Added generic EC scalar rwnaf encoding for ec_nistp

[dkostic]

Issues:

N/A

Description of changes:

Scalar encoding for scalar multiplication for curves P-384 and P-521
was implemented for each curve separately and with hard-coded
parameters. This commit refactors the encoding function to be
generic and uses removes the hard-coded ones.

Call-outs:

Point out areas that need special attention or support during the review process. Discuss architecture or design changes.

Testing:

How is this change tested (unit tests, fuzz tests, etc.)? Are there any testing steps to be verified by the reviewer?

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.22%. Comparing base (88faaa2) to head (60221d8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1664      +/-   ##
==========================================
- Coverage   78.24%   78.22%   -0.02%     
==========================================
  Files         566      566              
  Lines       95185    95177       -8     
  Branches    13664    13663       -1     
==========================================
- Hits        74476    74454      -22     
- Misses      20116    20127      +11     
- Partials      593      596       +3     

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[justsmth]

Are any extra precautions needed to avoid the C-compiler attempting to optimize this logic?

static int16_t get_bit(const EC_SCALAR *in, size_t i) {
// |in->words| is an array of BN_ULONGs which can be either 8 or 4 bytes long.
#if defined(OPENSSL_64_BIT)
  OPENSSL_STATIC_ASSERT(sizeof(BN_ULONG) == 8, bn_ulong_not_eight_bytes);
  return (value_barrier_u64(in->words[i >> 6]) >> (i & 63)) & 1;
#else
  OPENSSL_STATIC_ASSERT(sizeof(BN_ULONG) == 4, bn_ulong_not_four_bytes);
  return (value_barrier_u32(in->words[i >> 5]) >> (i & 31)) & 1;
#endif
}