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piv: implement key metadata retrieval
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Add support for a YubiKey vendor extension to retrieve public metadata
(including the public key, PIN/Touch policies and whether or not the key
was hardware generated) for a given key slot.

Also use this new method to determine the pin policy, for YubiKeys that
support it. This is more general than using the attestation certificate,
as it also works for keys that have been generated offline and imported
into the hardware token.

Fixes #129
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@Merovius @ericchiang
Merovius authored and ericchiang committed Aug 31, 2023
1 parent 55b6465 commit 8c3a0ff
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Showing 4 changed files with 378 additions and 29 deletions.
169 changes: 150 additions & 19 deletions piv/key.go
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Expand Up @@ -476,6 +476,16 @@ const (
TouchPolicyCached
)

// Origin represents whether a key was generated on the hardware, or has been
// imported into it.
type Origin int

// Origins supported by this package.
const (
OriginGenerated Origin = iota + 1
OriginImported
)

const (
tagPINPolicy = 0xaa
tagTouchPolicy = 0xab
Expand All @@ -487,12 +497,34 @@ var pinPolicyMap = map[PINPolicy]byte{
PINPolicyAlways: 0x03,
}

var pinPolicyMapInv = map[byte]PINPolicy{
0x01: PINPolicyNever,
0x02: PINPolicyOnce,
0x03: PINPolicyAlways,
}

var touchPolicyMap = map[TouchPolicy]byte{
TouchPolicyNever: 0x01,
TouchPolicyAlways: 0x02,
TouchPolicyCached: 0x03,
}

var touchPolicyMapInv = map[byte]TouchPolicy{
0x01: TouchPolicyNever,
0x02: TouchPolicyAlways,
0x03: TouchPolicyCached,
}

var originMap = map[Origin]byte{
OriginGenerated: 0x01,
OriginImported: 0x02,
}

var originMapInv = map[byte]Origin{
0x01: OriginGenerated,
0x02: OriginImported,
}

var algorithmsMap = map[Algorithm]byte{
AlgorithmEC256: algECCP256,
AlgorithmEC384: algECCP384,
Expand All @@ -501,6 +533,14 @@ var algorithmsMap = map[Algorithm]byte{
AlgorithmRSA2048: algRSA2048,
}

var algorithmsMapInv = map[byte]Algorithm{
algECCP256: AlgorithmEC256,
algECCP384: AlgorithmEC384,
algEd25519: AlgorithmEd25519,
algRSA1024: AlgorithmRSA1024,
algRSA2048: AlgorithmRSA2048,
}

// AttestationCertificate returns the YubiKey's attestation certificate, which
// is unique to the key and signed by Yubico.
func (yk *YubiKey) AttestationCertificate() (*x509.Certificate, error) {
Expand Down Expand Up @@ -554,6 +594,92 @@ func ykAttest(tx *scTx, slot Slot) (*x509.Certificate, error) {
return cert, nil
}

// KeyInfo holds unprotected metadata about a key slot.
type KeyInfo struct {
Algorithm Algorithm
PINPolicy PINPolicy
TouchPolicy TouchPolicy
Origin Origin
PublicKey crypto.PublicKey
}

func (ki *KeyInfo) unmarshal(b []byte) error {
for len(b) > 0 {
var v asn1.RawValue
rest, err := asn1.Unmarshal(b, &v)
if err != nil {
return err
}
b = rest
if v.Class != 0 || v.IsCompound {
continue
}
var ok bool
switch v.Tag {
case 1:
if len(v.Bytes) != 1 {
return errors.New("invalid algorithm in response")
}
if ki.Algorithm, ok = algorithmsMapInv[v.Bytes[0]]; !ok {
return errors.New("unknown algorithm in response")
}
case 2:
if len(v.Bytes) != 2 {
return errors.New("invalid policy in response")
}
if ki.PINPolicy, ok = pinPolicyMapInv[v.Bytes[0]]; !ok {
return errors.New("unknown PIN policy in response")
}
if ki.TouchPolicy, ok = touchPolicyMapInv[v.Bytes[1]]; !ok {
return errors.New("unknown touch policy in response")
}
case 3:
if len(v.Bytes) != 1 {
return errors.New("invalid origin in response")
}
if ki.Origin, ok = originMapInv[v.Bytes[0]]; !ok {
return errors.New("unknown origin in response")
}
case 4:
ki.PublicKey, err = decodePublic(v.Bytes, ki.Algorithm)
if err != nil {
return fmt.Errorf("parse public key: %w", err)
}
default:
// TODO: According to the Yubico website, we get two more fields,
// if we pass 0x80 or 0x81 as slots:
// 1. Default value (for PIN/PUK and management key): Whether the
// default value is used.
// 2. Retries (for PIN/PUK): The number of retries remaining
// However, it seems the reference implementation does not expect
// these and can not parse them out:
// https://github.com/Yubico/yubico-piv-tool/blob/yubico-piv-tool-2.3.1/lib/util.c#L1529
// For now, we just ignore them.
}
}
return nil
}

// KeyInfo returns public information about the given key slot. It is only
// supported by YubiKeys with a version >= 5.3.0.
func (yk *YubiKey) KeyInfo(slot Slot) (KeyInfo, error) {
// https://developers.yubico.com/PIV/Introduction/Yubico_extensions.html#_get_metadata
cmd := apdu{
instruction: insGetMetadata,
param1: 0x00,
param2: byte(slot.Key),
}
resp, err := yk.tx.Transmit(cmd)
if err != nil {
return KeyInfo{}, fmt.Errorf("command failed: %w", err)
}
var ki KeyInfo
if err := ki.unmarshal(resp); err != nil {
return KeyInfo{}, err
}
return ki, nil
}

// Certificate returns the certifiate object stored in a given slot.
//
// If a certificate hasn't been set in the provided slot, the returned error
Expand Down Expand Up @@ -709,10 +835,20 @@ func ykGenerateKey(tx *scTx, slot Slot, o Key) (crypto.PublicKey, error) {
return nil, fmt.Errorf("command failed: %w", err)
}

// https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-73-4.pdf#page=95
obj, _, err := unmarshalASN1(resp, 1, 0x49)
if err != nil {
return nil, fmt.Errorf("unmarshal response: %v", err)
}

return decodePublic(obj, o.Algorithm)
}

func decodePublic(b []byte, alg Algorithm) (crypto.PublicKey, error) {
var curve elliptic.Curve
switch o.Algorithm {
switch alg {
case AlgorithmRSA1024, AlgorithmRSA2048:
pub, err := decodeRSAPublic(resp)
pub, err := decodeRSAPublic(b)
if err != nil {
return nil, fmt.Errorf("decoding rsa public key: %v", err)
}
Expand All @@ -722,15 +858,15 @@ func ykGenerateKey(tx *scTx, slot Slot, o Key) (crypto.PublicKey, error) {
case AlgorithmEC384:
curve = elliptic.P384()
case AlgorithmEd25519:
pub, err := decodeEd25519Public(resp)
pub, err := decodeEd25519Public(b)
if err != nil {
return nil, fmt.Errorf("decoding ed25519 public key: %v", err)
}
return pub, nil
default:
return nil, fmt.Errorf("unsupported algorithm")
}
pub, err := decodeECPublic(resp, curve)
pub, err := decodeECPublic(b, curve)
if err != nil {
return nil, fmt.Errorf("decoding ec public key: %v", err)
}
Expand Down Expand Up @@ -791,6 +927,13 @@ func (k KeyAuth) do(yk *YubiKey, pp PINPolicy, f func(tx *scTx) ([]byte, error))
}

func pinPolicy(yk *YubiKey, slot Slot) (PINPolicy, error) {
if supportsVersion(yk.Version(), 5, 3, 0) {
info, err := yk.KeyInfo(slot)
if err != nil {
return 0, fmt.Errorf("get key info: %v", err)
}
return info.PINPolicy, nil
}
cert, err := yk.Attest(slot)
if err != nil {
var e *apduErr
Expand Down Expand Up @@ -1180,11 +1323,7 @@ func unmarshalASN1(b []byte, class, tag int) (obj, rest []byte, err error) {

func decodeECPublic(b []byte, curve elliptic.Curve) (*ecdsa.PublicKey, error) {
// https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-73-4.pdf#page=95
r, _, err := unmarshalASN1(b, 1, 0x49)
if err != nil {
return nil, fmt.Errorf("unmarshal response: %v", err)
}
p, _, err := unmarshalASN1(r, 2, 0x06)
p, _, err := unmarshalASN1(b, 2, 0x06)
if err != nil {
return nil, fmt.Errorf("unmarshal points: %v", err)
}
Expand All @@ -1210,11 +1349,7 @@ func decodeECPublic(b []byte, curve elliptic.Curve) (*ecdsa.PublicKey, error) {
func decodeEd25519Public(b []byte) (ed25519.PublicKey, error) {
// Adaptation of
// https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-73-4.pdf#page=95
r, _, err := unmarshalASN1(b, 1, 0x49)
if err != nil {
return nil, fmt.Errorf("unmarshal response: %v", err)
}
p, _, err := unmarshalASN1(r, 2, 0x06)
p, _, err := unmarshalASN1(b, 2, 0x06)
if err != nil {
return nil, fmt.Errorf("unmarshal points: %v", err)
}
Expand All @@ -1226,11 +1361,7 @@ func decodeEd25519Public(b []byte) (ed25519.PublicKey, error) {

func decodeRSAPublic(b []byte) (*rsa.PublicKey, error) {
// https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-73-4.pdf#page=95
r, _, err := unmarshalASN1(b, 1, 0x49)
if err != nil {
return nil, fmt.Errorf("unmarshal response: %v", err)
}
mod, r, err := unmarshalASN1(r, 2, 0x01)
mod, r, err := unmarshalASN1(b, 2, 0x01)
if err != nil {
return nil, fmt.Errorf("unmarshal modulus: %v", err)
}
Expand Down

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