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Key Management Services

SFTPGo stores sensitive data such as Cloud account credentials or passphrases to derive per-object encryption keys. These data are stored as ciphertext and only loaded to RAM in plaintext when needed.

Supported Services for encryption and decryption

The secrets section of the kms configuration allows to configure how to encrypt and decrypt sensitive data. The following configuration parameters are available:

  • url defines the URI to the KMS service
  • master_key_path defines the absolute path to a file containing the master encryption key. This could be, for example, a docker secrets or a file protected with filesystem level permissions.

We use Go CDK to access several key management services in a portable way.

Local provider

If the url is empty SFTPGo uses local encryption for keeping secrets. Internally, it uses the NaCl secret box algorithm to perform encryption and authentication.

We first generate a random key, then the per-object encryption key is derived from this random key in the following way:

  1. a master key is provided: the encryption key is derived using the HMAC-based Extract-and-Expand Key Derivation Function (HKDF) as defined in RFC 5869
  2. no master key is provided: the encryption key is derived as simple hash of the random key. This is the default configuration.

For compatibility with SFTPGo versions 1.2.x and before we also support encryption based on AES-256-GCM. The data encrypted with this algorithm will never use the master key to keep backward compatibility.

Google Cloud Key Management Service

To use keys from Google Cloud Platform’s Key Management Service (GCP KMS) you have to use gcpkms as URL scheme like this:

gcpkms://projects/[PROJECT_ID]/locations/[LOCATION]/keyRings/[KEY_RING]/cryptoKeys/[KEY]

SFTPGo will use Application Default Credentials. See here for alternatives such as environment variables.

The URL host+path are used as the key resource ID; see here for more details.

If a master key is provided we first encrypt the plaintext data using the local provider and then we encrypt the resulting payload using the Cloud provider and store this ciphertext.

AWS Key Management Service

To use customer master keys from Amazon Web Service’s Key Management Service (AWS KMS) you have to use awskms as URL scheme. You can use the key’s ID, alias, or Amazon Resource Name (ARN) to identify the key. You should specify the region query parameter to ensure your application connects to the correct region.

Here are some examples:

  • By ID: awskms://1234abcd-12ab-34cd-56ef-1234567890ab?region=us-east-1
  • By alias: awskms://alias/ExampleAlias?region=us-east-1
  • By ARN: arn:aws:kms:us-east-1:111122223333:key/1234abcd-12ab-34bc-56ef-1234567890ab?region=us-east-1

SFTPGo will use the default AWS session. See AWS Session to learn about authentication alternatives such as environment variables.

If a master key is provided we first encrypt the plaintext data using the local provider and then we encrypt the resulting payload using the Cloud provider and store this ciphertext.

HashiCorp Vault

To use the transit secrets engine in Vault you have to use hashivault as URL scheme like this: hashivault://mykey.

The Vault server endpoint and authentication token are specified using the environment variables VAULT_SERVER_URL and VAULT_SERVER_TOKEN, respectively.

If a master key is provided we first encrypt the plaintext data using the local provider and then we encrypt the resulting payload using Vault and store this ciphertext.

Notes

  • The KMS configuration is global.
  • If you set a master key you will be unable to decrypt the data without this key and the SFTPGo users that need the data as plain text will be unable to login.
  • You can start using the local provider and then switch to an external one but you can't switch between external providers and still be able to decrypt the data encrypted using the previous provider.