一般我们在使用http或者socket上传或者下载文件的时候，经常会在完成之后经行一次MD5值得校验（尤其是在断点续传的时候用的更
多），校验MD5值是为了防止在传输的过程当中丢包或者数据包被篡改，在使用MD5之前呢我们应该先了解MD5的一些常识。MD5 百度百科

需要引用：#include <CommonCrypto/CommonDigest.h>

#define FileHashDefaultChunkSizeForReadingData 1024*8
CFStringRef FileMD5HashCreateWithPath(CFStringRef filePath,size_t chunkSizeForReadingData) {
    // Declare needed variables
    CFStringRef result = NULL;
    CFReadStreamRef readStream = NULL;
    // Get the file URL
    CFURLRef fileURL =
    CFURLCreateWithFileSystemPath(kCFAllocatorDefault,
                                  (CFStringRef)filePath,
                                  kCFURLPOSIXPathStyle,
                                  (Boolean)false);
    if (!fileURL) goto done;
    // Create and open the read stream
    readStream = CFReadStreamCreateWithFile(kCFAllocatorDefault,
                                            (CFURLRef)fileURL);
    if (!readStream) goto done;
    bool didSucceed = (bool)CFReadStreamOpen(readStream);
    if (!didSucceed) goto done;
    // Initialize the hash object
    CC_MD5_CTX hashObject;
    CC_MD5_Init(&hashObject);
    // Make sure chunkSizeForReadingData is valid
    if (!chunkSizeForReadingData) {
        chunkSizeForReadingData = FileHashDefaultChunkSizeForReadingData;
    }
    // Feed the data to the hash object
    bool hasMoreData = true;
    while (hasMoreData) {
        uint8_t buffer[chunkSizeForReadingData];
        CFIndex readBytesCount = CFReadStreamRead(readStream,(UInt8 *)buffer,(CFIndex)sizeof(buffer));
        if (readBytesCount == -1) break;
        if (readBytesCount == 0) {
            hasMoreData = false;
            continue;
        }
        CC_MD5_Update(&hashObject,(const void *)buffer,(CC_LONG)readBytesCount);
    }
    // Check if the read operation succeeded
    didSucceed = !hasMoreData;
    // Compute the hash digest
    unsigned char digest[CC_MD5_DIGEST_LENGTH];
    CC_MD5_Final(digest, &hashObject);
    // Abort if the read operation failed
    if (!didSucceed) goto done;
    // Compute the string result
    char hash[2 * sizeof(digest) + 1];
    for (size_t i = 0; i < sizeof(digest); ++i) {
        snprintf(hash + (2 * i), 3, "%02x", (int)(digest[i]));
    }
    result = CFStringCreateWithCString(kCFAllocatorDefault,(const char *)hash,kCFStringEncodingUTF8);
    
done:
    if (readStream) {
        CFReadStreamClose(readStream);
        CFRelease(readStream);
    }
    if (fileURL) {
        CFRelease(fileURL);
    }
    return result;
}

#pragma mark - class method

+(NSString*)getFileMD5WithPath:(NSString*)path
{
    return (__bridge_transfer NSString *)FileMD5HashCreateWithPath((__bridge CFStringRef)path, FileHashDefaultChunkSizeForReadingData);
}