Supplying Your Own Encryption Keys

Supplying Your Own Encryption Keys

Overview

By default, Google Cloud Storage encrypts all object data using Google-managed encryption keys and the AES256 encryption algorithm. However, you can also supply your own encryption keys, also known as customer-supplied encryption keys. Google Cloud Storage will not permanently store these keys on Google's servers or otherwise manage them.

gsutil accepts customer-supplied encryption and decryption keys for interacting with Google Cloud Storage objects using the JSON API. The keys are provided via the .boto configuration file like so:

[GSUtil]
encryption_key = ...
decryption_key1 = ...
decryption_key2 = ...

Each key is a RFC 4648 Base64-encoded string of 256 bits of data for use with the AES256 encryption algorithm.

Encryption Behavior

A single encryption_key may be specified in the .boto configuration file, and multiple decryption_keys may be specified.

If encryption_key exists in the .boto configuration file, gsutil ensures that data it writes or copies in Google Cloud Storage is encrypted with that key. If encryption_key is not supplied, gsutil ensures that all data it writes or copies instead uses Google-managed keys. WARNING: This means gsutil will replace customer-supplied encryption with Google-managed encryption if encryption_key is not specified but a matching decryption_key is specified.

Objects encrypted with customer-supplied encryption keys require the matching decryption key any time they are downloaded or copied (via the gsutil cat, cp, mv, or rsync commands). Viewing the CRC32C or MD5 hashes of such objects (via the ls -L or stat commands) also requires the matching decryption key.

If a matching key exists in the .boto configuration, gsutil provides it as needed in requests to Google Cloud Storage and operates on the decrypted results. gsutil never stores encrypted data on your local disk.

gsutil automatically detects the correct customer-supplied encryption key to use for a cloud object by comparing the key's SHA256 hash against the hash of the customer-supplied encryption key. gsutil considers the configured encryption key and up to 100 decryption keys when searching for a match. Decryption keys must be listed in the boto configuration file in ascending numerical order starting with 1. For example, in the following configuration:

decryption_key1 = ...
decryption_key9 = ...
decryption_key10 = ...
decryption_key11 = ...

decryption_keys 9, 10, and 11 will be ignored because no values for decryption_keys 2 through 8 are provided.

Resumable Operations And Encryption Keys

If the encryption_key in your boto configuration file changes during a partially-completed write or copy operation (for example, if you re-run a gsutil cp object upload after hitting ^C or encountering a network timeout), gsutil will restart the partially-completed operation to ensure that the destination object is written with the new key.

Generating Encryption Keys

Generating a 256-bit RFC 4648 Base64-encoded string for use as an encryption key can be easily done with Python:

python -c 'import base64; import os;\
           print(base64.encodestring(os.urandom(32)))'

Managing Encryption Keys

Because Google does not store customer-supplied encryption keys, if you lose your customer-supplied encryption key, you will permanently lose access to all of your data encrypted with that key. Therefore, it is recommended that you back up each encryption key to a secure location. The .boto configuration file should never be the only place where your key is stored.

Also, when you create a customer-supplied encryption key, anyone who has the key and access to your objects can read those objects' data. Take precautions to ensure that your encryption keys are not shared with untrusted parties.

Rotating Keys

To rotate keys, you can change your encryption_key configuration value to a decryption_key configuration value and then use a new value for the encryption_key. Then you can use the rewrite command to rotate keys in the cloud without downloading and re-uploading the data. For example, if your initial configuration is:

# Old encryption key
encryption_key = keyA...

You can change it the configuration to:

# New encryption key
encryption_key = keyB...
# Encryption key prior to rotation
decryption_key1 = keyA...

and rotate the encryption key on an object by running:

gsutil rewrite gs://bucket/object temp-file

Performance Implications For Customer-Supplied Encryption Keys

Because gsutil must retrieve the SHA256 hash of an encryption key for comparison with decryption keys in the .boto configuration file, gsutil performs an additional metadata GET request for each object encrypted with a customer-supplied encryption key. In particular, performing a long listing via gsutil ls -L requires a GET request for each object that is encrypted with a customer-supplied encryption key. Therefore, listing such objects with the -L flag will require one operation per object, which will be substantially slower than listing objects encrypted with Google-owned keys.

Security Implications For Customer-Supplied Encryption Keys

gsutil always sends encryption keys over HTTPS, so your keys will never be visible on the network. However, the keys are present in your .boto configuration file as well as in the memory of the machine executing gsutil. Therefore, if this file or the machine are compromised, your encryption keys should also be considered compromised, and you should immediately perform key rotation for all objects encrypted with the compromised keys.

Xml Api Unsupported

gsutil does not support using the XML API to interact with encrypted objects, and will use the JSON API if any encryption_key or decryption_keys are specified in configuration.

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