fix(saml): improve error handling (#8928)

# Which Problems Are Solved

There are multiple issues with the metadata and error handling of SAML:
- When providing a SAML metadata for an IdP, which cannot be processed,
the error will only be noticed once a user tries to use the IdP.
- Parsing for metadata with any other encoding than UTF-8 fails.
- Metadata containing an enclosing EntitiesDescriptor around
EntityDescriptor cannot be parsed.
- Metadata's `validUntil` value is always set to 48 hours, which causes
issues on external providers, if processed from a manual down/upload.
- If a SAML response cannot be parsed, only a generic "Authentication
failed" error is returned, the cause is hidden to the user and also to
actions.

# How the Problems Are Solved

- Return parsing errors after create / update and retrieval of an IdP in
the API.
- Prevent the creation and update of an IdP in case of a parsing
failure.
- Added decoders for encodings other than UTF-8 (including ASCII,
windows and ISO, [currently
supported](efd25daf28/encoding/ianaindex/ianaindex.go (L156)))
- Updated parsing to handle both `EntitiesDescriptor` and
`EntityDescriptor` as root element
- `validUntil` will automatically set to the certificate's expiration
time
- Unwrapped the hidden error to be returned. The Login UI will still
only provide a mostly generic error, but action can now access the
underlying error.

# Additional Changes

None

# Additional Context

reported by a customer

(cherry picked from commit ffe9570776)

internal/idp/providers/saml/saml.go

@@ -1,15 +1,19 @@
 package saml
 
 import (
+	"bytes"
 	"context"
 	"crypto/rsa"
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/xml"
+	"io"
 	"net/url"
+	"time"
 
 	"github.com/crewjam/saml"
 	"github.com/crewjam/saml/samlsp"
+	"golang.org/x/text/encoding/ianaindex"
 
 	"github.com/zitadel/zitadel/internal/domain"
 	"github.com/zitadel/zitadel/internal/idp"
@@ -104,6 +108,41 @@ func WithEntityID(entityID string) ProviderOpts {
 	}
 }
 
+// ParseMetadata parses the metadata with the provided XML encoding and returns the EntityDescriptor
+func ParseMetadata(metadata []byte) (*saml.EntityDescriptor, error) {
+	entityDescriptor := new(saml.EntityDescriptor)
+	reader := bytes.NewReader(metadata)
+	decoder := xml.NewDecoder(reader)
+	decoder.CharsetReader = func(charset string, reader io.Reader) (io.Reader, error) {
+		enc, err := ianaindex.IANA.Encoding(charset)
+		if err != nil {
+			return nil, err
+		}
+		return enc.NewDecoder().Reader(reader), nil
+	}
+	if err := decoder.Decode(entityDescriptor); err != nil {
+		if err.Error() == "expected element type <EntityDescriptor> but have <EntitiesDescriptor>" {
+			// reset reader to start of metadata so we can try to parse it as an EntitiesDescriptor
+			if _, err := reader.Seek(0, io.SeekStart); err != nil {
+				return nil, err
+			}
+			entities := &saml.EntitiesDescriptor{}
+			if err := decoder.Decode(entities); err != nil {
+				return nil, err
+			}
+
+			for i, e := range entities.EntityDescriptors {
+				if len(e.IDPSSODescriptors) > 0 {
+					return &entities.EntityDescriptors[i], nil
+				}
+			}
+			return nil, zerrors.ThrowInternal(nil, "SAML-Ejoi3r2", "no entity found with IDPSSODescriptor")
+		}
+		return nil, err
+	}
+	return entityDescriptor, nil
+}
+
 func New(
 	name string,
 	rootURLStr string,
@@ -112,8 +151,8 @@ func New(
 	key []byte,
 	options ...ProviderOpts,
 ) (*Provider, error) {
-	entityDescriptor := new(saml.EntityDescriptor)
-	if err := xml.Unmarshal(metadata, entityDescriptor); err != nil {
+	entityDescriptor, err := ParseMetadata(metadata)
+	if err != nil {
 		return nil, err
 	}
 	keyPair, err := tls.X509KeyPair(certificate, key)
@@ -180,6 +219,7 @@ func (p *Provider) GetSP() (*samlsp.Middleware, error) {
 	if p.binding != "" {
 		sp.Binding = p.binding
 	}
+	sp.ServiceProvider.MetadataValidDuration = time.Until(sp.ServiceProvider.Certificate.NotAfter)
 	return sp, nil
 }