Base64編碼詳解

Base64編碼過程：將二進制序列轉換爲Base64字符序列(ASCII碼序列)。

 

1、標準Base64字符表

 

2、Base64編碼示例

編碼字符串"Hello!!"，輸出結果"SGVsbG8hIQ=="

 

3、Base64編碼

原理：一個字節包括8位二進制， 然而字符表總共才64個字符，用6位二進制徹底容納64個字符，因此每6位二進制轉換爲對應的Base64字符。

編碼過程：第一次讀6位二進制，該子節剩餘的2位轉到下一次操做。剛好最少3個字節(24位)能轉換成4個Base64字符，因此新數據的長度爲原來數據3分之4倍。

特殊狀況處理：當原來數據的字節數不是3的倍數時，若是除3餘1時，規定在編碼後添加2個"="，若是除3餘2時，規定在編碼後添加1個"＝"。

 

4、Base64編碼本身實現

+(NSString*)base64fromData:(NSData*)originData
{
    const uint8_t* input = (const uint8_t*)[originData bytes]; NSInteger originLength = [originData length]; static char table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="; NSMutableData* encodeData = [NSMutableData dataWithLength:((originLength + 2) / 3) * 4]; uint8_t* output = (uint8_t*)encodeData.mutableBytes; NSInteger i; for (i=0; i < originLength; i += 3) { NSInteger value = 0; NSInteger j; for (j = i; j < (i + 3); j++) { value <<= 8; if (j < originLength) { value |= (0xFF & input[j]); } } NSInteger theIndex = (i / 3) * 4; output[theIndex + 0] = table[(value >> 18) & 0x3F]; output[theIndex + 1] = table[(value >> 12) & 0x3F]; output[theIndex + 2] = (i + 1) < originLength ? table[(value >> 6) & 0x3F] : '='; output[theIndex + 3] = (i + 2) < originLength ? table[(value >> 0) & 0x3F] : '='; } return [[NSString alloc] initWithData:encodeData encoding:NSASCIIStringEncoding]; }

 

5、NSData官方提供的Base64編碼接口

/* Create an NSData from a Base-64 encoded NSString using the given options. By default, returns nil when the input is not recognized as valid Base-64.
*/
- (nullable instancetype)initWithBase64EncodedString:(NSString *)base64String options:(NSDataBase64DecodingOptions)options NS_AVAILABLE(10_9, 7_0);

/* Create a Base-64 encoded NSString from the receiver's contents using the given options.
*/
- (NSString *)base64EncodedStringWithOptions:(NSDataBase64EncodingOptions)options NS_AVAILABLE(10_9, 7_0);

/* Create an NSData from a Base-64, UTF-8 encoded NSData. By default, returns nil when the input is not recognized as valid Base-64.
*/
- (nullable instancetype)initWithBase64EncodedData:(NSData *)base64Data options:(NSDataBase64DecodingOptions)options NS_AVAILABLE(10_9, 7_0);

/* Create a Base-64, UTF-8 encoded NSData from the receiver's contents using the given options.
*/
- (NSData *)base64EncodedDataWithOptions:(NSDataBase64EncodingOptions)options NS_AVAILABLE(10_9, 7_0);