Add functions for DER parsing a BIT STRING.

net/der/parse_values.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_DER_PARSE_VALUES_H_
 #define NET_DER_PARSE_VALUES_H_
 
 #include "base/compiler_specific.h"
 #include "net/base/net_export.h"
 #include "net/der/input.h"
@@ skipping 12 matching lines @@
 NET_EXPORT bool ParseBoolRelaxed(const Input& in, bool* out) WARN_UNUSED_RESULT;
 
 // Reads a DER-encoded ASN.1 INTEGER value from |in| and puts the resulting
 // value in |out|. ASN.1 INTEGERs are arbitrary precision; this function is
 // provided as a convenience when the caller knows that the value is unsigned
 // and is between 0 and 2^63-1. This function does not support the full range of
 // uint64_t. This function returns false if the value is too big to fit in a
 // uint64_t, is negative, or if there is an error reading the integer.
 NET_EXPORT bool ParseUint64(const Input& in, uint64_t* out) WARN_UNUSED_RESULT;
 
+// Reads a DER-encoded ASN.1 BIT STRING value from |in| and puts the resulting
+// octet string into |bytes|, and the number of unused bits into |unused_bits|.
+// Returns true on success, otherwise returns false and does not modify the
+// out-parameters.
+//
+// On success it can be assumed that:
+//   * |*unused_bits| < 8
+//   * The final byte of |bytes| has its |*unused_bits| least significant bits
+//     set to 0.
+NET_EXPORT bool ParseBitString(const Input& in,
+                               Input* bytes,
+                               uint8_t* unused_bits) WARN_UNUSED_RESULT;

[nharper, 2015/07/21 23:23:47]

Should unused_bits be a size_t* instead? I realize that it's on the wire as a
uint8_t, but a size_t makes more sense semantically. (This would apply to the
methods in parser.h as well.)

[eroman, 2015/07/22 17:05:23]

I would argue for a uint8_t on the grounds that it is sufficient to represent
the needed range.

A size_t in my mind implies a length/offset in memory, which is not the case
here. It may also be overkill to use a (potentially) 64-bit number to represent
a total of 8 total values :)

+
 struct GeneralizedTime {
   uint16_t year;
   uint8_t month;
   uint8_t day;
   uint8_t hours;
   uint8_t minutes;
   uint8_t seconds;
 };
 
 NET_EXPORT_PRIVATE bool operator<(const GeneralizedTime& lhs,
@@ skipping 17 matching lines @@
 // DER rules - it follows the rules from RFC5280, which does not allow for
 // fractional seconds.
 NET_EXPORT bool ParseGeneralizedTime(const Input& in,
                                      GeneralizedTime* out) WARN_UNUSED_RESULT;
 
 }  // namespace der
 
 }  // namespace net
 
 #endif  // NET_DER_PARSE_VALUES_H_