Ecdsa

New API:
`VerifyingKey.precompute()` supports `lazy` argument to delay precomputation
to the first time the key is used to verify a signature.

Doc fixes:
Documentation for the `VerifyingKey.precompute()` method.

Bug fix:
Make created signatures correct when the hash used is bigger than the curve
order bit size and the curve order is not a multiple of 8 (this affects
only users of custom curves or hashes with output larger than 512 bits).

Performance:
Speed up library load time by calculating the generator point multiplication
tables the first time the points are used, not when they are initialised.

Maintenance:
Include Python 3.9 in CI testing.
Test coverage for the `VerifyingKey.precompute()` method.
Small speed-ups for the test suite.

New features:
Support for reading and writing private keys in PKCS8 format.

New API:
`to_pem` and `to_der` now accept new parameter, `format`, to specify
the format of the encoded files, either the default, legacy `ssleay`, or
the new `pkcs8` to use PKCS8. Note that only unencrypted PKCS8 files are
supported.
Add `allow_truncate` to `verify` in `VerifyingKey`, it defaults to True,
when specified as False, the use of large hashes smaller than curves will be
disallowed (as it was in 0.14.1 and earlier).

Bug fix:
Correctly calculate signatures for private keys equal to n-1.
Make `PointJacobi` and thus `SigningKey` and `VerifyingKey` pickleable.

Doc fixes:
`to_pem` functions return `bytes` not `str`, document them as such.
`from_pem` and `from_pem` in `SigningKey` returns `SigningKey`, document them
as such.

Maintenance:
Ensure that version checks will work with Python 4.
Format the source with black.
Fix uses of `assert_` in the test suite.
Use newer Ubuntu in Travis to test against OpenSSL 1.1.1 (and thus
test the interoperability of ECDH code in Travis).

Bug fixes:
`from curves import *` will now correctly import `BRAINPOOLP256r1` and
`BRAINPOOLP320r1` curves.

New features:
ECDH operations have a public explicit API.
Large hashes are now supported with small curves (e.g. SHA-256 can be used
with NIST192p).
`VerifyingKey` now supports the `precompute()` method to further speed up
signature verification with the given instance of the key.

New API:
`VerifyingKey`, `SigningKey`, `Public_key`, `Private_key` and
`CurveFp` now have `__eq__` methods.
`ecdsa.ecdh` module and `ECDH` class.
`PointJacobi` added.
`VerifyingKey.verify_digest`, `SigningKey.sign_digest` and
`SigningKey.sign_digest_deterministic` methods now accept the `allow_truncate`
argument to enable the use of hashes larger than the curve order.
`VerifyingKey` `from_pem` and `from_der` now accept `hashfunc` parameter
like other `from*` methods.
`VerifyingKey` has the `precompute` method now.
`VerifyingKey.from_public_point` may now not perform validation of public
point when `validate_point=False` argument is passed to the method.
`CurveFp` constructor now accepts the `h` parameter - the cofactor of the
elliptic curve, it's used for the selection of algorithm of public point
verification.

Performance:
`randrange` now will perform much fewer calls to the system random number
generator.
`PointJacobi` introduced and used as the underlying implementation; speeds up
the library by a factor of about 20.
The library has now optional dependencies on `gmpy` and `gmpy2`. When they are
available, the elliptic curve calculations will be about 3 times faster.

Maintenance:
expected minimum version of `six` module (1.9.0) is now specified explicitly
in `setup.py` and tested against.
Significantly faster test suite execution.

Remove the obsolete `six.py` file from wheel

Bug fixes:
Strict checking of DER requirements when parsing SEQUENCE, INTEGER,
OBJECT IDENTIFIER and BITSTRING objects.
DER parsers now consistently raise `UnexpectedDER` exception on malformed DER
encoded byte strings.
Make sure that both malformed and invalid signatures raise `BadSignatureError`.
Ensure that all `SigningKey` and `VerifyingKey` methods that should accept
bytes-like objects actually do accept them (also avoid copying input strings).
Make `SigningKey.sign_digest_deterministic` use default object hashfunc when
none was provided.
`encode_integer` now works for large integers.
Make `encode_oid` and `remove_object` correctly handle OBJECT IDENTIFIERs
with large second sub-identifier and padding in encoded sub-identifiers.

New features:
Deterministic signature methods now accept the `extra_entropy` parameter to
further randomise the selection of `k` (the nonce) for signature, as specified
in RFC6979.
Recovery of the public key from signature is now supported.
Support for SEC1/X9.62 formatted keys, all three encodings are supported:
"uncompressed", "compressed" and "hybrid". Both string, and PEM/DER will
automatically accept them, if the size of the key matches the curve.
Benchmarking application now provides performance numbers that are easier to
compare against OpenSSL.
Support for all Brainpool curves (non-twisted).

New API:
`CurveFp`: `__str__` is now supported.
`SigningKey.sign_deterministic`, `SigningKey.sign_digest_deterministic` and
`generate_k`: extra_entropy parameter was added
`Signature.recover_public_keys` was added
`VerifyingKey.from_public_key_recovery` and
`VerifyingKey.from_public_key_recovery_with_digest` were added
`VerifyingKey.to_string`: `encoding` parameter was added
`VerifyingKey.to_der` and `SigningKey.to_der`: `point_encoding` parameter was
added.
`encode_bitstring`: `unused` parameter was added
`remove_bitstring`: `expect_unused` parameter was added
`SECP256k1` is now part of `curves` `*` import
`Curves`: `__repr__` is now supported
`VerifyingKey`: `__repr__` is now supported

Deprecations:
Python 2.5 is not supported anymore - dead code removal.
`from ecdsa.keys import *` will now import only objects defined in that module.
Trying to decode a malformed point using `VerifyingKey.from_string`
will rise now the `MalformedPointError` exception (that inherits from
`AssertionError` but is not it).
Multiple functions in `numbertheory` are considered deprecated: `phi`,
`carmichael`, `carmichael_of_factorized`, `carmichael_of_ppower`,
`order_mod`, `largest_factor_relatively_prime`, `kinda_order_mod`. They will
now emit `DeprecationWarning` when used. Run the application or test suite
with `-Wd` option or with `PYTHONWARNINGS=default` environment variable to
verify if those methods are not used. They will be removed completely in a
future release.
`encode_bitstring` and `decode_bitstring` expect the number of unused
bits to be passed as an argument now. They will emit `DeprecationWarning`
if they are used in the deprecated way.
modular_exp: will emit `DeprecationWarning`

Hardening:
Deterministic signatures now verify that the signature won't leak private
key through a very unlikely selection of `k` value (the nonce).
Nonce bit size hiding was added (hardening against Minerva attack). Please
note that it DOES NOT make the library secure against side-channel attacks
(timing attacks).

Performance:
The public key in key generation is not verified twice now, making key
generation and private key reading about 33% faster.
Microoptimisation to `inverse_mod` function, increasing performance by about
40% for all operations.

Maintenance:
Extended test coverage to newer python versions.
Fixes to examples in README.md: correct commands, more correct code (now works
on Python 3).
Stopped bundling `six`
Moved sources into `src` subdirectory
Made benchmarking script standalone (runnable either with `tox -e speed`, or
after installation, with `python speed.py`)
Now test coverage reported to coveralls is branch coverage, not line coverage
Autodetection of curves supported by OpenSSL (test suite compatibility with
Fedora OpenSSL package).
More readable error messages (exceptions) in `der` module.
Documentation to `VerifyingKey`, `SigningKey` and signature encoder/decoder
functions added.
Added measuring and verifying condition coverage to Continuous Integration.
Big clean-up of the test suite, use pytest parametrisation and hypothesis
for better test coverage and more precise failure reporting.
Use platform-provided `math.gcd`, when provided.

Fix CVE-2019-14853 - possible DoS caused by malformed signature decoding and
signature malleability.

Also harden key decoding from string and DER encodings.