Implement FIPSCapable (#245)

* implement FIPSProvider

* Update openssl.go

Co-authored-by: Copilot <[email protected]>

* fix TestFIPSCapable

* improve func docs

---------

Co-authored-by: Copilot <[email protected]>

openssl.go

@@ -134,6 +134,37 @@ func FIPS() bool {
 	}
 }
 
+// FIPSCapable returns true if the provider used by default matches the `fips=yes` query.
+// It is useful for checking whether OpenSSL is capable of running in FIPS mode regardless
+// of whether FIPS mode is explicitly enabled. For example, Azure Linux 3 doesn't set the
+// `fips=yes` query in the default properties, but sets the default provider to be SCOSSL,
+// which is FIPS-capable.
+//
+// Considerations:
+//   - Multiple calls to FIPSCapable can return different values if [SetFIPS] is called in between.
+//   - Can return true even if [FIPS] returns false, because [FIPS] also checks whether
+//     the default properties contain `fips=yes`.
+//   - When using OpenSSL 3, will always return true if [FIPS] returns true.
+//   - When using OpenSSL 1, Will always return the same value as [FIPS].
+//   - OpenSSL 3 doesn't provide a way to know if a provider is FIPS-capable. This function uses
+//     some heuristics that should be treated as an implementation detail that may change in the future.
+func FIPSCapable() bool {
+	if FIPS() {
+		return true
+	}
+	if vMajor == 3 {
+		// Load the provider with and without the `fips=yes` query.
+		// If the providers are the same, then the default provider is FIPS-capable.
+		provFIPS := sha256Provider(propFIPS)
+		if provFIPS == nil {
+			return false
+		}
+		provDefault := sha256Provider(nil)
+		return provFIPS == provDefault
+	}
+	return false
+}
+
 // isProviderAvailable checks if the provider with the given name is available.
 // This function is used in export_test.go, but must be defined here as test files can't access C functions.
 func isProviderAvailable(name string) bool {
@@ -193,16 +224,22 @@ func SetFIPS(enable bool) error {
 	}
 }
 
-// proveSHA256 checks if the SHA-256 algorithm is available
+// sha256Provider returns the provider for the SHA-256 algorithm
 // using the given properties.
-func proveSHA256(props *C.char) bool {
+func sha256Provider(props *C.char) C.GO_OSSL_PROVIDER_PTR {
 	md := C.go_openssl_EVP_MD_fetch(nil, algorithmSHA256, props)
 	if md == nil {
 		C.go_openssl_ERR_clear_error()
-		return false
+		return nil
 	}
-	C.go_openssl_EVP_MD_free(md)
-	return true
+	defer C.go_openssl_EVP_MD_free(md)
+	return C.go_openssl_EVP_MD_get0_provider(md)
+}
+
+// proveSHA256 checks if the SHA-256 algorithm is available
+// using the given properties.
+func proveSHA256(props *C.char) bool {
+	return sha256Provider(props) != nil
 }
 
 // noescape hides a pointer from escape analysis. noescape is

openssl_test.go

@@ -62,6 +62,7 @@ func TestMain(m *testing.M) {
 	_ = openssl.SetFIPS(true) // Skip the error as we still want to run the tests on machines without FIPS support.
 	fmt.Println("OpenSSL version:", openssl.VersionText())
 	fmt.Println("FIPS enabled:", openssl.FIPS())
+	fmt.Println("FIPS capable:", openssl.FIPSCapable())
 	status := m.Run()
 	for range 5 {
 		// Run GC a few times to avoid false positives in leak detection.
@@ -133,3 +134,15 @@ func TestSetFIPS(t *testing.T) {
 		t.Skip("FIPS mode is not supported")
 	}
 }
+
+func TestFIPSCapable(t *testing.T) {
+	got := openssl.FIPSCapable()
+	want := openssl.FIPS()
+	if !want && openssl.SymCryptProviderAvailable() {
+		// The SymCrypt provider is FIPS-capable.
+		want = true
+	}
+	if got != want {
+		t.Fatalf("FIPSCapable mismatch: want %v, got %v", want, got)
+	}
+}