[feat] add fallback on trust sources for ssl verification

With the merge of platformTrust in twisted, the situation for cert chain
verification in linux improved a lot.

This patch implements fallbacks to do the following:

- Try to use whatever trust sources are found in the system. This means
that if ca-certificates is installed, pyopenssl will have a valid set of
root certificates and verification will likely work (twisted uses
platformTrust for this).

- If that fails, try to use certifi. We could/should depend on that from
now on, *but* it's not packaged before stretch.

- So, I'm not deprecating its usage right now, but this one should be
the last cacert.pem bundle that we ship with leap.common.

- If the cacert.pem from leap.common fails to be found, well, there's
nothing you can do. Your TOFU attempt with a cert coming from the
CArtel will fail.

Most of this MR should be sent as a patch upstream, see https://twistedmatrix.com/trac/ticket/6934
Also related: https://twistedmatrix.com/trac/ticket/9209

I think proper testing will depend on merging https://github.com/pyca/pyopenssl/pull/473

- Resolves: #8958
- Release: 0.6.0

1 files changed, 116 insertions, 0 deletions

diff --git a/src/leap/common/EFFchain.pem b/src/leap/common/EFFchain.pem
new file mode 100644
index 0000000..15a79d8
--- /dev/null
+++ b/src/leap/common/EFFchain.pem
@@ -0,0 +1,116 @@
+CONNECTED(00000003)
+---
+Certificate chain
+ 0 s:/CN=eff.org
+   i:/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
+-----BEGIN CERTIFICATE-----
+MIIGVTCCBT2gAwIBAgISAx9kTOWisGpqooJ4k5cVH4SGMA0GCSqGSIb3DQEBCwUA
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+z+9Sior3hfcLNdGZUZWpd7GQMKKoEbDPm8GubFTvcPfivu8I9Lc5428=
+-----END CERTIFICATE-----
+ 1 s:/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
+   i:/O=Digital Signature Trust Co./CN=DST Root CA X3
+-----BEGIN CERTIFICATE-----
+MIIEkjCCA3qgAwIBAgIQCgFBQgAAAVOFc2oLheynCDANBgkqhkiG9w0BAQsFADA/
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+KOqkqm57TH2H3eDJAkSnh6/DNFu0Qg==
+-----END CERTIFICATE-----
+---
+Server certificate
+subject=/CN=eff.org
+issuer=/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
+---
+No client certificate CA names sent
+Peer signing digest: SHA512
+Server Temp Key: ECDH, P-256, 256 bits
+---
+SSL handshake has read 3728 bytes and written 302 bytes
+Verification: OK
+---
+New, TLSv1.2, Cipher is ECDHE-RSA-AES256-GCM-SHA384
+Server public key is 4096 bit
+Secure Renegotiation IS supported
+Compression: NONE
+Expansion: NONE
+No ALPN negotiated
+SSL-Session:
+    Protocol  : TLSv1.2
+    Cipher    : ECDHE-RSA-AES256-GCM-SHA384
+    Session-ID: D448EDCC480EF1C06FE70E6D8B4B868431F613E1D4308FC9A40A0DAA3460C06B
+    Session-ID-ctx: 
+    Master-Key: 59C06531A77DCF1603ABC42B700311866C7D85A3154D9F733F1E671BC59C0287C81CBD9FD39516871CD434BD939379A0
+    PSK identity: None
+    PSK identity hint: None
+    SRP username: None
+    TLS session ticket lifetime hint: 600 (seconds)
+    TLS session ticket:
+    0000 - f8 47 e4 94 45 bd d3 df-61 76 8f b5 98 a1 b8 b5   .G..E...av......
+    0010 - 56 6d 1f 59 43 5e 5e c5-2a 90 66 66 3e 6e b4 45   Vm.YC^^.*.ff>n.E
+    0020 - 7d 15 76 d9 cc 6a d4 d5-db 26 bf d5 e5 4a 5e 9a   }.v..j...&...J^.
+    0030 - 96 ce 88 00 23 64 36 6e-1a 26 7b 94 6b da c4 95   ....#d6n.&{.k...
+    0040 - 96 49 1e 96 5e 34 35 3c-38 e1 0c 3e 41 57 64 1f   .I..^45<8..>AWd.
+    0050 - b0 fe 09 0b 3b f6 bf 8a-f5 6c 54 6e bc 63 35 4e   ....;....lTn.c5N
+    0060 - 3a 37 27 64 8f a4 0c 5b-4f 6b f3 17 a1 9a 12 be   :7'd...[Ok......
+    0070 - 4f 75 8d aa 40 01 58 83-be db 07 77 38 8c 04 ff   Ou..@.X....w8...
+    0080 - f2 58 f0 36 ba dc 74 39-1f 14 16 57 8a ac d2 e9   .X.6..t9...W....
+    0090 - 98 4d 30 2f ce 5e 5b d8-1e 66 96 bd f8 a9 eb 04   .M0/.^[..f......
+    00a0 - 35 14 c0 ad 84 7e 93 28-10 22 8c 7e 50 d2 ca ca   5....~.(.".~P...
+
+    Start Time: 1499775511
+    Timeout   : 7200 (sec)
+    Verify return code: 0 (ok)
+    Extended master secret: no
+---