AB#2159 Feat/cli/fetch measurements (#301)

Signed-off-by: Fabian Kammel <fk@edgeless.systems>

internal/config/measurements.go

@@ -1,9 +1,17 @@
 package config
 
 import (
+	"bytes"
+	"context"
 	"encoding/base64"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"net/url"
 
 	"github.com/edgelesssys/constellation/internal/attestation/vtpm"
+	"github.com/edgelesssys/constellation/internal/sigstore"
+	"gopkg.in/yaml.v2"
 )
 
 type Measurements map[uint32][]byte
@@ -30,6 +38,36 @@ var (
 	}
 )
 
+// FetchAndVerify fetches measurement and signature files via provided URLs,
+// using client for download. The publicKey is used to verify the measurements.
+func (m *Measurements) FetchAndVerify(ctx context.Context, client *http.Client, measurementsURL *url.URL, signatureURL *url.URL, publicKey []byte) error {
+	measurements, err := getFromURL(ctx, client, measurementsURL)
+	if err != nil {
+		return err
+	}
+	signature, err := getFromURL(ctx, client, signatureURL)
+	if err != nil {
+		return err
+	}
+	if err := sigstore.VerifySignature(measurements, signature, publicKey); err != nil {
+		return err
+	}
+	if err := yaml.NewDecoder(bytes.NewReader(measurements)).Decode(&m); err != nil {
+		return err
+	}
+	return nil
+}
+
+// CopyFrom copies over all values from other. Overwriting existing values,
+// but keeping not specified values untouched.
+func (m Measurements) CopyFrom(other Measurements) {
+	for idx := range other {
+		m[idx] = other[idx]
+	}
+}
+
+// MarshalYAML overwrites the default behaviour of writing out []byte not as
+// single bytes, but as a single base64 encoded string.
 func (m Measurements) MarshalYAML() (interface{}, error) {
 	base64Map := make(map[uint32]string)
 
@@ -40,6 +78,8 @@ func (m Measurements) MarshalYAML() (interface{}, error) {
 	return base64Map, nil
 }
 
+// UnmarshalYAML overwrites the default behaviour of reading []byte not as
+// single bytes, but as a single base64 encoded string.
 func (m *Measurements) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	base64Map := make(map[uint32]string)
 	err := unmarshal(base64Map)
@@ -57,3 +97,24 @@ func (m *Measurements) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	}
 	return nil
 }
+
+func getFromURL(ctx context.Context, client *http.Client, sourceURL *url.URL) ([]byte, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, sourceURL.String(), http.NoBody)
+	if err != nil {
+		return []byte{}, err
+	}
+
+	resp, err := client.Do(req)
+	if err != nil {
+		return []byte{}, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return []byte{}, fmt.Errorf("http status code: %d", resp.StatusCode)
+	}
+	content, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return []byte{}, err
+	}
+	return content, nil
+}