Accumulo 2.x Documentation  >>  Security  >>  On Disk Encryption

On Disk Encryption

For an additional layer of security, Accumulo can encrypt files stored on disk. On Disk encryption was reworked for 2.0, making it easier to configure and more secure. The files that can be encrypted include: RFiles and Write Ahead Logs (WALs). NOTE: This feature is considered experimental. For more information, see the notes below.


To encrypt all tables on disk, encryption must be enabled before an Accumulo instance is initialized. If on disk encryption is enabled on an existing cluster, only files created after it is enabled will be encrypted (root and metadata tables will not be encrypted in this case) and existing data won’t be encrypted until compaction. To configure on disk encryption, add the instance.crypto.service property to your file. The value of this property is the class name of the service which will perform crypto on RFiles and WALs.

Out of the box, Accumulo provides the AESCryptoService for basic encryption needs. This class provides AES encryption with Galois/Counter Mode (GCM) for RFiles and Cipher Block Chaining (CBC) mode for WALs. The additional property below is required by this crypto service to be set using the instance.crypto.opts.* prefix.


This property tells the crypto service where to find the file containing the key encryption key. The key file can be 16 or 32 bytes. For example, openssl can be used to create a random 32 byte key:

openssl rand -out /path/to/keyfile 32

Initializing Accumulo after these instance properties are set, will enable on disk encryption across your entire cluster.

Custom Crypto

The new crypto interface for 2.0 allows for easier custom implementation of encryption and decryption. Your class only has to implement the CryptoService interface to work with Accumulo. The interface has 3 methods:

  void init(Map<String,String> conf) throws CryptoException;
  FileEncrypter getFileEncrypter(CryptoEnvironment environment);
  FileDecrypter getFileDecrypter(CryptoEnvironment environment);

The init method is where you will initialize any resources required for crypto and will get called once per Tablet Server. The getFileEncrypter method requires implementation of a FileEncrypter for encryption and the getFileDecrypter method requires implementation of a FileDecrypter for decryption. The CryptoEnvironment passed into these methods will provide the scope of the crypto. The FileEncrypter has two methods:

  OutputStream encryptStream(OutputStream outputStream) throws CryptoService.CryptoException;
  byte[] getDecryptionParameters();

The encryptStream method performs the encryption on the provided OutputStream and returns an OutputStream, most likely wrapped in at least one other OutputStream. The getDecryptionParameters returns a byte array of anything that will be required to perform decryption. The FileDecrypter only has one method:

  InputStream decryptStream(InputStream inputStream) throws CryptoService.CryptoException;

For more help getting started see AESCryptoService.

Things to keep in mind

The on disk encryption configured here is only for RFiles and Write Ahead Logs (WALs). The majority of data in Accumulo is written to disk with these files but there are a few scenarios that can take place where data will be unencrypted, even with the crypto service enabled.

Sorted WALs

If a tablet server is killed with WALs enabled, Accumulo will create temporary sorted WALs during recovery that are unencrypted.
These files will only contain recent data that has not been compacted but will be written to the disk unencrypted. Once recovery is finished, these unencrypted files will be removed.

Data in Memory & Logs

For queries, data is decrypted when read from RFiles and cached in memory. This means that data is unencrypted in memory while Accumulo is running. Depending on the situation, this also means that some data can be printed to logs. A stacktrace being logged during an exception is one example. Accumulo developers have made sure not to expose data protected by authorizations during logging but its the additional data that gets encrypted on disk that could be exposed in a log file.

Bulk Import

There are 2 ways to create RFiles for bulk ingest: with the RFile API and during Map Reduce using AccumuloFileOutputFormat.
The RFile API allows passing in the configuration properties for encryption mentioned above. The AccumuloFileOutputFormat does not allow for encryption of RFiles so any data bulk imported through this process will be unencrypted.


Accumulo stores a lot of metadata about the cluster in Zookeeper. Keep in mind that this metadata does not get encrypted with On Disk encryption enabled.

GCM performance

The AESCryptoService uses GCM mode for RFiles. Java 9 introduced GHASH hardware support used by GCM.

A test was performed on a VM with 4 2.3GHz processors and 16GB of RAM. The test encrypted and decrypted arrays of size 131072 bytes 1000000 times. The results are as follows:

Java 9 GCM times:
    Time spent encrypting:        209.210s
    Time spent decrypting:        276.800s
Java 8 GCM times:
    Time spent encrypting:        2,818.440s
    Time spent decrypting:        2,883.960s

As you can see, there is a significant performance hit when running without the GHASH CPU instruction. It is advised Java 9 or later be used when enabling encryption.

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