EC key support

[czocher]

This change add EC key support for simple-mail-forwarder.
Also fixes a little typo.
Solves #50

[huan]

It looks good to me, I'll merge it first.

However, please test it after the new docker image had been published and make sure it works as you expected.

Thank you very much for bring this new feature to the SMF!

[huan]

@czocher I had just fixed a bug introduced from this PR that broke the unit testing.

[czocher]

@huan I'm extremely sorry, haven't realized there were unit tests in this project!

[huan]

Of course we have! ;-)

[petslane]

I think this PR has some problems...

I used letsencrypt to generate fullchain.pem and privkey.pem and used them as smtp.cert and smtp.key in folder that is mouted as /etc/postfix/cert/ in container.

When starting latest/1.1 image, then server starts correctly, no errors in logs, but when configuring smtp server in gmail, then I get error:

Authentication failed. Please check your username/password.
Server returned error: "TLS Negotiation failed, the certificate doesn't match the host., code: 0"

Tested certs with https://www.checktls.com/TestReceiver and cert test fails:

Details about cert fail:

	Cert VALIDATION ERROR(S): self signed certificate
So email is encrypted but the recipient domain is not verified
Cert Hostname DOES NOT VERIFY (mail.example.com != simple-mail-forwarder.com)
So email is encrypted but the host is not verified
cert not revoked by OCSP
Data:
  Version: n/a
  Serial Number: 99:7b:00:b0:9a:d6:ff:88
  Validity:
    Not Before: Sep  7 15:05:26 2020 GMT
    Not After: Sep  5 15:05:26 2030 GMT
  Issuer:
    commonName =	simple-mail-forwarder.com
    countryName =	US
    localityName =	L
    organizationName =	O
    stateOrProvinceName =	Matrix
  Subject:
    commonName =	simple-mail-forwarder.com
    countryName =	US
    localityName =	L
    organizationName =	O
    stateOrProvinceName =	Matrix
  Subject Public Key Info:
    Public Key Algorithm: id-ecPublicKey
    Public Key Bits: (384 bit)
    Modulus:
      49:F6:54:70:FF:D0:4F:75:A9:A3:53:BE:E5:FB:74:44
[...]
      B
    Exponent: n/a
    Key:
      -----BEGIN PUBLIC KEY-----
      MHYwEAYHKoZIzj0CAQYFK4EEACIDYgAEn2VHD/0E91qaNTvuX7dET+ZcuzKLg3ZV
[...]
      -----END PUBLIC KEY-----
  X509v3 Extensions:
Digest: 8aa3ee4ff9e1a701ee47b0c17a291228b03341e5ef7076f01ebfcad1aea369a2
Signature: ecdsa-with-SHA256
  6d:61:69:6c:2d:66:6f:72:77:61:72:64:65:72:2e:63
[...]
Fingerprints:
  sha256$8aa3ee4ff9e1a701ee47b0c17a291228b03341e5ef7076f01ebfcad1aea369a2
[...]
  sha512$pub$163ac8d3d075fe6a910e956a58bbe122fc06f21d662b14ec39bacfcad18c97f370415df2fc14ef6732c2c383f439d9b4a5a7437a4280f1d7f2bbb4bc4c9ea359
Certificate:
  -----BEGIN CERTIFICATE-----
  MIIB4TCCAWcCCQCZewCwmtb/iDAKBggqhkjOPQQDAjBaMQswCQYDVQQGEwJVUzEP
[...]
  E13NIJM=
  -----END CERTIFICATE-----

After starting container, I noticed that smtp.ec.cert and smtp.ec.key files were also generated. I do not know what exactly are they, but I think these are not needed.

Stopped container, deleted smtp.ec.cert and smtp.ec.key files, changed image to 1.0.0 and started container. Now smtp.ec.cert and smtp.ec.key files were not created and all tests pass on https://www.checktls.com/TestReceiver. Also, now gmail allows it to use as SMTP.

[ggilestro]

The following works for me:

cp ../../archive/mydomain/fullchain2.pem smtp.cert
cp ../../archive/mydomain/fullchain2.pem smtp.ec.cert
cp ../../archive/mydomain/privkey2.pem smtp.ec.key
cp ../../archive/mydomain/privkey2.pem smtp.key

Basically, the ec files are just a copy of the non-ec files.

[petslane]

I can confirm that cp smtp.cert smtp.ec.cert && cp smtp.key smtp.ec.key fixes latest image.

If these "ec" cert/key are required when using letsencrypt (or any other certs?), then I think README should explicitly mention that these files must be copied there.