// 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 {
const char kBase64Chars[] =
"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.
//
// (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. Padding at the
// end of the data is performed using the '=' character. Since all base
// 64 input is an integral number of octets, only the following cases
// can arise:
// Base64 encodes each three bytes of input into four bytes of output.
size_t len = (input_len / 3) * 4;
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;
}
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;
// 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 = limit_dest - cur_dest;
szsrc = 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 = (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 (cur_dest - dest);
}
} // namespace strings_internal
ABSL_NAMESPACE_END
} // namespace absl
