Fix AES IV reuse - drop support for CTR and ECB - address CVE-2017-1000246

[c00kiemon5ter]

Discussion on #417

Always generate a random IV for AES operations.

Quoting @obi1kenobi:

Initialization vector reuse like this is a security concern, since it leaks
information about the encrypted data to attackers, regardless of the
encryption mode used.
Instead of relying on a fixed, randomly-generated IV, it would be better to
randomly-generate a new IV for every encryption operation.

• Drop AESCipher CTR and ECB support
• Fix AESCipher IV reuse
• Address CVE-2017-1000246

Fixes #417

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[codecov-io]

Codecov Report

Merging #519 into master will increase coverage by 0.23%.
The diff coverage is 77.86%.

@@            Coverage Diff             @@
##           master     #519      +/-   ##
==========================================
+ Coverage   65.19%   65.42%   +0.23%     
==========================================
  Files         100      103       +3     
  Lines       25660    25681      +21     
==========================================
+ Hits        16729    16802      +73     
+ Misses       8931     8879      -52

Impacted Files	Coverage Δ
src/saml2/authn.py	0% <0%> (ø)	⬆️
src/saml2/cryptography/pki.py	100% <100%> (ø)
src/saml2/sigver.py	70.57% <100%> (-0.24%)	⬇️
src/saml2/aes.py	100% <100%> (+100%)	⬆️
src/saml2/cert.py	91.08% <100%> (-3.44%)	⬇️
src/saml2/server.py	72.47% <75%> (-0.13%)	⬇️
src/saml2/cryptography/symmetric.py	78.46% <78.46%> (ø)
src/saml2/cryptography/asymmetric.py	90.47% <90.47%> (ø)
src/saml2/saml.py	92.08% <0%> (-0.21%)	⬇️
... and 10 more

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