parse_values.cc

// 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.

#include "net/der/parse_values.h"

#include <tuple>

#include "base/logging.h"

namespace net {

namespace der {

namespace {

bool ParseBoolInternal(const Input& in, bool* out, bool relaxed) {
  // According to ITU-T X.690 section 8.2, a bool is encoded as a single octet
  // where the octet of all zeroes is FALSE and a non-zero value for the octet
  // is TRUE.
  if (in.Length() != 1)
    return false;
  ByteReader data(in);
  uint8_t byte;
  if (!data.ReadByte(&byte))
    return false;
  if (byte == 0) {
    *out = false;
    return true;
  }
  // ITU-T X.690 section 11.1 specifies that for DER, the TRUE value must be
  // encoded as an octet of all ones.
  if (byte == 0xff || relaxed) {
    *out = true;
    return true;
  }
  return false;
}

// Reads a positive decimal number with |digits| digits and stores it in
// |*out|. This function does not check that the type of |*out| is large
// enough to hold 10^digits - 1; the caller must choose an appropriate type
// based on the number of digits they wish to parse.
template <typename UINT>
bool DecimalStringToUint(ByteReader& in, size_t digits, UINT* out) {
  UINT value = 0;
  for (size_t i = 0; i < digits; ++i) {
    uint8_t digit;
    if (!in.ReadByte(&digit)) {
      return false;
    }
    if (digit < '0' || digit > '9') {
      return false;
    }
    value = (value * 10) + (digit - '0');
  }
  *out = value;
  return true;
}

// Checks that the values in a GeneralizedTime struct are valid. This involves
// checking that the year is 4 digits, the month is between 1 and 12, the day
// is a day that exists in that month (following current leap year rules),
// hours are between 0 and 23, minutes between 0 and 59, and seconds between
// 0 and 60 (to allow for leap seconds; no validation is done that a leap
// second is on a day that could be a leap second).
bool ValidateGeneralizedTime(const GeneralizedTime& time) {
  if (time.month < 1 || time.month > 12)
    return false;
  if (time.day < 1)
    return false;
  if (time.hours < 0 || time.hours > 23)
    return false;
  if (time.minutes < 0 || time.minutes > 59)
    return false;
  // Leap seconds are allowed.
  if (time.seconds < 0 || time.seconds > 60)
    return false;

  // validate upper bound for day of month
  switch (time.month) {
    case 4:
    case 6:
    case 9:
    case 11:
      if (time.day > 30)
        return false;
      break;
    case 1:
    case 3:
    case 5:
    case 7:
    case 8:
    case 10:
    case 12:
      if (time.day > 31)
        return false;
      break;
    case 2:
      if (time.year % 4 == 0 &&
          (time.year % 100 != 0 || time.year % 400 == 0)) {
        if (time.day > 29)
          return false;
      } else {
        if (time.day > 28)
          return false;
      }
      break;
    default:
      NOTREACHED();
      return false;
  }
  return true;
}

// Returns the number of bytes of numeric precision in a DER encoded INTEGER
// value. |in| must be a valid DER encoding of an INTEGER for this to work.
//
// Normally the precision of the number is exactly in.Length(). However when
// encoding positive numbers using DER it is possible to have a leading zero
// (to prevent number from being interpreted as negative).
//
// For instance a 160-bit positive number might take 21 bytes to encode. This
// function will return 20 in such a case.
size_t GetUnsignedIntegerLength(const Input& in) {
  der::ByteReader reader(in);
  uint8_t first_byte;
  if (!reader.ReadByte(&first_byte))
    return 0;  // Not valid DER  as |in| was empty.

  if (first_byte == 0 && in.Length() > 1)
    return in.Length() - 1;
  return in.Length();
}

}  // namespace

bool ParseBool(const Input& in, bool* out) {
  return ParseBoolInternal(in, out, false /* relaxed */);
}

// BER interprets any non-zero value as true, while DER requires a bool to
// have either all bits zero (false) or all bits one (true). To support
// malformed certs, we recognized the BER encoding instead of failing to
// parse.
bool ParseBoolRelaxed(const Input& in, bool* out) {
  return ParseBoolInternal(in, out, true /* relaxed */);
}

// ITU-T X.690 section 8.3.2 specifies that an integer value must be encoded
// in the smallest number of octets. If the encoding consists of more than
// one octet, then the bits of the first octet and the most significant bit
// of the second octet must not be all zeroes or all ones.
bool IsValidInteger(const Input& in, bool* negative) {
  der::ByteReader reader(in);
  uint8_t first_byte;

  if (!reader.ReadByte(&first_byte))
    return false;  // Empty inputs are not allowed.

  uint8_t second_byte;
  if (reader.ReadByte(&second_byte)) {
    if ((first_byte == 0x00 || first_byte == 0xFF) &&
        (first_byte & 0x80) == (second_byte & 0x80)) {
      // Not a minimal encoding.
      return false;
    }
  }

  *negative = (first_byte & 0x80) == 0x80;
  return true;
}

bool ParseUint64(const Input& in, uint64_t* out) {
  // Reject non-minimally encoded numbers and negative numbers.
  bool negative;
  if (!IsValidInteger(in, &negative) || negative)
    return false;

  // Reject (non-negative) integers whose value would overflow the output type.
  if (GetUnsignedIntegerLength(in) > sizeof(*out))
    return false;

  ByteReader reader(in);
  uint8_t data;
  uint64_t value = 0;

  while (reader.ReadByte(&data)) {
    value <<= 8;
    value |= data;
  }
  *out = value;
  return true;
}

bool ParseUint8(const Input& in, uint8_t* out) {
  // TODO(eroman): Implement this more directly.
  uint64_t value;
  if (!ParseUint64(in, &value))
    return false;

  if (value > 0xFF)
    return false;

  *out = static_cast<uint8_t>(value);
  return true;
}

BitString::BitString(const Input& bytes, uint8_t unused_bits)
    : bytes_(bytes), unused_bits_(unused_bits) {
  DCHECK_LT(unused_bits, 8);
  DCHECK(unused_bits == 0 || bytes.Length() != 0);
  // The unused bits must be zero.
  DCHECK(bytes.Length() == 0 ||
         (bytes.UnsafeData()[bytes.Length() - 1] & ((1u << unused_bits) - 1)) ==
             0);
}

bool BitString::AssertsBit(size_t bit_index) const {
  // Index of the byte that contains the bit.
  size_t byte_index = bit_index / 8;

  // If the bit is outside of the bitstring, by definition it is not
  // asserted.
  if (byte_index >= bytes_.Length())
    return false;

  // Within a byte, bits are ordered from most significant to least significant.
  // Convert |bit_index| to an index within the |byte_index| byte, measured from
  // its least significant bit.
  uint8_t bit_index_in_byte = 7 - (bit_index - byte_index * 8);

  // BIT STRING parsing already guarantees that unused bits in a byte are zero
  // (otherwise it wouldn't be valid DER). Therefore it isn't necessary to check
  // |unused_bits_|
  uint8_t byte = bytes_.UnsafeData()[byte_index];
  return 0 != (byte & (1 << bit_index_in_byte));
}

bool ParseBitString(const Input& in, BitString* out) {
  ByteReader reader(in);

  // From ITU-T X.690, section 8.6.2.2 (applies to BER, CER, DER):
  //
  // The initial octet shall encode, as an unsigned binary integer with
  // bit 1 as the least significant bit, the number of unused bits in the final
  // subsequent octet. The number shall be in the range zero to seven.
  uint8_t unused_bits;
  if (!reader.ReadByte(&unused_bits))
    return false;
  if (unused_bits > 7)
    return false;

  Input bytes;
  if (!reader.ReadBytes(reader.BytesLeft(), &bytes))
    return false;  // Not reachable.

  // Ensure that unused bits in the last byte are set to 0.
  if (unused_bits > 0) {
    // From ITU-T X.690, section 8.6.2.3 (applies to BER, CER, DER):
    //
    // If the bitstring is empty, there shall be no subsequent octets,
    // and the initial octet shall be zero.
    if (bytes.Length() == 0)
      return false;
    uint8_t last_byte = bytes.UnsafeData()[bytes.Length() - 1];

    // From ITU-T X.690, section 11.2.1 (applies to CER and DER, but not BER):
    //
    // Each unused bit in the final octet of the encoding of a bit string value
    // shall be set to zero.
    uint8_t mask = 0xFF >> (8 - unused_bits);
    if ((mask & last_byte) != 0)
      return false;
  }

  *out = BitString(bytes, unused_bits);
  return true;
}

bool operator<(const GeneralizedTime& lhs, const GeneralizedTime& rhs) {
  return std::tie(lhs.year, lhs.month, lhs.day, lhs.hours, lhs.minutes,
                  lhs.seconds) < std::tie(rhs.year, rhs.month, rhs.day,
                                          rhs.hours, rhs.minutes, rhs.seconds);
}

bool operator>(const GeneralizedTime& lhs, const GeneralizedTime& rhs) {
  return rhs < lhs;
}

bool operator<=(const GeneralizedTime& lhs, const GeneralizedTime& rhs) {
  return !(lhs > rhs);
}

bool operator>=(const GeneralizedTime& lhs, const GeneralizedTime& rhs) {
  return !(lhs < rhs);
}

// A UTC Time in DER encoding should be YYMMDDHHMMSSZ, but some CAs encode
// the time following BER rules, which allows for YYMMDDHHMMZ. If the length
// is 11, assume it's YYMMDDHHMMZ, and in converting it to a GeneralizedTime,
// add in the seconds (set to 0).
bool ParseUTCTimeRelaxed(const Input& in, GeneralizedTime* value) {
  ByteReader reader(in);
  GeneralizedTime time;
  if (!DecimalStringToUint(reader, 2, &time.year) ||
      !DecimalStringToUint(reader, 2, &time.month) ||
      !DecimalStringToUint(reader, 2, &time.day) ||
      !DecimalStringToUint(reader, 2, &time.hours) ||
      !DecimalStringToUint(reader, 2, &time.minutes)) {
    return false;
  }

  // Try to read the 'Z' at the end. If we read something else, then for it to
  // be valid the next bytes should be seconds (and then followed by 'Z').
  uint8_t zulu;
  ByteReader zulu_reader = reader;
  if (!zulu_reader.ReadByte(&zulu))
    return false;
  if (zulu == 'Z' && !zulu_reader.HasMore()) {
    time.seconds = 0;
    *value = time;
  } else {
    if (!DecimalStringToUint(reader, 2, &time.seconds))
      return false;
    if (!reader.ReadByte(&zulu) || zulu != 'Z' || reader.HasMore())
      return false;
  }

  if (time.year < 50) {
    time.year += 2000;
  } else {
    time.year += 1900;
  }
  if (!ValidateGeneralizedTime(time))
    return false;
  *value = time;
  return true;
}

bool ParseUTCTime(const Input& in, GeneralizedTime* value) {
  ByteReader reader(in);
  GeneralizedTime time;
  if (!DecimalStringToUint(reader, 2, &time.year) ||
      !DecimalStringToUint(reader, 2, &time.month) ||
      !DecimalStringToUint(reader, 2, &time.day) ||
      !DecimalStringToUint(reader, 2, &time.hours) ||
      !DecimalStringToUint(reader, 2, &time.minutes) ||
      !DecimalStringToUint(reader, 2, &time.seconds)) {
    return false;
  }
  uint8_t zulu;
  if (!reader.ReadByte(&zulu) || zulu != 'Z' || reader.HasMore())
    return false;
  if (time.year < 50) {
    time.year += 2000;
  } else {
    time.year += 1900;
  }
  if (!ValidateGeneralizedTime(time))
    return false;
  *value = time;
  return true;
}

bool ParseGeneralizedTime(const Input& in, GeneralizedTime* value) {
  ByteReader reader(in);
  GeneralizedTime time;
  if (!DecimalStringToUint(reader, 4, &time.year) ||
      !DecimalStringToUint(reader, 2, &time.month) ||
      !DecimalStringToUint(reader, 2, &time.day) ||
      !DecimalStringToUint(reader, 2, &time.hours) ||
      !DecimalStringToUint(reader, 2, &time.minutes) ||
      !DecimalStringToUint(reader, 2, &time.seconds)) {
    return false;
  }
  uint8_t zulu;
  if (!reader.ReadByte(&zulu) || zulu != 'Z' || reader.HasMore())
    return false;
  if (!ValidateGeneralizedTime(time))
    return false;
  *value = time;
  return true;
}

}  // namespace der

}  // namespace net