B2 cloud supports Server-Side Encryption. All read and write operations provided by b2sdk accept encryption settings as an optional argument. Not supplying this argument means relying on bucket defaults - for SSE-B2 and for no encryption. In case of SSE-C, providing a decryption key is required for successful downloading and copying.
Methods and classes that require encryption settings all accept an EncryptionSetting object, which holds information about present or desired encryption (mode, algorithm, key, key_id). Some, like copy operations, accept two sets of settings (for source and for destination). Sync, however, accepts an EncryptionSettingsProvider object, which is an EncryptionSetting factory, yielding them based on file metadata. For details, see
High security: use unique keys
B2 cloud does not promote or discourage either reusing encryption keys or using unique keys for SSE-C.
In applications requiring enhanced security, using unique key per file is a good strategy. b2sdk follows a convention,
that makes managing such keys easier: EncryptionSetting holds a key identifier, aside from the key itself. This key
identifier is saved in the metadata of all files uploaded, created or copied via b2sdk methods using SSE-C,
under sse_c_key_id in fileInfo. This allows developers to create key managers that map those ids to keys, stored
securely in a file or a database. Implementing such managers, and linking them to
implementations (necessary for using Sync) is outside of the scope of this library.
There is, however, a convention to such managers that authors of this library strongly suggest: if a manager needs to generate a new key-key_id pair for uploading, it’s best to commit this data to the underlying storage before commencing the upload. The justification of such approach is: should the key-key_id pair be committed to permanent storage after completing an IO operation, committing could fail after successfully upload the data. This data, however, is now just a random blob, that can never be read, since the key to decrypting it is lost.
This approach comes an overhead: to download a file, its fileInfo has to be known. This means that fetching metadata is required before downloading.
Moderate security: a single key with many ids
Should the application’s infrastructure require a single key (or a limited set of keys) to be used in a bucket, authors of this library recommend generating a unique key identifier for each file anyway (even though these unique identifiers all point to the same key value). This obfuscates confidential metadata from malicious users, like which files are encrypted with the same key, the total number of different keys, etc.