path: root/Foundation/GTMBase64.m

//
//  GTMBase64.m
//
//  Copyright 2006-2008 Google Inc.
//
//  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
// 
//  http://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.
//

#import "GTMBase64.h"
#import "GTMDefines.h"

static const char *kBase64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
static const char *kWebSafeBase64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
static const char kBase64PaddingChar = '=';
static const char kBase64InvalidChar = 99;

static const char kBase64DecodeChars[] = {
  // This array was generated by the following code:
  // #include <sys/time.h>
  // #include <stdlib.h>
  // #include <string.h>
  // main()
  // {
  //   static const char Base64[] =
  //     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
  //   char *pos;
  //   int idx, i, j;
  //   printf("    ");
  //   for (i = 0; i < 255; i += 8) {
  //     for (j = i; j < i + 8; j++) {
  //       pos = strchr(Base64, j);
  //       if ((pos == NULL) || (j == 0))
  //         idx = 99;
  //       else
  //         idx = pos - Base64;
  //       if (idx == 99)
  //         printf(" %2d,     ", idx);
  //       else
  //         printf(" %2d/*%c*/,", idx, j);
  //     }
  //     printf("\n    ");
  //   }
  // }
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      62/*+*/, 99,      99,      99,      63/*/ */,
  52/*0*/, 53/*1*/, 54/*2*/, 55/*3*/, 56/*4*/, 57/*5*/, 58/*6*/, 59/*7*/,
  60/*8*/, 61/*9*/, 99,      99,      99,      99,      99,      99,
  99,       0/*A*/,  1/*B*/,  2/*C*/,  3/*D*/,  4/*E*/,  5/*F*/,  6/*G*/,
  7/*H*/,  8/*I*/,  9/*J*/, 10/*K*/, 11/*L*/, 12/*M*/, 13/*N*/, 14/*O*/,
  15/*P*/, 16/*Q*/, 17/*R*/, 18/*S*/, 19/*T*/, 20/*U*/, 21/*V*/, 22/*W*/,
  23/*X*/, 24/*Y*/, 25/*Z*/, 99,      99,      99,      99,      99,
  99,      26/*a*/, 27/*b*/, 28/*c*/, 29/*d*/, 30/*e*/, 31/*f*/, 32/*g*/,
  33/*h*/, 34/*i*/, 35/*j*/, 36/*k*/, 37/*l*/, 38/*m*/, 39/*n*/, 40/*o*/,
  41/*p*/, 42/*q*/, 43/*r*/, 44/*s*/, 45/*t*/, 46/*u*/, 47/*v*/, 48/*w*/,
  49/*x*/, 50/*y*/, 51/*z*/, 99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99
};

static const char kWebSafeBase64DecodeChars[] = {
  // This array was generated by the following code:
  // #include <sys/time.h>
  // #include <stdlib.h>
  // #include <string.h>
  // main()
  // {
  //   static const char Base64[] =
  //     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
  //   char *pos;
  //   int idx, i, j;
  //   printf("    ");
  //   for (i = 0; i < 255; i += 8) {
  //     for (j = i; j < i + 8; j++) {
  //       pos = strchr(Base64, j);
  //       if ((pos == NULL) || (j == 0))
  //         idx = 99;
  //       else
  //         idx = pos - Base64;
  //       if (idx == 99)
  //         printf(" %2d,     ", idx);
  //       else
  //         printf(" %2d/*%c*/,", idx, j);
  //     }
  //     printf("\n    ");
  //   }
  // }
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      62/*-*/, 99,      99,
  52/*0*/, 53/*1*/, 54/*2*/, 55/*3*/, 56/*4*/, 57/*5*/, 58/*6*/, 59/*7*/,
  60/*8*/, 61/*9*/, 99,      99,      99,      99,      99,      99,
  99,       0/*A*/,  1/*B*/,  2/*C*/,  3/*D*/,  4/*E*/,  5/*F*/,  6/*G*/,
  7/*H*/,  8/*I*/,  9/*J*/, 10/*K*/, 11/*L*/, 12/*M*/, 13/*N*/, 14/*O*/,
  15/*P*/, 16/*Q*/, 17/*R*/, 18/*S*/, 19/*T*/, 20/*U*/, 21/*V*/, 22/*W*/,
  23/*X*/, 24/*Y*/, 25/*Z*/, 99,      99,      99,      99,      63/*_*/,
  99,      26/*a*/, 27/*b*/, 28/*c*/, 29/*d*/, 30/*e*/, 31/*f*/, 32/*g*/,
  33/*h*/, 34/*i*/, 35/*j*/, 36/*k*/, 37/*l*/, 38/*m*/, 39/*n*/, 40/*o*/,
  41/*p*/, 42/*q*/, 43/*r*/, 44/*s*/, 45/*t*/, 46/*u*/, 47/*v*/, 48/*w*/,
  49/*x*/, 50/*y*/, 51/*z*/, 99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99,
  99,      99,      99,      99,      99,      99,      99,      99
};


// Tests a character to see if it's a whitespace character.
//
// Returns:
//   YES if the character is a whitespace character.
//   NO if the character is not a whitespace character.
//
GTM_INLINE BOOL IsSpace(unsigned char c) {
  // we use our own mapping here because we don't want anything w/ locale
  // support.
  static BOOL kSpaces[256] = {
    0, 0, 0, 0, 0, 0, 0, 0, 0, 1,  // 0-9
    1, 1, 1, 1, 0, 0, 0, 0, 0, 0,  // 10-19
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 20-29
    0, 0, 1, 0, 0, 0, 0, 0, 0, 0,  // 30-39
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 40-49
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 50-59
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 60-69
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 70-79
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 80-89
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 90-99
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 100-109
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 110-119
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 120-129
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 130-139
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 140-149
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 150-159
    1, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 160-169
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 170-179
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 180-189
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 190-199
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 200-209
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 210-219
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 220-229
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 230-239
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 240-249
    0, 0, 0, 0, 0, 1,              // 250-255
  };
  return kSpaces[c];
}

// Calculate how long the data will be once it's base64 encoded.
//
// Returns:
//   The guessed encoded length for a source length
//
GTM_INLINE NSUInteger CalcEncodedLength(NSUInteger srcLen, BOOL padded) {
  NSUInteger intermediate_result = 8 * srcLen + 5;
  NSUInteger len = intermediate_result / 6;
  if (padded) {
    len = ((len + 3) / 4) * 4;
  }
  return len;
}

// Tries to calculate how long the data will be once it's base64 decoded.
// Unlike the above, this is always an upperbound, since the source data
// could have spaces and might end with the padding characters on them.
//
// Returns:
//   The guessed decoded length for a source length
//
GTM_INLINE NSUInteger GuessDecodedLength(NSUInteger srcLen) {
  return (srcLen + 3) / 4 * 3;
}


@interface GTMBase64 (PrivateMethods)

+(NSData *)baseEncode:(const void *)bytes
               length:(NSUInteger)length
              charset:(const char *)charset
               padded:(BOOL)padded;

+(NSData *)baseDecode:(const void *)bytes
               length:(NSUInteger)length
              charset:(const char*)charset
       requirePadding:(BOOL)requirePadding;

+(NSUInteger)baseEncode:(const char *)srcBytes
                 srcLen:(NSUInteger)srcLen
              destBytes:(char *)destBytes
                destLen:(NSUInteger)destLen
                charset:(const char *)charset
                 padded:(BOOL)padded;

+(NSUInteger)baseDecode:(const char *)srcBytes
                 srcLen:(NSUInteger)srcLen
              destBytes:(char *)destBytes
                destLen:(NSUInteger)destLen
                charset:(const char *)charset
         requirePadding:(BOOL)requirePadding;

@end


@implementation GTMBase64

//
// Standard Base64 (RFC) handling
//

+(NSData *)encodeData:(NSData *)data {
  return [self baseEncode:[data bytes]
                   length:[data length]
                  charset:kBase64EncodeChars
                   padded:YES];
}

+(NSData *)decodeData:(NSData *)data {
  return [self baseDecode:[data bytes]
                   length:[data length]
                  charset:kBase64DecodeChars
           requirePadding:YES];
}

+(NSData *)encodeBytes:(const void *)bytes length:(NSUInteger)length {
  return [self baseEncode:bytes
                   length:length
                  charset:kBase64EncodeChars
                   padded:YES];
}

+(NSData *)decodeBytes:(const void *)bytes length:(NSUInteger)length {
  return [self baseDecode:bytes
                   length:length
                  charset:kBase64DecodeChars
           requirePadding:YES];
}

+(NSString *)stringByEncodingData:(NSData *)data {
  NSString *result = nil;
  NSData *converted = [self baseEncode:[data bytes]
                                length:[data length]
                               charset:kBase64EncodeChars
                                padded:YES];
  if (converted) {
    result = [[[NSString alloc] initWithData:converted
                                    encoding:NSASCIIStringEncoding] autorelease];
  }
  return result;
}

+(NSString *)stringByEncodingBytes:(const void *)bytes length:(NSUInteger)length {
  NSString *result = nil;
  NSData *converted = [self baseEncode:bytes
                                length:length
                               charset:kBase64EncodeChars
                                padded:YES];
  if (converted) {
    result = [[[NSString alloc] initWithData:converted
                                    encoding:NSASCIIStringEncoding] autorelease];
  }
  return result;
}

+(NSData *)decodeString:(NSString *)string {
  NSData *result = nil;
  NSData *data = [string dataUsingEncoding:NSASCIIStringEncoding];
  if (data) {
    result = [self baseDecode:[data bytes]
                       length:[data length]
                      charset:kBase64DecodeChars
               requirePadding:YES];
  }
  return result;
}

//
// Modified Base64 encoding so the results can go onto urls.
//
// The changes are in the characters generated and also the result isn't
// padded to a multiple of 4.
// Must use the matching call to encode/decode, won't interop with the
// RFC versions.
//

+(NSData *)webSafeEncodeData:(NSData *)data
                      padded:(BOOL)padded {
  return [self baseEncode:[data bytes]
                   length:[data length]
                  charset:kWebSafeBase64EncodeChars
                   padded:padded];
}

+(NSData *)webSafeDecodeData:(NSData *)data {
  return [self baseDecode:[data bytes]
                   length:[data length]
                  charset:kWebSafeBase64DecodeChars
           requirePadding:NO];
}

+(NSData *)webSafeEncodeBytes:(const void *)bytes
                       length:(NSUInteger)length
                       padded:(BOOL)padded {
  return [self baseEncode:bytes
                   length:length
                  charset:kWebSafeBase64EncodeChars
                   padded:padded];
}

+(NSData *)webSafeDecodeBytes:(const void *)bytes length:(NSUInteger)length {
  return [self baseDecode:bytes
                   length:length
                  charset:kWebSafeBase64DecodeChars
           requirePadding:NO];
}

+(NSString *)stringByWebSafeEncodingData:(NSData *)data
                                  padded:(BOOL)padded {
  NSString *result = nil;
  NSData *converted = [self baseEncode:[data bytes]
                                length:[data length]
                               charset:kWebSafeBase64EncodeChars
                                padded:padded];
  if (converted) {
    result = [[[NSString alloc] initWithData:converted
                                    encoding:NSASCIIStringEncoding] autorelease];
  }
  return result;
}

+(NSString *)stringByWebSafeEncodingBytes:(const void *)bytes
                                   length:(NSUInteger)length
                                   padded:(BOOL)padded {
  NSString *result = nil;
  NSData *converted = [self baseEncode:bytes
                                length:length
                               charset:kWebSafeBase64EncodeChars
                                padded:padded];
  if (converted) {
    result = [[[NSString alloc] initWithData:converted
                                    encoding:NSASCIIStringEncoding] autorelease];
  }
  return result;
}

+(NSData *)webSafeDecodeString:(NSString *)string {
  NSData *result = nil;
  NSData *data = [string dataUsingEncoding:NSASCIIStringEncoding];
  if (data) {
    result = [self baseDecode:[data bytes]
                       length:[data length]
                      charset:kWebSafeBase64DecodeChars
               requirePadding:NO];
  }
  return result;
}

@end

@implementation GTMBase64 (PrivateMethods)

//
// baseEncode:length:charset:padded:
//
// Does the common lifting of creating the dest NSData.  it creates & sizes the
// data for the results.  |charset| is the characters to use for the encoding
// of the data.  |padding| controls if the encoded data should be padded to a
// multiple of 4.
//
// Returns:
//   an autorelease NSData with the encoded data, nil if any error.
//
+(NSData *)baseEncode:(const void *)bytes
               length:(NSUInteger)length
              charset:(const char *)charset
               padded:(BOOL)padded {
  // how big could it be?
  NSUInteger maxLength = CalcEncodedLength(length, padded);
  // make space
  NSMutableData *result = [NSMutableData data];
  [result setLength:maxLength];
  // do it
  NSUInteger finalLength = [self baseEncode:bytes
                                        srcLen:length
                                     destBytes:[result mutableBytes]
                                       destLen:[result length]
                                       charset:charset
                                        padded:padded];
  if (finalLength) {
    _GTMDevAssert(finalLength == maxLength, @"how did we calc the length wrong?");
  } else {
    // shouldn't happen, this means we ran out of space
    result = nil;
  }
  return result;
}

//
// baseDecode:length:charset:requirePadding:
//
// Does the common lifting of creating the dest NSData.  it creates & sizes the
// data for the results.  |charset| is the characters to use for the decoding
// of the data.
//
// Returns:
//   an autorelease NSData with the decoded data, nil if any error.
//
//
+(NSData *)baseDecode:(const void *)bytes
               length:(NSUInteger)length
              charset:(const char *)charset
       requirePadding:(BOOL)requirePadding {
  // could try to calculate what it will end up as
  NSUInteger maxLength = GuessDecodedLength(length);
  // make space
  NSMutableData *result = [NSMutableData data];
  [result setLength:maxLength];
  // do it
  NSUInteger finalLength = [self baseDecode:bytes
                                     srcLen:length
                                  destBytes:[result mutableBytes]
                                    destLen:[result length]
                                    charset:charset
                             requirePadding:requirePadding];
  if (finalLength) {
    if (finalLength != maxLength) {
      // resize down to how big it was
      [result setLength:finalLength];
    }
  } else {
    // either an error in the args, or we ran out of space
    result = nil;
  }
  return result;
}

//
// baseEncode:srcLen:destBytes:destLen:charset:padded:
//
// Encodes the buffer into the larger.  returns the length of the encoded
// data, or zero for an error.
// |charset| is the characters to use for the encoding
// |padded| tells if the result should be padded to a multiple of 4.
//
// Returns:
//   the length of the encoded data.  zero if any error.
//
+(NSUInteger)baseEncode:(const char *)srcBytes
                 srcLen:(NSUInteger)srcLen
              destBytes:(char *)destBytes
                destLen:(NSUInteger)destLen
                charset:(const char *)charset
                 padded:(BOOL)padded {
  if (!srcLen || !destLen || !srcBytes || !destBytes) {
    return 0;
  }

  char *curDest = destBytes;
  const unsigned char *curSrc = (const unsigned char *)(srcBytes);
  
  // Three bytes of data encodes to four characters of cyphertext.
  // So we can pump through three-byte chunks atomically.
  while (srcLen > 2) {
    // space?
    _GTMDevAssert(destLen >= 4, @"our calc for encoded length was wrong");
    curDest[0] = charset[curSrc[0] >> 2];
    curDest[1] = charset[((curSrc[0] & 0x03) << 4) + (curSrc[1] >> 4)];
    curDest[2] = charset[((curSrc[1] & 0x0f) << 2) + (curSrc[2] >> 6)];
    curDest[3] = charset[curSrc[2] & 0x3f];

    curDest += 4;
    curSrc += 3;
    srcLen -= 3;
    destLen -= 4;
  }

  // now deal with the tail (<=2 bytes)
  switch (srcLen) {
    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.
      _GTMDevAssert(destLen >= 2, @"our calc for encoded length was wrong");
      curDest[0] = charset[curSrc[0] >> 2];
      curDest[1] = charset[(curSrc[0] & 0x03) << 4];
      curDest += 2;
      destLen -= 2;
      if (padded) {
        _GTMDevAssert(destLen >= 2, @"our calc for encoded length was wrong");
        curDest[0] = kBase64PaddingChar;
        curDest[1] = kBase64PaddingChar;
        curDest += 2;
      }
      break;
    case 2:
      // Two bytes left: this encodes to three characters, and (optionally)
      // one pad character to round out the four-character cypherblock.
      _GTMDevAssert(destLen >= 3, @"our calc for encoded length was wrong");
      curDest[0] = charset[curSrc[0] >> 2];
      curDest[1] = charset[((curSrc[0] & 0x03) << 4) + (curSrc[1] >> 4)];
      curDest[2] = charset[(curSrc[1] & 0x0f) << 2];
      curDest += 3;
      destLen -= 3;
      if (padded) {
        _GTMDevAssert(destLen >= 1, @"our calc for encoded length was wrong");
        curDest[0] = kBase64PaddingChar;
        curDest += 1;
      }
      break;
  }
  // return the length
  return (curDest - destBytes);
}

//
// baseDecode:srcLen:destBytes:destLen:charset:requirePadding:
//
// Decodes the buffer into the larger.  returns the length of the decoded
// data, or zero for an error.
// |charset| is the character decoding buffer to use
//
// Returns:
//   the length of the encoded data.  zero if any error.
//
+(NSUInteger)baseDecode:(const char *)srcBytes
                 srcLen:(NSUInteger)srcLen
              destBytes:(char *)destBytes
                destLen:(NSUInteger)destLen
                charset:(const char *)charset
         requirePadding:(BOOL)requirePadding {
  if (!srcLen || !destLen || !srcBytes || !destBytes) {
    return 0;
  }

  int decode;
  NSUInteger destIndex = 0;
  int state = 0;
  char ch = 0;
  while (srcLen-- && (ch = *srcBytes++) != 0)  {
    if (IsSpace(ch))  // Skip whitespace
      continue;
    
    if (ch == kBase64PaddingChar)
      break;
    
    decode = charset[(unsigned int)ch];
    if (decode == kBase64InvalidChar)
      return 0;
    
    // Four cyphertext characters decode to three bytes.
    // Therefore we can be in one of four states.
    switch (state) {
      case 0:
        // We're at the beginning of a four-character cyphertext block.
        // This sets the high six bits of the first byte of the
        // plaintext block.
        _GTMDevAssert(destIndex < destLen, @"our calc for decoded length was wrong");
        destBytes[destIndex] = decode << 2;
        state = 1;
        break;
      case 1:
        // We're one character into a four-character cyphertext block.
        // This sets the low two bits of the first plaintext byte,
        // and the high four bits of the second plaintext byte.
        _GTMDevAssert((destIndex+1) < destLen, @"our calc for decoded length was wrong");
        destBytes[destIndex] |= decode >> 4;
        destBytes[destIndex+1] = (decode & 0x0f) << 4;
        destIndex++;
        state = 2;
        break;
      case 2:
        // We're two characters into a four-character cyphertext block.
        // This sets the low four bits of the second plaintext
        // byte, and the high two bits of the third plaintext byte.
        // However, if this is the end of data, and those two
        // bits are zero, it could be that those two bits are
        // leftovers from the encoding of data that had a length
        // of two mod three.
        _GTMDevAssert((destIndex+1) < destLen, @"our calc for decoded length was wrong");
        destBytes[destIndex] |= decode >> 2;
        destBytes[destIndex+1] = (decode & 0x03) << 6;
        destIndex++;
        state = 3;
        break;
      case 3:
        // We're at the last character of a four-character cyphertext block.
        // This sets the low six bits of the third plaintext byte.
        _GTMDevAssert(destIndex < destLen, @"our calc for decoded length was wrong");
        destBytes[destIndex] |= decode;
        destIndex++;
        state = 0;
        break;
    }
  }
  
  // We are done decoding Base-64 chars.  Let's see if we ended
  //      on a byte boundary, and/or with erroneous trailing characters.
  if (ch == kBase64PaddingChar) {               // We got a pad char
    if ((state == 0) || (state == 1)) {
      return 0;  // Invalid '=' in first or second position
    }
    if (srcLen == 0) {
      if (state == 2) { // We run out of input but we still need another '='
        return 0;
      }
      // Otherwise, we are in state 3 and only need this '='
    } else {
      if (state == 2) {  // need another '='
        while ((ch = *srcBytes++) && (srcLen-- > 0)) {
          if (!IsSpace(ch))
            break;
        }
        if (ch != kBase64PaddingChar) {
          return 0;
        }
      }
      // state = 1 or 2, check if all remain padding is space
      while ((ch = *srcBytes++) && (srcLen-- > 0)) {
        if (!IsSpace(ch)) {
          return 0;
        }
      }
    }
  } else {
    // We ended by seeing the end of the string.
    
    if (requirePadding) {
      // If we require padding, then anything but state 0 is an error.
      if (state != 0) {
        return 0;
      }
    } else {
      // Make sure we have no partial bytes lying around.  Note that we do not
      // require trailing '=', so states 2 and 3 are okay too.
      if (state == 1) {
        return 0;
      }
    }
  }
  
  // If then next piece of output was valid and got written to it means we got a
  // very carefully crafted input that appeared valid but contains some trailing
  // bits past the real length, so just toss the thing.
  if ((destIndex < destLen) &&
      (destBytes[destIndex] != 0)) {
    return 0;
  }
  
  return destIndex;
}

@end