Add additional verification of AK certificate

acme/challenge.go

@@ -578,6 +578,13 @@ func doTPMAttestationFormat(ctx context.Context, prov Provisioner, ch *Challenge
 		return nil, NewErrorISE("no root CA bundle available to verify the attestation certificate")
 	}
 
+	// verify that the AK certificate was signed by a trusted root,
+	// chained to by the intermediates provided by the client. As part
+	// of building the verified certificate chain, the signature over the
+	// AK certificate is checked to be a valid signature of one of the
+	// provided intermediates. Signatures over the intermediates are in
+	// turn also verified to be valid signatures from one of the trusted
+	// roots.
 	verifiedChains, err := akCert.Verify(x509.VerifyOptions{
 		Roots:         roots,
 		Intermediates: intermediates,
@@ -588,6 +595,11 @@ func doTPMAttestationFormat(ctx context.Context, prov Provisioner, ch *Challenge
 		return nil, WrapError(ErrorBadAttestationStatementType, err, "x5c is not valid")
 	}
 
+	// validate additional AK certificate requirements
+	if err := validateAKCertificate(akCert); err != nil {
+		return nil, WrapError(ErrorBadAttestationStatementType, err, "AK certificate is not valid")
+	}
+
 	// TODO(hs): implement revocation check; Verify() doesn't perform CRL check nor OCSP lookup.
 
 	sans, err := x509util.ParseSubjectAlternativeNames(akCert)
@@ -697,6 +709,93 @@ func doTPMAttestationFormat(ctx context.Context, prov Provisioner, ch *Challenge
 	return data, nil
 }
 
+var (
+	oidExtensionExtendedKeyUsage = asn1.ObjectIdentifier{2, 5, 29, 37}
+	oidTCGKpAIKCertificate       = asn1.ObjectIdentifier{2, 23, 133, 8, 3}
+)
+
+// validateAKCertifiate validates the X.509 AK certificate to be
+// in accordance with the required properties. The requirements come from:
+// https://www.w3.org/TR/webauthn-2/#sctn-tpm-cert-requirements.
+//
+// 	- Version MUST be set to 3.
+// 	- Subject field MUST be set to empty.
+//	- The Subject Alternative Name extension MUST be set as defined
+//	  in [TPMv2-EK-Profile] section 3.2.9.
+//	- The Extended Key Usage extension MUST contain the OID 2.23.133.8.3
+//	  ("joint-iso-itu-t(2) internationalorganizations(23) 133 tcg-kp(8) tcg-kp-AIKCertificate(3)").
+//	- The Basic Constraints extension MUST have the CA component set to false.
+//	- An Authority Information Access (AIA) extension with entry id-ad-ocsp
+// 	  and a CRL Distribution Point extension [RFC5280] are both OPTIONAL as
+//	  the status of many attestation certificates is available through metadata
+// 	  services. See, for example, the FIDO Metadata Service.
+
+func validateAKCertificate(c *x509.Certificate) error {
+	if c.Version != 3 {
+		return fmt.Errorf("AK certificate has invalid version %d; only version 3 is allowed", c.Version)
+	}
+	if c.Subject.String() != "" {
+		return fmt.Errorf("AK certificate subject must be empty; got %s", c.Subject)
+	}
+	if err := validateAKCertificateSubjectAlternativeNames(c); err != nil {
+		return err
+	}
+	if err := validateAKCertificateExtendedKeyUsage(c); err != nil {
+		return err
+	}
+	if c.IsCA {
+		return errors.New("AK certificate must not be a CA")
+	}
+
+	return nil
+}
+
+func validateAKCertificateSubjectAlternativeNames(c *x509.Certificate) error {
+	return nil // TODO(hs): remove this early return when we require AK certificates to set these
+
+	sans, err := x509util.ParseSubjectAlternativeNames(c)
+	if err != nil {
+		return fmt.Errorf("failed parsing AK certificate Subject Alternative Names: %w", err)
+	}
+
+	details := sans.TPMHardwareDetails
+	manufacturer, model, version := details.Manufacturer, details.Model, details.Version
+
+	switch {
+	case manufacturer == "":
+		return errors.New("missing TPM manufacturer")
+	case model == "":
+		return errors.New("missing TPM model")
+	case version == "":
+		return errors.New("missing TPM version")
+	}
+
+	return nil
+}
+
+// validateAKCertificateExtendedKeyUsage checks if the AK certificate
+// has the "tcg-kp-AIKCertificate" Extended Key Usage set.
+func validateAKCertificateExtendedKeyUsage(c *x509.Certificate) error {
+	var (
+		valid = false
+		ekus  []asn1.ObjectIdentifier
+	)
+	for _, ext := range c.Extensions {
+		if ext.Id.Equal(oidExtensionExtendedKeyUsage) {
+			if _, err := asn1.Unmarshal(ext.Value, &ekus); err != nil || !ekus[0].Equal(oidTCGKpAIKCertificate) {
+				return errors.New("AK certificate is missing Extended Key Usage value tcg-kp-AIKCertificate (2.23.133.8.3)")
+			}
+			valid = true
+		}
+	}
+
+	if !valid {
+		return errors.New("AK certificate is missing Extended Key Usage extension")
+	}
+
+	return nil
+}
+
 // Apple Enterprise Attestation Root CA from
 // https://www.apple.com/certificateauthority/private/
 const appleEnterpriseAttestationRootCA = `-----BEGIN CERTIFICATE-----

acme/challenge_tpmsimulator_test.go

@@ -93,7 +93,9 @@ func mustAttestTPM(t *testing.T, keyAuthorization string, permanentIdentifiers [
 	// create template and sign certificate for the AK public key
 	keyID := generateKeyID(t, eks[0].Public())
 	template := &x509.Certificate{
-		PublicKey: akp.Public,
+		PublicKey:          akp.Public,
+		IsCA:               false,
+		UnknownExtKeyUsage: []asn1.ObjectIdentifier{oidTCGKpAIKCertificate},
 	}
 	if len(permanentIdentifiers) == 0 {
 		template.URIs = []*url.URL{
@@ -456,7 +458,9 @@ func Test_doTPMAttestationFormat(t *testing.T) {
 	// create template and sign certificate for the AK public key
 	keyID := generateKeyID(t, eks[0].Public())
 	template := &x509.Certificate{
-		PublicKey: akp.Public,
+		PublicKey:          akp.Public,
+		IsCA:               false,
+		UnknownExtKeyUsage: []asn1.ObjectIdentifier{oidTCGKpAIKCertificate},
 		URIs: []*url.URL{
 			{Scheme: "urn", Opaque: "ek:sha256:" + base64.StdEncoding.EncodeToString(keyID)},
 		},

go.mod

@@ -29,7 +29,7 @@ require (
 	github.com/urfave/cli v1.22.12
 	go.mozilla.org/pkcs7 v0.0.0-20210826202110-33d05740a352
 	go.step.sm/cli-utils v0.7.5
-	go.step.sm/crypto v0.28.1-0.20230403133050-c4ef6cdd0c34
+	go.step.sm/crypto v0.28.1-0.20230404230013-4e7c14d93fcc
 	go.step.sm/linkedca v0.19.0
 	golang.org/x/crypto v0.7.0
 	golang.org/x/exp v0.0.0-20230310171629-522b1b587ee0
@@ -58,7 +58,7 @@ require (
 	github.com/Masterminds/goutils v1.1.1 // indirect
 	github.com/Masterminds/semver/v3 v3.2.0 // indirect
 	github.com/ThalesIgnite/crypto11 v1.2.5 // indirect
-	github.com/aws/aws-sdk-go v1.44.231 // indirect
+	github.com/aws/aws-sdk-go v1.44.235 // indirect
 	github.com/cenkalti/backoff/v3 v3.0.0 // indirect
 	github.com/cespare/xxhash v1.1.0 // indirect
 	github.com/cespare/xxhash/v2 v2.2.0 // indirect
@@ -108,7 +108,7 @@ require (
 	github.com/kylelemons/godebug v1.1.0 // indirect
 	github.com/manifoldco/promptui v0.9.0 // indirect
 	github.com/mattn/go-colorable v0.1.8 // indirect
-	github.com/mattn/go-isatty v0.0.13 // indirect
+	github.com/mattn/go-isatty v0.0.16 // indirect
 	github.com/mgutz/ansi v0.0.0-20200706080929-d51e80ef957d // indirect
 	github.com/miekg/pkcs11 v1.1.1 // indirect
 	github.com/mitchellh/copystructure v1.2.0 // indirect
@@ -118,7 +118,6 @@ require (
 	github.com/peterbourgon/diskv/v3 v3.0.1 // indirect
 	github.com/pkg/browser v0.0.0-20210911075715-681adbf594b8 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
-	github.com/rogpeppe/go-internal v1.9.0 // indirect
 	github.com/russross/blackfriday/v2 v2.1.0 // indirect
 	github.com/ryanuber/go-glob v1.0.0 // indirect
 	github.com/schollz/jsonstore v1.1.0 // indirect
@@ -132,10 +131,9 @@ require (
 	golang.org/x/oauth2 v0.6.0 // indirect
 	golang.org/x/sys v0.6.0 // indirect
 	golang.org/x/text v0.8.0 // indirect
-	golang.org/x/time v0.0.0-20220411224347-583f2d630306 // indirect
+	golang.org/x/time v0.1.0 // indirect
 	google.golang.org/appengine v1.6.7 // indirect
 	google.golang.org/genproto v0.0.0-20230306155012-7f2fa6fef1f4 // indirect
-	gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )
 

go.sum

@@ -160,8 +160,8 @@ github.com/aws/aws-sdk-go v1.23.20/go.mod h1:KmX6BPdI08NWTb3/sm4ZGu5ShLoqVDhKgpi
 github.com/aws/aws-sdk-go v1.25.11/go.mod h1:KmX6BPdI08NWTb3/sm4ZGu5ShLoqVDhKgpiN924inxo=
 github.com/aws/aws-sdk-go v1.27.0/go.mod h1:KmX6BPdI08NWTb3/sm4ZGu5ShLoqVDhKgpiN924inxo=
 github.com/aws/aws-sdk-go v1.37.0/go.mod h1:hcU610XS61/+aQV88ixoOzUoG7v3b31pl2zKMmprdro=
-github.com/aws/aws-sdk-go v1.44.231 h1:wH/ihcZzBv8F443PyRoUogWnEdDp1KYtSew7ji9LNIY=
+github.com/aws/aws-sdk-go v1.44.235 h1:5MS1ZW1Pr27mmHFqqjuXYwGMlNTW/g6DqU5ekamPMeU=
-github.com/aws/aws-sdk-go v1.44.231/go.mod h1:aVsgQcEevwlmQ7qHE9I3h+dtQgpqhFB+i8Phjh7fkwI=
+github.com/aws/aws-sdk-go v1.44.235/go.mod h1:aVsgQcEevwlmQ7qHE9I3h+dtQgpqhFB+i8Phjh7fkwI=
 github.com/aws/aws-sdk-go-v2 v0.18.0/go.mod h1:JWVYvqSMppoMJC0x5wdwiImzgXTI9FuZwxzkQq9wy+g=
 github.com/aybabtme/rgbterm v0.0.0-20170906152045-cc83f3b3ce59/go.mod h1:q/89r3U2H7sSsE2t6Kca0lfwTK8JdoNGS/yzM/4iH5I=
 github.com/benbjohnson/clock v1.0.3/go.mod h1:bGMdMPoPVvcYyt1gHDf4J2KE153Yf9BuiUKYMaxlTDM=
@@ -647,7 +647,6 @@ github.com/kr/logfmt v0.0.0-20140226030751-b84e30acd515/go.mod h1:+0opPa2QZZtGFB
 github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
 github.com/kr/pretty v0.2.0/go.mod h1:ipq/a2n7PKx3OHsz4KJII5eveXtPO4qwEXGdVfWzfnI=
 github.com/kr/pretty v0.2.1/go.mod h1:ipq/a2n7PKx3OHsz4KJII5eveXtPO4qwEXGdVfWzfnI=
-github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
 github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
 github.com/kr/pty v1.1.8/go.mod h1:O1sed60cT9XZ5uDucP5qwvh+TE3NnUj51EiZO/lmSfw=
 github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
@@ -687,8 +686,8 @@ github.com/mattn/go-isatty v0.0.9/go.mod h1:YNRxwqDuOph6SZLI9vUUz6OYw3QyUt7WiY2y
 github.com/mattn/go-isatty v0.0.10/go.mod h1:qgIWMr58cqv1PHHyhnkY9lrL7etaEgOFcMEpPG5Rm84=
 github.com/mattn/go-isatty v0.0.11/go.mod h1:PhnuNfih5lzO57/f3n+odYbM4JtupLOxQOAqxQCu2WE=
 github.com/mattn/go-isatty v0.0.12/go.mod h1:cbi8OIDigv2wuxKPP5vlRcQ1OAZbq2CE4Kysco4FUpU=
-github.com/mattn/go-isatty v0.0.13 h1:qdl+GuBjcsKKDco5BsxPJlId98mSWNKqYA+Co0SC1yA=
+github.com/mattn/go-isatty v0.0.16 h1:bq3VjFmv/sOjHtdEhmkEV4x1AJtvUvOJ2PFAZ5+peKQ=
-github.com/mattn/go-isatty v0.0.13/go.mod h1:cbi8OIDigv2wuxKPP5vlRcQ1OAZbq2CE4Kysco4FUpU=
+github.com/mattn/go-isatty v0.0.16/go.mod h1:kYGgaQfpe5nmfYZH+SKPsOc2e4SrIfOl2e/yFXSvRLM=
 github.com/mattn/go-runewidth v0.0.2/go.mod h1:LwmH8dsx7+W8Uxz3IHJYH5QSwggIsqBzpuz5H//U1FU=
 github.com/mattn/go-runewidth v0.0.7/go.mod h1:H031xJmbD/WCDINGzjvQ9THkh0rPKHF+m2gUSrubnMI=
 github.com/mattn/go-runewidth v0.0.9/go.mod h1:H031xJmbD/WCDINGzjvQ9THkh0rPKHF+m2gUSrubnMI=
@@ -744,6 +743,7 @@ github.com/nats-io/nkeys v0.1.3/go.mod h1:xpnFELMwJABBLVhffcfd1MZx6VsNRFpEugbxzi
 github.com/nats-io/nuid v1.0.1/go.mod h1:19wcPz3Ph3q0Jbyiqsd0kePYG7A95tJPxeL+1OSON2c=
 github.com/newrelic/go-agent/v3 v3.21.0 h1:KpkoW6PnSVzEDEO0W/C9LZEZZGwAb+a9g5DN8ifvt4Y=
 github.com/newrelic/go-agent/v3 v3.21.0/go.mod h1:rT6ZUxJc5rQbWLyCtjqQCOcfb01lKRFbc1yMQkcboWM=
+github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e h1:fD57ERR4JtEqsWbfPhv4DMiApHyliiK5xCTNVSPiaAs=
 github.com/nishanths/predeclared v0.0.0-20200524104333-86fad755b4d3/go.mod h1:nt3d53pc1VYcphSCIaYAJtnPYnr3Zyn8fMq2wvPGPso=
 github.com/oklog/oklog v0.3.2/go.mod h1:FCV+B7mhrz4o+ueLpx+KqkyXRGMWOYEvfiXtdGtbWGs=
 github.com/oklog/run v1.0.0/go.mod h1:dlhp/R75TPv97u0XWUtDeV/lRKWPKSdTuV0TZvrmrQA=
@@ -834,8 +834,6 @@ github.com/rogpeppe/fastuuid v0.0.0-20150106093220-6724a57986af/go.mod h1:XWv6So
 github.com/rogpeppe/fastuuid v1.1.0/go.mod h1:jVj6XXZzXRy/MSR5jhDC/2q6DgLz+nrA6LYCDYWNEvQ=
 github.com/rogpeppe/fastuuid v1.2.0/go.mod h1:jVj6XXZzXRy/MSR5jhDC/2q6DgLz+nrA6LYCDYWNEvQ=
 github.com/rogpeppe/go-internal v1.3.0/go.mod h1:M8bDsm7K2OlrFYOpmOWEs/qY81heoFRclV5y23lUDJ4=
-github.com/rogpeppe/go-internal v1.9.0 h1:73kH8U+JUqXU8lRuOHeVHaa/SZPifC7BkcraZVejAe8=
-github.com/rogpeppe/go-internal v1.9.0/go.mod h1:WtVeX8xhTBvf0smdhujwtBcq4Qrzq/fJaraNFVN+nFs=
 github.com/rs/cors v1.7.0/go.mod h1:gFx+x8UowdsKA9AchylcLynDq+nNFfI8FkUZdN/jGCU=
 github.com/rs/cors v1.8.0/go.mod h1:EBwu+T5AvHOcXwvZIkQFjUN6s8Czyqw12GL/Y0tUyRM=
 github.com/rs/xid v1.2.1/go.mod h1:+uKXf+4Djp6Md1KODXJxgGQPKngRmWyn10oCKFzNHOQ=
@@ -1024,8 +1022,8 @@ go.opentelemetry.io/proto/otlp v0.7.0/go.mod h1:PqfVotwruBrMGOCsRd/89rSnXhoiJIqe
 go.step.sm/cli-utils v0.7.5 h1:jyp6X8k8mN1B0uWJydTid0C++8tQhm2kaaAdXKQQzdk=
 go.step.sm/cli-utils v0.7.5/go.mod h1:taSsY8haLmXoXM3ZkywIyRmVij/4Aj0fQbNTlJvv71I=
 go.step.sm/crypto v0.9.0/go.mod h1:+CYG05Mek1YDqi5WK0ERc6cOpKly2i/a5aZmU1sfGj0=
-go.step.sm/crypto v0.28.1-0.20230403133050-c4ef6cdd0c34 h1:sByVwgjzB3A0MXtLyMbBd/AgvsCxglPm2PLvDhdJqAE=
+go.step.sm/crypto v0.28.1-0.20230404230013-4e7c14d93fcc h1:Z55pL1dV3YfM+bXu1MRSFXttCdD9MCX1A8y4V/Uh9U8=
-go.step.sm/crypto v0.28.1-0.20230403133050-c4ef6cdd0c34/go.mod h1:PFmyUJUvF5YDVokruSlTDcENBD/QBDpBiV2zInfsgV0=
+go.step.sm/crypto v0.28.1-0.20230404230013-4e7c14d93fcc/go.mod h1:9cgcaYvZf4vJo1vzBbb6uS6ydzUqvfZX6C7GRd0rAzY=
 go.step.sm/linkedca v0.19.0 h1:xuagkR35wrJI2gnu6FAM+q3VmjwsHScvGcJsfZ0GdsI=
 go.step.sm/linkedca v0.19.0/go.mod h1:b7vWPrHfYLEOTSUZitFEcztVCpTc+ileIN85CwEAluM=
 go.uber.org/atomic v1.3.2/go.mod h1:gD2HeocX3+yG+ygLZcrzQJaqmWj9AIm7n08wl/qW/PE=
@@ -1292,6 +1290,7 @@ golang.org/x/sys v0.0.0-20220209214540-3681064d5158/go.mod h1:oPkhp1MJrh7nUepCBc
 golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20220715151400-c0bba94af5f8/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20220811171246-fbc7d0a398ab/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.1.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.2.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.4.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
@@ -1327,8 +1326,8 @@ golang.org/x/time v0.0.0-20191024005414-555d28b269f0/go.mod h1:tRJNPiyCQ0inRvYxb
 golang.org/x/time v0.0.0-20200416051211-89c76fbcd5d1/go.mod h1:tRJNPiyCQ0inRvYxbN9jk5I+vvW/OXSQhTDSoE431IQ=
 golang.org/x/time v0.0.0-20200630173020-3af7569d3a1e/go.mod h1:tRJNPiyCQ0inRvYxbN9jk5I+vvW/OXSQhTDSoE431IQ=
 golang.org/x/time v0.0.0-20210220033141-f8bda1e9f3ba/go.mod h1:tRJNPiyCQ0inRvYxbN9jk5I+vvW/OXSQhTDSoE431IQ=
-golang.org/x/time v0.0.0-20220411224347-583f2d630306 h1:+gHMid33q6pen7kv9xvT+JRinntgeXO2AeZVd0AWD3w=
+golang.org/x/time v0.1.0 h1:xYY+Bajn2a7VBmTM5GikTmnK8ZuX8YgnQCqZpbBNtmA=
-golang.org/x/time v0.0.0-20220411224347-583f2d630306/go.mod h1:tRJNPiyCQ0inRvYxbN9jk5I+vvW/OXSQhTDSoE431IQ=
+golang.org/x/time v0.1.0/go.mod h1:tRJNPiyCQ0inRvYxbN9jk5I+vvW/OXSQhTDSoE431IQ=
 golang.org/x/tools v0.0.0-20180221164845-07fd8470d635/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 golang.org/x/tools v0.0.0-20180828015842-6cd1fcedba52/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
@@ -1576,8 +1575,7 @@ gopkg.in/alecthomas/kingpin.v2 v2.2.6/go.mod h1:FMv+mEhP44yOT+4EoQTLFTRgOQ1FBLks
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
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