Algorithm ES512 really was named correctly prior to issue #219

[douganger]

Issue #219 requested that the algorithm ES512 be renamed to ES521, citing RFC4492. The reporter correctly noted that the curve is named P-521, not P-512.

However, RFC4492 addresses the names of standard curves, not algorithm specifications for JSON Web Signatures or JSON Web Tokens.

In RFC7518, the table under section 3.1 on page 6 specifies ES512 as the "alg" Param Value for ECDSA using P-521 and SHA-512.

Currently, both options are accepted, but there is a comment in algorithms.py that ES512 is only left for backward compatibility after a fix. ES512 is in fact the correct value.

[jpadilla]

@douganger great catch.

https://tools.ietf.org/html/rfc7518#section-3.1

3.1.  "alg" (Algorithm) Header Parameter Values for JWS

   The table below is the set of "alg" (algorithm) Header Parameter
   values defined by this specification for use with JWS, each of which
   is explained in more detail in the following sections:

   +--------------+-------------------------------+--------------------+
   | "alg" Param  | Digital Signature or MAC      | Implementation     |
   | Value        | Algorithm                     | Requirements       |
   +--------------+-------------------------------+--------------------+
   | HS256        | HMAC using SHA-256            | Required           |
   | HS384        | HMAC using SHA-384            | Optional           |
   | HS512        | HMAC using SHA-512            | Optional           |
   | RS256        | RSASSA-PKCS1-v1_5 using       | Recommended        |
   |              | SHA-256                       |                    |
   | RS384        | RSASSA-PKCS1-v1_5 using       | Optional           |
   |              | SHA-384                       |                    |
   | RS512        | RSASSA-PKCS1-v1_5 using       | Optional           |
   |              | SHA-512                       |                    |
   | ES256        | ECDSA using P-256 and SHA-256 | Recommended+       |
   | ES384        | ECDSA using P-384 and SHA-384 | Optional           |
   | ES512        | ECDSA using P-521 and SHA-512 | Optional           |
   | PS256        | RSASSA-PSS using SHA-256 and  | Optional           |
   |              | MGF1 with SHA-256             |                    |
   | PS384        | RSASSA-PSS using SHA-384 and  | Optional           |
   |              | MGF1 with SHA-384             |                    |
   | PS512        | RSASSA-PSS using SHA-512 and  | Optional           |
   |              | MGF1 with SHA-512             |                    |
   | none         | No digital signature or MAC   | Optional           |
   |              | performed                     |                    |
   +--------------+-------------------------------+--------------------+

   The use of "+" in the Implementation Requirements column indicates
   that the requirement strength is likely to be increased in a future
   version of the specification.

[jpadilla]

Addressing this for v2 #470

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