azure: add SAS authentication option

changelog/unreleased/issue-2295

@@ -0,0 +1,14 @@
+Enhancement: Allow use of SAS token to authenticate to Azure
+
+Previously restic only supported AccountKeys to authenticate to Azure
+storage accounts, which necessitates giving a significant amount of
+access.
+
+We added support for Azure SAS tokens which are a more fine-grained
+and time-limited manner of granting access. Set the `AZURE_ACCOUNT_NAME`
+and `AZURE_ACCOUNT_SAS` environment variables to use a SAS token for
+authentication. Note that if `AZURE_ACCOUNT_KEY` is set, it will take
+preference.
+
+https://github.com/restic/restic/issues/2295
+https://github.com/restic/restic/pull/3661

cmd/restic/global.go

@@ -596,6 +596,10 @@ func parseConfig(loc location.Location, opts options.Options) (interface{}, erro
 			cfg.AccountKey = options.NewSecretString(os.Getenv("AZURE_ACCOUNT_KEY"))
 		}
 
+		if cfg.AccountSAS.String() == "" {
+			cfg.AccountSAS = options.NewSecretString(os.Getenv("AZURE_ACCOUNT_SAS"))
+		}
+
 		if err := opts.Apply(loc.Scheme, &cfg); err != nil {
 			return nil, err
 		}

doc/030_preparing_a_new_repo.rst

@@ -499,6 +499,17 @@ account name and key as follows:
     $ export AZURE_ACCOUNT_NAME=<ACCOUNT_NAME>
     $ export AZURE_ACCOUNT_KEY=<SECRET_KEY>
 
+or
+
+.. code-block:: console
+
+    $ export AZURE_ACCOUNT_NAME=<ACCOUNT_NAME>
+    $ export AZURE_ACCOUNT_SAS=<SAS_TOKEN>
+
+With the later form, ensure your ``SAS_TOKEN`` does not start with a leading
+``?``. If the generated token starts with a leading ``?`` it is safe to just
+delete the first character (the ``?``) before use.
+
 Afterwards you can initialize a repository in a container called ``foo`` in the
 root path like this:
 

doc/040_backup.rst

@@ -593,6 +593,7 @@ environment variables. The following lists these environment variables:
 
     AZURE_ACCOUNT_NAME                  Account name for Azure
     AZURE_ACCOUNT_KEY                   Account key for Azure
+    AZURE_ACCOUNT_SAS                   Shared access signatures (SAS) for Azure
 
     GOOGLE_PROJECT_ID                   Project ID for Google Cloud Storage
     GOOGLE_APPLICATION_CREDENTIALS      Application Credentials for Google Cloud Storage (e.g. $HOME/.config/gs-secret-restic-key.json)

internal/backend/azure/azure.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"crypto/md5"
 	"encoding/base64"
+	"fmt"
 	"hash"
 	"io"
 	"net/http"
@@ -39,10 +40,29 @@ var _ restic.Backend = &Backend{}
 
 func open(cfg Config, rt http.RoundTripper) (*Backend, error) {
 	debug.Log("open, config %#v", cfg)
-
-	client, err := storage.NewBasicClient(cfg.AccountName, cfg.AccountKey.Unwrap())
-	if err != nil {
-		return nil, errors.Wrap(err, "NewBasicClient")
+	var client storage.Client
+	var err error
+	if cfg.AccountKey.String() != "" {
+		// We have an account key value, find the BlobServiceClient
+		// from with a BasicClient
+		debug.Log(" - using account key")
+		client, err = storage.NewBasicClient(cfg.AccountName, cfg.AccountKey.Unwrap())
+		if err != nil {
+			return nil, errors.Wrap(err, "NewBasicClient")
+		}
+	} else if cfg.AccountSAS.String() != "" {
+		// Get the client using the SAS Token as authentication, this
+		// is longer winded than above because the SDK wants a URL for the Account
+		// if your using a SAS token, and not just the account name
+		// we (as per the SDK ) assume the default Azure portal.
+		url := fmt.Sprintf("https://%s.blob.core.windows.net/", cfg.AccountName)
+		debug.Log(" - using sas token")
+		client, err = storage.NewAccountSASClientFromEndpointToken(url, cfg.AccountSAS.Unwrap())
+		if err != nil {
+			return nil, errors.Wrap(err, "NewAccountSASClientFromEndpointToken")
+		}
+	} else {
+		return nil, errors.New("no azure authentication information found")
 	}
 
 	client.HTTPClient = &http.Client{Transport: rt}

internal/backend/azure/config.go

@@ -12,6 +12,7 @@ import (
 // server.
 type Config struct {
 	AccountName string
+	AccountSAS  options.SecretString
 	AccountKey  options.SecretString
 	Container   string
 	Prefix      string