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According to RFC4055 a NULL parameter MUST be present in the signature
algorithm. This patch adds the NULL value to the Signature Algorithm
parameters in the signingParamsForPrivateKey function for RSA based keys.

Section 2.1 states:

"There are two possible encodings for the AlgorithmIdentifier
parameters field associated with these object identifiers.  The two
alternatives arise from the loss of the OPTIONAL associated with the
algorithm identifier parameters when the 1988 syntax for
AlgorithmIdentifier was translated into the 1997 syntax.  Later the
OPTIONAL was recovered via a defect report, but by then many people
thought that algorithm parameters were mandatory.  Because of this
history some implementations encode parameters as a NULL element
while others omit them entirely.  The correct encoding is to omit the
parameters field; however, when RSASSA-PSS and RSAES-OAEP were
defined, it was done using the NULL parameters rather than absent
parameters.

All implementations MUST accept both NULL and absent parameters as
legal and equivalent encodings.

To be clear, the following algorithm identifiers are used when a NULL
parameter MUST be present:

   sha1Identifier AlgorithmIdentifier ::= { id-sha1, NULL }
   sha224Identifier AlgorithmIdentifier ::= { id-sha224, NULL }
   sha256Identifier AlgorithmIdentifier ::= { id-sha256, NULL }
   sha384Identifier AlgorithmIdentifier ::= { id-sha384, NULL }
   sha512Identifier AlgorithmIdentifier ::= { id-sha512, NULL }"

This CL has been discussed at: http://golang.org/cl/177610043

Change-Id: Ic782161938b287f34f64ef5eb1826f0d936f2f71
Reviewed-on: https://go-review.googlesource.com/2256

Reviewed-by: Adam Langley <agl@golang.org>