mcias/vendor/github.com/google/go-tpm/tpm2/sessions.go

package tpm2

import (
	"bytes"
	"crypto/aes"
	"crypto/cipher"
	"crypto/hmac"
	"crypto/rand"
	"encoding/binary"
	"fmt"

	"github.com/google/go-tpm/tpm2/transport"
)

// Session represents a session in the TPM.
type Session interface {
	// Initializes the session, if needed. Has no effect if not needed or
	// already done. Some types of sessions may need to be initialized
	// just-in-time, e.g., to support calling patterns that help the user
	// securely authorize their actions without writing a lot of code.
	Init(tpm transport.TPM) error
	// Cleans up the session, if needed.
	// Some types of session need to be cleaned up if the command failed,
	// again to support calling patterns that help the user securely
	// authorize their actions without writing a lot of code.
	CleanupFailure(tpm transport.TPM) error
	// The last nonceTPM for this session.
	NonceTPM() TPM2BNonce
	// Updates nonceCaller to a new random value.
	NewNonceCaller() error
	// Computes the authorization HMAC for the session.
	// If this is the first authorization session for a command, and
	// there is another session (or sessions) for parameter
	// decryption and/or encryption, then addNonces contains the
	// nonceTPMs from each of them, respectively (see Part 1, 19.6.5)
	Authorize(cc TPMCC, parms, addNonces []byte, names []TPM2BName, authIndex int) (*TPMSAuthCommand, error)
	// Validates the response for the session.
	// Updates NonceTPM for the session.
	Validate(rc TPMRC, cc TPMCC, parms []byte, names []TPM2BName, authIndex int, auth *TPMSAuthResponse) error
	// Returns true if this is an encryption session.
	IsEncryption() bool
	// Returns true if this is a decryption session.
	IsDecryption() bool
	// If this session is used for parameter decryption, encrypts the
	// parameter. Otherwise, does not modify the parameter.
	Encrypt(parameter []byte) error
	// If this session is used for parameter encryption, encrypts the
	// parameter. Otherwise, does not modify the parameter.
	Decrypt(parameter []byte) error
	// Returns the handle value of this session.
	Handle() TPMHandle
}

// CPHash calculates the TPM command parameter hash for a given Command.
// N.B. Authorization sessions on handles are ignored, but names aren't.
func CPHash[R any](alg TPMIAlgHash, cmd Command[R, *R]) (*TPM2BDigest, error) {
	cc := cmd.Command()
	names, err := cmdNames(cmd)
	if err != nil {
		return nil, err
	}
	parms, err := cmdParameters(cmd, nil)
	if err != nil {
		return nil, err
	}
	digest, err := cpHash(alg, cc, names, parms)
	if err != nil {
		return nil, err
	}
	return &TPM2BDigest{
		Buffer: digest,
	}, nil
}

// pwSession represents a password-pseudo-session.
type pwSession struct {
	auth []byte
}

// PasswordAuth assembles a password pseudo-session with the given auth value.
func PasswordAuth(auth []byte) Session {
	return &pwSession{
		auth: auth,
	}
}

// Init is not required and has no effect for a password session.
func (s *pwSession) Init(_ transport.TPM) error { return nil }

// Cleanup is not required and has no effect for a password session.
func (s *pwSession) CleanupFailure(_ transport.TPM) error { return nil }

// NonceTPM normally returns the last nonceTPM value from the session.
// Since a password session is a pseudo-session with the auth value stuffed
// in where the HMAC should go, this is not used.
func (s *pwSession) NonceTPM() TPM2BNonce { return TPM2BNonce{} }

// NewNonceCaller updates the nonceCaller for this session.
// Password sessions don't have nonces.
func (s *pwSession) NewNonceCaller() error { return nil }

// Computes the authorization structure for the session.
func (s *pwSession) Authorize(_ TPMCC, _, _ []byte, _ []TPM2BName, _ int) (*TPMSAuthCommand, error) {
	return &TPMSAuthCommand{
		Handle:     TPMRSPW,
		Nonce:      TPM2BNonce{},
		Attributes: TPMASession{},
		Authorization: TPM2BData{
			Buffer: s.auth,
		},
	}, nil
}

// Validates the response session structure for the session.
func (s *pwSession) Validate(_ TPMRC, _ TPMCC, _ []byte, _ []TPM2BName, _ int, auth *TPMSAuthResponse) error {
	if len(auth.Nonce.Buffer) != 0 {
		return fmt.Errorf("expected empty nonce in response auth to PW session, got %x", auth.Nonce)
	}
	expectedAttrs := TPMASession{
		ContinueSession: true,
	}
	if auth.Attributes != expectedAttrs {
		return fmt.Errorf("expected only ContinueSession in response auth to PW session, got %v", auth.Attributes)
	}
	if len(auth.Authorization.Buffer) != 0 {
		return fmt.Errorf("expected empty HMAC in response auth to PW session, got %x", auth.Authorization)
	}
	return nil
}

// IsEncryption returns true if this is an encryption session.
// Password sessions can't be used for encryption.
func (s *pwSession) IsEncryption() bool { return false }

// IsDecryption returns true if this is a decryption session.
// Password sessions can't be used for decryption.
func (s *pwSession) IsDecryption() bool { return false }

// If this session is used for parameter decryption, encrypts the
// parameter. Otherwise, does not modify the parameter.
// Password sessions can't be used for decryption.
func (s *pwSession) Encrypt(_ []byte) error { return nil }

// If this session is used for parameter encryption, encrypts the
// parameter. Otherwise, does not modify the parameter.
// Password sessions can't be used for encryption.
func (s *pwSession) Decrypt(_ []byte) error { return nil }

// Handle returns the handle value associated with this session.
// In the case of a password session, this is always TPM_RS_PW.
func (s *pwSession) Handle() TPMHandle { return TPMRSPW }

// cpHash calculates the TPM command parameter hash.
// cpHash = hash(CC || names || parms)
func cpHash(alg TPMIAlgHash, cc TPMCC, names []TPM2BName, parms []byte) ([]byte, error) {
	ha, err := alg.Hash()
	if err != nil {
		return nil, err
	}
	h := ha.New()
	binary.Write(h, binary.BigEndian, cc)
	for _, name := range names {
		h.Write(name.Buffer)
	}
	h.Write(parms)
	return h.Sum(nil), nil
}

// rpHash calculates the TPM response parameter hash.
// rpHash = hash(RC || CC || parms)
func rpHash(alg TPMIAlgHash, rc TPMRC, cc TPMCC, parms []byte) ([]byte, error) {
	ha, err := alg.Hash()
	if err != nil {
		return nil, err
	}
	h := ha.New()
	binary.Write(h, binary.BigEndian, rc)
	binary.Write(h, binary.BigEndian, cc)
	h.Write(parms)
	return h.Sum(nil), nil
}

// sessionOptions represents extra options used when setting up an HMAC or policy session.
type sessionOptions struct {
	auth        []byte
	password    bool
	bindHandle  TPMIDHEntity
	bindName    TPM2BName
	bindAuth    []byte
	saltHandle  TPMIDHObject
	saltPub     TPMTPublic
	attrs       TPMASession
	symmetric   TPMTSymDef
	trialPolicy bool
}

// defaultOptions represents the default options used when none are provided.
func defaultOptions() sessionOptions {
	return sessionOptions{
		symmetric: TPMTSymDef{
			Algorithm: TPMAlgNull,
		},
		bindHandle: TPMRHNull,
		saltHandle: TPMRHNull,
	}
}

// AuthOption is an option for setting up an auth session variadically.
type AuthOption func(*sessionOptions)

// Auth uses the session to prove knowledge of the object's auth value.
func Auth(auth []byte) AuthOption {
	return func(o *sessionOptions) {
		o.auth = auth
	}
}

// Password is a policy-session-only option that specifies to provide the
// object's auth value in place of the authorization HMAC when authorizing.
// For HMAC sessions, has the same effect as using Auth.
// Deprecated: This is not recommended and is only provided for completeness;
// use Auth instead.
func Password(auth []byte) AuthOption {
	return func(o *sessionOptions) {
		o.auth = auth
		o.password = true
	}
}

// Bound specifies that this session's session key should depend on the auth
// value of the given object.
func Bound(handle TPMIDHEntity, name TPM2BName, auth []byte) AuthOption {
	return func(o *sessionOptions) {
		o.bindHandle = handle
		o.bindName = name
		o.bindAuth = auth
	}
}

// Salted specifies that this session's session key should depend on an
// encrypted seed value using the given public key.
// 'handle' must refer to a loaded RSA or ECC key.
func Salted(handle TPMIDHObject, pub TPMTPublic) AuthOption {
	return func(o *sessionOptions) {
		o.saltHandle = handle
		o.saltPub = pub
	}
}

// parameterEncryptiontpm2ion specifies whether the session-encrypted
// parameters are encrypted on the way into the TPM, out of the TPM, or both.
type parameterEncryptiontpm2ion int

const (
	// EncryptIn specifies a decrypt session.
	EncryptIn parameterEncryptiontpm2ion = 1 + iota
	// EncryptOut specifies an encrypt session.
	EncryptOut
	// EncryptInOut specifies a decrypt+encrypt session.
	EncryptInOut
)

// AESEncryption uses the session to encrypt the first parameter sent to/from
// the TPM.
// Note that only commands whose first command/response parameter is a 2B can
// support session encryption.
func AESEncryption(keySize TPMKeyBits, dir parameterEncryptiontpm2ion) AuthOption {
	return func(o *sessionOptions) {
		o.attrs.Decrypt = (dir == EncryptIn || dir == EncryptInOut)
		o.attrs.Encrypt = (dir == EncryptOut || dir == EncryptInOut)
		o.symmetric = TPMTSymDef{
			Algorithm: TPMAlgAES,
			KeyBits: NewTPMUSymKeyBits(
				TPMAlgAES,
				TPMKeyBits(keySize),
			),
			Mode: NewTPMUSymMode(
				TPMAlgAES,
				TPMAlgCFB,
			),
		}
	}
}

// Audit uses the session to compute extra HMACs.
// An Audit session can be used with GetSessionAuditDigest to obtain attestation
// over a sequence of commands.
func Audit() AuthOption {
	return func(o *sessionOptions) {
		o.attrs.Audit = true
	}
}

// AuditExclusive is like an audit session, but even more powerful.
// This allows an audit session to additionally indicate that no other auditable
// commands were executed other than the ones described by the audit hash.
func AuditExclusive() AuthOption {
	return func(o *sessionOptions) {
		o.attrs.Audit = true
		o.attrs.AuditExclusive = true
	}
}

// Trial indicates that the policy session should be in trial-mode.
// This allows using the TPM to calculate policy hashes.
// This option has no effect on non-Policy sessions.
func Trial() AuthOption {
	return func(o *sessionOptions) {
		o.trialPolicy = true
	}
}

// hmacSession generally implements the HMAC session.
type hmacSession struct {
	sessionOptions
	hash       TPMIAlgHash
	nonceSize  int
	handle     TPMHandle
	sessionKey []byte
	// last nonceCaller
	nonceCaller TPM2BNonce
	// last nonceTPM
	nonceTPM TPM2BNonce
}

// HMAC sets up a just-in-time HMAC session that is used only once.
// A real session is created, but just in time and it is flushed when used.
func HMAC(hash TPMIAlgHash, nonceSize int, opts ...AuthOption) Session {
	// Set up a one-off session that knows the auth value.
	sess := hmacSession{
		sessionOptions: defaultOptions(),
		hash:           hash,
		nonceSize:      nonceSize,
		handle:         TPMRHNull,
	}
	for _, opt := range opts {
		opt(&sess.sessionOptions)
	}
	return &sess
}

// HMACSession sets up a reusable HMAC session that needs to be closed.
func HMACSession(t transport.TPM, hash TPMIAlgHash, nonceSize int, opts ...AuthOption) (s Session, close func() error, err error) {
	// Set up a not-one-off session that knows the auth value.
	sess := hmacSession{
		sessionOptions: defaultOptions(),
		hash:           hash,
		nonceSize:      nonceSize,
		handle:         TPMRHNull,
	}
	for _, opt := range opts {
		opt(&sess.sessionOptions)
	}
	// This session is reusable and is closed with the function we'll
	// return.
	sess.sessionOptions.attrs.ContinueSession = true

	// Initialize the session.
	if err := sess.Init(t); err != nil {
		return nil, nil, err
	}

	closer := func() error {
		_, err := (&FlushContext{FlushHandle: sess.handle}).Execute(t)
		return err
	}

	return &sess, closer, nil
}

// getEncryptedSalt creates a salt value for salted sessions.
// Returns the encrypted salt and plaintext salt, or an error value.
func getEncryptedSalt(pub TPMTPublic) (*TPM2BEncryptedSecret, []byte, error) {
	key, err := ImportEncapsulationKey(&pub)
	if err != nil {
		return nil, nil, err
	}
	salt, encSalt, err := CreateEncryptedSalt(rand.Reader, key)
	if err != nil {
		return nil, nil, err
	}
	return &TPM2BEncryptedSecret{
		Buffer: encSalt,
	}, salt, nil
}

// Init initializes the session, just in time, if needed.
func (s *hmacSession) Init(t transport.TPM) error {
	if s.handle != TPMRHNull {
		// Session is already initialized.
		return nil
	}

	// Get a high-quality nonceCaller for our use.
	// Store it with the session object for later reference.
	s.nonceCaller = TPM2BNonce{
		Buffer: make([]byte, s.nonceSize),
	}
	if _, err := rand.Read(s.nonceCaller.Buffer); err != nil {
		return err
	}

	// Start up the actual auth session.
	sasCmd := StartAuthSession{
		TPMKey:      s.saltHandle,
		Bind:        s.bindHandle,
		NonceCaller: s.nonceCaller,
		SessionType: TPMSEHMAC,
		Symmetric:   s.symmetric,
		AuthHash:    s.hash,
	}
	var salt []byte
	if s.saltHandle != TPMRHNull {
		var err error
		var encSalt *TPM2BEncryptedSecret
		encSalt, salt, err = getEncryptedSalt(s.saltPub)
		if err != nil {
			return err
		}
		sasCmd.EncryptedSalt = *encSalt
	}
	sasRsp, err := sasCmd.Execute(t)
	if err != nil {
		return err
	}
	s.handle = TPMHandle(sasRsp.SessionHandle.HandleValue())
	s.nonceTPM = sasRsp.NonceTPM
	// Part 1, 19.6
	ha, err := s.hash.Hash()
	if err != nil {
		return err
	}
	if s.bindHandle != TPMRHNull || len(salt) != 0 {
		var authSalt []byte
		authSalt = append(authSalt, s.bindAuth...)
		authSalt = append(authSalt, salt...)
		s.sessionKey = KDFa(ha, authSalt, "ATH", s.nonceTPM.Buffer, s.nonceCaller.Buffer, ha.Size()*8)
	}
	return nil
}

// Cleanup cleans up the session, if needed.
func (s *hmacSession) CleanupFailure(t transport.TPM) error {
	// The user is already responsible to clean up this session.
	if s.attrs.ContinueSession {
		return nil
	}
	fc := FlushContext{FlushHandle: s.handle}
	if _, err := fc.Execute(t); err != nil {
		return err
	}
	s.handle = TPMRHNull
	return nil
}

// NonceTPM returns the last nonceTPM value from the session.
// May be nil, if the session hasn't been initialized yet.
func (s *hmacSession) NonceTPM() TPM2BNonce { return s.nonceTPM }

// To avoid a circular dependency on gotpm by tpm2, implement a
// tiny serialization by hand for TPMASession here
func attrsToBytes(attrs TPMASession) []byte {
	var res byte
	if attrs.ContinueSession {
		res |= (1 << 0)
	}
	if attrs.AuditExclusive {
		res |= (1 << 1)
	}
	if attrs.AuditReset {
		res |= (1 << 2)
	}
	if attrs.Decrypt {
		res |= (1 << 5)
	}
	if attrs.Encrypt {
		res |= (1 << 6)
	}
	if attrs.Audit {
		res |= (1 << 7)
	}
	return []byte{res}
}

// computeHMAC computes an authorization HMAC according to various equations in
// Part 1.
// This applies to both commands and responses.
// The value of key depends on whether the session is bound and/or salted.
// pHash cpHash for a command, or an rpHash for a response.
// nonceNewer in a command is the new nonceCaller sent in the command session packet.
// nonceNewer in a response is the new nonceTPM sent in the response session packet.
// nonceOlder in a command is the last nonceTPM sent by the TPM for this session.
// This may be when the session was created, or the last time it was used.
// nonceOlder in a response is the corresponding nonceCaller sent in the command.
func computeHMAC(alg TPMIAlgHash, key, pHash, nonceNewer, nonceOlder, addNonces []byte, attrs TPMASession) ([]byte, error) {
	ha, err := alg.Hash()
	if err != nil {
		return nil, err
	}
	mac := hmac.New(ha.New, key)
	mac.Write(pHash)
	mac.Write(nonceNewer)
	mac.Write(nonceOlder)
	mac.Write(addNonces)
	mac.Write(attrsToBytes(attrs))
	return mac.Sum(nil), nil
}

// Trim trailing zeros from the auth value. Part 1, 19.6.5, Note 2
// Does not allocate a new underlying byte array.
func hmacKeyFromAuthValue(auth []byte) []byte {
	key := auth
	for i := len(key) - 1; i >= 0; i-- {
		if key[i] == 0 {
			key = key[:i]
		}
	}
	return key
}

// NewNonceCaller updates the nonceCaller for this session.
func (s *hmacSession) NewNonceCaller() error {
	_, err := rand.Read(s.nonceCaller.Buffer)
	return err
}

// Authorize computes the authorization structure for the session.
// Unlike the TPM spec, authIndex is zero-based.
func (s *hmacSession) Authorize(cc TPMCC, parms, addNonces []byte, names []TPM2BName, authIndex int) (*TPMSAuthCommand, error) {
	if s.handle == TPMRHNull {
		// Session is not initialized.
		return nil, fmt.Errorf("session not initialized")
	}

	// Part 1, 19.6
	// HMAC key is (sessionKey || auth) unless this session is authorizing
	// its bind target
	var hmacKey []byte
	hmacKey = append(hmacKey, s.sessionKey...)
	if len(s.bindName.Buffer) == 0 || authIndex >= len(names) || !bytes.Equal(names[authIndex].Buffer, s.bindName.Buffer) {
		hmacKey = append(hmacKey, hmacKeyFromAuthValue(s.auth)...)
	}

	// Compute the authorization HMAC.
	cph, err := cpHash(s.hash, cc, names, parms)
	if err != nil {
		return nil, err
	}
	hmac, err := computeHMAC(s.hash, hmacKey, cph, s.nonceCaller.Buffer, s.nonceTPM.Buffer, addNonces, s.attrs)
	if err != nil {
		return nil, err
	}
	result := TPMSAuthCommand{
		Handle:     s.handle,
		Nonce:      s.nonceCaller,
		Attributes: s.attrs,
		Authorization: TPM2BData{
			Buffer: hmac,
		},
	}
	return &result, nil
}

// Validate validates the response session structure for the session.
// It updates nonceTPM from the TPM's response.
func (s *hmacSession) Validate(rc TPMRC, cc TPMCC, parms []byte, names []TPM2BName, authIndex int, auth *TPMSAuthResponse) error {
	// Track the new nonceTPM for the session.
	s.nonceTPM = auth.Nonce
	// Track the session being automatically flushed.
	if !auth.Attributes.ContinueSession {
		s.handle = TPMRHNull
	}

	// Part 1, 19.6
	// HMAC key is (sessionKey || auth) unless this session is authorizing
	// its bind target
	var hmacKey []byte
	hmacKey = append(hmacKey, s.sessionKey...)
	if len(s.bindName.Buffer) == 0 || authIndex >= len(names) || !bytes.Equal(names[authIndex].Buffer, s.bindName.Buffer) {
		hmacKey = append(hmacKey, hmacKeyFromAuthValue(s.auth)...)
	}

	// Compute the authorization HMAC.
	rph, err := rpHash(s.hash, rc, cc, parms)
	if err != nil {
		return err
	}
	mac, err := computeHMAC(s.hash, hmacKey, rph, s.nonceTPM.Buffer, s.nonceCaller.Buffer, nil, auth.Attributes)
	if err != nil {
		return err
	}
	// Compare the HMAC (constant time)
	if !hmac.Equal(mac, auth.Authorization.Buffer) {
		return fmt.Errorf("incorrect authorization HMAC")
	}
	return nil
}

// IsEncryption returns true if this is an encryption session.
func (s *hmacSession) IsEncryption() bool {
	return s.attrs.Encrypt
}

// IsDecryption returns true if this is a decryption session.
func (s *hmacSession) IsDecryption() bool {
	return s.attrs.Decrypt
}

// Encrypt decrypts the parameter in place, if this session is used for
// parameter decryption. Otherwise, it does not modify the parameter.
func (s *hmacSession) Encrypt(parameter []byte) error {
	if !s.IsDecryption() {
		return nil
	}
	// Only AES-CFB is supported.
	bits, err := s.symmetric.KeyBits.AES()
	if err != nil {
		return err
	}
	keyBytes := *bits / 8
	keyIVBytes := int(keyBytes) + 16
	var sessionValue []byte
	sessionValue = append(sessionValue, s.sessionKey...)
	sessionValue = append(sessionValue, s.auth...)
	ha, err := s.hash.Hash()
	if err != nil {
		return err
	}
	keyIV := KDFa(ha, sessionValue, "CFB", s.nonceCaller.Buffer, s.nonceTPM.Buffer, keyIVBytes*8)
	key, err := aes.NewCipher(keyIV[:keyBytes])
	if err != nil {
		return err
	}
	stream := cipher.NewCFBEncrypter(key, keyIV[keyBytes:])
	stream.XORKeyStream(parameter, parameter)
	return nil
}

// Decrypt encrypts the parameter in place, if this session is used for
// parameter encryption. Otherwise, it does not modify the parameter.
func (s *hmacSession) Decrypt(parameter []byte) error {
	if !s.IsEncryption() {
		return nil
	}
	// Only AES-CFB is supported.
	bits, err := s.symmetric.KeyBits.AES()
	if err != nil {
		return err
	}
	keyBytes := *bits / 8
	keyIVBytes := int(keyBytes) + 16
	// Part 1, 21.1
	var sessionValue []byte
	sessionValue = append(sessionValue, s.sessionKey...)
	sessionValue = append(sessionValue, s.auth...)
	ha, err := s.hash.Hash()
	if err != nil {
		return err
	}
	keyIV := KDFa(ha, sessionValue, "CFB", s.nonceTPM.Buffer, s.nonceCaller.Buffer, keyIVBytes*8)
	key, err := aes.NewCipher(keyIV[:keyBytes])
	if err != nil {
		return err
	}
	stream := cipher.NewCFBDecrypter(key, keyIV[keyBytes:])
	stream.XORKeyStream(parameter, parameter)
	return nil
}

// Handle returns the handle value of the session.
// If the session is created with HMAC (instead of HMACSession) this will be
// TPM_RH_NULL.
func (s *hmacSession) Handle() TPMHandle {
	return s.handle
}

// PolicyCallback represents an object's policy in the form of a function.
// This function makes zero or more TPM policy commands and returns error.
type PolicyCallback = func(tpm transport.TPM, handle TPMISHPolicy, nonceTPM TPM2BNonce) error

// policySession generally implements the policy session.
type policySession struct {
	sessionOptions
	hash       TPMIAlgHash
	nonceSize  int
	handle     TPMHandle
	sessionKey []byte
	// last nonceCaller
	nonceCaller TPM2BNonce
	// last nonceTPM
	nonceTPM TPM2BNonce
	callback *PolicyCallback
}

// Policy sets up a just-in-time policy session that created each time it's
// needed.
// Each time the policy is created, the callback is invoked to authorize the
// session.
// A real session is created, but just in time, and it is flushed when used.
func Policy(hash TPMIAlgHash, nonceSize int, callback PolicyCallback, opts ...AuthOption) Session {
	// Set up a one-off session that knows the auth value.
	sess := policySession{
		sessionOptions: defaultOptions(),
		hash:           hash,
		nonceSize:      nonceSize,
		handle:         TPMRHNull,
		callback:       &callback,
	}
	for _, opt := range opts {
		opt(&sess.sessionOptions)
	}
	return &sess
}

// PolicySession opens a policy session that needs to be closed.
// The caller is responsible to call whichever policy commands they want in the
// session.
// Note that the TPM resets a policy session after it is successfully used.
func PolicySession(t transport.TPM, hash TPMIAlgHash, nonceSize int, opts ...AuthOption) (s Session, close func() error, err error) {
	// Set up a not-one-off session that knows the auth value.
	sess := policySession{
		sessionOptions: defaultOptions(),
		hash:           hash,
		nonceSize:      nonceSize,
		handle:         TPMRHNull,
	}
	for _, opt := range opts {
		opt(&sess.sessionOptions)
	}

	// This session is reusable and is closed with the function we'll
	// return.
	sess.sessionOptions.attrs.ContinueSession = true

	// Initialize the session.
	if err := sess.Init(t); err != nil {
		return nil, nil, err
	}

	closer := func() error {
		_, err := (&FlushContext{sess.handle}).Execute(t)
		return err
	}

	return &sess, closer, nil
}

// Init initializes the session, just in time, if needed.
func (s *policySession) Init(t transport.TPM) error {
	if s.handle != TPMRHNull {
		// Session is already initialized.
		return nil
	}

	// Get a high-quality nonceCaller for our use.
	// Store it with the session object for later reference.
	s.nonceCaller = TPM2BNonce{
		Buffer: make([]byte, s.nonceSize),
	}
	if _, err := rand.Read(s.nonceCaller.Buffer); err != nil {
		return err
	}

	sessType := TPMSEPolicy
	if s.sessionOptions.trialPolicy {
		sessType = TPMSETrial
	}

	// Start up the actual auth session.
	sasCmd := StartAuthSession{
		TPMKey:      s.saltHandle,
		Bind:        s.bindHandle,
		NonceCaller: s.nonceCaller,
		SessionType: sessType,
		Symmetric:   s.symmetric,
		AuthHash:    s.hash,
	}
	var salt []byte
	if s.saltHandle != TPMRHNull {
		var err error
		var encSalt *TPM2BEncryptedSecret
		encSalt, salt, err = getEncryptedSalt(s.saltPub)
		if err != nil {
			return err
		}
		sasCmd.EncryptedSalt = *encSalt
	}
	sasRsp, err := sasCmd.Execute(t)
	if err != nil {
		return err
	}
	s.handle = TPMHandle(sasRsp.SessionHandle.HandleValue())
	s.nonceTPM = sasRsp.NonceTPM
	// Part 1, 19.6
	if s.bindHandle != TPMRHNull || len(salt) != 0 {
		var authSalt []byte
		authSalt = append(authSalt, s.bindAuth...)
		authSalt = append(authSalt, salt...)
		ha, err := s.hash.Hash()
		if err != nil {
			return err
		}
		s.sessionKey = KDFa(ha, authSalt, "ATH", s.nonceTPM.Buffer, s.nonceCaller.Buffer, ha.Size()*8)
	}

	// Call the callback to execute the policy, if needed
	if s.callback != nil {
		if err := (*s.callback)(t, s.handle, s.nonceTPM); err != nil {
			return fmt.Errorf("executing policy: %w", err)
		}
	}

	return nil
}

// CleanupFailure cleans up the session, if needed.
func (s *policySession) CleanupFailure(t transport.TPM) error {
	// The user is already responsible to clean up this session.
	if s.attrs.ContinueSession {
		return nil
	}
	fc := FlushContext{FlushHandle: s.handle}
	if _, err := fc.Execute(t); err != nil {
		return err
	}
	s.handle = TPMRHNull
	return nil
}

// NonceTPM returns the last nonceTPM value from the session.
// May be nil, if the session hasn't been initialized yet.
func (s *policySession) NonceTPM() TPM2BNonce { return s.nonceTPM }

// NewNonceCaller updates the nonceCaller for this session.
func (s *policySession) NewNonceCaller() error {
	_, err := rand.Read(s.nonceCaller.Buffer)
	return err
}

// Authorize computes the authorization structure for the session.
func (s *policySession) Authorize(cc TPMCC, parms, addNonces []byte, names []TPM2BName, _ int) (*TPMSAuthCommand, error) {
	if s.handle == TPMRHNull {
		// Session is not initialized.
		return nil, fmt.Errorf("session not initialized")
	}

	var hmac []byte
	if s.password {
		hmac = s.auth
	} else {
		// Part 1, 19.6
		// HMAC key is (sessionKey || auth).
		var hmacKey []byte
		hmacKey = append(hmacKey, s.sessionKey...)
		hmacKey = append(hmacKey, hmacKeyFromAuthValue(s.auth)...)

		// Compute the authorization HMAC.
		cph, err := cpHash(s.hash, cc, names, parms)
		if err != nil {
			return nil, err
		}
		hmac, err = computeHMAC(s.hash, hmacKey, cph, s.nonceCaller.Buffer, s.nonceTPM.Buffer, addNonces, s.attrs)
		if err != nil {
			return nil, err
		}
	}

	result := TPMSAuthCommand{
		Handle:     s.handle,
		Nonce:      s.nonceCaller,
		Attributes: s.attrs,
		Authorization: TPM2BData{
			Buffer: hmac,
		},
	}
	return &result, nil
}

// Validate valitades the response session structure for the session.
// Updates nonceTPM from the TPM's response.
func (s *policySession) Validate(rc TPMRC, cc TPMCC, parms []byte, _ []TPM2BName, _ int, auth *TPMSAuthResponse) error {
	// Track the new nonceTPM for the session.
	s.nonceTPM = auth.Nonce
	// Track the session being automatically flushed.
	if !auth.Attributes.ContinueSession {
		s.handle = TPMRHNull
	}

	if s.password {
		// If we used a password, expect no nonce and no response HMAC.
		if len(auth.Nonce.Buffer) != 0 {
			return fmt.Errorf("expected empty nonce in response auth to PW policy, got %x", auth.Nonce)
		}
		if len(auth.Authorization.Buffer) != 0 {
			return fmt.Errorf("expected empty HMAC in response auth to PW policy, got %x", auth.Authorization)
		}
	} else {
		// Part 1, 19.6
		// HMAC key is (sessionKey || auth).
		var hmacKey []byte
		hmacKey = append(hmacKey, s.sessionKey...)
		hmacKey = append(hmacKey, hmacKeyFromAuthValue(s.auth)...)
		// Compute the authorization HMAC.
		rph, err := rpHash(s.hash, rc, cc, parms)
		if err != nil {
			return err
		}
		mac, err := computeHMAC(s.hash, hmacKey, rph, s.nonceTPM.Buffer, s.nonceCaller.Buffer, nil, auth.Attributes)
		if err != nil {
			return err
		}
		// Compare the HMAC (constant time)
		if !hmac.Equal(mac, auth.Authorization.Buffer) {
			return fmt.Errorf("incorrect authorization HMAC")
		}
	}
	return nil
}

// IsEncryption returns true if this is an encryption session.
func (s *policySession) IsEncryption() bool {
	return s.attrs.Encrypt
}

// IsDecryption returns true if this is a decryption session.
func (s *policySession) IsDecryption() bool {
	return s.attrs.Decrypt
}

// Encrypt encrypts the parameter in place, if this session is used for
// parameter decryption. Otherwise, it does not modify the parameter.
func (s *policySession) Encrypt(parameter []byte) error {
	if !s.IsDecryption() {
		return nil
	}
	// Only AES-CFB is supported.
	bits, err := s.symmetric.KeyBits.AES()
	if err != nil {
		return err
	}
	keyBytes := *bits / 8
	keyIVBytes := int(keyBytes) + 16
	var sessionValue []byte
	sessionValue = append(sessionValue, s.sessionKey...)
	sessionValue = append(sessionValue, s.auth...)
	ha, err := s.hash.Hash()
	if err != nil {
		return err
	}
	keyIV := KDFa(ha, sessionValue, "CFB", s.nonceCaller.Buffer, s.nonceTPM.Buffer, keyIVBytes*8)
	key, err := aes.NewCipher(keyIV[:keyBytes])
	if err != nil {
		return err
	}
	stream := cipher.NewCFBEncrypter(key, keyIV[keyBytes:])
	stream.XORKeyStream(parameter, parameter)
	return nil
}

// Decrypt decrypts the parameter in place, if this session is used for
// parameter encryption. Otherwise, it does not modify the parameter.
func (s *policySession) Decrypt(parameter []byte) error {
	if !s.IsEncryption() {
		return nil
	}
	// Only AES-CFB is supported.
	bits, err := s.symmetric.KeyBits.AES()
	if err != nil {
		return err
	}
	keyBytes := *bits / 8
	keyIVBytes := int(keyBytes) + 16
	// Part 1, 21.1
	var sessionValue []byte
	sessionValue = append(sessionValue, s.sessionKey...)
	sessionValue = append(sessionValue, s.auth...)
	ha, err := s.hash.Hash()
	if err != nil {
		return err
	}
	keyIV := KDFa(ha, sessionValue, "CFB", s.nonceTPM.Buffer, s.nonceCaller.Buffer, keyIVBytes*8)
	key, err := aes.NewCipher(keyIV[:keyBytes])
	if err != nil {
		return err
	}
	stream := cipher.NewCFBDecrypter(key, keyIV[keyBytes:])
	stream.XORKeyStream(parameter, parameter)
	return nil
}

// Handle returns the handle value of the session.
// If the session is created with Policy (instead of PolicySession) this will be
// TPM_RH_NULL.
func (s *policySession) Handle() TPMHandle {
	return s.handle
}