問題原因：

At least there are special circumstances for which this unreliability kicks in.

Comparing [a hash] and [b hash] of two different NSString is safe when:

• the strings' length is shorter or equal to 96 characters.
• [a length] is different to [b length].
• the concatinated first, middle, and last 32 characters of a differ to the concatinated components of b.

  Otherwise every difference between the first and the middle 32 chars, as well as every difference between the middle and the last 32 characters, are not used while producing the [NSString hash] value.

  解決方法：

  For my case, the solution was pretty easy. I made use of a sha1 hash, which produced
  a fingerprint from the whole string instead of pieces only. I got mine inspired by
  the SHA1 method of Saurabh Sharma

  lang:objc SHA1

  • (NSString *)sha1 { NSData *data = [self dataUsingEncoding:NSUTF8StringEncoding]; uint8t digest[CCSHA1DIGESTLENGTH];

    CC_SHA1(data.bytes, data.length, digest);

    NSMutableString *output = [NSMutableString stringWithCapacity:CCSHA1DIGEST_LENGTH * 2];

    for (int i = 0; i < CCSHA1DIGEST_LENGTH; i++) { [output appendFormat:@"%02x", digest[i]]; }

    return output; }

  The SHA1 was for the name of a cached file, generated by giving the complete URL of the resource to be cached. Works perfect. I already filed a pull request for

  http://www.makebetterthings.com/iphone/how-to-get-md5-and-sha1-in-objective-c-ios-sdk/
  

  PS：

  How to get md5 and SHA1 in objective c (iOS sdk)

  Calculating the md5 and sha1 hash in iOS sdk is pretty simple -

  Step 1 – The very first thing you need to do is import CommonCrypto’s CommonDigest.h

  #import 
  

  Step 2 – Here is the real code for calculating SHA1 and MD5 hash -

  SHA1 -

  -(NSString*) sha1:(NSString*)input
  {
   const char *cstr = [input cStringUsingEncoding:NSUTF8StringEncoding];
   NSData *data = [NSData dataWithBytes:cstr length:input.length];
  
   uint8_t digest[CC_SHA1_DIGEST_LENGTH];
  
   CC_SHA1(data.bytes, data.length, digest);
  
   NSMutableString* output = [NSMutableString stringWithCapacity:CC_SHA1_DIGEST_LENGTH * 2];
  
   for(int i = 0; i < CC_SHA1_DIGEST_LENGTH; i++)
   [output appendFormat:@"%02x", digest[i]];
  
   return output;
  
  }
  

  MD5 -

  - (NSString *) md5:(NSString *) input
  {
   const char *cStr = [input UTF8String];
   unsigned char digest[16];
   CC_MD5( cStr, strlen(cStr), digest ); // This is the md5 call
  
   NSMutableString *output = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
  
   for(int i = 0; i < CC_MD5_DIGEST_LENGTH; i++)
   [output appendFormat:@"%02x", digest[i]];
  
   return  output;
  
  }
  

  Hope it will help someone!!