IOS常用加密算法

1 通过简单的URLENCODE ＋ BASE64编码防止数据明文传输
2 对普通请求、返回数据，生成MD5校验（MD5中加入动态密钥），进行数据完整性（简单防篡改，安全性较低，优点：快速）校验。
3 对于重要数据，使用RSA进行数字签名，起到防篡改作用。
4 对于比较敏感的数据，如用户信息（登陆、注册等），客户端发送使用RSA加密，服务器返回使用DES(AES)加密。
原因：客户端发送之所以使用RSA加密，是因为RSA解密需要知道服务器私钥，而服务器私钥一般盗取难度较大；如果使用DES的话，可以通过破解客户端获取密钥，安全性较低。而服务器返回之所以使用DES，是因为不管使用DES还是RSA，密钥（或私钥）都存储在客户端，都存在被破解的风险，因此，需要采用动态密钥，而RSA的密钥生成比较复杂，不太适合动态密钥，并且RSA速度相对较慢，所以选用DES）

把相关算法的代码也贴一下吧（其实使用一些成熟的第三方库或许会来得更加简单，不过自己写，自由点）。注，这里的大部分加密算法都是参考一些现有成熟的算法，或者直接拿来用的。
1、MD5
//因为是使用category，所以木有参数传入啦

-(NSString ) stringFromMD5 {
  if(self == nil || [self length] == ) {
  return nil;
}
  const char value = [self UTF8String];
  unsigned char outputBuffer[CC_MD5_DIGEST_LENGTH];
  CC_MD5(value, strlen(value), outputBuffer);
  NSMutableString outputString = [[NSMutableString alloc] initWithCapacity:CC_MD5_DIGEST_LENGTH  2];
  for(NSInteger count = ; count < CC_MD5_DIGEST_LENGTH; count++){
  [outputString appendFormat:@"%02x",outputBuffer[count]];
  }
  return [outputString autorelease];
}

2、Base64

+ (NSString ) base64EncodeData: (NSData ) objData {
const unsigned char objRawData = [objData bytes];
char objPointer;
char strResult;

// Get the Raw Data length and ensure we actually have data
int intLength = [objData length];
if (intLength == ) return nil;

// Setup the String-based Result placeholder and pointer within that placeholder
strResult = (char )calloc(((intLength + 2) / 3) 4, sizeof(char));
objPointer = strResult;

// Iterate through everything
while (intLength > 2) { // keep going until we have less than 24 bits
objPointer++ = _base64EncodingTable[objRawData[] >> 2];
objPointer++ = _base64EncodingTable[((objRawData[] & 0x03) << 4) + (objRawData[1] >> 4)];
objPointer++ = _base64EncodingTable[((objRawData[1] & 0x0f) << 2) + (objRawData[2] >> 6)];
objPointer++ = _base64EncodingTable[objRawData[2] & 0x3f];

// we just handled 3 octets (24 bits) of data
objRawData += 3;
intLength -= 3;
}

// now deal with the tail end of things
if (intLength != ) {
objPointer++ = _base64EncodingTable[objRawData[] >> 2];
if (intLength > 1) {
objPointer++ = _base64EncodingTable[((objRawData[] & 0x03) << 4) + (objRawData[1] >> 4)];
objPointer++ = _base64EncodingTable[(objRawData[1] & 0x0f) << 2];
objPointer++ = '=';
} else {
objPointer++ = _base64EncodingTable[(objRawData[] & 0x03) << 4];
objPointer++ = '=';
objPointer++ = '=';
}
}

// Terminate the string-based result
objPointer = '\0';

  NSString rstStr = [NSString stringWithCString:strResult encoding:NSASCIIStringEncoding];
  free(objPointer);
  return rstStr;
}

3、AES
-(NSData) EncryptAES: (NSString ) key {
  char keyPtr[kCCKeySizeAES256+1];
  bzero(keyPtr, sizeof(keyPtr));

  [key getCString:keyPtr maxLength:sizeof(keyPtr) encoding:NSUTF8StringEncoding];

  NSUInteger dataLength = [self length];

  size_t bufferSize = dataLength + kCCBlockSizeAES128;
  void buffer = malloc(bufferSize);

  size_t numBytesEncrypted = ;
  CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, kCCAlgorithmAES128,
  kCCOptionPKCS7Padding | kCCOptionECBMode,
  keyPtr, kCCBlockSizeAES128,
  NULL,
  [self bytes], dataLength,
  buffer, bufferSize,
  &numBytesEncrypted);
  if (cryptStatus == kCCSuccess) {
  return [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];
  }

  free(buffer);
  return nil;
}

4、RSA

- (NSData ) encryptWithData:(NSData )content {
  size_t plainLen = [content length];
  if (plainLen > maxPlainLen) {
  NSLog(@"content(%ld) is too long, must < %ld", plainLen, maxPlainLen);
  return nil;
  }

  void plain = malloc(plainLen);
  [content getBytes:plain
     length:plainLen];

  size_t cipherLen = 128; // currently RSA key length is set to 128 bytes
  void cipher = malloc(cipherLen);

  OSStatus returnCode = SecKeyEncrypt(publicKey, kSecPaddingPKCS1, plain,
  plainLen, cipher, &cipherLen);

  NSData result = nil;
  if (returnCode != ) {
  NSLog(@"SecKeyEncrypt fail. Error Code: %ld", returnCode);
  }
  else {
  result = [NSData dataWithBytes:cipher
  length:cipherLen];
  }

  free(plain);
  free(cipher);

  return result;
}

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IOS常用加密算法