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tka: implement credential signatures (key material delegation)

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This will be needed to support preauth-keys with network lock in the future,
so getting the core mechanics out of the way now.

Signed-off-by: Tom DNetto <tom@tailscale.com>
This commit is contained in:
Tom DNetto
2022-08-29 13:53:33 -07:00
committed by Tom
parent 490acdefb6
commit be95aebabd
4 changed files with 154 additions and 28 deletions
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80
tka/sig.go
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@@ -33,6 +33,19 @@ const (
// SigRotation signature and sign it again with their rotation key. That
// way, SigRotation nesting should only be 2 deep in the common case.
SigRotation
// SigCredential describes a signature over a specifi public key, signed
// by a key in the tailnet key authority referenced by the specified keyID.
// In effect, SigCredential delegates the ability to make a signature to
// a different public/private key pair.
//
// It is intended that a different public/private key pair be generated
// for each different SigCredential that is created. Implementors must
// take care that the private side is only known to the entity that needs
// to generate the wrapping SigRotation signature, and it is immediately
// discarded after use.
//
// SigCredential is expected to be nested in a SigRotation signature.
SigCredential
)
func (s SigKind) String() string {
@@ -43,6 +56,8 @@ func (s SigKind) String() string {
return "direct"
case SigRotation:
return "rotation"
case SigCredential:
return "credential"
default:
return fmt.Sprintf("Sig?<%d>", int(s))
}
@@ -53,8 +68,9 @@ func (s SigKind) String() string {
type NodeKeySignature struct {
// SigKind identifies the variety of signature.
SigKind SigKind `cbor:"1,keyasint"`
// Pubkey identifies the public key which is being authorized.
Pubkey []byte `cbor:"2,keyasint"`
// Pubkey identifies the key.NodePublic which is being authorized.
// SigCredential signatures do not use this field.
Pubkey []byte `cbor:"2,keyasint,omitempty"`
// KeyID identifies which key in the tailnet key authority should
// be used to verify this signature. Only set for SigDirect and
@@ -69,19 +85,23 @@ type NodeKeySignature struct {
// used as Pubkey. Only used for SigRotation signatures.
Nested *NodeKeySignature `cbor:"5,keyasint,omitempty"`
// RotationPubkey specifies the ed25519 public key which may sign a
// SigRotation signature, which embeds this one.
// WrappingPubkey specifies the ed25519 public key which must be used
// to sign a Signature which embeds this one.
//
// Intermediate SigRotation signatures may omit this value to use the
// parent one.
RotationPubkey []byte `cbor:"6,keyasint,omitempty"`
// For SigRotation signatures multiple levels deep, intermediate
// signatures may omit this value, in which case the parent WrappingPubkey
// is used.
//
// SigCredential signatures use this field to specify the public key
// they are certifying, following the usual semanticsfor WrappingPubkey.
WrappingPubkey []byte `cbor:"6,keyasint,omitempty"`
}
// rotationPublic returns the public key which must sign a SigRotation
// signature that embeds this signature, if any.
func (s NodeKeySignature) rotationPublic() (pub ed25519.PublicKey, ok bool) {
if len(s.RotationPubkey) > 0 {
return ed25519.PublicKey(s.RotationPubkey), true
// wrappingPublic returns the public key which must sign a signature which
// embeds this one, if any.
func (s NodeKeySignature) wrappingPublic() (pub ed25519.PublicKey, ok bool) {
if len(s.WrappingPubkey) > 0 {
return ed25519.PublicKey(s.WrappingPubkey), true
}
switch s.SigKind {
@@ -89,7 +109,7 @@ func (s NodeKeySignature) rotationPublic() (pub ed25519.PublicKey, ok bool) {
if s.Nested == nil {
return nil, false
}
return s.Nested.rotationPublic()
return s.Nested.wrappingPublic()
default:
return nil, false
@@ -138,15 +158,18 @@ func (s *NodeKeySignature) Unserialize(data []byte) error {
return dec.Unmarshal(data, s)
}
// verifySignature checks that the NodeKeySignature is authentic, certified
// by the given verificationKey, and authorizes the given nodeKey.
// verifySignature checks that the NodeKeySignature is authentic & certified
// by the given verificationKey. Additionally, SigDirect and SigRotation
// signatures are checked to ensure they authorize the given nodeKey.
func (s *NodeKeySignature) verifySignature(nodeKey key.NodePublic, verificationKey Key) error {
nodeBytes, err := nodeKey.MarshalBinary()
if err != nil {
return fmt.Errorf("marshalling pubkey: %v", err)
}
if !bytes.Equal(nodeBytes, s.Pubkey) {
return errors.New("signature does not authorize nodeKey")
if s.SigKind != SigCredential {
nodeBytes, err := nodeKey.MarshalBinary()
if err != nil {
return fmt.Errorf("marshalling pubkey: %v", err)
}
if !bytes.Equal(nodeBytes, s.Pubkey) {
return errors.New("signature does not authorize nodeKey")
}
}
sigHash := s.SigHash()
@@ -157,7 +180,7 @@ func (s *NodeKeySignature) verifySignature(nodeKey key.NodePublic, verificationK
}
// Verify the signature using the nested rotation key.
verifyPub, ok := s.Nested.rotationPublic()
verifyPub, ok := s.Nested.wrappingPublic()
if !ok {
return errors.New("missing rotation key")
}
@@ -167,15 +190,22 @@ func (s *NodeKeySignature) verifySignature(nodeKey key.NodePublic, verificationK
// Recurse to verify the signature on the nested structure.
var nestedPub key.NodePublic
if err := nestedPub.UnmarshalBinary(s.Nested.Pubkey); err != nil {
return fmt.Errorf("nested pubkey: %v", err)
// SigCredential signatures certify an indirection key rather than a node
// key, so theres no need to check the node key.
if s.Nested.SigKind != SigCredential {
if err := nestedPub.UnmarshalBinary(s.Nested.Pubkey); err != nil {
return fmt.Errorf("nested pubkey: %v", err)
}
}
if err := s.Nested.verifySignature(nestedPub, verificationKey); err != nil {
return fmt.Errorf("nested: %v", err)
}
return nil
case SigDirect:
case SigDirect, SigCredential:
if s.Nested != nil {
return fmt.Errorf("invalid signature: signatures of type %v cannot nest another signature", s.SigKind)
}
switch verificationKey.Kind {
case Key25519:
if ed25519consensus.Verify(ed25519.PublicKey(verificationKey.Public), sigHash[:], s.Signature) {