Repository: go-webauthn/webauthn (go)

Issue:
Make protocol error wrap error that caused it.
### Description

I'm trying to authenticate a user using discoverable login.

If credentials are not found in the database I want to tell the user to remove credentials which they just tried to use.
For that I return my sentinel `ErrCredentialsNotFound` from `DiscoverableUserHandler` so I can extract it down the line and tell the user it doesn't exist. However my sentinel error doesn't get wrapped inside `FinishDiscoverableLogin` but being turned into a string in `ValidatePasskeyLogin` in [login.go](https://github.com/go-webauthn/webauthn/blob/b9a233f627c94835e5ad3d2bc4a63dd55140580b/webauthn/login.go#L253)

But even if it was wrapped with for example `fmt.Errorf` it wouldn't help because `protocol.Error`'s field `Details` [is a string](https://github.com/go-webauthn/webauthn/blob/b9a233f627c94835e5ad3d2bc4a63dd55140580b/protocol/errors.go#L77)

```go
var webAuthnUserId uint64
creds, err := a.webAuthn.FinishDiscoverableLogin(func(rawID, userHandle []byte) (user webauthn.User, err error) {
    credsId := strconv.FormatUint(idBytesToUint64(rawID), 10)
    rawUser, err := a.GetWebAuthnUserByWebauthnID(ctx, idBytesToUint64(userHandle))
    webAuthnUserId = idBytesToUint64(userHandle)
    if err != nil {
        return nil, fmt.Errorf("failed to fetch user: %w", err)
    }
                                                                                                                  
    creds, err := a.GetUserWebAuthnCredentialsById(ctx, idBytesToUint64(userHandle), credsId)
    if errors.Is(err, sql.ErrNoRows) {
        return nil, ErrCredentialsNotFound
    }
                                                                                                                  
    if err != nil {
        return nil, fmt.Errorf("failed to fetch user credentials: %w", err)
    }
    rawUser.credentials = []webauthn.Credential{creds}
                                                                                                                  
    return rawUser, nil
}, session, r)

if errors.Is(err, ErrCredentialsNotFound) {
    return "", time.Time{}, errors.Join(ErrInvalidCredentials, err)
}
```
What I want is to have an ability to unwrap `protocol.Error` to get errors that caused it.
Perhaps changing `Details` field to being an `error` type and adding `Unwrap` method would be a good decision to keep consistency with `golang` ecosystem. And according to your readme you are at version 0 and breaking changes should be expected.

As per my case I can work around it by declaring a flag outside my handler function and setting it inside when I encounter a sentinel error just like I'm setting `webAuthnUserId`. But it would be better if it was inside of the error returned from `FinishDiscoverableLogin`.

### Use Case

_No response_

### Documentation

_No response_

Pull request description:
feat(protocol): enhance errors
This adds significant improvements to the MDS3 errors and general errors helping identify the root cause of a validation errors as well as many general errors via additional information or unwrapping the inner error.

Closes #365

Patch:
diff --git a/README.md b/README.md
index 202c177..82cd186 100644
--- a/README.md
+++ b/README.md
@@ -237,7 +237,7 @@ func beginLogin() {
 	allowList := make([]protocol.CredentialDescriptor, 1)
 	allowList[0] = protocol.CredentialDescriptor{
 		CredentialID: credentialToAllowID,
-		Type: protocol.CredentialType("public-key"),
+		Type: protocol.PublicKeyCredentialType,
 	}
 
 	user := datastore.GetUser() // Get the user  
diff --git a/metadata/const.go b/metadata/const.go
index 97a5ad4..76e6a33 100644
--- a/metadata/const.go
+++ b/metadata/const.go
@@ -20,15 +20,15 @@ const (
 )
 
 var (
-	errIntermediateCertRevoked = &MetadataError{
+	errIntermediateCertRevoked = &Error{
 		Type:    "intermediate_revoked",
 		Details: "Intermediate certificate is on issuers revocation list",
 	}
-	errLeafCertRevoked = &MetadataError{
+	errLeafCertRevoked = &Error{
 		Type:    "leaf_revoked",
 		Details: "Leaf certificate is on issuers revocation list",
 	}
-	errCRLUnavailable = &MetadataError{
+	errCRLUnavailable = &Error{
 		Type:    "crl_unavailable",
 		Details: "Certificate revocation list is unavailable",
 	}
diff --git a/metadata/status.go b/metadata/status.go
index 158e37c..f20d716 100644
--- a/metadata/status.go
+++ b/metadata/status.go
@@ -44,17 +44,17 @@ func ValidateStatusReports(reports []StatusReport, desired, undesired []Authenti
 	case len(present) == 0 && len(absent) == 0:
 		return nil
 	case len(present) != 0 && len(absent) == 0:
-		return &MetadataError{
+		return &Error{
 			Type:    "invalid_status",
 			Details: fmt.Sprintf("The following undesired status reports were present: %s", strings.Join(present, ", ")),
 		}
 	case len(present) == 0 && len(absent) != 0:
-		return &MetadataError{
+		return &Error{
 			Type:    "invalid_status",
 			Details: fmt.Sprintf("The following desired status reports were absent: %s", strings.Join(absent, ", ")),
 		}
 	default:
-		return &MetadataError{
+		return &Error{
 			Type:    "invalid_status",
 			Details: fmt.Sprintf("The following undesired status reports were present: %s; the following desired status reports were absent: %s", strings.Join(present, ", "), strings.Join(absent, ", ")),
 		}
diff --git a/metadata/types.go b/metadata/types.go
index 1562b63..dcc8913 100644
--- a/metadata/types.go
+++ b/metadata/types.go
@@ -299,7 +299,7 @@ const (
 	ALG_KEY_COSE PublicKeyAlgAndEncoding = "cose"
 )
 
-type MetadataError struct {
+type Error struct {
 	// Short name for the type of error that has occurred.
 	Type string `json:"type"`
 
@@ -310,8 +310,8 @@ type MetadataError struct {
 	DevInfo string `json:"debug"`
 }
 
-func (err *MetadataError) Error() string {
-	return err.Details
+func (e *Error) Error() string {
+	return e.Details
 }
 
 // Clock is an interface used to implement clock functionality in various metadata areas.
diff --git a/protocol/assertion.go b/protocol/assertion.go
index 50cb500..f3caa91 100644
--- a/protocol/assertion.go
+++ b/protocol/assertion.go
@@ -54,8 +54,10 @@ func ParseCredentialRequestResponse(response *http.Request) (*ParsedCredentialAs
 		return nil, ErrBadRequest.WithDetails("No response given")
 	}
 
-	defer response.Body.Close()
-	defer io.Copy(io.Discard, response.Body)
+	defer func(request *http.Request) {
+		_, _ = io.Copy(io.Discard, request.Body)
+		_ = request.Body.Close()
+	}(response)
 
 	return ParseCredentialRequestResponseBody(response.Body)
 }
@@ -67,7 +69,7 @@ func ParseCredentialRequestResponseBody(body io.Reader) (par *ParsedCredentialAs
 	var car CredentialAssertionResponse
 
 	if err = decodeBody(body, &car); err != nil {
-		return nil, ErrBadRequest.WithDetails("Parse error for Assertion").WithInfo(err.Error())
+		return nil, ErrBadRequest.WithDetails("Parse error for Assertion").WithInfo(err.Error()).WithError(err)
 	}
 
 	return car.Parse()
@@ -79,7 +81,7 @@ func ParseCredentialRequestResponseBytes(data []byte) (par *ParsedCredentialAsse
 	var car CredentialAssertionResponse
 
 	if err = decodeBytes(data, &car); err != nil {
-		return nil, ErrBadRequest.WithDetails("Parse error for Assertion").WithInfo(err.Error())
+		return nil, ErrBadRequest.WithDetails("Parse error for Assertion").WithInfo(err.Error()).WithError(err)
 	}
 
 	return car.Parse()
@@ -95,20 +97,18 @@ func (car CredentialAssertionResponse) Parse() (par *ParsedCredentialAssertionDa
 	}
 
 	if _, err = base64.RawURLEncoding.DecodeString(car.ID); err != nil {
-		return nil, ErrBadRequest.WithDetails("CredentialAssertionResponse with ID not base64url encoded")
+		return nil, ErrBadRequest.WithDetails("CredentialAssertionResponse with ID not base64url encoded").WithError(err)
 	}
 
-	if car.Type != "public-key" {
+	if car.Type != string(PublicKeyCredentialType) {
 		return nil, ErrBadRequest.WithDetails("CredentialAssertionResponse with bad type")
 	}
 
 	var attachment AuthenticatorAttachment
 
-	switch car.AuthenticatorAttachment {
-	case "platform":
-		attachment = Platform
-	case "cross-platform":
-		attachment = CrossPlatform
+	switch att := AuthenticatorAttachment(car.AuthenticatorAttachment); att {
+	case Platform, CrossPlatform:
+		attachment = att
 	}
 
 	par = &ParsedCredentialAssertionData{
@@ -129,7 +129,7 @@ func (car CredentialAssertionResponse) Parse() (par *ParsedCredentialAssertionDa
 	}
 
 	if err = par.Response.AuthenticatorData.Unmarshal(car.AssertionResponse.AuthenticatorData); err != nil {
-		return nil, ErrParsingData.WithDetails("Error unmarshalling auth data")
+		return nil, ErrParsingData.WithDetails("Error unmarshalling auth data").WithError(err)
 	}
 
 	return par, nil
@@ -143,9 +143,9 @@ func (p *ParsedCredentialAssertionData) Verify(storedChallenge string, relyingPa
 	// Steps 4 through 6 in verifying the assertion data (https://www.w3.org/TR/webauthn/#verifying-assertion) are
 	// "assertive" steps, i.e. "Let JSONtext be the result of running UTF-8 decode on the value of cData."
 	// We handle these steps in part as we verify but also beforehand
-
+	//
 	// Handle steps 7 through 10 of assertion by verifying stored data against the Collected Client Data
-	// returned by the authenticator
+	// returned by the authenticator.
 	validError := p.Response.CollectedClientData.Verify(storedChallenge, AssertCeremony, rpOrigins, rpTopOrigins, rpTopOriginsVerify)
 	if validError != nil {
 		return validError
@@ -189,12 +189,12 @@ func (p *ParsedCredentialAssertionData) Verify(storedChallenge string, relyingPa
 	}
 
 	if err != nil {
-		return ErrAssertionSignature.WithDetails(fmt.Sprintf("Error parsing the assertion public key: %+v", err))
+		return ErrAssertionSignature.WithDetails(fmt.Sprintf("Error parsing the assertion public key: %+v", err)).WithError(err)
 	}
 
 	valid, err := webauthncose.VerifySignature(key, sigData, p.Response.Signature)
 	if !valid || err != nil {
-		return ErrAssertionSignature.WithDetails(fmt.Sprintf("Error validating the assertion signature: %+v", err))
+		return ErrAssertionSignature.WithDetails(fmt.Sprintf("Error validating the assertion signature: %+v", err)).WithError(err)
 	}
 
 	return nil
diff --git a/protocol/authenticator.go b/protocol/authenticator.go
index 82bd910..4cd636a 100644
--- a/protocol/authenticator.go
+++ b/protocol/authenticator.go
@@ -349,7 +349,7 @@ func (a *AuthenticatorData) unmarshalAttestedData(rawAuthData []byte) (err error
 
 	a.AttData.CredentialPublicKey, err = unmarshalCredentialPublicKey(rawAuthData[55+idLength:])
 	if err != nil {
-		return ErrBadRequest.WithDetails(fmt.Sprintf("Could not unmarshal Credential Public Key: %v", err))
+		return ErrBadRequest.WithDetails(fmt.Sprintf("Could not unmarshal Credential Public Key: %v", err)).WithError(err)
 	}
 
 	return nil
diff --git a/protocol/client.go b/protocol/client.go
index 7e8356a..e5c2167 100644
--- a/protocol/client.go
+++ b/protocol/client.go
@@ -43,13 +43,15 @@ type TokenBinding struct {
 type TokenBindingStatus string
 
 const (
-	// Indicates token binding was used when communicating with the
+	// Present indicates token binding was used when communicating with the
 	// Relying Party. In this case, the id member MUST be present.
 	Present TokenBindingStatus = "present"
-	// Indicates token binding was used when communicating with the
+
+	// Supported indicates token binding was used when communicating with the
 	// negotiated when communicating with the Relying Party.
 	Supported TokenBindingStatus = "supported"
-	// Indicates token binding not supported
+
+	// NotSupported indicates token binding not supported
 	// when communicating with the Relying Party.
 	NotSupported TokenBindingStatus = "not-supported"
 )
@@ -109,7 +111,7 @@ func (c *CollectedClientData) Verify(storedChallenge string, ceremony CeremonyTy
 	var fqOrigin string
 
 	if fqOrigin, err = FullyQualifiedOrigin(c.Origin); err != nil {
-		return ErrParsingData.WithDetails("Error decoding clientData origin as URL")
+		return ErrParsingData.WithDetails("Error decoding clientData origin as URL").WithError(err)
 	}
 
 	found := false
@@ -144,7 +146,7 @@ func (c *CollectedClientData) Verify(storedChallenge string, ceremony CeremonyTy
 			)
 
 			if fqTopOrigin, err = FullyQualifiedOrigin(c.TopOrigin); err != nil {
-				return ErrParsingData.WithDetails("Error decoding clientData topOrigin as URL")
+				return ErrParsingData.WithDetails("Error decoding clientData topOrigin as URL").WithError(err)
 			}
 
 			switch rpTopOriginsVerify {
diff --git a/protocol/credential.go b/protocol/credential.go
index 5c9f6b0..7b18a1d 100644
--- a/protocol/credential.go
+++ b/protocol/credential.go
@@ -79,7 +79,7 @@ func ParseCredentialCreationResponseBody(body io.Reader) (pcc *ParsedCredentialC
 	var ccr CredentialCreationResponse
 
 	if err = decodeBody(body, &ccr); err != nil {
-		return nil, ErrBadRequest.WithDetails("Parse error for Registration").WithInfo(err.Error())
+		return nil, ErrBadRequest.WithDetails("Parse error for Registration").WithInfo(err.Error()).WithError(err)
 	}
 
 	return ccr.Parse()
@@ -91,7 +91,7 @@ func ParseCredentialCreationResponseBytes(data []byte) (pcc *ParsedCredentialCre
 	var ccr CredentialCreationResponse
 
 	if err = decodeBytes(data, &ccr); err != nil {
-		return nil, ErrBadRequest.WithDetails("Parse error for Registration").WithInfo(err.Error())
+		return nil, ErrBadRequest.WithDetails("Parse error for Registration").WithInfo(err.Error()).WithError(err)
 	}
 
 	return ccr.Parse()
@@ -115,7 +115,7 @@ func (ccr CredentialCreationResponse) Parse() (pcc *ParsedCredentialCreationData
 		return nil, ErrBadRequest.WithDetails("Parse error for Registration").WithInfo("Missing type")
 	}
 
-	if ccr.PublicKeyCredential.Credential.Type != "public-key" {
+	if ccr.PublicKeyCredential.Credential.Type != string(PublicKeyCredentialType) {
 		return nil, ErrBadRequest.WithDetails("Parse error for Registration").WithInfo("Type not public-key")
 	}
 
diff --git a/protocol/errors.go b/protocol/errors.go
index 6d134d6..d09b80e 100644
--- a/protocol/errors.go
+++ b/protocol/errors.go
@@ -9,6 +9,38 @@ type Error struct {
 
 	// Information to help debug the error.
 	DevInfo string `json:"debug"`
+
+	// Inner error.
+	Err error `json:"-"`
+}
+
+func (e *Error) Error() string {
+	return e.Details
+}
+
+func (e *Error) Unwrap() error {
+	return e.Err
+}
+
+func (e *Error) WithDetails(details string) *Error {
+	err := *e
+	err.Details = details
+
+	return &err
+}
+
+func (e *Error) WithInfo(info string) *Error {
+	err := *e
+	err.DevInfo = info
+
+	return &err
+}
+
+func (e *Error) WithError(err error) *Error {
+	errCopy := *e
+	errCopy.Err = err
+
+	return &errCopy
 }
 
 var (
@@ -40,6 +72,10 @@ var (
 		Type:    "invalid_attestation",
 		Details: "Invalid attestation data",
 	}
+	ErrMetadata = &Error{
+		Type:    "invalid_metadata",
+		Details: "",
+	}
 	ErrAttestationFormat = &Error{
 		Type:    "invalid_attestation",
 		Details: "Invalid attestation format",
@@ -69,21 +105,3 @@ var (
 		Details: "This field is not yet supported by this library",
 	}
 )
-
-func (e *Error) Error() string {
-	return e.Details
-}
-
-func (e *Error) WithDetails(details string) *Error {
-	err := *e
-	err.Details = details
-
-	return &err
-}
-
-func (e *Error) WithInfo(info string) *Error {
-	err := *e
-	err.DevInfo = info
-
-	return &err
-}
diff --git a/protocol/metadata.go b/protocol/metadata.go
index a7e6651..da48cb8 100644
--- a/protocol/metadata.go
+++ b/protocol/metadata.go
@@ -2,6 +2,7 @@ package protocol
 
 import (
 	"context"
+	"crypto/x509"
 	"fmt"
 
 	"github.com/google/uuid"
@@ -9,17 +10,18 @@ import (
 	"github.com/go-webauthn/webauthn/metadata"
 )
 
-func ValidateMetadata(ctx context.Context, aaguid uuid.UUID, mds metadata.Provider) (err error) {
+func ValidateMetadata(ctx context.Context, mds metadata.Provider, aaguid uuid.UUID, attestationType string, x5cs []any) (protoErr *Error) {
 	if mds == nil {
 		return nil
 	}
 
 	var (
 		entry *metadata.Entry
+		err   error
 	)
 
 	if entry, err = mds.GetEntry(ctx, aaguid); err != nil {
-		return err
+		return ErrMetadata.WithInfo(fmt.Sprintf("Failed to validate authenticator metadata for Authenticator Attestation GUID '%s'. Error occurred retreiving the metadata entry: %+v", aaguid, err))
 	}
 
 	if entry == nil {
@@ -28,17 +30,97 @@ func ValidateMetadata(ctx context.Context, aaguid uuid.UUID, mds metadata.Provid
 		}
 
 		if mds.GetValidateEntry(ctx) {
-			return fmt.Errorf("error occurred performing authenticator entry validation: AAGUID entry has not been registered with the metadata service")
+			return ErrMetadata.WithInfo(fmt.Sprintf("Failed to validate authenticator metadata for Authenticator Attestation GUID '%s'. The authenticator has no registered metadata.", aaguid))
 		}
 
 		return nil
 	}
 
+	if mds.GetValidateAttestationTypes(ctx) {
+		found := false
+
+		for _, atype := range entry.MetadataStatement.AttestationTypes {
+			if string(atype) == attestationType {
+				found = true
+
+				break
+			}
+		}
+
+		if !found {
+			return ErrMetadata.WithInfo(fmt.Sprintf("Failed to validate authenticator metadata for Authenticator Attestation GUID '%s'. The attestation type '%s' is not known to be used by this authenticator.", aaguid.String(), attestationType))
+		}
+	}
+
 	if mds.GetValidateStatus(ctx) {
 		if err = mds.ValidateStatusReports(ctx, entry.StatusReports); err != nil {
-			return fmt.Errorf("error occurred performing authenticator status validation: %w", err)
+			return ErrMetadata.WithInfo(fmt.Sprintf("Failed to validate authenticator metadata for Authenticator Attestation GUID '%s'. Error occurred validating the authenticator status: %+v", aaguid, err))
+		}
+	}
+
+	if mds.GetValidateTrustAnchor(ctx) {
+		if len(x5cs) == 0 {
+			return nil
+		}
+
+		var (
+			x5c, parsed *x509.Certificate
+			x5cis       []*x509.Certificate
+			raw         []byte
+			ok          bool
+		)
+
+		for i, x5cAny := range x5cs {
+			if raw, ok = x5cAny.([]byte); !ok {
+				return ErrMetadata.WithDetails(fmt.Sprintf("Failed to parse attestation certificate from x5c during attestation validation for Authenticator Attestation GUID '%s'.", aaguid)).WithInfo(fmt.Sprintf("The %s certificate in the attestation was type '%T' but '[]byte' was expected", loopOrdinalNumber(i), x5cAny))
+			}
+
+			if parsed, err = x509.ParseCertificate(raw); err != nil {
+				return ErrMetadata.WithDetails(fmt.Sprintf("Failed to parse attestation certificate from x5c during attestation validation for Authenticator Attestation GUID '%s'.", aaguid)).WithInfo(fmt.Sprintf("Error returned from x509.ParseCertificate: %+v", err)).WithError(err)
+			}
+
+			if x5c == nil {
+				x5c = parsed
+			} else {
+				x5cis = append(x5cis, parsed)
+			}
+		}
+
+		if attestationType == string(metadata.AttCA) {
+			if protoErr = tpmParseAIKAttCA(x5c, x5cis); protoErr != nil {
+				return ErrMetadata.WithDetails(protoErr.Details).WithInfo(protoErr.DevInfo).WithError(protoErr)
+			}
+		}
+
+		if x5c != nil && x5c.Subject.CommonName != x5c.Issuer.CommonName {
+			if !entry.MetadataStatement.AttestationTypes.HasBasicFull() {
+				return ErrMetadata.WithDetails(fmt.Sprintf("Failed to validate attestation statement signature during attestation validation for Authenticator Attestation GUID '%s'. Attestation was provided in the full format but the authenticator doesn't support the full attestation format.", aaguid))
+			}
+
+			if _, err = x5c.Verify(entry.MetadataStatement.Verifier(x5cis)); err != nil {
+				return ErrMetadata.WithDetails(fmt.Sprintf("Failed to validate attestation statement signature during attestation validation for Authenticator Attestation GUID '%s'. The attestation certificate could not be verified due to an error validating the trust chain agaisnt the Metadata Service.", aaguid)).WithError(err)
+			}
 		}
 	}
 
 	return nil
 }
+
+func loopOrdinalNumber(n int) string {
+	n++
+
+	if n > 9 && n < 20 {
+		return fmt.Sprintf("%dth", n)
+	}
+
+	switch n % 10 {
+	case 1:
+		return fmt.Sprintf("%dst", n)
+	case 2:
+		return fmt.Sprintf("%dnd", n)
+	case 3:
+		return fmt.Sprintf("%drd", n)
+	default:
+		return fmt.Sprintf("%dth", n)
+	}
+}
diff --git a/webauthn/login.go b/webauthn/login.go
index acd1e26..9c73b34 100644
--- a/webauthn/login.go
+++ b/webauthn/login.go
@@ -186,7 +186,7 @@ func WithLoginRelyingPartyID(id string) LoginOption {
 // WithChallenge overrides the default random challenge with a user supplied value.
 // In order to prevent replay attacks, the challenges MUST contain enough entropy to make guessing them infeasible.
 // Challenges SHOULD therefore be at least 16 bytes long.
-// This function is EXPERIMENTAL and can be removed without warning. 
+// This function is EXPERIMENTAL and can be removed without warning.
 //
 // Specification: §13.4.3. Cryptographic Challenges (https://www.w3.org/TR/webauthn/#sctn-cryptographic-challenges)
 func WithChallenge(challenge []byte) LoginOption {
@@ -250,7 +250,7 @@ func (webauthn *WebAuthn) ValidatePasskeyLogin(handler DiscoverableUserHandler,
 	}
 
 	if user, err = handler(parsedResponse.RawID, parsedResponse.Response.UserHandle); err != nil {
-		return nil, nil, protocol.ErrBadRequest.WithDetails(fmt.Sprintf("Failed to lookup Client-side Discoverable Credential: %s", err))
+		return nil, nil, protocol.ErrBadRequest.WithDetails(fmt.Sprintf("Failed to lookup Client-side Discoverable Credential: %s", err)).WithError(err)
 	}
 
 	if credential, err = webauthn.validateLogin(user, session, parsedResponse); err != nil {
@@ -344,11 +344,13 @@ func (webauthn *WebAuthn) validateLogin(user User, session SessionData, parsedRe
 		var aaguid uuid.UUID
 
 		if aaguid, err = uuid.FromBytes(credential.Authenticator.AAGUID); err != nil {
-			return nil, protocol.ErrBadRequest.WithDetails("Failed to decode AAGUID").WithInfo(fmt.Sprintf("Error occurred decoding AAGUID from the credential record: %s", err))
+			return nil, protocol.ErrBadRequest.WithDetails("Failed to decode AAGUID").WithInfo(fmt.Sprintf("Error occurred decoding AAGUID from the credential record: %s", err)).WithError(err)
 		}
 
-		if err = protocol.ValidateMetadata(context.Background(), aaguid, webauthn.Config.MDS); err != nil {
-			return nil, protocol.ErrBadRequest.WithDetails("Failed to validate credential record metadata").WithInfo(fmt.Sprintf("Error occurred validating authenticator metadata from the credential record: %s", err))
+		var protoErr *protocol.Error
+
+		if protoErr = protocol.ValidateMetadata(context.Background(), webauthn.Config.MDS, aaguid, credential.AttestationType, nil); protoErr != nil {
+			return nil, protocol.ErrBadRequest.WithDetails("Failed to validate credential record metadata").WithInfo(protoErr.DevInfo).WithError(protoErr)
 		}
 	}
 


Test patch:
diff --git a/metadata/metadata_test.go b/metadata/metadata_test.go
index b720fdf..b0bfb80 100644
--- a/metadata/metadata_test.go
+++ b/metadata/metadata_test.go
@@ -82,7 +82,7 @@ func TestConformanceMetadataTOCParsing(t *testing.T) {
 	var (
 		res  *http.Response
 		blob *PayloadJSON
-		me   *MetadataError
+		me   *Error
 	)
 
 	for _, endpoint := range endpoints {
diff --git a/protocol/assertion_test.go b/protocol/assertion_test.go
index 2731850..c93f467 100644
--- a/protocol/assertion_test.go
+++ b/protocol/assertion_test.go
@@ -44,7 +44,7 @@ func TestParseCredentialRequestResponse(t *testing.T) {
 				ParsedPublicKeyCredential: ParsedPublicKeyCredential{
 					ParsedCredential: ParsedCredential{
 						ID:   "AI7D5q2P0LS-Fal9ZT7CHM2N5BLbUunF92T8b6iYC199bO2kagSuU05-5dZGqb1SP0A0lyTWng",
-						Type: "public-key",
+						Type: string(PublicKeyCredentialType),
 					},
 					RawID: byteID,
 					ClientExtensionResults: map[string]any{
@@ -74,7 +74,7 @@ func TestParseCredentialRequestResponse(t *testing.T) {
 				Raw: CredentialAssertionResponse{
 					PublicKeyCredential: PublicKeyCredential{
 						Credential: Credential{
-							Type: "public-key",
+							Type: string(PublicKeyCredentialType),
 							ID:   "AI7D5q2P0LS-Fal9ZT7CHM2N5BLbUunF92T8b6iYC199bO2kagSuU05-5dZGqb1SP0A0lyTWng",
 						},
 						RawID: byteID,
diff --git a/protocol/attestation.go b/protocol/attestation.go
index be30e92..144fd7c 100644
--- a/protocol/attestation.go
+++ b/protocol/attestation.go
@@ -3,7 +3,6 @@ package protocol
 import (
 	"context"
 	"crypto/sha256"
-	"crypto/x509"
 	"encoding/json"
 	"fmt"
 
@@ -95,18 +94,18 @@ func (ccr *AuthenticatorAttestationResponse) Parse() (p *ParsedAttestationRespon
 	p = &ParsedAttestationResponse{}
 
 	if err = json.Unmarshal(ccr.ClientDataJSON, &p.CollectedClientData); err != nil {
-		return nil, ErrParsingData.WithInfo(err.Error())
+		return nil, ErrParsingData.WithInfo(err.Error()).WithError(err)
 	}
 
 	if err = webauthncbor.Unmarshal(ccr.AttestationObject, &p.AttestationObject); err != nil {
-		return nil, ErrParsingData.WithInfo(err.Error())
+		return nil, ErrParsingData.WithInfo(err.Error()).WithError(err)
 	}
 
 	// Step 8. Perform CBOR decoding on the attestationObject field of the AuthenticatorAttestationResponse
 	// structure to obtain the attestation statement format fmt, the authenticator data authData, and
 	// the attestation statement attStmt.
 	if err = p.AttestationObject.AuthData.Unmarshal(p.AttestationObject.RawAuthData); err != nil {
-		return nil, fmt.Errorf("error decoding auth data: %v", err)
+		return nil, err
 	}
 
 	if !p.AttestationObject.AuthData.Flags.HasAttestedCredentialData() {
@@ -144,12 +143,12 @@ func (a *AttestationObject) VerifyAttestation(clientDataHash []byte, mds metadat
 	// list of registered WebAuthn Attestation Statement Format Identifier
 	// values is maintained in the IANA registry of the same name
 	// [WebAuthn-Registries] (https://www.w3.org/TR/webauthn/#biblio-webauthn-registries).
-
+	//
 	// Since there is not an active registry yet, we'll check it against our internal
 	// Supported types.
-
+	//
 	// But first let's make sure attestation is present. If it isn't, we don't need to handle
-	// any of the following steps
+	// any of the following steps.
 	if AttestationFormat(a.Format) == AttestationFormatNone {
 		if len(a.AttStatement) != 0 {
 			return ErrAttestationFormat.WithInfo("Attestation format none with attestation present")
@@ -173,12 +172,11 @@ func (a *AttestationObject) VerifyAttestation(clientDataHash []byte, mds metadat
 
 	var (
 		aaguid uuid.UUID
-		entry  *metadata.Entry
 	)
 
 	if len(a.AuthData.AttData.AAGUID) != 0 {
 		if aaguid, err = uuid.FromBytes(a.AuthData.AttData.AAGUID); err != nil {
-			return ErrInvalidAttestation.WithInfo("Error occurred parsing AAGUID during attestation validation").WithDetails(err.Error())
+			return ErrInvalidAttestation.WithInfo("Error occurred parsing AAGUID during attestation validation").WithDetails(err.Error()).WithError(err)
 		}
 	}
 
@@ -186,105 +184,10 @@ func (a *AttestationObject) VerifyAttestation(clientDataHash []byte, mds metadat
 		return nil
 	}
 
-	ctx := context.Background()
-
-	if entry, err = mds.GetEntry(ctx, aaguid); err != nil {
-		return ErrInvalidAttestation.WithInfo(fmt.Sprintf("Error occurred retrieving metadata entry during attestation validation: %+v", err)).WithDetails(fmt.Sprintf("Error occurred looking up entry for AAGUID %s", aaguid.String()))
-	}
-
-	if entry == nil {
-		if aaguid == uuid.Nil && mds.GetValidateEntryPermitZeroAAGUID(ctx) {
-			return nil
-		}
-
-		if mds.GetValidateEntry(ctx) {
-			return ErrInvalidAttestation.WithDetails(fmt.Sprintf("AAGUID %s not found in metadata during attestation validation", aaguid.String()))
-		}
+	var protoErr *Error
 
-		return nil
-	}
-
-	if mds.GetValidateAttestationTypes(ctx) {
-		found := false
-
-		for _, atype := range entry.MetadataStatement.AttestationTypes {
-			if string(atype) == attestationType {
-				found = true
-
-				break
-			}
-		}
-
-		if !found {
-			return ErrInvalidAttestation.WithDetails(fmt.Sprintf("Authenticator with invalid attestation type encountered during attestation validation. The attestation type '%s' is not known to be used by AAGUID '%s'", attestationType, aaguid.String()))
-		}
-	}
-
-	if mds.GetValidateStatus(ctx) {
-		if err = mds.ValidateStatusReports(ctx, entry.StatusReports); err != nil {
-			return ErrInvalidAttestation.WithDetails(fmt.Sprintf("Authenticator with invalid status encountered during attestation validation. %s", err.Error()))
-		}
-	}
-
-	if mds.GetValidateTrustAnchor(ctx) {
-		if x5cs == nil {
-			return nil
-		}
-
-		var (
-			x5c, parsed *x509.Certificate
-			x5cis       []*x509.Certificate
-			raw         []byte
-			ok          bool
-		)
-
-		if len(x5cs) == 0 {
-			return ErrInvalidAttestation.WithDetails("Unable to parse attestation certificate from x5c during attestation validation").WithInfo("The attestation had no certificates")
-		}
-
-		for _, x5cAny := range x5cs {
-			if raw, ok = x5cAny.([]byte); !ok {
-				return ErrInvalidAttestation.WithDetails("Unable to parse attestation certificate from x5c during attestation validation").WithInfo(fmt.Sprintf("The first certificate in the attestation was type '%T' but '[]byte' was expected", x5cs[0]))
-			}
-
-			if parsed, err = x509.ParseCertificate(raw); err != nil {
-				return ErrInvalidAttestation.WithDetails("Unable to parse attestation certificate from x5c during attestation validation").WithInfo(fmt.Sprintf("Error returned from x509.ParseCertificate: %+v", err))
-			}
-
-			if x5c == nil {
-				x5c = parsed
-			} else {
-				x5cis = append(x5cis, parsed)
-			}
-		}
-
-		if attestationType == string(metadata.AttCA) {
-			if err = tpmParseSANExtension(x5c); err != nil {
-				return err
-			}
-
-			if err = tpmRemoveEKU(x5c); err != nil {
-				return err
-			}
-
-			for _, parent := range x5cis {
-				if err = tpmRemoveEKU(parent); err != nil {
-					return err
-				}
-			}
-		}
-
-		if x5c != nil && x5c.Subject.CommonName != x5c.Issuer.CommonName {
-			if !entry.MetadataStatement.AttestationTypes.HasBasicFull() {
-				return ErrInvalidAttestation.WithDetails("Unable to validate attestation statement signature during attestation validation: attestation with full attestation from authenticator that does not support full attestation")
-			}
-
-			verifier := entry.MetadataStatement.Verifier(x5cis)
-
-			if _, err = x5c.Verify(verifier); err != nil {
-				return ErrInvalidAttestation.WithDetails(fmt.Sprintf("Unable to validate attestation signature statement during attestation validation: invalid certificate chain from MDS: %v", err))
-			}
-		}
+	if protoErr = ValidateMetadata(context.Background(), mds, aaguid, attestationType, x5cs); protoErr != nil {
+		return ErrInvalidAttestation.WithInfo(fmt.Sprintf("Error occurred validating metadata during attestation validation: %+v", protoErr)).WithDetails(protoErr.DevInfo).WithError(protoErr)
 	}
 
 	return nil
diff --git a/protocol/attestation_androidkey.go b/protocol/attestation_androidkey.go
index 6e387b3..7bfff7e 100644
--- a/protocol/attestation_androidkey.go
+++ b/protocol/attestation_androidkey.go
@@ -65,25 +65,25 @@ func verifyAndroidKeyFormat(att AttestationObject, clientDataHash []byte, _ meta
 
 	attCert, err := x509.ParseCertificate(attCertBytes)
 	if err != nil {
-		return "", nil, ErrAttestationFormat.WithDetails(fmt.Sprintf("Error parsing certificate from ASN.1 data: %+v", err))
+		return "", nil, ErrAttestationFormat.WithDetails(fmt.Sprintf("Error parsing certificate from ASN.1 data: %+v", err)).WithError(err)
 	}
 
 	coseAlg := webauthncose.COSEAlgorithmIdentifier(alg)
 	if err = attCert.CheckSignature(webauthncose.SigAlgFromCOSEAlg(coseAlg), signatureData, sig); err != nil {
-		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Signature validation error: %+v\n", err))
+		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Signature validation error: %+v\n", err)).WithError(err)
 	}
 
 	// Verify that the public key in the first certificate in x5c matches the credentialPublicKey in the attestedCredentialData in authenticatorData.
 	pubKey, err := webauthncose.ParsePublicKey(att.AuthData.AttData.CredentialPublicKey)
 	if err != nil {
-		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error parsing public key: %+v\n", err))
+		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error parsing public key: %+v\n", err)).WithError(err)
 	}
 
 	e := pubKey.(webauthncose.EC2PublicKeyData)
 
 	valid, err = e.Verify(signatureData, sig)
 	if err != nil || !valid {
-		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error parsing public key: %+v\n", err))
+		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error parsing public key: %+v\n", err)).WithError(err)
 	}
 
 	// §8.4.3. Verify that the attestationChallenge field in the attestation certificate extension data is identical to clientDataHash.
@@ -105,11 +105,11 @@ func verifyAndroidKeyFormat(att AttestationObject, clientDataHash []byte, _ meta
 	decoded := keyDescription{}
 
 	if _, err = asn1.Unmarshal(attExtBytes, &decoded); err != nil {
-		return "", nil, ErrAttestationFormat.WithDetails("Unable to parse Android key attestation certificate extensions")
+		return "", nil, ErrAttestationFormat.WithDetails("Unable to parse Android key attestation certificate extensions").WithError(err)
 	}
 
 	// Verify that the attestationChallenge field in the attestation certificate extension data is identical to clientDataHash.
-	if 0 != bytes.Compare(decoded.AttestationChallenge, clientDataHash) {
+	if bytes.Compare(decoded.AttestationChallenge, clientDataHash) != 0 {
 		return "", nil, ErrAttestationFormat.WithDetails("Attestation challenge not equal to clientDataHash")
 	}
 
diff --git a/protocol/attestation_androidkey_test.go b/protocol/attestation_androidkey_test.go
index bdec9bc..ba9d94d 100644
--- a/protocol/attestation_androidkey_test.go
+++ b/protocol/attestation_androidkey_test.go
@@ -54,14 +54,10 @@ func TestVerifyAndroidKeyFormat(t *testing.T) {
 				t.Errorf("verifyAndroidKeyFormat() error = %v, wantErr %v", err, tt.wantErr)
 				return
 			}
+
 			if got != tt.want {
 				t.Errorf("verifyAndroidKeyFormat() got = %v, want %v", got, tt.want)
 			}
-			/*
-			if !reflect.DeepEqual(got1, tt.want1) {
-				t.Errorf("verifySafetyNetFormat() got1 = %v, want %v", got1, tt.want1)
-			}
-			*/
 		})
 	}
 }
diff --git a/protocol/attestation_apple.go b/protocol/attestation_apple.go
index 9b80b28..e2b3e9e 100644
--- a/protocol/attestation_apple.go
+++ b/protocol/attestation_apple.go
@@ -34,8 +34,7 @@ func init() {
 func verifyAppleFormat(att AttestationObject, clientDataHash []byte, _ metadata.Provider) (string, []any, error) {
 	// Step 1. Verify that attStmt is valid CBOR conforming to the syntax defined
 	// above and perform CBOR decoding on it to extract the contained fields.
-
-	// If x5c is not present, return an error
+	// If x5c is not present, return an error.
 	x5c, x509present := att.AttStatement[stmtX5C].([]any)
 	if !x509present {
 		// Handle Basic Attestation steps for the x509 Certificate
@@ -49,7 +48,7 @@ func verifyAppleFormat(att AttestationObject, clientDataHash []byte, _ metadata.
 
 	credCert, err := x509.ParseCertificate(credCertBytes)
 	if err != nil {
-		return "", nil, ErrAttestationFormat.WithDetails(fmt.Sprintf("Error parsing certificate from ASN.1 data: %+v", err))
+		return "", nil, ErrAttestationFormat.WithDetails(fmt.Sprintf("Error parsing certificate from ASN.1 data: %+v", err)).WithError(err)
 	}
 
 	// Step 2. Concatenate authenticatorData and clientDataHash to form nonceToHash.
@@ -74,7 +73,7 @@ func verifyAppleFormat(att AttestationObject, clientDataHash []byte, _ metadata.
 	decoded := AppleAnonymousAttestation{}
 
 	if _, err = asn1.Unmarshal(attExtBytes, &decoded); err != nil {
-		return "", nil, ErrAttestationFormat.WithDetails("Unable to parse apple attestation certificate extensions")
+		return "", nil, ErrAttestationFormat.WithDetails("Unable to parse apple attestation certificate extensions").WithError(err)
 	}
 
 	if !bytes.Equal(decoded.Nonce, nonce[:]) {
@@ -85,7 +84,7 @@ func verifyAppleFormat(att AttestationObject, clientDataHash []byte, _ metadata.
 	// TODO: Probably move this part to webauthncose.go
 	pubKey, err := webauthncose.ParsePublicKey(att.AuthData.AttData.CredentialPublicKey)
 	if err != nil {
-		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error parsing public key: %+v\n", err))
+		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error parsing public key: %+v\n", err)).WithError(err)
 	}
 
 	credPK := pubKey.(webauthncose.EC2PublicKeyData)
@@ -104,7 +103,8 @@ func verifyAppleFormat(att AttestationObject, clientDataHash []byte, _ metadata.
 	return string(metadata.AnonCA), x5c, nil
 }
 
-// Apple has not yet publish schema for the extension(as of JULY 2021.)
+// AppleAnonymousAttestation represents the attestation format for Apple, who have not yet published a schema for the
+// extension (as of JULY 2021.)
 type AppleAnonymousAttestation struct {
 	Nonce []byte `asn1:"tag:1,explicit"`
 }
diff --git a/protocol/attestation_packed.go b/protocol/attestation_packed.go
index a46a47f..8f299eb 100644
--- a/protocol/attestation_packed.go
+++ b/protocol/attestation_packed.go
@@ -37,10 +37,8 @@ func init() {
 func verifyPackedFormat(att AttestationObject, clientDataHash []byte, _ metadata.Provider) (string, []any, error) {
 	// Step 1. Verify that attStmt is valid CBOR conforming to the syntax defined
 	// above and perform CBOR decoding on it to extract the contained fields.
-
 	// Get the alg value - A COSEAlgorithmIdentifier containing the identifier of the algorithm
 	// used to generate the attestation signature.
-
 	alg, present := att.AttStatement[stmtAlgorithm].(int64)
 	if !present {
 		return string(AttestationFormatPacked), nil, ErrAttestationFormat.WithDetails("Error retrieving alg value")
@@ -83,7 +81,7 @@ func handleBasicAttestation(signature, clientDataHash, authData, aaguid []byte,
 
 		ct, err := x509.ParseCertificate(cb)
 		if err != nil {
-			return "", x5c, ErrAttestationFormat.WithDetails(fmt.Sprintf("Error parsing certificate from ASN.1 data: %+v", err))
+			return "", x5c, ErrAttestationFormat.WithDetails(fmt.Sprintf("Error parsing certificate from ASN.1 data: %+v", err)).WithError(err)
 		}
 
 		if ct.NotBefore.After(time.Now()) || ct.NotAfter.Before(time.Now()) {
@@ -100,12 +98,12 @@ func handleBasicAttestation(signature, clientDataHash, authData, aaguid []byte,
 
 	attCert, err := x509.ParseCertificate(attCertBytes)
 	if err != nil {
-		return "", x5c, ErrAttestationFormat.WithDetails(fmt.Sprintf("Error parsing certificate from ASN.1 data: %+v", err))
+		return "", x5c, ErrAttestationFormat.WithDetails(fmt.Sprintf("Error parsing certificate from ASN.1 data: %+v", err)).WithError(err)
 	}
 
 	coseAlg := webauthncose.COSEAlgorithmIdentifier(alg)
 	if err = attCert.CheckSignature(webauthncose.SigAlgFromCOSEAlg(coseAlg), signatureData, signature); err != nil {
-		return "", x5c, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Signature validation error: %+v\n", err))
+		return "", x5c, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Signature validation error: %+v\n", err)).WithError(err)
 	}
 
 	// Step 2.2 Verify that attestnCert meets the requirements in §8.2.1 Packed attestation statement certificate requirements.
@@ -202,10 +200,6 @@ func handleECDAAAttestation(signature, clientDataHash, ecdaaKeyID []byte) (strin
 }
 
 func handleSelfAttestation(alg int64, pubKey, authData, clientDataHash, signature []byte) (string, []any, error) {
-	// §4.1 Validate that alg matches the algorithm of the credentialPublicKey in authenticatorData.
-
-	// §4.2 Verify that sig is a valid signature over the concatenation of authenticatorData and
-	// clientDataHash using the credential public key with alg.
 	verificationData := append(authData, clientDataHash...)
 
 	key, err := webauthncose.ParsePublicKey(pubKey)
@@ -213,6 +207,7 @@ func handleSelfAttestation(alg int64, pubKey, authData, clientDataHash, signatur
 		return "", nil, ErrAttestationFormat.WithDetails(fmt.Sprintf("Error parsing the public key: %+v\n", err))
 	}
 
+	// §4.1 Validate that alg matches the algorithm of the credentialPublicKey in authenticatorData.
 	switch k := key.(type) {
 	case webauthncose.OKPPublicKeyData:
 		err = verifyKeyAlgorithm(k.Algorithm, alg)
@@ -228,6 +223,8 @@ func handleSelfAttestation(alg int64, pubKey, authData, clientDataHash, signatur
 		return "", nil, err
 	}
 
+	// §4.2 Verify that sig is a valid signature over the concatenation of authenticatorData and
+	// clientDataHash using the credential public key with alg.
 	valid, err := webauthncose.VerifySignature(key, verificationData, signature)
 	if !valid && err == nil {
 		return "", nil, ErrInvalidAttestation.WithDetails("Unable to verify signature")
diff --git a/protocol/attestation_packed_test.go b/protocol/attestation_packed_test.go
index 7f22266..11ad9b2 100644
--- a/protocol/attestation_packed_test.go
+++ b/protocol/attestation_packed_test.go
@@ -67,10 +67,10 @@ func Test_verifyPackedFormat(t *testing.T) {
 				return
 			}
 
+			// TODO: Consider doing something with the second return value from verifyPackedFormat, x5c.
 			if got != tt.want {
 				t.Errorf("verifyPackedFormat() got = %v, want %v", got, tt.want)
 			}
-			// TODO: Consider doing something with the second return value from verifyPackedFormat, x5c.
 		})
 	}
 }
diff --git a/protocol/attestation_safetynet.go b/protocol/attestation_safetynet.go
index 849cda1..7355c97 100644
--- a/protocol/attestation_safetynet.go
+++ b/protocol/attestation_safetynet.go
@@ -85,18 +85,19 @@ func verifySafetyNetFormat(att AttestationObject, clientDataHash []byte, mds met
 		}
 
 		cert, err := x509.ParseCertificate(o[:n])
+
 		return cert.PublicKey, err
 	})
 
 	if err != nil {
-		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error finding cert issued to correct hostname: %+v", err))
+		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error finding cert issued to correct hostname: %+v", err)).WithError(err)
 	}
 
 	// marshall the JWT payload into the safetynet response json
 	var safetyNetResponse SafetyNetResponse
 
 	if err = mapstructure.Decode(token.Claims, &safetyNetResponse); err != nil {
-		return "", nil, ErrAttestationFormat.WithDetails(fmt.Sprintf("Error parsing the SafetyNet response: %+v", err))
+		return "", nil, ErrAttestationFormat.WithDetails(fmt.Sprintf("Error parsing the SafetyNet response: %+v", err)).WithError(err)
 	}
 
 	// §8.5.3 Verify that the nonce in the response is identical to the Base64 encoding of the SHA-256 hash of the concatenation
@@ -105,7 +106,7 @@ func verifySafetyNetFormat(att AttestationObject, clientDataHash []byte, mds met
 
 	nonceBytes, err := base64.StdEncoding.DecodeString(safetyNetResponse.Nonce)
 	if !bytes.Equal(nonceBuffer[:], nonceBytes) || err != nil {
-		return "", nil, ErrInvalidAttestation.WithDetails("Invalid nonce for in SafetyNet response")
+		return "", nil, ErrInvalidAttestation.WithDetails("Invalid nonce for in SafetyNet response").WithError(err)
 	}
 
 	// §8.5.4 Let attestationCert be the attestation certificate (https://www.w3.org/TR/webauthn/#attestation-certificate)
@@ -114,18 +115,18 @@ func verifySafetyNetFormat(att AttestationObject, clientDataHash []byte, mds met
 
 	n, err := base64.StdEncoding.Decode(l, []byte(certChain[0].(string)))
 	if err != nil {
-		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error finding cert issued to correct hostname: %+v", err))
+		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error finding cert issued to correct hostname: %+v", err)).WithError(err)
 	}
 
 	attestationCert, err := x509.ParseCertificate(l[:n])
 	if err != nil {
-		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error finding cert issued to correct hostname: %+v", err))
+		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error finding cert issued to correct hostname: %+v", err)).WithError(err)
 	}
 
 	// §8.5.5 Verify that attestationCert is issued to the hostname "attest.android.com"
 	err = attestationCert.VerifyHostname("attest.android.com")
 	if err != nil {
-		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error finding cert issued to correct hostname: %+v", err))
+		return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error finding cert issued to correct hostname: %+v", err)).WithError(err)
 	}
 
 	// §8.5.6 Verify that the ctsProfileMatch attribute in the payload of response is true.
diff --git a/protocol/attestation_tpm.go b/protocol/attestation_tpm.go
index 873e864..fb71c7a 100644
--- a/protocol/attestation_tpm.go
+++ b/protocol/attestation_tpm.go
@@ -72,7 +72,7 @@ func verifyTPMFormat(att AttestationObject, clientDataHash []byte, _ metadata.Pr
 	// is identical to the credentialPublicKey in the attestedCredentialData in authenticatorData.
 	pubArea, err := tpm2.DecodePublic(pubAreaBytes)
 	if err != nil {
-		return "", nil, ErrAttestationFormat.WithDetails("Unable to decode TPMT_PUBLIC in attestation statement")
+		return "", nil, ErrAttestationFormat.WithDetails("Unable to decode TPMT_PUBLIC in attestation statement").WithError(err)
 	}
 
 	key, err := webauthncose.ParsePublicKey(att.AuthData.AttData.CredentialPublicKey)
@@ -115,6 +115,7 @@ func verifyTPMFormat(att AttestationObject, clientDataHash []byte, _ metadata.Pr
 	// 3/4 Verify that extraData is set to the hash of attToBeSigned using the hash algorithm employed in "alg".
 	h := webauthncose.HasherFromCOSEAlg(coseAlg)
 	h.Write(attToBeSigned)
+
 	if !bytes.Equal(certInfo.ExtraData, h.Sum(nil)) {
 		return "", nil, ErrAttestationFormat.WithDetails("ExtraData is not set to hash of attToBeSigned")
 	}
@@ -219,23 +220,24 @@ func verifyTPMFormat(att AttestationObject, clientDataHash []byte, _ metadata.Pr
 	return string(metadata.AttCA), x5c, err
 }
 
+// forEachSAN loops through the TPM SAN extension.
+//
+// RFC 5280, 4.2.1.6
+// SubjectAltName ::= GeneralNames
+//
+// GeneralNames ::= SEQUENCE SIZE (1..MAX) OF GeneralName
+//
+//	GeneralName ::= CHOICE {
+//	     otherName                       [0]     OtherName,
+//	     rfc822Name                      [1]     IA5String,
+//	     dNSName                         [2]     IA5String,
+//	     x400Address                     [3]     ORAddress,
+//	     directoryName                   [4]     Name,
+//	     ediPartyName                    [5]     EDIPartyName,
+//	     uniformResourceIdentifier       [6]     IA5String,
+//	     iPAddress                       [7]     OCTET STRING,
+//	     registeredID                    [8]     OBJECT IDENTIFIER }
 func forEachSAN(extension []byte, callback func(tag int, data []byte) error) error {
-	// RFC 5280, 4.2.1.6
-
-	// SubjectAltName ::= GeneralNames
-	//
-	// GeneralNames ::= SEQUENCE SIZE (1..MAX) OF GeneralName
-	//
-	// GeneralName ::= CHOICE {
-	//      otherName                       [0]     OtherName,
-	//      rfc822Name                      [1]     IA5String,
-	//      dNSName                         [2]     IA5String,
-	//      x400Address                     [3]     ORAddress,
-	//      directoryName                   [4]     Name,
-	//      ediPartyName                    [5]     EDIPartyName,
-	//      uniformResourceIdentifier       [6]     IA5String,
-	//      iPAddress                       [7]     OCTET STRING,
-	//      registeredID                    [8]     OBJECT IDENTIFIER }
 	var seq asn1.RawValue
 
 	rest, err := asn1.Unmarshal(extension, &seq)
@@ -284,13 +286,16 @@ func parseSANExtension(value []byte) (manufacturer string, model string, version
 		case nameTypeDN:
 			tpmDeviceAttributes := pkix.RDNSequence{}
 			_, err := asn1.Unmarshal(data, &tpmDeviceAttributes)
+
 			if err != nil {
 				return err
 			}
+
 			for _, rdn := range tpmDeviceAttributes {
 				if len(rdn) == 0 {
 					continue
 				}
+
 				for _, atv := range rdn {
 					value, ok := atv.Value.(string)
 					if !ok {
@@ -300,15 +305,18 @@ func parseSANExtension(value []byte) (manufacturer string, model string, version
 					if atv.Type.Equal(tcgAtTpmManufacturer) {
 						manufacturer = strings.TrimPrefix(value, "id:")
 					}
+
 					if atv.Type.Equal(tcgAtTpmModel) {
 						model = value
 					}
+
 					if atv.Type.Equal(tcgAtTpmVersion) {
 						version = strings.TrimPrefix(value, "id:")
 					}
 				}
 			}
 		}
+
 		return nil
 	})
 
@@ -359,21 +367,40 @@ func isValidTPMManufacturer(id string) bool {
 	return false
 }
 
-func tpmParseSANExtension(attestation *x509.Certificate) (err error) {
+func tpmParseAIKAttCA(x5c *x509.Certificate, x5cis []*x509.Certificate) (err *Error) {
+	if err = tpmParseSANExtension(x5c); err != nil {
+		return err
+	}
+
+	if err = tpmRemoveEKU(x5c); err != nil {
+		return err
+	}
+
+	for _, parent := range x5cis {
+		if err = tpmRemoveEKU(parent); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func tpmParseSANExtension(attestation *x509.Certificate) (protoErr *Error) {
 	var (
 		manufacturer, model, version string
+		err                          error
 	)
 
 	for _, ext := range attestation.Extensions {
 		if ext.Id.Equal(oidExtensionSubjectAltName) {
 			if manufacturer, model, version, err = parseSANExtension(ext.Value); err != nil {
-				return ErrInvalidAttestation.WithDetails("Authenticator with invalid AIK SAN data encountered during attestation validation.").WithInfo(fmt.Sprintf("Error occurred parsing SAN extension: %s", err.Error()))
+				return ErrInvalidAttestation.WithDetails("Authenticator with invalid Authenticator Identity Key SAN data encountered during attestation validation.").WithInfo(fmt.Sprintf("Error occurred parsing SAN extension: %s", err.Error())).WithError(err)
 			}
 		}
 	}
 
 	if manufacturer == "" || model == "" || version == "" {
-		return ErrAttestationFormat.WithDetails("Invalid SAN data in AIK certificate")
+		return ErrAttestationFormat.WithDetails("Invalid SAN data in AIK certificate.")
 	}
 
 	var unhandled []asn1.ObjectIdentifier
@@ -404,7 +431,7 @@ type tpmBasicConstraints struct {
 }
 
 // Remove extension key usage to avoid ExtKeyUsage check failure.
-func tpmRemoveEKU(x5c *x509.Certificate) error {
+func tpmRemoveEKU(x5c *x509.Certificate) *Error {
 	var (
 		unknown []asn1.ObjectIdentifier
 		hasAiK  bool
@@ -425,7 +452,7 @@ func tpmRemoveEKU(x5c *x509.Certificate) error {
 	}
 
 	if !hasAiK {
-		return ErrAttestationFormat.WithDetails("AIK certificate missing EKU")
+		return ErrAttestationFormat.WithDetails("Attestation Identity Key certificate missing required Extended Key Usage.")
 	}
 
 	x5c.UnknownExtKeyUsage = unknown
diff --git a/protocol/attestation_tpm_test.go b/protocol/attestation_tpm_test.go
index b784882..f122924 100644
--- a/protocol/attestation_tpm_test.go
+++ b/protocol/attestation_tpm_test.go
@@ -30,6 +30,7 @@ func TestTPMAttestationVerificationSuccess(t *testing.T) {
 
 			attestationType, _, err := verifyTPMFormat(pcc.Response.AttestationObject, clientDataHash[:], nil)
 			require.NoError(t, err)
+
 			if err != nil {
 				t.Fatalf("Not valid: %+v", err)
 			}
diff --git a/protocol/attestation_u2f.go b/protocol/attestation_u2f.go
index 211aab7..3b858f5 100644
--- a/protocol/attestation_u2f.go
+++ b/protocol/attestation_u2f.go
@@ -76,7 +76,7 @@ func verifyU2FFormat(att AttestationObject, clientDataHash []byte, _ metadata.Pr
 
 	attCert, err := x509.ParseCertificate(asn1Bytes)
 	if err != nil {
-		return "", nil, ErrAttestationFormat.WithDetails("Error parsing certificate from ASN.1 data into certificate")
+		return "", nil, ErrAttestationFormat.WithDetails("Error parsing certificate from ASN.1 data into certificate").WithError(err)
 	}
 
 	// Step 2.3
diff --git a/protocol/credential_test.go b/protocol/credential_test.go
index b4e7ba8..17febb1 100644
--- a/protocol/credential_test.go
+++ b/protocol/credential_test.go
@@ -41,7 +41,7 @@ func TestParseCredentialCreationResponse(t *testing.T) {
 				ParsedPublicKeyCredential: ParsedPublicKeyCredential{
 					ParsedCredential: ParsedCredential{
 						ID:   "6xrtBhJQW6QU4tOaB4rrHaS2Ks0yDDL_q8jDC16DEjZ-VLVf4kCRkvl2xp2D71sTPYns-exsHQHTy3G-zJRK8g",
-						Type: "public-key",
+						Type: string(PublicKeyCredentialType),
 					},
 					RawID: byteID,
 					ClientExtensionResults: AuthenticationExtensionsClientOutputs{
@@ -74,7 +74,7 @@ func TestParseCredentialCreationResponse(t *testing.T) {
 				Raw: CredentialCreationResponse{
 					PublicKeyCredential: PublicKeyCredential{
 						Credential: Credential{
-							Type: "public-key",
+							Type: string(PublicKeyCredentialType),
 							ID:   "6xrtBhJQW6QU4tOaB4rrHaS2Ks0yDDL_q8jDC16DEjZ-VLVf4kCRkvl2xp2D71sTPYns-exsHQHTy3G-zJRK8g",
 						},
 						RawID: byteID,
@@ -182,7 +182,7 @@ func TestParsedCredentialCreationData_Verify(t *testing.T) {
 				ParsedPublicKeyCredential: ParsedPublicKeyCredential{
 					ParsedCredential: ParsedCredential{
 						ID:   "6xrtBhJQW6QU4tOaB4rrHaS2Ks0yDDL_q8jDC16DEjZ-VLVf4kCRkvl2xp2D71sTPYns-exsHQHTy3G-zJRK8g",
-						Type: "public-key",
+						Type: string(PublicKeyCredentialType),
 					},
 					RawID: byteID,
 				},
@@ -210,7 +210,7 @@ func TestParsedCredentialCreationData_Verify(t *testing.T) {
 				Raw: CredentialCreationResponse{
 					PublicKeyCredential: PublicKeyCredential{
 						Credential: Credential{
-							Type: "public-key",
+							Type: string(PublicKeyCredentialType),
 							ID:   "6xrtBhJQW6QU4tOaB4rrHaS2Ks0yDDL_q8jDC16DEjZ-VLVf4kCRkvl2xp2D71sTPYns-exsHQHTy3G-zJRK8g",
 						},
 						RawID: byteID,
diff --git a/protocol/options_test.go b/protocol/options_test.go
index e3fe3e7..456a59f 100644
--- a/protocol/options_test.go
+++ b/protocol/options_test.go
@@ -27,7 +27,7 @@ func TestPublicKeyCredentialRequestOptions_GetAllowedCredentialIDs(t *testing.T)
 				Timeout:   60,
 				AllowedCredentials: []CredentialDescriptor{
 					{
-						Type: "public-key", CredentialID: []byte("1234"), Transport: []AuthenticatorTransport{"usb"},
+						Type: PublicKeyCredentialType, CredentialID: []byte("1234"), Transport: []AuthenticatorTransport{"usb"},
 					},
 				},
 				RelyingPartyID:   "test.org",