header parameter
name or algorithm value SHOULD either be defined in the IANA JSON Web
Signature Header Parameters registry or be defined as a URI that
contains a collision resistant namespace. In each case, the definer
of the name or value MUST take reasonable precautions to make sure
they are in control of the part of the namespace they use to define
the header parameter name.
New header parameters should be introduced sparingly, as they can
result in non-interoperable JWSs.
4.3. Private Header Parameter Names
A producer and consumer of a JWS may agree to any header parameter
name that is not a Reserved Name Section 4.1 or a Public Name
Section 4.2. Unlike Public Names, these private names are subject to
collision and should be used with caution.
New header parameters should be introduced sparingly, as they can
result in non-interoperable JWSs.
5. Rules for Creating and Validating a JWS
To create a JWS, one MUST follow these steps:
1. Create the payload content to be encoded as the Decoded JWS
Payload Input.
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2. Base64url encode the Decoded JWS Payload Input. This encoding
becomes the JWS Payload Input.
3. Create a JSON object containing a set of desired header
parameters. Note that white space is explicitly allowed in the
representation and no canonicalization is performed before
encoding.
4. Translate this JSON object's Unicode code points into UTF-8, as
defined in RFC 3629 [RFC3629].
5. Base64url encode the UTF-8 representation of this JSON object as
defined in this specification (without padding). This encoding
becomes the JWS Header Input.
6. Compute the JWS Crypto Output in the manner defined for the
particular algorithm being used. The JWS Signing Input is always
the concatenation of the JWS Header Input, a period ('.')
character, and the JWS Payload Input. The "alg" header parameter
MUST be present in the JSON Header Input, with the algorithm
value accurately representing the algorithm used to construct the
JWS Crypto Input.
When validating a JWS, the following steps MUST be taken. If any of
the listed steps fails, then the signed content MUST be rejected.
1. The JWS Payload Input MUST be successfully base64url decoded
following the restriction given in this specification that no
padding characters have been used.
2. The JWS Header Input MUST be successfully base64url decoded
following the restriction given in this specification that no
padding characters have been used.
3. The Decoded JWS Header Input MUST be completely valid JSON syntax
conforming to RFC 4627 [RFC4627].
4. The JWS Crypto Output MUST be successfully base64url decoded
following the restriction given in this specification that no
padding characters have been used.
5. The JWS Header Input MUST be validated to only include parameters
and values whose syntax and semantics are both understood and
supported.
6. The JWS Crypto Output MUST be successfully validated against the
JWS Header Input and JWS Payload Input in the manner defined for
the algorithm being used, which MUST be accurately represented by
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the value of the "alg" header parameter, which MUST be present.
Processing a JWS inevitably requires comparing known strings to
values in the header. For example, in checking what the algorithm
is, the Unicode string encoding "alg" will be checked against the
member names in the Decoded JWS Header Input to see if there is a
matching header parameter name. A similar process occurs when
determining if the value of the "alg" header parameter represents a
supported algorithm. Comparing Unicode strings, however, has
significant security implications, as per Section 9.
Comparisons between JSON strings and other Unicode strings MUST be
performed as specified below:
1. Remove any JSON applied escaping to produce an array of Unicode
code points.
2. Unicode Normalization [USA15] MUST NOT be applied at any point to
either the JSON string or to the string it is to be compared
against.
3. Comparisons between the two strings MUST be performed as a
Unicode code point to code point equality comparison.
6. Base64url encoding as used by JWSs
JWSs make use of the base64url encoding as defined in RFC 4648
[RFC4648]. As allowed by Section 3.2 of the RFC, this specification
mandates that base64url encoding when used with JWSs MUST NOT use
padding. The reason for this restriction is that the padding
character ('=') is not URL safe.
For notes on implementing base64url encoding without padding, see
Appendix B.
7. Signing JWSs with Cryptographic Algorithms
JWSs use specific cryptographic algorithms to sign the contents of
the JWS Header Input and the JWS Payload Input. The use of the
following algorithms for producing JWSs is defined in this section.
The table below is the list of "alg" header parameter values reserved
by this specification, each of which is explained in more detail in
the following sections:
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+--------------------+----------------------------------------------+
| Alg Parameter | Algorithm |
| Value | |
+--------------------+----------------------------------------------+
| HS256 | HMAC using SHA-256 hash algorithm |
| HS384 | HMAC using SHA-384 hash algorithm |
| HS512 | HMAC using SHA-512 hash algorithm |
| RS256 | RSA using SHA-256 hash algorithm |
| RS384 | RSA using SHA-384 hash algorithm |
| RS512 | RSA using SHA-512 hash algorithm |
| ES256 | ECDSA using P-256 curve and SHA-256 hash |
| | algorithm |
| ES384 | ECDSA using P-384 curve and SHA-384 hash |
| | algorithm |
| ES512 | ECDSA using P-521 curve and SHA-512 hash |
| | algorithm |
+--------------------+----------------------------------------------+
Table 3: JSON Web Signature Reserved Algorithm Values
Of these algorithms, only HMAC SHA-256 and RSA SHA-256 MUST be
implemented by conforming implementations. It is RECOMMENDED that
implementations also support the ECDSA P-256 SHA-256 algorithm.
Support for other algorithms is OPTIONAL.
The signed content for a JWS is the same for all algorithms: the
concatenation of the JWS Header Input, a period ('.') character, and
the JWS Payload Input. This character sequence is referred to as the
JWS Signing Input. Note that if the JWS represents a JWT, this
corresponds to the portion of the JWT representation preceding the
second period character. The UTF-8 representation of the JWS Signing
Input is passed to the respective signing algorithms.
7.1. Creating a JWS with HMAC SHA-256
Hash based Message Authentication Codes (HMACs) enable one to use a
secret plus a cryptographic hash function to generate a Message
Authentication Code (MAC). This can be used to demonstrate that the
MAC matches the hashed content, in this case the JWS Signing Input,
which therefore demonstrates that whoever generated the MAC was in
possession of the secret.
The algorithm for implementing and validating HMACs is provided in
RFC 2104 [RFC2104]. Although any HMAC can be used with JWSs, this
section defines the use of the SHA-256 cryptographic hash function as
defined in FIPS 180-3 [FIPS.180-3]. The reserved "alg" header
parameter value "HS256" is used in the JWS Header Input to indicate
that the JWS Crypto Output contains a base64url encoded HMAC SHA-256
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HMAC value.
The HMAC SHA-256 MAC is generated as follows:
1. Apply the HMAC SHA-256 algorithm to the UTF-8 representation of
the JWS Signing Input using the shared key to produce an HMAC.
2. Base64url encode the HMAC as defined in this document.
The output is the JWS Crypto Output for that JWS.
The HMAC SHA-256 MAC for a JWS is validated as follows:
1. Apply the HMAC SHA-256 algorithm to the UTF-8 representation of
the JWS Signing Input of the JWS using the shared key.
2. Base64url encode the previously generated HMAC as defined in this
document.
3. If the JWS Crypto Output and the previously calculated value
exactly match, then one has confirmation that the key was used to
generate the HMAC on the JWS and that the contents of the JWS
have not be tampered with.
4. If the validation fails, the signed content MUST be rejected.
Signing with the HMAC SHA-384 and HMAC SHA-512 algorithms is
performed identically to the procedure for HMAC SHA-256 - just with
correspondingly longer key and result values.
7.2. Creating a JWS with RSA SHA-256
This section defines the use of the RSASSA-PKCS1-v1_5 signature
algorithm as defined in RFC 3447 [RFC3447], Section 8.2 (commonly
known as PKCS#1), using SHA-256 as the hash function. Note that the
use of the RSASSA-PKCS1-v1_5 algorithm is described in FIPS 186-3
[FIPS.186-3], Section 5.5, as is the SHA-256 cryptographic hash
function, which is defined in FIPS 180-3 [FIPS.180-3]. The reserved
"alg" header parameter value "RS256" is used in the JWS Header Input
to indicate that the JWS Crypto Output contains an RSA SHA-256
signature.
A 2048-bit or longer key length MUST be used with this algorithm.
The RSA SHA-256 signature is generated as follows:
1. Let K be the signer's RSA private key and let M be the UTF-8
representation of the JWS Signing Input.
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2. Compute the octet string S = RSASSA-PKCS1-V1_5-SIGN (K, M) using
SHA-256 as the hash function.
3. Base64url encode the octet string S, as defined in this document.
The output is the JWS Crypto Output for that JWS.
The RSA SHA-256 signature for a JWS is validated as follows:
1. Take the JWS Crypto Output and base64url decode it into an octet
string S. If decoding fails, then the signed content MUST be
rejected.
2. Let M be the UTF-8 representation of the JWS Signing Input and
let (n, e) be the public key corresponding to the private key
used by the signer.
3. Validate the signature with RSASSA-PKCS1-V1_5-VERIFY ((n, e), M,
S) using SHA-256 as the hash function.
4. If the validation fails, the signed content MUST be rejected.
Signing with the RSA SHA-384 and RSA SHA-512 algorithms is performed
identically to the procedure for RSA SHA-256 - just with
correspondingly longer key and result values.
7.3. Creating a JWS with ECDSA P-256 SHA-256
The Elliptic Curve Digital Signature Algorithm (ECDSA) is defined by
FIPS 186-3 [FIPS.186-3]. ECDSA provides for the use of Elliptic
Curve cryptography, which is able to provide equivalent security to
RSA cryptography but using shorter key lengths and with greater
processing speed. This means that ECDSA signatures will be
substantially smaller in terms of length than equivalently strong RSA
Digital Signatures.
This specification defines the use of ECDSA with the P-256 curve and
the SHA-256 cryptographic hash function. The P-256 curve is also
defined in FIPS 186-3. The reserved "alg" header parameter value
"ES256" is used in the JWS Header Input to indicate that the JWS
Crypto Output contains an ECDSA P-256 SHA-256 signature.
A JWS is signed with an ECDSA P-256 SHA-256 signature as follows:
1. Generate a digital signature of the UTF-8 representation of the
JWS Signing Input using ECDSA P-256 SHA-256 with the desired
private key. The output will be the EC point (R, S), where R and
S are unsigned integers.
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2. Turn R and S into byte arrays in big endian order. Each array
will be 32 bytes long.
3. Concatenate the two byte arrays in the order R and then S.
4. Base64url encode the 64 byte array as defined in this
specification.
The output is the JWS Crypto Output for the JWS.
The ECDSA P-256 SHA-256 signature for a JWS is validated as follows:
1. Take the JWS Crypto Output and base64url decode it into a byte
array. If decoding fails, the signed content MUST be rejected.
2. The output of the base64url decoding MUST be a 64 byte array.
3. Split the 64 byte array into two 32 byte arrays. The first array
will be R and the second S. Remember that the byte arrays are in
big endian byte order; please check the ECDSA validator in use to
see what byte order it requires.
4. Submit the UTF-8 representation of the JWS Signing Input, R, S
and the public key (x, y) to the ECDSA P-256 SHA-256 validator.
5. If the validation fails, the signed content MUST be rejected.
The ECDSA validator will then determine if the digital signature is
valid, given the inputs. Note that ECDSA digital signature contains
a value referred to as K, which is a random number generated for each
digital signature instance. This means that two ECDSA digital
signatures using exactly the same input parameters will output
different signatures because their K values will be different. The
consequence of this is that one must validate an ECDSA signature by
submitting the previously specified inputs to an ECDSA validator.
Signing with the ECDSA P-384 SHA-384 and ECDSA P-521 SHA-512
algorithms is performed identically to the procedure for ECDSA P-256
SHA-256 - just with correspondingly longer key and result values.
7.4. Additional Algorithms
Additional algorithms MAY be used to protect JWSs with corresponding
"alg" header parameter values being defined to refer to them. New
"alg" header parameter values SHOULD either be defined in the IANA
JSON Web Signature Algorithms registry or be a URI that contains a
collision resistant namespace. In particular, the use of algorithm
identifiers defined in XML DSIG [RFC3275] and related specifications
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is permitted.
8. IANA Considerations
This specification calls for:
o A new IANA registry entitled "JSON Web Signature Header
Parameters" for reserved header parameter names is defined in
Section 4.1. Inclusion in the registry is RFC Required in the RFC
5226 [RFC5226] sense for reserved JWS header parameter names that
are intended to be interoperable between implementations. The
registry will just record the reserved header parameter name and a
pointer to the RFC that defines it. This specification defines
inclusion of the header parameter names defined in Table 1.
o A new IANA registry entitled "JSON Web Signature Algorithms" for
reserved values used with the "alg" header parameter values is
defined in Section 7.4. Inclusion in the registry is RFC Required
in the RFC 5226 [RFC5226] sense. The registry will just record
the "alg" value and a pointer to the RFC that defines it. This
specification defines inclusion of the algorithm values defined in
Table 3.
9. Security Considerations
TBD: Lots of work to do here. We need to remember to look into any
issues relating to security and JSON parsing. One wonders just how
secure most JSON parsing libraries are. Were they ever hardened for
security scenarios? If not, what kind of holes does that open up?
Also, we need to walk through the JSON standard and see what kind of
issues we have especially around comparison of names. For instance,
comparisons of header parameter names and other parameters must occur
after they are unescaped. Need to also put in text about: Importance
of keeping secrets secret. Rotating keys. Strengths and weaknesses
of the different algorithms.
TBD: Need to put in text about why strict JSON validation is
necessary. Basically, that if malformed JSON is received then the
intent of the sender is impossible to reliably discern.
9.1. Unicode Comparison Security Issues
Header parameter names in JWSs are Unicode strings. For security
reasons, the representations of these names must be compared verbatim
after performing any escape processing (as per RFC 4627 [RFC4627],
Section 2.5).
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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").
JSON strings MAY 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.
10. Open Issues and Things To Be Done (TBD)
The following items remain to be done in this draft (and related
drafts):
o Consider whether there is a better term than "Digital Signature"
for the concept that includes both HMACs and digital signatures
using public keys.
o Consider whether we really want to allow private header parameter
names that are not registered with IANA and are not in collision-
resistant namespaces. Eventually this could result in interop
nightmares where you need to have different code to talk to
different endpoints that "knows" about each endpoints' private
parameters.
o Clarify the optional ability to provide type information in the
JWS header. Specifically, clarify the intended use of the "typ"
Header Parameter, whether it conveys syntax or semantics, and
indeed, whether this is the right approach. Also clarify the
relationship between these type values and MIME [RFC2045] types.
o Clarify the semantics of the "kid" (key ID) header parameter.
Open issues include: What happens if a kid header is received with
an unrecognized value? Is that an error? Should it be treated as
if it's empty? What happens if the header has a recognized value
but the value doesn't match the key associated with that value,
but it does match another key that is associated with the issuer?
Is that an error?
o The "x5t" parameter is currently specified as "a base64url encoded
SHA-256 thumbprint of the DER encoding of an X.509 certificate".
SHA-1 was traditionally used for certificate digests but
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collisions are possible to create and can be used for denial of
service attacks within multi-tenant services. We need to
understand the compatibility issues of using SHA-256 thumbprints
instead. We also likely want to specify the digest algorithm
explicitly.
o Several people have objected to the requirement for implementing
RSA SHA-256, some because they will only be using HMACs and
symmetric keys, and others because they only want to use ECDSA
when using asymmetric keys, either for security or key length
reasons, or both. I believe therefore, that we should consider
changing the MUST for RSA SHA-256 to RECOMMENDED.
o Since RFC 3447 Section 8 explicitly calls for people NOT to adopt
RSASSA-PKCS1 for new applications and instead requests that people
transition to RSASSA-PSS, we probably need some Security
Considerations text explaining why RSASSA-PKCS1 is being used
(it's what's commonly implemented) and what the potential
consequences are.
o Generalize the normative text on signing algorithms so that the
descriptions apply equally to the use of various key lengths - not
just HMAC SHA-256, RSA SHA-256, and ECDSA P-256 SHA-256.
o Add a table cross-referencing the algorithm name strings used in
standard software packages and specifications.
o Add Security Considerations text on timing attacks.
o Finish the Security Considerations section.
o Sort out what to do with the IANA registries if this is first
standardized as an OpenID specification.
o Write the related specification for encoding public keys using
JSON, as per the agreement documented at
http://self-issued.info/?p=390. This will be used by the "jku"
(JSON Key URL) header parameter.
o Write the companion encryption specification, per the agreements
documented at http://self-issued.info/?p=378.
11. References
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11.1. Normative References
[FIPS.180-3]
National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-3, October 2008.
[FIPS.186-3]
National Institute of Standards and Technology, "Digital
Signature Standard (DSS)", FIPS PUB 186-3, June 2009.
[JWT] Jones, M., Balfanz, D., Bradley, J., Goland, Y., Panzer,
J., Sakimura, N., and P. Tarjan, "JSON Web Token (JWT)",
March 2011.
[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the
Internet: Timestamps", RFC 3339, July 2002.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
Jones, et al. Expires September 29, 2011 [Page 19]
Internet-Draft JSON Web Signature (JWS) March 2011
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[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, May 2008.
[USA15] Davis, M., Whistler, K., and M. Duerst, "Unicode
Normalization Forms", Unicode Standard Annex 15, 09 2009.
11.2. Informative References
[CanvasApp]
Facebook, "Canvas Applications", 2010.
[JSS] Bradley, J. and N. Sakimura (editor), "JSON Simple Sign",
September 2010.
[JWE] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Encryption (JWE)", March 2011.
[MagicSignatures]
Panzer (editor), J., Laurie, B., and D. Balfanz, "Magic
Signatures", August 2010.
[RFC3275] Eastlake, D., Reagle, J., and D. Solo, "(Extensible Markup
Language) XML-Signature Syntax and Processing", RFC 3275,
March 2002.
Appendix A. JWS Examples
This section provides several examples of JWSs. While these examples
all represent JSON Web Tokens (JWTs) [JWT], note that the payload can
be any base64url encoded content.
A.1. JWS using HMAC SHA-256
A.1.1. Encoding
The following example JSON header object declares that the data
structure is a JSON Web Token (JWT) [JWT] and the JWS Signing Input
is signed using the HMAC SHA-256 algorithm. Note that white space is
explicitly allowed in Decoded JWS Header Input strings and no
canonicalization is performed before encoding.
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{"typ":"JWT",
"alg":"HS256"}
The following byte array contains the UTF-8 characters for the
Decoded JWS Header Input:
[123, 34, 116, 121, 112, 34, 58, 34, 74, 87, 84, 34, 44, 13, 10, 32,
34, 97, 108, 103, 34, 58, 34, 72, 83, 50, 53, 54, 34, 125]
Base64url encoding this UTF-8 representation yields this JWS Header
Input value:
eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9
The Decoded JWS Payload Input used in this example follows. (Note
that the payload can be any base64url encoded content, and need not
be a base64url encoded JSON object.)
{"iss":"joe",
"exp":1300819380,
"http://example.com/is_root":true}
The following byte array contains the UTF-8 characters for the
Decoded JWS Payload Input:
[123, 34, 105, 115, 115, 34, 58, 34, 106, 111, 101, 34, 44, 13, 10,
32, 34, 101, 120, 112, 34, 58, 49, 51, 48, 48, 56, 49, 57, 51, 56,
48, 44, 13, 10, 32, 34, 104, 116, 116, 112, 58, 47, 47, 101, 120, 97,
109, 112, 108, 101, 46, 99, 111, 109, 47, 105, 115, 95, 114, 111,
111, 116, 34, 58, 116, 114, 117, 101, 125]
Base64url encoding the above yields the JWS Payload Input value:
eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFtcGxlLmNvbS9pc19yb290Ijp0cnVlfQ
Concatenating the JWS Header Input, a period character, and the JWS
Payload Input yields this JWS Signing Input value (with line breaks
for display purposes only):
eyJ0eXAiOiJKV1QiLA0KICJhbGciOiJIUzI1NiJ9
.
eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFtcGxlLmNvbS9pc19yb290Ijp0cnVlfQ
The UTF-8 representation of the JWS Signing Input is the following
byte array:
[101, 121, 74, 48, 101, 88, 65, 105, 79, 105, 74, 75, 86, 49, 81,
105, 76, 65, 48, 75, 73, 67, 74, 104, 98, 71, 99, 105, 79, 105, 74,
73, 85, 122, 73, 49, 78, 105, 74, 57, 46, 101, 121, 74, 112, 99, 51,
77, 105, 79, 105, 74, 113, 98, 50, 85, 105, 76, 65, 48, 75, 73, 67,
74, 108, 101, 72, 65, 105, 79, 106, 69, 122, 77, 68, 65, 52, 77, 84,
107, 122, 79, 68, 65, 115, 68, 81, 111, 103, 73, 109, 104, 48, 100,
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72, 65, 54, 76, 121, 57, 108, 101, 71, 70, 116, 99, 71, 120, 108, 76,
109, 78, 118, 98, 83, 57, 112, 99, 49, 57, 121, 98, 50, 57, 48, 73,
106, 112, 48, 99, 110, 86, 108, 102, 81]
HMACs are generated using keys. This example uses the key
represented by the following byte array:
[3, 35, 53, 75, 43, 15, 165, 188, 131, 126, 6, 101, 119, 123, 166,
143, 90, 179, 40, 230, 240, 84, 201, 40, 169, 15, 132, 178, 210, 80,
46, 191, 211, 251, 90, 146, 210, 6, 71, 239, 150, 138, 180, 195, 119,
98, 61, 34, 61, 46, 33, 114, 5, 46, 79, 8, 192, 205, 154, 245, 103,
208, 128, 163]
Running the HMAC SHA-256 algorithm on the UTF-8 representation of the
JWS Signing Input with this key yields the following byte array:
[116, 24, 223, 180, 151, 153, 224, 37, 79, 250, 96, 125, 216, 173,
187, 186, 22, 212, 37, 77, 105, 214, 191, 240, 91, 88, 5, 88, 83,
132, 141, 121]
Base64url encoding the above HMAC output yields the JWS Crypto Output
value:
dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk
A.1.2. Decoding
Decoding the JWS first requires removing the base64url encoding from
the JWS Header Input, the JWS Payload Input, and the JWS Crypto
Output. We base64url decode the inputs per Section 6 and turn them
into the corresponding byte arrays. We translate the header input
byte array containing UTF-8 encoded characters into the Decoded JWS
Header Input string.
A.1.3. Validating
Next we validate the decoded results. Since the "alg" parameter in
the header is "HS256", we validate the HMAC SHA-256 signature
contained in the JWS Crypto Output. If any of the validation steps
fail, the signed content MUST be rejected.
First, we validate that the decoded JWS Header Input string is legal
JSON.
To validate the signature, we repeat the previous process of using
the correct key and the UTF-8 representation of the JWS Signing Input
as input to a SHA-256 HMAC function and then taking the output and
determining if it matches the Decoded JWS Crypto Output. If it
matches exactly, the signature has been validated.
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A.2. JWS using RSA SHA-256
A.2.1. Encoding
The Decoded JWS Header Input in this example is different from the
previous example in two ways: First, because a different algorithm is
being used, the "alg" value is different. Second, for illustration
purposes only, the optional "typ" parameter is not used. (This
difference is not related to the signature algorithm employed.) The
Decoded JWS Header Input used is:
{"alg":"RS256"}
The following byte array contains the UTF-8 characters for the
Decoded JWS Header Input:
[123, 34, 97, 108, 103, 34, 58, 34, 82, 83, 50, 53, 54, 34, 125]
Base64url encoding this UTF-8 representation yields this JWS Header
Input value:
eyJhbGciOiJSUzI1NiJ9
The Decoded JWS Payload Input used in this example, which follows, is
the same as in the previous example. Since the JWS Payload Input
will therefore be the same, its computation is not repeated here.
{"iss":"joe",
"exp":1300819380,
"http://example.com/is_root":true}
Concatenating the JWS Header Input, a period character, and the JWS
Payload Input yields this JWS Signing Input value (with line breaks
for display purposes only):
eyJhbGciOiJSUzI1NiJ9
.
eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFtcGxlLmNvbS9pc19yb290Ijp0cnVlfQ
The UTF-8 representation of the JWS Signing Input is the following
byte array:
[101, 121, 74, 104, 98, 71, 99, 105, 79, 105, 74, 83, 85, 122, 73,
49, 78, 105, 74, 57, 46, 101, 121, 74, 112, 99, 51, 77, 105, 79, 105,
74, 113, 98, 50, 85, 105, 76, 65, 48, 75, 73, 67, 74, 108, 101, 72,
65, 105, 79, 106, 69, 122, 77, 68, 65, 52, 77, 84, 107, 122, 79, 68,
65, 115, 68, 81, 111, 103, 73, 109, 104, 48, 100, 72, 65, 54, 76,
121, 57, 108, 101, 71, 70, 116, 99, 71, 120, 108, 76, 109, 78, 118,
98, 83, 57, 112, 99, 49, 57, 121, 98, 50, 57, 48, 73, 106, 112, 48,
99, 110, 86, 108, 102, 81]
The RSA key consists of a public part (n, e), and a private exponent
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d. The values of the RSA key used in this example, presented as the
byte arrays representing big endian integers are:
+-----------+-------------------------------------------------------+
| Parameter | Value |
| Name | |
+-----------+-------------------------------------------------------+
| n | [161, 248, 22, 10, 226, 227, 201, 180, 101, 206, 141, |
| | 45, 101, 98, 99, 54, 43, 146, 125, 190, 41, 225, 240, |
| | 36, 119, 252, 22, 37, 204, 144, 161, 54, 227, 139, |
| | 217, 52, 151, 197, 182, 234, 99, 221, 119, 17, 230, |
| | 124, 116, 41, 249, 86, 176, 251, 138, 143, 8, 154, |
| | 220, 75, 105, 137, 60, 193, 51, 63, 83, 237, 208, 25, |
| | 184, 119, 132, 37, 47, 236, 145, 79, 228, 133, 119, |
| | 105, 89, 75, 234, 66, 128, 211, 44, 15, 85, 191, 98, |
| | 148, 79, 19, 3, 150, 188, 110, 155, 223, 110, 189, |
| | 210, 189, 163, 103, 142, 236, 160, 198, 104, 247, 1, |
| | 179, 141, 191, 251, 56, 200, 52, 44, 226, 254, 109, |
| | 39, 250, 222, 74, 90, 72, 116, 151, 157, 212, 185, |
| | 207, 154, 222, 196, 199, 91, 5, 133, 44, 44, 15, 94, |
| | 248, 165, 193, 117, 3, 146, 249, 68, 232, 237, 100, |
| | 193, 16, 198, 182, 71, 96, 154, 164, 120, 58, 235, |
| | 156, 108, 154, 215, 85, 49, 48, 80, 99, 139, 131, |
| | 102, 92, 111, 111, 122, 130, 163, 150, 112, 42, 31, |
| | 100, 27, 130, 211, 235, 242, 57, 34, 25, 73, 31, 182, |
| | 134, 135, 44, 87, 22, 245, 10, 248, 53, 141, 154, |
| | 139, 157, 23, 195, 64, 114, 143, 127, 135, 216, 154, |
| | 24, 216, 252, 171, 103, 173, 132, 89, 12, 46, 207, |
| | 117, 147, 57, 54, 60, 7, 3, 77, 111, 96, 111, 158, |
| | 33, 224, 84, 86, 202, 229, 233, 161] |
| e | [1, 0, 1] |
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| d | [18, 174, 113, 164, 105, 205, 10, 43, 195, 126, 82, |
| | 108, 69, 0, 87, 31, 29, 97, 117, 29, 100, 233, 73, |
| | 112, 123, 98, 89, 15, 157, 11, 165, 124, 150, 60, 64, |
| | 30, 63, 207, 47, 44, 211, 189, 236, 136, 229, 3, 191, |
| | 198, 67, 155, 11, 40, 200, 47, 125, 55, 151, 103, 31, |
| | 82, 19, 238, 216, 193, 90, 37, 216, 213, 206, 160, 2, |
| | 94, 227, 171, 46, 139, 127, 121, 33, 111, 198, 59, |
| | 234, 86, 39, 83, 180, 6, 68, 198, 161, 81, 39, 217, |
| | 178, 149, 69, 64, 160, 187, 225, 163, 5, 86, 152, 45, |
| | 78, 159, 222, 95, 100, 37, 241, 77, 75, 113, 52, 65, |
| | 181, 93, 199, 59, 155, 74, 237, 204, 146, 172, 227, |
| | 146, 126, 55, 245, 125, 12, 253, 94, 117, 129, 250, |
| | 81, 44, 143, 73, 97, 169, 235, 11, 128, 248, 168, 7, |
| | 70, 114, 138, 85, 255, 70, 71, 31, 52, 37, 6, 59, |
| | 157, 83, 100, 47, 94, 222, 30, 132, 214, 19, 8, 26, |
| | 250, 92, 34, 208, 81, 40, 91, 214, 59, 148, 59, 86, |
| | 93, 137, 138, 5, 104, 84, 19, 229, 60, 60, 108, 101, |
| | 37, 255, 31, 227, 78, 61, 220, 112, 240, 213, 100, |
| | 80, 253, 164, 139, 161, 46, 16, 78, 157, 235, 159, |
| | 184, 24, 129, 225, 196, 189, 242, 93, 146, 71, 244, |
| | 80, 200, 101, 146, 121, 104, 231, 115, 52, 244, 65, |
| | 79, 117, 167, 80, 225, 57, 84, 110, 58, 138, 115, |
| | 157] |
+-----------+-------------------------------------------------------+
The RSA private key (n, d) is then passed to the RSA signing
function, which also takes the hash type, SHA-256, and the UTF-8
representation of the JWS Signing Input as inputs. The result of the
signature is a byte array S, which represents a big endian integer.
In this example, S is:
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+--------+----------------------------------------------------------+
| Result | Value |
| Name | |
+--------+----------------------------------------------------------+
| S | [112, 46, 33, 137, 67, 232, 143, 209, 30, 181, 216, 45, |
| | 191, 120, 69, 243, 65, 6, 174, 27, 129, 255, 247, 115, |
| | 17, 22, 173, 209, 113, 125, 131, 101, 109, 66, 10, 253, |
| | 60, 150, 238, 221, 115, 162, 102, 62, 81, 102, 104, 123, |
| | 0, 11, 135, 34, 110, 1, 135, 237, 16, 115, 249, 69, 229, |
| | 130, 173, 252, 239, 22, 216, 90, 121, 142, 232, 198, |
| | 109, 219, 61, 184, 151, 91, 23, 208, 148, 2, 190, 237, |
| | 213, 217, 217, 112, 7, 16, 141, 178, 129, 96, 213, 248, |
| | 4, 12, 167, 68, 87, 98, 184, 31, 190, 127, 249, 217, 46, |
| | 10, 231, 111, 36, 242, 91, 51, 187, 230, 244, 74, 230, |
| | 30, 177, 4, 10, 203, 32, 4, 77, 62, 249, 18, 142, 212, |
| | 1, 48, 121, 91, 212, 189, 59, 65, 238, 202, 208, 102, |
| | 171, 101, 25, 129, 253, 228, 141, 247, 127, 55, 45, 195, |
| | 139, 159, 175, 221, 59, 239, 177, 139, 93, 163, 204, 60, |
| | 46, 176, 47, 158, 58, 65, 214, 18, 202, 173, 21, 145, |
| | 18, 115, 160, 95, 35, 185, 232, 56, 250, 175, 132, 157, |
| | 105, 132, 41, 239, 90, 30, 136, 121, 130, 54, 195, 212, |
| | 14, 96, 69, 34, 165, 68, 200, 242, 122, 122, 45, 184, 6, |
| | 99, 209, 108, 247, 202, 234, 86, 222, 64, 92, 178, 33, |
| | 90, 69, 178, 194, 85, 102, 181, 90, 193, 167, 72, 160, |
| | 112, 223, 200, 163, 42, 70, 149, 67, 208, 25, 238, 251, |
| | 71] |
+--------+----------------------------------------------------------+
Base64url encoding the signature produces this value for the JWS
Crypto Output:
cC4hiUPoj9Eetdgtv3hF80EGrhuB__dzERat0XF9g2VtQgr9PJbu3XOiZj5RZmh7AAuHIm4Bh-0Qc_lF5YKt_O8W2Fp5jujGbds9uJdbF9CUAr7t1dnZcAcQjbKBYNX4BAynRFdiuB--f_nZLgrnbyTyWzO75vRK5h6xBArLIARNPvkSjtQBMHlb1L07Qe7K0GarZRmB_eSN9383LcOLn6_dO--xi12jzDwusC-eOkHWEsqtFZESc6BfI7noOPqvhJ1phCnvWh6IeYI2w9QOYEUipUTI8np6LbgGY9Fs98rqVt5AXLIhWkWywlVmtVrBp0igcN_IoypGlUPQGe77Rw
A.2.2. Decoding
Decoding the JWS from this example requires processing the JWS Header
Input and JWS Payload Input exactly as done in the first example.
A.2.3. Validating
Since the "alg" parameter in the header is "RS256", we validate the
RSA SHA-256 signature contained in the JWS Crypto Output. If any of
the validation steps fail, the signed content MUST be rejected.
First, we validate that the decoded JWS Header Input string is legal
JSON.
Validating the JWS Crypto Output is a little different from the
previous example. First, we base64url decode the JWS Crypto Output
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to produce a signature S to check. We then pass (n, e), S and the
UTF-8 representation of the JWS Signing Input to an RSA signature
verifier that has been configured to use the SHA-256 hash function.
A.3. JWS using ECDSA P-256 SHA-256
A.3.1. Encoding
The Decoded JWS Header Input for this example differs from the
previous example because a different algorithm is being used. The
Decoded JWS Header Input used is:
{"alg":"ES256"}
The following byte array contains the UTF-8 characters for the
Decoded JWS Header Input:
[123, 34, 97, 108, 103, 34, 58, 34, 69, 83, 50, 53, 54, 34, 125]
Base64url encoding this UTF-8 representation yields this JWS Header
Input value:
eyJhbGciOiJFUzI1NiJ9
The Decoded JWS Payload Input used in this example, which follows, is
the same as in the previous examples. Since the JWS Payload Input
will therefore be the same, its computation is not repeated here.
{"iss":"joe",
"exp":1300819380,
"http://example.com/is_root":true}
Concatenating the JWS Header Input, a period character, and the JWS
Payload Input yields this JWS Signing Input value (with line breaks
for display purposes only):
eyJhbGciOiJFUzI1NiJ9
.
eyJpc3MiOiJqb2UiLA0KICJleHAiOjEzMDA4MTkzODAsDQogImh0dHA6Ly9leGFtcGxlLmNvbS9pc19yb290Ijp0cnVlfQ
The UTF-8 representation of the JWS Signing Input is the following
byte array:
[101, 121, 74, 104, 98, 71, 99, 105, 79, 105, 74, 70, 85, 122, 73,
49, 78, 105, 74, 57, 46, 101, 121, 74, 112, 99, 51, 77, 105, 79, 105,
74, 113, 98, 50, 85, 105, 76, 65, 48, 75, 73, 67, 74, 108, 101, 72,
65, 105, 79, 106, 69, 122, 77, 68, 65, 52, 77, 84, 107, 122, 79, 68,
65, 115, 68, 81, 111, 103, 73, 109, 104, 48, 100, 72, 65, 54, 76,
121, 57, 108, 101, 71, 70, 116, 99, 71, 120, 108, 76, 109, 78, 118,
98, 83, 57, 112, 99, 49, 57, 121, 98, 50, 57, 48, 73, 106, 112, 48,
99, 110, 86, 108, 102, 81]
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The ECDSA key consists of a public part, the EC point (x, y), and a
private part d. The values of the ECDSA key used in this example,
presented as the byte arrays representing big endian integers are:
+-----------+-------------------------------------------------------+
| Parameter | Value |
| Name | |
+-----------+-------------------------------------------------------+
| x | [127, 205, 206, 39, 112, 246, 196, 93, 65, 131, 203, |
| | 238, 111, 219, 75, 123, 88, 7, 51, 53, 123, 233, 239, |
| | 19, 186, 207, 110, 60, 123, 209, 84, 69] |
| y | [199, 241, 68, 205, 27, 189, 155, 126, 135, 44, 223, |
| | 237, 185, 238, 185, 244, 179, 105, 93, 110, 169, 11, |
| | 36, 173, 138, 70, 35, 40, 133, 136, 229, 173] |
| d | [142, 155, 16, 158, 113, 144, 152, 191, 152, 4, 135, |
| | 223, 31, 93, 119, 233, 203, 41, 96, 110, 190, 210, |
| | 38, 59, 95, 87, 194, 19, 223, 132, 244, 178] |
+-----------+-------------------------------------------------------+
The ECDSA private part d is then passed to an ECDSA signing function,
which also takes the curve type, P-256, the hash type, SHA-256, and
the UTF-8 representation of the JWS Signing Input as inputs. The
result of the signature is the EC point (R, S), where R and S are
unsigned integers. In this example, the R and S values, given as
byte arrays representing big endian integers are:
+--------+----------------------------------------------------------+
| Result | Value |
| Name | |
+--------+----------------------------------------------------------+
| R | [14, 209, 33, 83, 121, 99, 108, 72, 60, 47, 127, 21, 88, |
| | 7, 212, 2, 163, 178, 40, 3, 58, 249, 124, 126, 23, 129, |
| | 154, 195, 22, 158, 166, 101] |
| S | [197, 10, 7, 211, 140, 60, 112, 229, 216, 241, 45, 175, |
| | 8, 74, 84, 128, 166, 101, 144, 197, 242, 147, 80, 154, |
| | 143, 63, 127, 138, 131, 163, 84, 213] |
+--------+----------------------------------------------------------+
Concatenating the S array to the end of the R array and base64url
encoding the result produces this value for the JWS Crypto Output:
DtEhU3ljbEg8L38VWAfUAqOyKAM6-Xx-F4GawxaepmXFCgfTjDxw5djxLa8ISlSApmWQxfKTUJqPP3-Kg6NU1Q
A.3.2. Decoding
Decoding the JWS from this example requires processing the JWS Header
Input and JWS Payload Input exactly as done in the first example.
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A.3.3. Validating
Since the "alg" parameter in the header is "ES256", we validate the
ECDSA P-256 SHA-256 signature contained in the JWS Crypto Output. If
any of the validation steps fail, the signed content MUST be
rejected.
First, we validate that the decoded JWS Header Input string is legal
JSON.
Validating the JWS Crypto Output is a little different from the first
example. First, we base64url decode the JWS Crypto Output as in the
previous examples but we then need to split the 64 member byte array
that must result into two 32 byte arrays, the first R and the second
S. We then pass (x, y), (R, S) and the UTF-8 representation of the
JWS Signing Input to an ECDSA signature verifier that has been
configured to use the P-256 curve with the SHA-256 hash function.
As explained in Section 7.3, the use of the k value in ECDSA means
that we cannot validate the correctness of the signature in the same
way we validated the correctness of the HMAC. Instead,
implementations MUST use an ECDSA validator to validate the
signature.
Appendix B. Notes on implementing base64url encoding without padding
This appendix describes how to implement base64url encoding and
decoding functions without padding based upon standard base64
encoding and decoding functions that do use padding.
To be concrete, example C# code implementing these functions is shown
below. Similar code could be used in other languages.
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static string base64urlencode(byte [] arg)
{
string s = Convert.ToBase64String(arg); // Standard base64 encoder
s = s.Split('=')[0]; // Remove any trailing '='s
s = s.Replace('+', '-'); // 62nd char of encoding
s = s.Replace('/', '_'); // 63rd char of encoding
return s;
}
static byte [] base64urldecode(string arg)
{
string s = arg;
s = s.Replace('-', '+'); // 62nd char of encoding
s = s.Replace('_', '/'); // 63rd char of encoding
switch (s.Length % 4) // Pad with trailing '='s
{
case 0: break; // No pad chars in this case
case 2: s += "=="; break; // Two pad chars
case 3: s += "="; break; // One pad char
default: throw new System.Exception(
"Illegal base64url string!");
}
return Convert.FromBase64String(s); // Standard base64 decoder
}
As per the example code above, the number of '=' padding characters
that needs to be added to the end of a base64url encoded string
without padding to turn it into one with padding is a deterministic
function of the length of the encoded string. Specifically, if the
length mod 4 is 0, no padding is added; if the length mod 4 is 2, two
'=' padding characters are added; if the length mod 4 is 3, one '='
padding character is added; if the length mod 4 is 1, the input is
malformed.
An example correspondence between unencoded and encoded values
follows. The byte sequence below encodes into the string below,
which when decoded, reproduces the byte sequence.
3 236 255 224 193
A-z_4ME
Appendix C. Acknowledgements
Solutions for signing JSON content were previously explored by Magic
Signatures [MagicSignatures], JSON Simple Sign [JSS], and Canvas
Applications [CanvasApp], all of which influenced this draft.
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Appendix D. Document History
-00
o Created first signature draft using content split from
draft-jones-json-web-token-01. This split introduced no semantic
changes.
Authors' Addresses
Michael B. Jones
Microsoft
Email: mbj@microsoft.com
URI: http://self-issued.info/
Dirk Balfanz
Google
Email: balfanz@google.com
John Bradley
independent
Email: ve7jtb@ve7jtb.com
Yaron Y. Goland
Microsoft
Email: yarong@microsoft.com
John Panzer
Google
Email: jpanzer@google.com
Nat Sakimura
Nomura Research Institute
Email: n-sakimura@nri.co.jp
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Paul Tarjan
Facebook
Email: pt@fb.com
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