As of https://github.com/btcsuite/btcd/pull/1193, decompressPoint now
validates that the point is on the curve. The x and y cooordinates are
also implicitly <= P, since the modular reduction is applied to both
before the method returns. The checks are moved so that they are still
applied when parsing an uncompressed pubkey, as the checks are not
redundant in that path.
This commit adds a new function to btcec: IsCompressedPubKey. This
function returns true iff the passed serialized public key is encoded
in compressed format.
Profiles discovered that lookups into the signature cache included an
expensive comparison to the stored `sigInfo` struct. This lookup had the
potential to be more expensive than directly verifying the signature
itself!
In addition, evictions were rather expensive because they involved
reading from /dev/urandom, or equivalent, for each eviction once the
signature cache was full as well as potentially iterating over every
item in the cache in the worst-case.
To remedy this poor performance several changes have been made:
* Change the lookup key to the fixed sized 32-byte signature hash
* Perform a full equality check only if there is a cache hit which
results in a significant speed up for both insertions and existence
checks
* Override entries in the case of a colliding hash on insert Add an
* .IsEqual() method to the Signature and PublicKey types in the
btcec package to facilitate easy equivalence testing
* Allocate the signature cache map with the max number of entries in
order to avoid unnecessary map re-sizes/allocations
* Optimize evictions from the signature cache Delete the first entry
* seen which is safe from manipulation due to
the pre image resistance of the hash function
* Double the default maximum number of entries within the signature
cache due to the reduction in the size of a cache entry
* With this eviction scheme, removals are effectively O(1)
Fixes#575.
