Key Management Server, KeyProvider, EDEKs

A new cluster service is required to store, manage, and access encryption keys: the Hadoop Key Management Server (KMS). The KMS is a proxy that interfaces with a backing key store on behalf of HDFS daemons and clients. Both the backing key store and the KMS implement the Hadoop KeyProvider client API. See the KMS documentation for more information.

In the KeyProvider API, each encryption key has a unique key name. Because keys can be rolled, a key can have multiple key versions, where each key version has its own key material (the actual secret bytes used during encryption and decryption). An encryption key can be fetched by either its key name, returning the latest version of the key, or by a specific key version.

The KMS implements additional functionality which enables creation and decryption of encrypted encryption keys (EEKs). Creation and decryption of EEKs happens entirely on the KMS. Importantly, the client requesting creation or decryption of an EEK never handles the EEK's encryption key. To create a new EEK, the KMS generates a new random key, encrypts it with the specified key, and returns the EEK to the client. To decrypt an EEK, the KMS checks that the user has access to the encryption key, uses it to decrypt the EEK, and returns the decrypted encryption key.

In the context of HDFS encryption, EEKs are encrypted data encryption keys (EDEKs), where a data encryption key (DEK) is what is used to encrypt and decrypt file data. Typically, the key store is configured to only allow end users access to the keys used to encrypt DEKs. This means that EDEKs can be safely stored and handled by HDFS, since the HDFS user will not have access to EDEK encryption keys.

Encryption zones

For transparent encryption, we introduce a new abstraction to HDFS: the encryption zone. An encryption zone is a special directory whose contents will be transparently encrypted upon write and transparently decrypted upon read. Each encryption zone is associated with a single encryption zone key which is specified when the zone is created. Each file within an encryption zone has its own unique EDEK.

When creating a new file in an encryption zone, the NameNode asks the KMS to generate a new EDEK encrypted with the encryption zone's key. The EDEK is then stored persistently as part of the file's metadata on the NameNode.

When reading a file within an encryption zone, the NameNode provides the client with the file's EDEK and the encryption zone key version used to encrypt the EDEK. The client then asks the KMS to decrypt the EDEK, which involves checking that the client has permission to access the encryption zone key version. Assuming that is successful, the client uses the DEK to decrypt the file's contents.

All of the above steps for the read and write path happen automatically through interactions between the DFSClient, the NameNode, and the KMS.

Access to encrypted file data and metadata is controlled by normal HDFS filesystem permissions. This means that if HDFS is compromised (for example, by gaining unauthorized access to an HDFS superuser account), a malicious user only gains access to ciphertext and encrypted keys. However, since access to encryption zone keys is controlled by a separate set of permissions on the KMS and key store, this does not pose a security threat.

Configuring the cluster KeyProvider

Selecting an encryption algorithm and codec

Namenode configuration

createZone

Usage: [-createZone -keyName <keyName> -path <path>]

Create a new encryption zone.

listZones

Usage: [-listZones]

List all encryption zones. Requires superuser permissions.

Hardware access exploits

These exploits assume that attacker has gained physical access to hard drives from cluster machines, i.e. datanodes and namenodes.

1. Access to swap files of processes containing data encryption keys.
   • By itself, this does not expose cleartext, as it also requires access to encrypted block files.
   • This can be mitigated by disabling swap, using encrypted swap, or using mlock to prevent keys from being swapped out.
2. Access to encrypted block files.
   • By itself, this does not expose cleartext, as it also requires access to DEKs.

Root access exploits

These exploits assume that attacker has gained root shell access to cluster machines, i.e. datanodes and namenodes. Many of these exploits cannot be addressed in HDFS, since a malicious root user has access to the in-memory state of processes holding encryption keys and cleartext. For these exploits, the only mitigation technique is carefully restricting and monitoring root shell access.

1. Access to encrypted block files.
   • By itself, this does not expose cleartext, as it also requires access to encryption keys.
2. Dump memory of client processes to obtain DEKs, delegation tokens, cleartext.
   • No mitigation.
3. Recording network traffic to sniff encryption keys and encrypted data in transit.
   • By itself, insufficient to read cleartext without the EDEK encryption key.
4. Dump memory of datanode process to obtain encrypted block data.
   • By itself, insufficient to read cleartext without the DEK.
5. Dump memory of namenode process to obtain encrypted data encryption keys.
   • By itself, insufficient to read cleartext without the EDEK's encryption key and encrypted block files.

HDFS admin exploits

These exploits assume that the attacker has compromised HDFS, but does not have root or hdfs user shell access.

1. Access to encrypted block files.
   • By itself, insufficient to read cleartext without the EDEK and EDEK encryption key.
2. Access to encryption zone and encrypted file metadata (including encrypted data encryption keys), via -fetchImage.
   • By itself, insufficient to read cleartext without EDEK encryption keys.

Rogue user exploits

A rogue user can collect keys to which they have access, and use them later to decrypt encrypted data. This can be mitigated through periodic key rolling policies.