HmacSHA256

#import <CommonCrypto/CommonDigest.h>  
#import <CommonCrypto/CommonHMAC.h>  
  
@implementation HmacUtils  
  
/** 
 *  加密方式,MAC算法: HmacSHA256 
 * 
 *  @param plaintext 要加密的文本 
 *  @param key       秘钥 
 * 
 *  @return 加密后的字符串 
 */  
+ (NSString *)hmac:(NSString *)plaintext withKey:(NSString *)key  
{  
    const charchar *cKey  = [key cStringUsingEncoding:NSASCIIStringEncoding];  
    const charchar *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 charchar *buffer = (const unsigned charchar *)[HMACData bytes];  
    NSMutableString *HMAC = [NSMutableString stringWithCapacity:HMACData.length * 2];  
    for (int i = 0; i < HMACData.length; ++i){  
        [HMAC appendFormat:@"%02x", buffer[i]];  
    }  
      
    return HMAC;  
}  
  
@end