哈希值用作表示大量数据的固定大小的唯一值。数据的少量更改会在哈希值中产生不可预知的大量更改。
sha256 的作用就是防止数据被篡改
上代码

#import <CommonCrypto/CommonDigest.h>
#import <CommonCrypto/CommonHMAC.h>
- (NSString *)hmac:(NSString *)plaintext withKey:(NSString *)key
{
    const char *cKey  = [key cStringUsingEncoding:NSASCIIStringEncoding];
    const char *cData = [plaintext cStringUsingEncoding:NSASCIIStringEncoding];
    unsigned char cHMAC[CC_SHA256_DIGEST_LENGTH];
    CCHmac(kCCHmacAlgSHA256, cKey, strlen(cKey), cData, strlen(cData), cHMAC);
    NSData *HMACData = [NSData dataWithBytes:cHMAC length:sizeof(cHMAC)];
    const unsigned char *buffer = (const unsigned char *)[HMACData bytes];
    NSMutableString *HMAC = [NSMutableString stringWithCapacity:HMACData.length * 2];
    for (int i = 0; i < HMACData.length; ++i){
        [HMAC appendFormat:@"%02x", buffer[i]];
    }
    
    return HMAC;
}