By using non-DER or invalid encodings outside the signed portion of a
certificate the fingerprint can be changed without breaking the signature.
Although no details of the signed portion of the certificate can be changed
this can cause problems with some applications: e.g. those using the
certificate fingerprint for blacklists.

1. Reject signatures with non zero unused bits.

If the BIT STRING containing the signature has non zero unused bits reject the
signature. All current signature algorithms require zero unused bits.

2. Check certificate algorithm consistency.

Check the AlgorithmIdentifier inside TBS matches the one in the certificate
signature. NB: this will result in signature failure errors for some broken
certificates.

3. Check DSA/ECDSA signatures use DER.

Reencode DSA/ECDSA signatures and compare with the original received signature.
Return an error if there is a mismatch.

This will reject various cases including garbage after signature (thanks to
Antti Karjalainen and Tuomo Untinen from the Codenomicon CROSS program for
discovering this case) and use of BER or invalid ASN.1 INTEGERs (negative or
with leading zeroes).

CVE-2014-8275

(Imported from upstream's 85cfc188c0 and
4c52816d35)

Change-Id: Ic901aea8ea6457df27dc542a11c30464561e322b
Reviewed-on: https://boringssl-review.googlesource.com/2783
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>