OpenSSL DH backend implementation [Second attempt] (#2914)

* Start of OpenSSL DH backend implementation

* Supporting DH in MultiBackend

* DHBackend has dh_parameters_supported method

* Removed DHParametersWithNumbers and DHPrivateKeyWithNumbers from documentation

* Removed ExchangeContext. exchange is a method of DHPrivateKeyWithSerialization

* PEP8 fixes

* Fixed TestDH.test_bad_tls_exchange

* Fixed generate_private_key reference in dh documentation

* test DH multibackend support

* testing DH coversion to serialized

* Validating that we receive serialized class in test_generate_dh

* Testing DH exchange symmetric key padding

* struct DH is now opaqued

* PEP8 fixes

* Testing load_dh_private_numbers throws ValueError when DH_check fails

* Using openssl_assert

* Passing keywords arguments in DH key exchange example

* test_dh::test_bad_tls_exchange now uses pre calculated parameters

* TestDH - Add test that the computed secret is equivalent to the definition by comparing with secret computed in pure python

* Add missing generator parameter to DHBackend interface docs.

* Include parameter type in DHBackend abc docs.

* Add docs for dh.generate_parameters function

* Remove the dh Numbers section, and move the DHNumbers class docs to where they are first used.

* Add note of big endian byte packing to DH exchange method.

* DH documentation updates.

Add single sentence overview with wikipedia link.
Add paragraph on assembling using Numbers objects.
Add link to backend interface docs.
First section was all indented, I think by mistake.

* Add exchange method to DHPrivateKey abstract base class.

* Small tweaks to DH documentation - remove Provider.

* Add endian to dictionary

* Use utils.int_from_bytes in test_tls_exchange_algorithm

* Removed duplicate line

* Change dh.rst exchange algorithm from doctest to code-block

The example in the Diffie-Hellman exhange algorithm is using
2048 bits key. Generating the parameters of 2048 takes long
time. This caused the automated tests to fail. In order to
pass the tests we change the example to code-block so it
will not run in the doc tests.

* Fix dh docs

* Document the generator in DHBackend relevant methods

* Fix dh tests

* use DHparams_dup

* Fix key type to unsigned char as expected by DH_compute_key

* Validate that DH generator is 2 or 5

* test dh exchange using botan vectors

* group all numbers classes

* Simplify _DHPrivateKey

* Rename test with serialized to numbers

* Move bad exchange params to external vector file

* update exchange versionadded to 1.7

* Make key_size bit accurate

* Change botan link

* Added CHANGELOG entry

2 files changed, 59 insertions, 0 deletions

diff --git a/vectors/cryptography_vectors/asymmetric/DH/bad_exchange.txt b/vectors/cryptography_vectors/asymmetric/DH/bad_exchange.txt
new file mode 100644
index 00000000..c511ce3d
--- /dev/null
+++ b/vectors/cryptography_vectors/asymmetric/DH/bad_exchange.txt
@@ -0,0 +1,22 @@
+# These are pairs of DH vectors that an exchange between them will result in an error.
+
+# This pair of vector will result in ValueError as the exchange. That is because Y2 > P1.
+COUNT = 0
+G = 2
+P1 = 6982449264326893170076800712455263291469349829090516222210222668518102590295128975951345863513171140301173103075603894877550567268976511279764959149528443
+X1 = 4801532380147672323646734353223620022391392516566390315327913029035641903575640872665137366552076226566411004476255204566867879551828094289388660251782046
+Y1 = 821604160094281686420547900781786073619010872682627891698497892534032059614878030140061466421083272893526509531465029773964398063523169243002396742005311
+P2 = 11080956217961768232353507616608271011094948914260968400815080208586343913901873894476784420098894723332990692686674731674801476861138954727825965908678667
+X2 = 5711420683916244371478141649351505929365347129399136675372566081882611630063783016816795794055507751345814825076698378102524000583805069503046990368030659
+Y2 = 10250982112542385918017381009520473894895926030138509814084625750094204966649199270766083680909922864972425983868488727218698464441106269212018923589462134
+
+# This pair of vector exchange will result in two different symmetric keys.
+# That is because they are not generated from the same parameters.
+COUNT = 1
+G = 2
+P1 = 12900013872746249964533588299424217034480441280987975922107615584280973803458959936181179967371408335732783334903224087948303561132833270378314994720245923
+X1 = 6071003937964305313717247598692016437334013449238587402845410045585785082471492220556199473955398532734458121110746407155944589178844230676809305984951446
+Y1 = 7991113599078333875847794507844575617505022450446398197075320323186801365513151014983321390753605753306200081479128423955529753566986512451925943417370573
+P2 = 8230910504630293046781610423436023785570058813983426764127680163386079097481556242295408030406562540441574901920011043362099679138268556410914150106172603
+X2 = 5094233318592725701435923671489794134941722294242428364605342252104678038235605245715640898911801472906659807096135548788171923289152060442445002898139883
+Y2 = 3153667608586790035405826466812972271982421015353174890600325108812327867424805742934205854052751840807361048259220540363086632109209018127565889223022306
diff --git a/vectors/cryptography_vectors/asymmetric/DH/vec.txt b/vectors/cryptography_vectors/asymmetric/DH/vec.txt
new file mode 100644
index 00000000..4ee8e903
--- /dev/null
+++ b/vectors/cryptography_vectors/asymmetric/DH/vec.txt
@@ -0,0 +1,37 @@
+# This vectors are taken from https://raw.githubusercontent.com/randombit/botan/master/src/tests/data/pubkey/dh.vec.
+# converted to fit NIST format.
+
+COUNT = 0
+P = 58458002095536094658683755258523362961421200751439456159756164191494576279467
+G = 2
+X = 46205663093589612668746163860870963912226379131190812163519349848291472898748
+Y = 26821400572298074358375073922715498403273583367617402781946773132088456286733
+K = 5D9A64F9E54B011381308CF462C207CB0DB7630EAB026E06E5B893041207DBD8
+
+COUNT = 1
+P = 7080941971697125115953429172307253449997092375902849066092516886443770423993013931939664664691599157495586618571486777257251370592538466283944520569755243
+G = 2
+X = 558545918073450953822828294657166871085534488182588754070944297274702844791239790350137385125663944726718987047123606999344398260535747142299581818644894
+Y = 5414004572904348025296832268296927639985617596751048805496391472266617693297043686410755975231136915922983393130810278045355076974349430254905203521469281
+K = 65F79BCC47862E02DDE775B8FC7B1D0CBA094B753E502D49A4468687681F178CEE98016210E136E80255FC8FDECC4D38D91EBC82C94B17652BDC7569AC383F39
+
+COUNT = 2
+P = 13136945886549419892672364204240698856868140920222753719410945199521492957942501101571382423757107983263697942121022398653040312294171805755003420937008819
+G = 2
+X = 7234683127321298509925294581253140833264699266098978220501763131351358687985896274862103003239800546011982186510097828911316488529426006616714526458066271
+Y = 2279687933022692843565305738851942847753122181957871349712298819158246913703861338250576252594546190393887657989733709994119777023140405518896999362296894
+K = 290BF0265BAAD40A5853BAF3CC18B315982FF282C82D7332DA6A8826E789330D875F29F57D9A622DB7DDF156CF4440EDB8455BC32859FA2A178D42528FA64136
+
+COUNT = 3
+P = 7837101158566379575244981793821373232935960937791570311529087011989828139360157566031864344629391921990623396775060696298931110532512473525227009216442663
+G = 5
+X = 2271743810121848480269660476736588602223609425319496369070326649477151605521404690612976189919437998885287608828699462183553427369647044489290359586354073
+Y = 5553605535786829473948867606958960228544449485744361474243970893441441245322015477973193059017330938119851955120496598438033407790780406867070511553809237
+K = 4ED3B2ED6B3E8446D8E1C3D2BCA00FB939464874BB6750BDE5F5DACFE9F819597E96F6B1DBA27C785553383AE87188019734A160B6BA5396760B88EC1F2A0E92
+
+COUNT = 4
+P = 94601366105683233785857165617633883930888659900865798821537131281240413334589084848277586190796291421413056657355624982032535320546697702506255545831695999765620345337665948695835489391652130862575428732385880123143529399201847515289798598538556585982541405034303884158851145992650670977252891826481077576283
+G = 2
+X = 23484534159498465809072519330053257189446469650913804186985945596423260246286292600333967669376208639922768407894768298151128017428601886623219847603288857771541399565808620552396892534905511436715248032341612931260211253869255846963169818664325386727815821240629227196043893200670349600641190073530521475769
+Y = 5432307605192951130143195594341230106472453193740817014833299044466758796406314885816769577417181043800208414512454882219387455875605089197474590288847925182956692434090736024405561167965732783350054200615740891235826401928590081691818434389717991597889828540215480852496077254876423767132564403274562077989
+K = 2B6D9504C1D7ACAD9652CF79A6A0630EA9D19C197E908E992BC318BE2867FAE3C1AD5BE83E6E4AA1CBE0347774038F20ACD790DEA82B2A5862BF21FA4A1AB464AE985CA8AC4E5076AD0843E144C1305759BA047446A3A7F2426BE0C724F269009B54447B6A970876E5E4C613356805014D6060039C081AFB046863CA9BE9C848