Use ((x + 7) >> 3) instead of (x >> 3) when converting from bits to bytes.
This patch changes our bits-to-bytes conversion logic in the NSS
implementation of `tor_tls_cert_matches_key()` from using (x >> 3) to
((x + 7) >> 3) since DER bit-strings are allowed to contain a number of
bits that is not a multiple of 8.
Additionally, we add a comment on why we cannot use the
`DER_ConvertBitString()` macro from NSS, as we would potentially apply
the bits-to-bytes conversion logic twice, which would lead to an
insignificant amount of bytes being compared in
`SECITEM_ItemsAreEqual()` and thus turn the logic into being a
prefix match instead of a full match.
The `DER_ConvertBitString()` macro is defined in NSS as:
/*
** Macro to convert der decoded bit string into a decoded octet
** string. All it needs to do is fiddle with the length code.
*/
#define DER_ConvertBitString(item) \
{ \
(item)->len = ((item)->len + 7) >> 3; \
}
Thanks to Taylor Yu for spotting this problem.
This patch is part of the fix for TROVE-2020-001.
See: https://bugs.torproject.org/33119