<h1>Question：</h1>

在对keychain数据存储的时候，常常需要用到aes加密对数据进行处理，但貌似iOS5.0之后就找不到nsdata下面的aes加密方法了，于是，我们给nsdata新建一个category来做加密处理。

<h1>Show my code：</h1>

NSData+Encryption.h:

<pre>

import@interface NSData (Encryption)

• (NSData *)AES256EncryptWithKey:(NSString *)key;

• (NSData *)AES256DecryptWithKey:(NSString *)key;

@end

</pre>

NSData+Encryption.m:
<pre>

• (NSData *)AES256EncryptWithKey:(NSString *)key {
  // 'key' should be 32 bytes for AES256, will be null-padded otherwise

  char keyPtr[kCCKeySizeAES256+1]; // room for terminator (unused)

  bzero(keyPtr, sizeof(keyPtr)); // fill with zeroes (for padding)

  // fetch key data

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

  NSUInteger dataLength = [self length];

  //See the doc: For block ciphers, the output size will always be less than or

  //equal to the input size plus the size of one block.

  //That's why we need to add the size of one block here

  size_t bufferSize = dataLength + kCCBlockSizeAES128;

  void *buffer = malloc(bufferSize);

  size_t numBytesEncrypted = 0;

  CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, kCCAlgorithmAES128, kCCOptionPKCS7Padding,

  keyPtr, kCCKeySizeAES256,

  NULL /* initialization vector (optional) */,

  [self bytes], dataLength, /* input */

  buffer, bufferSize, /* output */

  &numBytesEncrypted);

  if (cryptStatus == kCCSuccess) {

  //the returned NSData takes ownership of the buffer and will free it on deallocation

  return [NSData dataWithBytesNoCopy:buffer length:numBytesEncrypted];

  }

  free(buffer); //free the buffer;

  return nil;

  }
  </pre>
  <pre>

• (NSData *)AES256DecryptWithKey:(NSString *)key {

// 'key' should be 32 bytes for AES256, will be null-padded otherwise

char keyPtr[kCCKeySizeAES256+1]; // room for terminator (unused)

bzero(keyPtr, sizeof(keyPtr)); // fill with zeroes (for padding)

// fetch key data

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

NSUInteger dataLength = [self length];

//See the doc: For block ciphers, the output size will always be less than or

//equal to the input size plus the size of one block.

//That's why we need to add the size of one block here

size_t bufferSize = dataLength + kCCBlockSizeAES128;

void *buffer = malloc(bufferSize);

size_t numBytesDecrypted = 0;

CCCryptorStatus cryptStatus = CCCrypt(kCCDecrypt, kCCAlgorithmAES128, kCCOptionPKCS7Padding,

keyPtr, kCCKeySizeAES256,

NULL /* initialization vector (optional) */,

[self bytes], dataLength, /* input */

buffer, bufferSize, /* output */

&numBytesDecrypted);

if (cryptStatus == kCCSuccess) {

//the returned NSData takes ownership of the buffer and will free it on deallocation

return [NSData dataWithBytesNoCopy:buffer length:numBytesDecrypted];

}

free(buffer); //free the buffer;

return nil;

}
</pre>

此时，NSData就可以找到对应的AES加密方法。

参考链接：
http://stackoverflow.com/questions/11482470/ios-5-data-encryption-aes-256-encryptwithkey-not-found