-(NSString*)md5{

const char *cStr = [self bytes];

unsigned char digest[CC_MD5_DIGEST_LENGTH];

CC_MD5( cStr, [self length], digest );

NSString* s = [NSString stringWithFormat: @"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",

  digest[0], digest[1], 

  digest[2], digest[3],

  digest[4], digest[5],

  digest[6], digest[7],

  digest[8], digest[9],

  digest[10], digest[11],

  digest[12], digest[13],

  digest[14], digest[15]];

return s;

}