-(NSString*) hmacForSecret:(NSString*)secret data:(NSString*)data {
    
    const char *cKey  = [secret cStringUsingEncoding:NSASCIIStringEncoding];
    const char *cData = [data cStringUsingEncoding:NSUTF8StringEncoding];
    unsigned char cHMAC[CC_SHA256_DIGEST_LENGTH];
    
    CCHmac(kCCHmacAlgSHA256, cKey, strlen(cKey), cData, strlen(cData), cHMAC);
    
    NSData *HMAC = [[NSData alloc] initWithBytes:cHMAC length:sizeof(cHMAC)];
    
     NSString *hash =  [HMAC base64EncodedString];
    
    return hash;
}

再创建一个NSData的category类名字自己随意取

.h文件代码
void *NewBase64Decode(const char *inputBuffer, size_t length, size_t *outputLength);
char *NewBase64Encode(const void *inputBuffer, size_t length, bool separateLines, size_t *outputLength);

@interface NSData (base64EncodedString)

+ (NSData *)dataFromBase64String:(NSString *)aString;
- (NSString *)base64EncodedString;
@end

.m文件代码
static unsigned char base64EncodeLookup[65] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
#define xx 65

static unsigned char base64DecodeLookup[256] =
{
    xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx,
    xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx,
    xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, 62, xx, xx, xx, 63,
    52, 53, 54, 55, 56, 57, 58, 59, 60, 61, xx, xx, xx, xx, xx, xx,
    xx,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
    15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, xx, xx, xx, xx, xx,
    xx, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
    41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, xx, xx, xx, xx, xx,
    xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx,
    xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx,
    xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx,
    xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx,
    xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx,
    xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx,
    xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx,
    xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx,
};

#define BINARY_UNIT_SIZE 3
#define BASE64_UNIT_SIZE 4

void *NewBase64Decode(const char *inputBuffer, size_t length, size_t *outputLength)
{
    if (length == -1)
    {
        length = strlen(inputBuffer);
    }
    
    size_t outputBufferSize = (length / BASE64_UNIT_SIZE) * BINARY_UNIT_SIZE;
    unsigned char *outputBuffer = (unsigned char *)malloc(outputBufferSize);
    
    size_t i = 0;
    size_t j = 0;
    while (i < length)
    {

        unsigned char accumulated[BASE64_UNIT_SIZE];
        size_t accumulateIndex = 0;
        while (i < length)
        {
            unsigned char decode = base64DecodeLookup[inputBuffer[i++]];
            if (decode != xx)
            {
                accumulated[accumulateIndex] = decode;
                accumulateIndex++;
                
                if (accumulateIndex == BASE64_UNIT_SIZE)
                {
                    break;
                }
            }
        }
        
        //
        // Store the 6 bits from each of the 4 characters as 3 bytes
        //
        outputBuffer[j] = (accumulated[0] << 2) | (accumulated[1] >> 4);
        outputBuffer[j + 1] = (accumulated[1] << 4) | (accumulated[2] >> 2);
        outputBuffer[j + 2] = (accumulated[2] << 6) | accumulated[3];
        j += accumulateIndex - 1;
    }
    
    if (outputLength)
    {
        *outputLength = j;
    }
    return outputBuffer;
}

char *NewBase64Encode(
                      const void *buffer,
                      size_t length,
                      bool separateLines,
                      size_t *outputLength)
{
    const unsigned char *inputBuffer = (const unsigned char *)buffer;
    
#define MAX_NUM_PADDING_CHARS 2
#define OUTPUT_LINE_LENGTH 64
#define INPUT_LINE_LENGTH ((OUTPUT_LINE_LENGTH / BASE64_UNIT_SIZE) * BINARY_UNIT_SIZE)
#define CR_LF_SIZE 2

    size_t outputBufferSize =
    ((length / BINARY_UNIT_SIZE)
     + ((length % BINARY_UNIT_SIZE) ? 1 : 0))
    * BASE64_UNIT_SIZE;
    if (separateLines)
    {
        outputBufferSize +=
        (outputBufferSize / OUTPUT_LINE_LENGTH) * CR_LF_SIZE;
    }

    outputBufferSize += 1;

    char *outputBuffer = (char *)malloc(outputBufferSize);
    if (!outputBuffer)
    {
        return NULL;
    }
    
    size_t i = 0;
    size_t j = 0;
    const size_t lineLength = separateLines ? INPUT_LINE_LENGTH : length;
    size_t lineEnd = lineLength;
    
    while (true)
    {
        if (lineEnd > length)
        {
            lineEnd = length;
        }
        
        for (; i + BINARY_UNIT_SIZE - 1 < lineEnd; i += BINARY_UNIT_SIZE)
        {
            outputBuffer[j++] = base64EncodeLookup[(inputBuffer[i] & 0xFC) >> 2];
            outputBuffer[j++] = base64EncodeLookup[((inputBuffer[i] & 0x03) << 4)
                                                   | ((inputBuffer[i + 1] & 0xF0) >> 4)];
            outputBuffer[j++] = base64EncodeLookup[((inputBuffer[i + 1] & 0x0F) << 2)
                                                   | ((inputBuffer[i + 2] & 0xC0) >> 6)];
            outputBuffer[j++] = base64EncodeLookup[inputBuffer[i + 2] & 0x3F];
        }
        
        if (lineEnd == length)
        {
            break;
        }
        outputBuffer[j++] = '\r';
        outputBuffer[j++] = '\n';
        lineEnd += lineLength;
    }
    
    if (i + 1 < length)
    {
        outputBuffer[j++] = base64EncodeLookup[(inputBuffer[i] & 0xFC) >> 2];
        outputBuffer[j++] = base64EncodeLookup[((inputBuffer[i] & 0x03) << 4)
                                               | ((inputBuffer[i + 1] & 0xF0) >> 4)];
        outputBuffer[j++] = base64EncodeLookup[(inputBuffer[i + 1] & 0x0F) << 2];
        outputBuffer[j++] = '=';
    }
    else if (i < length)
    {
        outputBuffer[j++] = base64EncodeLookup[(inputBuffer[i] & 0xFC) >> 2];
        outputBuffer[j++] = base64EncodeLookup[(inputBuffer[i] & 0x03) << 4];
        outputBuffer[j++] = '=';
        outputBuffer[j++] = '=';
    }
    outputBuffer[j] = 0;

    if (outputLength)
    {
        *outputLength = j;
    }
    return outputBuffer;
}

@implementation NSData (base64EncodedString)


+ (NSData *)dataFromBase64String:(NSString *)aString {
    
    NSData *data = [aString dataUsingEncoding:NSASCIIStringEncoding];
    size_t outputLength;
    void *outputBuffer = NewBase64Decode([data bytes], [data length], &outputLength);
    NSData *result = [NSData dataWithBytes:outputBuffer length:outputLength];
    free(outputBuffer);
    return result;
}


- (NSString *)base64EncodedString {
    
    size_t outputLength;
    char *outputBuffer = NewBase64Encode([self bytes], [self length], true, &outputLength);
    
    NSString *result = [[NSString alloc] initWithBytes:outputBuffer length:outputLength encoding:NSASCIIStringEncoding];
    free(outputBuffer);
    return result;
}
@end