diff --git a/acme/challenge.go b/acme/challenge.go
index 3efcf32d..d66b28a0 100644
--- a/acme/challenge.go
+++ b/acme/challenge.go
@@ -19,6 +19,7 @@ import (
 	"net/url"
 	"os"
 	"reflect"
+	"strconv"
 	"strings"
 	"time"
 
@@ -361,11 +362,19 @@ func deviceAttest01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose
 		if data.UDID != ch.Value && data.SerialNumber != ch.Value {
 			return storeError(ctx, db, ch, true, NewError(ErrorBadAttestationStatement, "permanent identifier does not match"))
 		}
+	case "step":
+		data, err := doStepAttestationFormat(ctx, ch, db, &att)
+		if err != nil {
+			return err
+		}
 
-		// TODO(mariano): debug - remove me
-		pem.Encode(os.Stderr, &pem.Block{
-			Type: "CERTIFICATE", Bytes: data.Certificate.Raw,
-		})
+		// Validate Apple's ClientIdentifier (Identifier.Value) with device
+		// identifiers.
+		//
+		// Note: We might want to use an external service for this.
+		if data.SerialNumber != ch.Value {
+			return storeError(ctx, db, ch, true, NewError(ErrorBadAttestationStatement, "permanent identifier does not match"))
+		}
 	default:
 		return storeError(ctx, db, ch, true, NewError(ErrorBadAttestationStatement, "unexpected attestation object format"))
 	}
@@ -479,6 +488,106 @@ func doAppleAttestationFormat(ctx context.Context, ch *Challenge, db DB, att *At
 	return data, nil
 }
 
+// Yubico PIV Root CA Serial 263751
+// https://developers.yubico.com/PIV/Introduction/piv-attestation-ca.pem
+const yubicoPIVRootCA = `-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----`
+
+// Serial number of the YubiKey, encoded as an integer.
+// https://developers.yubico.com/PIV/Introduction/PIV_attestation.html
+var oidYubicoSerialNumber = asn1.ObjectIdentifier{1, 3, 6, 1, 4, 1, 41482, 3, 7}
+
+type stepAttestationData struct {
+	Certificate  *x509.Certificate
+	SerialNumber string
+}
+
+func doStepAttestationFormat(ctx context.Context, ch *Challenge, db DB, att *AttestationObject) (*stepAttestationData, error) {
+	root, err := pemutil.ParseCertificate([]byte(yubicoPIVRootCA))
+	if err != nil {
+		return nil, WrapErrorISE(err, "error parsing apple enterprise ca")
+	}
+	roots := x509.NewCertPool()
+	roots.AddCert(root)
+
+	x5c, ok := att.AttStatement["x5c"].([]interface{})
+	if !ok {
+		return nil, storeError(ctx, db, ch, true, NewError(ErrorBadAttestationStatement, "x5c not present"))
+	}
+	if len(x5c) == 0 {
+		return nil, storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "x5c is empty"))
+	}
+
+	der, ok := x5c[0].([]byte)
+	if !ok {
+		return nil, storeError(ctx, db, ch, true, NewError(ErrorBadAttestationStatement, "x5c is malformed"))
+	}
+	leaf, err := x509.ParseCertificate(der)
+	if err != nil {
+		return nil, storeError(ctx, db, ch, true, WrapError(ErrorBadAttestationStatement, err, "x5c is malformed"))
+	}
+
+	pem.Encode(os.Stderr, &pem.Block{
+		Type: "CERTIFICATE", Bytes: leaf.Raw,
+	})
+
+	intermediates := x509.NewCertPool()
+	for _, v := range x5c[1:] {
+		der, ok = v.([]byte)
+		if !ok {
+			return nil, storeError(ctx, db, ch, true, NewError(ErrorBadAttestationStatement, "x5c is malformed"))
+		}
+		cert, err := x509.ParseCertificate(der)
+		if err != nil {
+			return nil, storeError(ctx, db, ch, true, WrapError(ErrorBadAttestationStatement, err, "x5c is malformed"))
+		}
+		intermediates.AddCert(cert)
+	}
+
+	if _, err := leaf.Verify(x509.VerifyOptions{
+		Intermediates: intermediates,
+		Roots:         roots,
+		CurrentTime:   time.Now().Truncate(time.Second),
+		KeyUsages:     []x509.ExtKeyUsage{x509.ExtKeyUsageAny},
+	}); err != nil {
+		return nil, storeError(ctx, db, ch, true, WrapError(ErrorBadAttestationStatement, err, "x5c is not valid"))
+	}
+
+	data := &stepAttestationData{
+		Certificate: leaf,
+	}
+	for _, ext := range leaf.Extensions {
+		switch {
+		case ext.Id.Equal(oidYubicoSerialNumber):
+			var serialNumber int
+			rest, err := asn1.Unmarshal(ext.Value, &serialNumber)
+			if err != nil || len(rest) > 0 {
+				return nil, storeError(ctx, db, ch, true, WrapError(ErrorBadAttestationStatement, err, "error parsing serial number"))
+			}
+			data.SerialNumber = strconv.Itoa(serialNumber)
+		}
+	}
+
+	return data, nil
+}
+
 // serverName determines the SNI HostName to set based on an acme.Challenge
 // for TLS-ALPN-01 challenges RFC8738 states that, if HostName is an IP, it
 // should be the ARPA address https://datatracker.ietf.org/doc/html/rfc8738#section-6.