Using Encryption Keys

Using 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 use one of two encryption key types - those that you supply, called customer-supplied encryption keys (CSEK), or those that you manage through Google Cloud KMS, called customer-managed encryption keys (CMEK). Google Cloud Storage does not permanently store CSEKs on Google's servers or otherwise manage them. You can read more about CMEKS at https://cloud.google.com/kms/docs.

gsutil accepts CSEKs 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.

gsutil also accepts CMEKs for encrypting objects using the JSON API. Note that if your goal is to use a CMEK to encrypt all newly-written objects in some bucket, you should set that bucket's default KMS key (see gsutil help kms). Alternatively, you can specify the desired CMEK in the .boto configuration file, but you need only to specify the encryption_key attribute:

[GSUtil]
encryption_key = projects/PROJECT_ID/locations/LOCATION/keyRings/KEYRING/cryptoKeys/KEYNAME

While decrypting a CSEK-encrypted object requires supplying the CSEK in one of the decryption_key attributes, this is not necessary for decrypting CMEK-encrypted objects because the name of the CMEK used to encrypt the object is stored in the object's metadata.

Note that if you'd like to specify CMEKs on a per-command basis without needing to edit your boto file, you may specify the key name as top-level boto option:

gsutil -o 'GSUtil:encryption_key=projects/PROJECT_ID/locations/LOCATION/keyRings/KEYRING/cryptoKeys/KEYNAME' \
       cp /some/local/file gs://my-bucket/

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 the destination bucket's default encryption type - if the bucket has a default KMS key set, that CMEK is used for encryption; if not, Google-managed encryption is used. WARNING: This means that when overwriting or rewriting a CSEK- or CMEK-encrypted object, if encryption_key is not specified, gsutil will replace customer-supplied or customer-managed encryption with Google-managed encryption (or customer-managed encryption if the bucket has a default KMS key set).

Objects encrypted with CSEKs 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 CSEK to use for a cloud object by comparing the key's SHA256 hash against the hash of the CSEK. 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 Customer-Supplied 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 Customer-Supplied Encryption Keys

Because Google does not store CSEKs, if you lose your CSEK, 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 CSEK, 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 CSEKs, 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 -k gs://bucket/object temp-file

You can also do this to apply different CMEKs, but you do not need to add CMEKs to decryption_key configuration values. Similarly, you can switch between CSEK and CMEK encryption depending on the type of key specified in the encryption_key configuration value.

Performance Implications For Encryption Keys

When performing an object listing, metadata for objects encrypted with a CSEK or CMEK will not include the objects' CRC32C or MD5 hashes. For gsutil commands that require these fields, such as gsutil ls -L, gsutil performs an additional metadata GET request for each object encrypted with a CSEK or CMEK. Therefore, listing such objects with the -L flag will require one additional 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 CSEKs 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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