oc 用DES加密+base64编码后 ，得到的字符串结尾有一小段乱码

oc 用DES加密+base64编码后 ，得到的字符串结尾有一小段乱码，而且很难复现，哪位大神懂，求助

base64是用GTMBase64，，DES加密方法如下：

• (NSString *)encryptDESUseCBCModel:(NSString *)clearText key:(NSString *)key vi:(NSString *)vi
  {
  NSData *data = [clearText dataUsingEncoding:NSUTF8StringEncoding allowLossyConversion:YES];

  unsigned char buffer[[data length]];
  memset(buffer, 0, sizeof(char));
  size_t numBytesEncrypted = 0;

  const void *vinitVec = (const void *) [vi UTF8String];

  CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt,
  kCCAlgorithmDES,
  kCCOptionPKCS7Padding /*| kCCOptionECBMode*/,
  [key UTF8String],
  kCCKeySizeDES,
  vinitVec,
  [data bytes],
  [data length],
  buffer,
  100000000,
  &numBytesEncrypted);

  NSString* plainText = nil;
  if (cryptStatus == kCCSuccess) {
  NSData *dataTemp = [NSData dataWithBytes:buffer length:(NSUInteger)numBytesEncrypted];
  plainText = [GTMBase64 stringByEncodingData:dataTemp];
  }else{
  TestLog(@"DES加密失败");
  }
  return plainText;
  }

NSString *jsonString = [parameters JSONString];
NSString *encodeJsonString = [EncryptUtil encryptDESUseCBCModel:jsonString key:kEncryptKey vi:kEncryptVi];