client: Add tests for server signature validation.

New tests to ensure that the correct error is returned when the server signature is invalid.

client/client_test.go

@@ -2,6 +2,8 @@ package client
 
 import (
 	"context"
+	"crypto/ed25519"
+	"encoding/base64"
 	"errors"
 	"net/http"
 	"net/http/httptest"
@@ -103,3 +105,77 @@ func TestErrorDetails(t *testing.T) {
 		})
 	}
 }
+
+// TestSignatureValidation ensures that responses with invalid signatures are
+// flagged.
+func TestSignatureValidation(t *testing.T) {
+
+	// Generate some test data for the valid signature case.
+	privKey := ed25519.NewKeyFromSeed([]byte("00000000000000000000000000000000"))
+	pubKey, _ := privKey.Public().(ed25519.PublicKey)
+	emptyJSON := []byte("{}")
+	validSig := base64.StdEncoding.EncodeToString(ed25519.Sign(privKey, emptyJSON))
+
+	tests := map[string]struct {
+		responseSig  string
+		expectErr    bool
+		expectErrStr string
+	}{
+		"valid signature": {
+			responseSig: validSig,
+			expectErr:   false,
+		},
+		"invalid signature": {
+			responseSig:  "1234",
+			expectErr:    true,
+			expectErrStr: "authenticate server response: invalid signature",
+		},
+		"no signature": {
+			responseSig:  "",
+			expectErr:    true,
+			expectErrStr: "authenticate server response: no signature provided",
+		},
+		"failed to decode signature": {
+			responseSig:  "0xp",
+			expectErr:    true,
+			expectErrStr: "authenticate server response: failed to decode signature: illegal base64 data at input byte 0",
+		},
+	}
+
+	for testName, testData := range tests {
+		t.Run(testName, func(t *testing.T) {
+
+			testServer := httptest.NewServer(http.HandlerFunc(func(res http.ResponseWriter, req *http.Request) {
+				res.Header().Add("VSP-Server-Signature", testData.responseSig)
+				res.WriteHeader(http.StatusOK)
+				_, err := res.Write(emptyJSON)
+				if err != nil {
+					t.Fatalf("writing response body failed: %v", err)
+				}
+			}))
+
+			client := Client{
+				URL:    testServer.URL,
+				PubKey: pubKey,
+				Log:    slog.Disabled,
+			}
+
+			var resp interface{}
+			err := client.do(context.TODO(), http.MethodGet, "", nil, &resp, nil)
+
+			testServer.Close()
+
+			if testData.expectErr {
+				if err.Error() != testData.expectErrStr {
+					t.Fatalf("client.do returned incorrect error, expected %q, got %q",
+						testData.expectErrStr, err.Error())
+				}
+			} else {
+				if err != nil {
+					t.Fatalf("client.do returned unexpected error: %v", err)
+				}
+			}
+
+		})
+	}
+}