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Rotating keys

Key rotation #

This feature is available in Acra Enterprise Edition only.

All cryptographic keys should have a definite time span – known as cryptoperiod – during which the key is authorised for usage. After cryptoperiod’s expiration, key should be replaced with a new one. This regularly performed procedure is known as key rotation.

Here we discuss how to approach and perform key rotation in Acra.

When to rotate keys #

There are many factors that determine an appropriate duration of the cryptoperiod, such as threat and risk model, encryption strength and risk of compromise. We recommend using a cryptoperiod of 1 year for data storage keys, which is consistent with current NIST recommendations (full publication, short version).

Key rotation is one of required security operations procedure, you should rotate all or certain storage keys in these cases:

  • if keys are about to expire (when they get near the end of their cryptoperiod);
  • if there’s a risk that encryption keys, transport protection or data are compromised;
  • if there’s a known fact that security incident occurred.

If you suspect a security breach that may have leaked the keys, it is wise to rotate potentially affected keys immediately and be ready to do it again after the breach has been confirmed and contained.

Check NIST SP 800-57 recommendations to learn more about key management best practices.

Which keys can be rotated #

Acra keeps a multitude of keys which require different approaches.

  • Acra Master Keys protect other keys in the keystore. Each Acra component has its own set of cryptographic keys which can be rotated independently. Currently, there’s no easy way to rotate Master keys.

  • Data storage keys protect the data stored in a datastore. They are rotated by generating a new storage keypair and distributing the public key.

    • If you’re using client-side encryption (data is encrypted on application side using AcraWriter), distribute public key to all your applications that encrypt data.

    • If you’re using only AcraServer or AcraTranslator, distribute public key to all instances of AcraServer or AcraTranslator or put them into shared keystore.

    The new key will apply to newly stored data, but the old data was encrypted with the previous key and is still unchanged. Depending on your key rotation strategy, you might need to re-encrypt data with new data storage key.

  • Transport keys authenticate component identities and provide transport encryption (TLS or Themis Secure Session). These keys are easily rotated.

    • TLS: generate new TLS certificate and revoke old one. Distribute new TLS certificates and private keys throughout your infrastructure.

    • Themis Secure Session keys: generate a new key pair and distribute the public key to appropriate peers. The new key will apply on the next connection.

Data re-encryption strategies #

There are different strategies of re-encrypting data after rotating data storage keys:

  • No re-encryption. Old data is retained as is, encrypted with old keys, only new data is encrypted with new keys. Acra decrypts old data with old keys and new data with new keys. This strategy should not be used if keys were compromised.

  • Full re-encryption. Old data is decrypted with old keys, then encrypted with new keys and replaced in storage. New data is encrypted with new keys. Acra decrypts all data with new keys. Old keys are unused and can be destroyed and/or marked as compromised. This strategy requires data re-encryption which increases system load.

  • Partial re-encryption. Re-encrypting data “piece by piece”, leaving old keys active for some amount of time. This strategy is the best one performance-wise, but requires careful planning.

Note that there are some differences in data re-encryption flow depending on selected cryptographic container type. AcraStructs use ephemeral asymmetric keys to enable applications to encrypt the data but never decrypt it (end-to-end encryption) - the private key stays on the AcraServer / AcraTranslator which are the only ones capable of decryption. In case of AcraStruct container you can be sure that: 1) even if application used outdated (rotated) key, the data stored in the database is encrypted with a fresh key; 2) it is impossible to find any correlation between encrypted data and any table row, as the re-encrypted data will look completely different.

AcraBlocks use symmetric keys stored on AcraServer / AcraTranslator for both encryption and decryption. AcraBlocks provide similar properties for encrypted data as mentioned for AcraStructs. The only difference is that application is not able to encrypt / decrypt data and operates only with a plaintext. However, it simplifies Acra integration into existing infrastructure, since no changes to application source code is required. Moreover, AcraBlocks provide faster data encryption/decryption comparing with AcraStructs.

How to rotate Acra keys #

acra-keymaker tool can be used to generate new keys. After that you will need to distribute the new public keys. Data storage keys may require additional attention.

Make sure you have set up the correct master key before using acra-keymaker. Please refer to the key generation instructions to learn about master keys.

Rotating data storage keys #

Run acra-keymaker to rotate the key for a specific client:

acra-keymaker --client_id=Alice --generate_acrawriter_keys
Note: acra-keymaker must be used on the machine that runs AcraServer or AcraTranslator. This ensures that the new private key never leaves the machine that will use it. If you’re using shared key storage, provide corresponded parameters to acra-keymaker to place the keys into shared storage.
  • If you’re using client-side encryption, remember to share newly generated public key with AcraWriters that need it.

  • If you’re using AcraServer in transparent proxy encryption mode or using AcraTranslator, starting from Acra 0.90.0, Acra uses “rotation without re-encryption” strategy by default. If you expect other data re-encryption strategy or you’re using older version of Acra, you need to re-encrypt your data.

Please refer to the key exchange guide to learn where the keys are stored and how to exchange them correctly and securely.

Rotating zone keys #

If you are using zones, run acra-addzone on AcraServer or AcraTranslator to generate a new zone with a new key:

acra-addzone

It will output the new zone ID and the corresponding public key:

{"id":"DDDDDDDDQHpbUSOgYTzqCktp","public_key":"VUVDMgAAAC3yMBGsAmK/wBXZkL8iBv/C+7hqoQtSZpYoi4fZYMafkJbWe2dL"}

Remember to share newly generated public key and zone ID with AcraWriters that need them.

Rotating transport keys #

This is useful only if you are using AcraConnector as transport encryption daemon to securely connect client application with AcraServer/AcraTranslator, and it uses Themis Secure Session as the transport encryption protocol.

Run acra-keymaker to rotate a specific transport key for a client:

acra-keymaker --client_id=Alice --generate_acraconnector_keys
Note: acra-keymaker must be used on the machine that runs the corresponding component. For example, AcraConnector keys must be generated only on the machine that runs AcraConnector. This ensures that the new private key never leaves the machine that will use it.

Use the following command-line parameters to rotate specific transport keys.

Parameter Key
--generate_acraconnector_keys AcraConnector transport keypair
--generate_acraserver_keys AcraServer transport keypair
--generate_acratranslator_keys AcraTranslator transport keypair

Remember to share newly generated public keys with peer components that need them:

  • AcraConnector public key → AcraServer/AcraTranslator
  • AcraServer public key → AcraConnector
  • AcraTranslator public key → AcraConnector

Please refer to the key exchange guide to learn where the keys are stored and how to exchange them correctly and securely.

Re-encrypting encrypted data #

After you have rotated storage keys, you may need to re-encrypt existing data with new keys.

Have a backup! #

As always with any database migrations, it is a good idea to make and check a fresh backup immediately before doing anything.

Full re-encryption #

In order to migrate the data you need to decrypt it with the old key and encrypt it back with the new one. Acra components will always use the new, current key when encrypting data. On decryption requests, the old key will be used if necessary to access the data (for Acra 0.90.0 and newer). The easiest way to re-encrypt the data is to query data through Acra and put it back right away.

Using AcraServer in transparent proxy encryption mode #

Note: This will work for Acra 0.90.0 and newer.

If you are using AcraServer in transparent proxy mode:

  1. Rotate the storage keys on AcraServer. It will keep the old keys.
  2. Query data from the storage through AcraServer for decryption. It will use old key to decrypt data.
  3. Push data back to the database through AcraServer. It will use new key to encrypt data.

Using AcraTranslator #

Note: This will work for Acra 0.90.0 and newer.

If you are using AcraTranslator, the idea is the same:

  1. Rotate the storage keys on AcraTranslator. It will keep the old keys.
  2. Query data from the storage, submit it to AcraTranslator for decryption.
  3. Encrypt the data with AcraTranslator again and put it back into the storage.

Using client-side encryption #

If you are using AcraConnector and AcraWriter:

  1. Make sure that you have the new public key for AcraWriter. This key will be used to encrypt the data.
  2. Connect to the database via AcraConnector. It still keeps the old key and knows how to decrypt the data.
  3. Run a migration script to fetch plaintext (decrypted) data via AcraConnector, encrypt it with AcraWriter, and store the updated data back.

The script may look like this if you are using Python:

import acrawriter

rows = db.query('SELECT id, column1, column2 FROM table')
for row in rows:
    column1 = acrawriter.create_acrastruct(
        data=row['column1'],
        acra_public_key=new_public_key)
    column2 = acrawriter.create_acrastruct(
        data=row['column2'],
        acra_public_key=new_public_key)
    db.execute(
        'UPDATE table SET column1 = $1, column2 = $2 WHERE id = $3',
        column1, column2, row['id'])

Remember that Acra does not encrypt all columns in the database. Only binary blobs can be encrypted (bytea type) so you need to select and update only the encrypted columns.

Migrating zoned data #

If you are using Zones, the queries will be more complicated to include the zone ID for decryption. AcraWriter also needs the new zone ID for encryption. For example, if the zone ID is stored in the same table, the Python script may look like this:

rows = db.query('SELECT id, zone_id, column1, zone_id, column2 FROM table')
for row in rows:
    column1 = acrawriter.create_acrastruct(
        data=row['column1'],
        acra_public_key=new_public_key,
        context=new_zone_ID)
    column2 = acrawriter.create_acrastruct(
        data=row['column2'],
        acra_public_key=new_public_key,
        context=new_zone_ID)
    db.execute(
        'UPDATE table \
         SET column1 = $1, column2 = $2, zone_id = $3 \
         WHERE id = $4',
        column1, column2, new_zone_ID, row['id'])

The old zone ID is queried before the encrypted column for AcraConnector. AcraWriter needs to be configured to use the new zone ID and public key received from acra-addzone utility. The new zone ID is also written into the database along with updated data.

If you are using zones with AcraTranslator, remember to submit zone ID with each request as required by the API you use.

Partial re-encryption #

You need to update each and every table and row encrypted with the old key (maybe many rows). You may want to proceed with migration incrementally, processing only some part of the data set at a time. Be sure to keep track of the data you have already migrated to avoid duplicating the work.

Each client and zone have their own storage key pair which can be rotated independently. This allows to spread out in time planned rotations, or may limit the effort in case the keys need to be rotated after a suspected security breach.

Verify encrypted data #

After you have completed full or partial data re-encryption, it is advisable to check that it was successful. Try quering the migrated data to confirm that AcraServer or AcraTranslator are able to decrypt it.