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client.go
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client.go
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package akscep
import (
"bytes"
"crypto"
"crypto/rand"
"crypto/rsa"
"crypto/x509"
"encoding/hex"
"fmt"
"io"
"io/ioutil"
insecurerand "math/rand"
"net/http"
"strings"
"time"
"github.com/chrisfenner/pkcs7"
forkx509 "github.com/google/certificate-transparency-go/x509"
"github.com/google/go-attestation/attest"
"github.com/google/go-tpm/tpm2"
"github.com/google/go-tpm/tpmutil"
)
type ClientContext struct {
tpm io.ReadWriteCloser
httpClient http.Client
requestID string
machineName string
scepKey *rsa.PrivateKey
scepCert *x509.Certificate
serviceURL string
raCert *x509.Certificate
ak *AK
akBundle akBundle
ek *attest.EK
}
func (cli *ClientContext) Close() {
tpm2.FlushContext(cli.tpm, cli.akBundle.handle)
cli.tpm.Close()
}
func unfork(cert *forkx509.Certificate) *x509.Certificate {
unforked, err := x509.ParseCertificate(cert.Raw)
if err != nil {
panic(err)
}
return unforked
}
func NewClientContext() (*ClientContext, error) {
machineName := randomMachineName()
// Fetch the EK
ek, err := getEK()
if err != nil {
return nil, err
}
httpClient := http.Client{
Timeout: 10 * time.Second,
}
b := make([]byte, 16)
rand.Reader.Read(b)
requestID := fmt.Sprintf("%x-%x-%x-%x-%x", b[0:4], b[4:6], b[6:8], b[8:10], b[10:16])
// TODO get the manufacturer id properly
serviceURL := serviceAddress("ifx", ek.Certificate.AuthorityKeyId)
cli := ClientContext{
httpClient: httpClient,
requestID: requestID,
machineName: machineName,
serviceURL: serviceURL,
ek: ek,
}
// Fetch the RA certificate from the service, we encrypt requests to it
// TODO: clean up the fact that we rely on the fact that getRA works on a
// partly constructed client
ra, err := (&cli).getRA(unfork(ek.Certificate))
if err != nil {
return nil, err
}
cli.raCert = ra
// Generate a new AK
ak, err := newAK()
if err != nil {
return nil, err
}
cli.ak = ak
tpm, err := tpm2.OpenTPM("/dev/tpmrm0")
if err != nil {
return nil, err
}
// TODO: don't leak this if there is an error in the next few lines
cli.tpm = tpm
bundle, err := ak.Generate(tpm)
if err != nil {
return nil, err
}
// TODO: don't leak this if there is an error in the next few lines
cli.akBundle = *bundle
// Generate the SCEP key for decrypting responses from the server
key, err := rsa.GenerateKey(rand.Reader, 2048)
if err != nil {
return nil, err
}
cli.scepKey = key
cert, _, err := MakeClientCert(key.Public(), key)
if err != nil {
return nil, err
}
cli.scepCert = cert
return &cli, nil
}
func randomMachineName() string {
var buf strings.Builder
buf.WriteString("DESKTOP-")
r := insecurerand.New(insecurerand.NewSource(time.Now().UnixNano()))
for i := 0; i < 7; i++ {
c := r.Intn(len(machNameCharSet))
buf.WriteByte(machNameCharSet[c])
}
return buf.String()
}
func serviceAddress(tpmManufacturerID string, ekid []byte) string {
return fmt.Sprintf("https://%s-keyid-%x.microsoftaik.azure.net/templates/Aik/scep", tpmManufacturerID, ekid)
}
func (cli *ClientContext) addSCEPHeaders(h http.Header) {
h.Add("Cache-Control", "no-cache")
h.Add("Connection", "Keep-Alive")
h.Add("Pragma", "no-cache")
h.Add("Content-Type", "application/x-pki-message")
// TODO: populate this with the same version number we put into the claims
h.Add("User-Agent", `Mozilla/4.0 (compatible; Win32; NDES client 10.0.19041.746/vb_release_svc_prod1)`)
// TODO: Populate this with real data from the TPM, assuming the service checks
h.Add("PlatformType", `TPM-Version:2.0 -Level:0-Revision:0.99-VendorID:'IFX '-Firmware:327680.278786`)
h.Add("x-ms-client-request-id", cli.requestID)
}
func (cli *ClientContext) getRA(ekCert *x509.Certificate) (*x509.Certificate, error) {
url := cli.serviceURL + "?operation=GetCACertChain&message=default"
req, err := http.NewRequest("GET", url, nil)
if err != nil {
return nil, err
}
cli.addSCEPHeaders(req.Header)
fmt.Printf("Connecting to AK Service...")
rsp, err := cli.httpClient.Do(req)
fmt.Printf("done\n")
if err != nil {
return nil, err
}
defer rsp.Body.Close()
data, err := ioutil.ReadAll(rsp.Body)
if err != nil {
return nil, err
}
serviceCerts, err := pkcs7.Parse(data)
if err != nil {
return nil, err
}
for _, serviceCert := range serviceCerts.Certificates {
if serviceCert.Subject.CommonName == "RA Encryption Certificate" {
return serviceCert, nil
}
}
return nil, fmt.Errorf("Service did not provide an RA encryption cert")
}
func (cli *ClientContext) getChallenge(chal *GetChallengeReqBuilder) ([]byte, error) {
chalReq, err := chal.Build()
if err != nil {
return nil, err
}
url := cli.serviceURL + "?operation=PKIOperation"
req, err := http.NewRequest("POST", url, bytes.NewReader(chalReq))
if err != nil {
return nil, err
}
cli.addSCEPHeaders(req.Header)
rsp, err := cli.httpClient.Do(req)
if err != nil {
return nil, err
}
defer rsp.Body.Close()
data, err := ioutil.ReadAll(rsp.Body)
if err != nil {
return nil, err
}
return data, nil
}
func (cli *ClientContext) getCert(b *GetCertReqBuilder) ([]byte, error) {
certReq, err := b.Build()
if err != nil {
return nil, err
}
url := cli.serviceURL + "?operation=PKIOperation"
req, err := http.NewRequest("POST", url, bytes.NewReader(certReq))
if err != nil {
return nil, err
}
cli.addSCEPHeaders(req.Header)
rsp, err := cli.httpClient.Do(req)
if err != nil {
return nil, err
}
defer rsp.Body.Close()
data, err := ioutil.ReadAll(rsp.Body)
if err != nil {
return nil, err
}
return data, nil
}
func getEK() (*attest.EK, error) {
tpm, err := attest.OpenTPM(nil)
if err != nil {
return nil, err
}
defer tpm.Close()
eks, err := tpm.EKs()
if err != nil {
return nil, err
}
if len(eks) == 0 {
return nil, fmt.Errorf("this TPM has no EK")
}
return &eks[0], nil
}
type AK struct {
subjectKeyId []byte
}
var machNameCharSet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
func newAK() (*AK, error) {
id := make([]byte, 20)
rand.Reader.Read(id)
return &AK{
subjectKeyId: id,
}, nil
}
func decodeHexOrPanic(in string) []byte {
data, err := hex.DecodeString(in)
if err != nil {
panic(err)
}
return data
}
type akBundle struct {
handle tpmutil.Handle
public, creationName, creationData, creationHash []byte
creationTicket tpm2.Ticket
template []byte
}
func (ak *AK) Generate(tpm io.ReadWriter) (*akBundle, error) {
template := tpm2.Public{
Type: tpm2.AlgRSA,
NameAlg: tpm2.AlgSHA256,
Attributes: tpm2.FlagFixedTPM | tpm2.FlagFixedParent | tpm2.FlagSensitiveDataOrigin |
tpm2.FlagUserWithAuth | tpm2.FlagNoDA | tpm2.FlagRestricted | tpm2.FlagSign,
AuthPolicy: decodeHexOrPanic("9dffcbf36c383ae699fb9868dc6dcb89" +
"d7153884be2803922c124158bfad22ae"),
RSAParameters: &tpm2.RSAParams{
Sign: &tpm2.SigScheme{
Alg: tpm2.AlgRSASSA,
Hash: tpm2.AlgSHA1,
},
KeyBits: 2048,
// Customize the input template with the random Subject Key ID
// This allows each AIK to be unique even though they are Primary
ModulusRaw: tpmutil.U16Bytes(ak.subjectKeyId),
},
}
encodedTemplate, err := template.Encode()
if err != nil {
return nil, err
}
fmt.Printf("Generating AK %x...", ak.subjectKeyId)
h, pub, cd, ch, tk, cn, err := tpm2.CreatePrimaryEx(tpm, tpm2.HandleOwner, tpm2.PCRSelection{}, "", "", template)
fmt.Printf("done\n")
if err != nil {
return nil, err
}
return &akBundle{
handle: h,
public: pub,
creationData: cd,
creationHash: ch,
creationTicket: tk,
creationName: cn,
template: encodedTemplate,
}, nil
}
func (cli *ClientContext) AttestationStatement() (*AttestationStatement, crypto.PublicKey, error) {
fmt.Printf("Certifying AK...")
attest, sig, err := tpm2.CertifyCreation(cli.tpm, "", cli.akBundle.handle, cli.akBundle.handle, []byte{}, cli.akBundle.creationHash, tpm2.SigScheme{Alg: tpm2.AlgNull}, cli.akBundle.creationTicket)
fmt.Printf("done\n")
if err != nil {
return nil, nil, err
}
public, err := tpm2.DecodePublic(cli.akBundle.public)
if err != nil {
return nil, nil, err
}
creationData, err := tpm2.DecodeCreationData(cli.akBundle.creationData)
if err != nil {
return nil, nil, err
}
att, err := tpm2.DecodeAttestationData(attest)
if err != nil {
return nil, nil, err
}
tpmPub, err := tpm2.DecodePublic(cli.akBundle.public)
if err != nil {
return nil, nil, err
}
tpmSig, err := tpm2.DecodeSignature(bytes.NewBuffer(sig))
if err != nil {
return nil, nil, err
}
pub, err := tpmPub.Key()
if err != nil {
return nil, nil, err
}
stmt := AttestationStatement{
Header: AttestationStatementHdr{
Magic: 0x5453414b,
Version: 1,
Platform: 2,
HeaderSize: 28,
},
IDBinding: IDBinding{
Public: public,
CreationAttestation: CreationAttestation{
CreationData: *creationData,
Attest: *att,
SignatureAlg: tpmSig.Alg,
Signature: *tpmSig.RSA,
},
},
}
return &stmt, pub, nil
}
// Default EK template defined in:
// https://trustedcomputinggroup.org/wp-content/uploads/Credential_Profile_EK_V2.0_R14_published.pdf
// From https://github.com/google/go-attestation/blob/0a3c6e82bfbdef476a1b6b9e6ac2a0fdcc79e821/attest/tpm.go#L74
var defaultEKTemplate = tpm2.Public{
Type: tpm2.AlgRSA,
NameAlg: tpm2.AlgSHA256,
Attributes: tpm2.FlagFixedTPM | tpm2.FlagFixedParent | tpm2.FlagSensitiveDataOrigin |
tpm2.FlagAdminWithPolicy | tpm2.FlagRestricted | tpm2.FlagDecrypt,
AuthPolicy: []byte{
0x83, 0x71, 0x97, 0x67, 0x44, 0x84,
0xB3, 0xF8, 0x1A, 0x90, 0xCC, 0x8D,
0x46, 0xA5, 0xD7, 0x24, 0xFD, 0x52,
0xD7, 0x6E, 0x06, 0x52, 0x0B, 0x64,
0xF2, 0xA1, 0xDA, 0x1B, 0x33, 0x14,
0x69, 0xAA,
},
RSAParameters: &tpm2.RSAParams{
Symmetric: &tpm2.SymScheme{
Alg: tpm2.AlgAES,
KeyBits: 128,
Mode: tpm2.AlgCFB,
},
KeyBits: 2048,
ModulusRaw: make([]byte, 256),
},
}
func (cli *ClientContext) DecryptChallenge(challenge *AttestationKeyChallenge) ([]byte, error) {
ek, _, err := tpm2.CreatePrimary(cli.tpm, tpm2.HandleEndorsement, tpm2.PCRSelection{}, "", "", defaultEKTemplate)
if err != nil {
return nil, fmt.Errorf("EK CreatePrimary failed: %v", err)
}
defer tpm2.FlushContext(cli.tpm, ek)
sessHandle, _, err := tpm2.StartAuthSession(
cli.tpm,
tpm2.HandleNull, /*tpmKey*/
tpm2.HandleNull, /*bindKey*/
make([]byte, 16), /*nonceCaller*/
nil, /*secret*/
tpm2.SessionPolicy,
tpm2.AlgNull,
tpm2.AlgSHA256)
if err != nil {
return nil, fmt.Errorf("creating session: %v", err)
}
defer tpm2.FlushContext(cli.tpm, sessHandle)
if _, err := tpm2.PolicySecret(cli.tpm, tpm2.HandleEndorsement, tpm2.AuthCommand{Session: tpm2.HandlePasswordSession, Attributes: tpm2.AttrContinueSession}, sessHandle, nil, nil, nil, 0); err != nil {
return nil, fmt.Errorf("tpm2.PolicySecret() failed: %v", err)
}
result, err := tpm2.ActivateCredentialUsingAuth(cli.tpm, []tpm2.AuthCommand{
{Session: tpm2.HandlePasswordSession, Attributes: tpm2.AttrContinueSession},
{Session: sessHandle, Attributes: tpm2.AttrContinueSession},
}, cli.akBundle.handle, ek, challenge.CredentialBlob, challenge.Secret)
return result, err
}
func (cli *ClientContext) unwrapGetChallengeResponse(rsp []byte) (*AttestationKeyChallenge, error) {
// Unwrap the outer PKCS7 signed blob
env, err := ParseGetChallengeRsp(rsp)
if err != nil {
return nil, err
}
// Decrypt the inner PKCS7 encrypted blob and parse the inner structure there
return env.DecryptChallenge(cli.scepCert, cli.scepKey)
}
func (cli *ClientContext) unwrapGetCertResponse(rsp []byte) ([]byte, error) {
// Unwrap the outer PKCS7 signed blob
env, err := ParseGetCertRsp(rsp)
if err != nil {
return nil, err
}
// Decrypt the inner PKCS7 encrypted PKCS7 signed certificate
decrypted, err := env.DecryptCert(cli.scepCert, cli.scepKey)
if err != nil {
return nil, err
}
// Unwrap the redundantly signed PKCS7 to return a nice x509
signed, err := pkcs7.Parse(decrypted)
if err != nil {
return nil, err
}
if len(signed.Certificates) < 1 {
return nil, fmt.Errorf("didn't get any certs back")
}
return signed.Certificates[0].Raw, nil
}
func (cli *ClientContext) GetAKCert() (template, cert []byte, err error) {
tpmInfo, err := ekTPMInfo(unfork(cli.ek.Certificate))
if err != nil {
return nil, nil, err
}
extraEKCerts, err := AdditionalEKCerts(unfork(cli.ek.Certificate))
if err != nil {
return nil, nil, fmt.Errorf("fetching EK intermediates: %w", err)
}
ekInfo, err := EncryptEKInfo(unfork(cli.ek.Certificate), cli.raCert)
if err != nil {
return nil, nil, err
}
attestation, public, err := cli.AttestationStatement()
if err != nil {
return nil, nil, err
}
csr := AttestationKeyCSR{
SubjectPublicKey: public,
SubjectKeyID: cli.ak.subjectKeyId,
OSVersion: "10.0.19043.2",
SCEPSignerHash: thumbprint(cli.scepCert),
MachineName: cli.machineName,
UserName: `WORKGROUP\` + cli.machineName + "$",
AttestationStatement: *attestation,
EncryptedEKCerts: ekInfo,
TPMInfo: *tpmInfo,
}
claims, err := GenerateAttestationKeyCSR(&csr)
if err != nil {
return nil, nil, err
}
challengeBuilder := GetChallengeReqBuilder{
Claims: claims,
ExtraEKCerts: extraEKCerts,
SignerCert: cli.scepCert,
SignerKey: cli.scepKey,
RecipientCert: cli.raCert,
}
fmt.Printf("Requesting EK challenge...")
getChallengeRsp, err := cli.getChallenge(&challengeBuilder)
fmt.Printf("done\n")
if err != nil {
return nil, nil, err
}
fmt.Printf("Unwrapping EK challenge outer PKCS7 layers...")
challenge, err := cli.unwrapGetChallengeResponse(getChallengeRsp)
fmt.Printf("done\n")
if err != nil {
return nil, nil, err
}
fmt.Printf("Decrypting EK challenge...")
decrypted, err := cli.DecryptChallenge(challenge)
fmt.Printf("done\n")
if err != nil {
return nil, nil, err
}
fmt.Printf("Decrypted challenge: %x\n", decrypted)
certBuilder := GetCertReqBuilder{
Challenge: challenge,
DecryptedChallenge: decrypted,
SignerCert: cli.scepCert,
SignerKey: cli.scepKey,
RecipientCert: cli.raCert,
}
fmt.Printf("Presenting decrypted EK challenge to server...")
encCert, err := cli.getCert(&certBuilder)
fmt.Printf("done\n")
if err != nil {
return nil, nil, err
}
fmt.Printf("Decrypting AK cert...")
certificate, err := cli.unwrapGetCertResponse(encCert)
fmt.Printf("done\n")
if err != nil {
return nil, nil, err
}
return cli.akBundle.template, certificate, err
}