Check if private 'd' paramter of EC JWKs is correctly padded

[Vladimir Dzhuvinov]

See http://tools.ietf.org/html/draft-ietf-jose-json-web-algorithms-20#section-6.2.2.1

[Vladimir Dzhuvinov]

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The length of [the d] octet string MUST be ceiling(log-base-2(n)/8) octets (where n is the order of the curve).

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Order of curve P-256 is BigInteger

115792089210356248762697446949407573529996955224135760342422259061068512044369

Computation seems infeasible, consider using table lookup.

[Vladimir Dzhuvinov]

The 'd' parameter is leading zero padded up the curve field size (the std P-256/384/512 curves have 'x', 'y' and 'd' parameters with the same length). The curve order - based calculation is not used. See commit fa37b29.