iOS: [NSString hash]出现同样的hash值问题

问题原因：

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:

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.

解决方法：

For my case, the solution was pretty easy. I made use of a sha1 hash, which produceda fingerprint from the whole string instead of pieces only. I got mine inspired bythe 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

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 <CommonCrypto/CommonDigest.h>

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!!

，当你能爱的时候就不要放弃爱