strings/internal/escaping.cc

// Copyright 2020 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "absl/strings/internal/escaping.h"

#include "absl/base/internal/endian.h"
#include "absl/base/internal/raw_logging.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace strings_internal {

// The two strings below provide maps from normal 6-bit characters to their
// base64-escaped equivalent.
// For the inverse case, see kUn(WebSafe)Base64 in the external
// escaping.cc.
ABSL_CONST_INIT const char kBase64Chars[] =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

ABSL_CONST_INIT const char kWebSafeBase64Chars[] =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";


size_t CalculateBase64EscapedLenInternal(size_t input_len, bool do_padding) {
  // Base64 encodes three bytes of input at a time. If the input is not
  // divisible by three, we pad as appropriate.
  //
  // Base64 encodes each three bytes of input into four bytes of output.
  size_t len = (input_len / 3) * 4;

  // Since all base 64 input is an integral number of octets, only the following
  // cases can arise:
  if (input_len % 3 == 0) {
    // (from https://tools.ietf.org/html/rfc3548)
    // (1) the final quantum of encoding input is an integral multiple of 24
    // bits; here, the final unit of encoded output will be an integral
    // multiple of 4 characters with no "=" padding,
  } else if (input_len % 3 == 1) {
    // (from https://tools.ietf.org/html/rfc3548)
    // (2) the final quantum of encoding input is exactly 8 bits; here, the
    // final unit of encoded output will be two characters followed by two
    // "=" padding characters, or
    len += 2;
    if (do_padding) {
      len += 2;
    }
  } else {  // (input_len % 3 == 2)
    // (from https://tools.ietf.org/html/rfc3548)
    // (3) the final quantum of encoding input is exactly 16 bits; here, the
    // final unit of encoded output will be three characters followed by one
    // "=" padding character.
    len += 3;
    if (do_padding) {
      len += 1;
    }
  }

  assert(len >= input_len);  // make sure we didn't overflow
  return len;
}

// ----------------------------------------------------------------------
//   Take the input in groups of 4 characters and turn each
//   character into a code 0 to 63 thus:
//           A-Z map to 0 to 25
//           a-z map to 26 to 51
//           0-9 map to 52 to 61
//           +(- for WebSafe) maps to 62
//           /(_ for WebSafe) maps to 63
//   There will be four numbers, all less than 64 which can be represented
//   by a 6 digit binary number (aaaaaa, bbbbbb, cccccc, dddddd respectively).
//   Arrange the 6 digit binary numbers into three bytes as such:
//   aaaaaabb bbbbcccc ccdddddd
//   Equals signs (one or two) are used at the end of the encoded block to
//   indicate that the text was not an integer multiple of three bytes long.
// ----------------------------------------------------------------------
size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
                            size_t szdest, const char* base64,
                            bool do_padding) {
  static const char kPad64 = '=';

  if (szsrc * 4 > szdest * 3) return 0;

  char* cur_dest = dest;
  const unsigned char* cur_src = src;

  char* const limit_dest = dest + szdest;
  const unsigned char* const limit_src = src + szsrc;

  // (from https://tools.ietf.org/html/rfc3548)
  // Special processing is performed if fewer than 24 bits are available
  // at the end of the data being encoded.  A full encoding quantum is
  // always completed at the end of a quantity.  When fewer than 24 input
  // bits are available in an input group, zero bits are added (on the
  // right) to form an integral number of 6-bit groups.
  //
  // If do_padding is true, padding at the end of the data is performed. This
  // output padding uses the '=' character.

  // Three bytes of data encodes to four characters of cyphertext.
  // So we can pump through three-byte chunks atomically.
  if (szsrc >= 3) {                    // "limit_src - 3" is UB if szsrc < 3.
    while (cur_src < limit_src - 3) {  // While we have >= 32 bits.
      uint32_t in = absl::big_endian::Load32(cur_src) >> 8;

      cur_dest[0] = base64[in >> 18];
      in &= 0x3FFFF;
      cur_dest[1] = base64[in >> 12];
      in &= 0xFFF;
      cur_dest[2] = base64[in >> 6];
      in &= 0x3F;
      cur_dest[3] = base64[in];

      cur_dest += 4;
      cur_src += 3;
    }
  }
  // To save time, we didn't update szdest or szsrc in the loop.  So do it now.
  szdest = static_cast<size_t>(limit_dest - cur_dest);
  szsrc = static_cast<size_t>(limit_src - cur_src);

  /* now deal with the tail (<=3 bytes) */
  switch (szsrc) {
    case 0:
      // Nothing left; nothing more to do.
      break;
    case 1: {
      // One byte left: this encodes to two characters, and (optionally)
      // two pad characters to round out the four-character cypherblock.
      if (szdest < 2) return 0;
      uint32_t in = cur_src[0];
      cur_dest[0] = base64[in >> 2];
      in &= 0x3;
      cur_dest[1] = base64[in << 4];
      cur_dest += 2;
      szdest -= 2;
      if (do_padding) {
        if (szdest < 2) return 0;
        cur_dest[0] = kPad64;
        cur_dest[1] = kPad64;
        cur_dest += 2;
        szdest -= 2;
      }
      break;
    }
    case 2: {
      // Two bytes left: this encodes to three characters, and (optionally)
      // one pad character to round out the four-character cypherblock.
      if (szdest < 3) return 0;
      uint32_t in = absl::big_endian::Load16(cur_src);
      cur_dest[0] = base64[in >> 10];
      in &= 0x3FF;
      cur_dest[1] = base64[in >> 4];
      in &= 0x00F;
      cur_dest[2] = base64[in << 2];
      cur_dest += 3;
      szdest -= 3;
      if (do_padding) {
        if (szdest < 1) return 0;
        cur_dest[0] = kPad64;
        cur_dest += 1;
        szdest -= 1;
      }
      break;
    }
    case 3: {
      // Three bytes left: same as in the big loop above.  We can't do this in
      // the loop because the loop above always reads 4 bytes, and the fourth
      // byte is past the end of the input.
      if (szdest < 4) return 0;
      uint32_t in =
          (uint32_t{cur_src[0]} << 16) + absl::big_endian::Load16(cur_src + 1);
      cur_dest[0] = base64[in >> 18];
      in &= 0x3FFFF;
      cur_dest[1] = base64[in >> 12];
      in &= 0xFFF;
      cur_dest[2] = base64[in >> 6];
      in &= 0x3F;
      cur_dest[3] = base64[in];
      cur_dest += 4;
      szdest -= 4;
      break;
    }
    default:
      // Should not be reached: blocks of 4 bytes are handled
      // in the while loop before this switch statement.
      ABSL_RAW_LOG(FATAL, "Logic problem? szsrc = %zu", szsrc);
      break;
  }
  return static_cast<size_t>(cur_dest - dest);
}

}  // namespace strings_internal
ABSL_NAMESPACE_END
}  // namespace absl