implement certificate posture check

shared/management/certposture/proof.go

@@ -0,0 +1,154 @@
+package certposture
+
+import (
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/sha256"
+	"crypto/sha512"
+	"crypto/x509"
+	"errors"
+	"fmt"
+	"time"
+)
+
+const (
+	SigAlgECDSASHA256  = "ecdsa-sha256"
+	SigAlgECDSASHA384  = "ecdsa-sha384"
+	SigAlgRSAPSSSHA256 = "rsa-pss-sha256"
+	SigAlgEd25519      = "ed25519"
+
+	proofDomain = "netbird-posture-cert-v1"
+	minRSABits  = 2048
+)
+
+var (
+	ErrUnsupportedKey   = errors.New("unsupported certificate key")
+	ErrEmptyChain       = errors.New("certificate chain is empty")
+	ErrSignatureInvalid = errors.New("certificate proof signature is invalid")
+	ErrSigAlgMismatch   = errors.New("signature algorithm does not match the certificate key")
+)
+
+// Proof is a client's demonstration that it holds the private key of the leaf
+// certificate in Chain, made by signing the challenge nonce bound to its peer key.
+type Proof struct {
+	Nonce     []byte
+	Chain     [][]byte
+	SigAlg    string
+	Signature []byte
+}
+
+// Sign produces the proof signature for a nonce using the leaf's private key. The key
+// never leaves the signer, which may be backed by a file, a TPM or an OS keystore.
+func Sign(signer crypto.Signer, nonce, peerKey []byte) (string, []byte, error) {
+	sigAlg, err := sigAlgFor(signer.Public())
+	if err != nil {
+		return "", nil, err
+	}
+
+	msg := proofMessage(nonce, peerKey)
+	var sig []byte
+	switch sigAlg {
+	case SigAlgECDSASHA256:
+		d := sha256.Sum256(msg)
+		sig, err = signer.Sign(rand.Reader, d[:], crypto.SHA256)
+	case SigAlgECDSASHA384:
+		d := sha512.Sum384(msg)
+		sig, err = signer.Sign(rand.Reader, d[:], crypto.SHA384)
+	case SigAlgRSAPSSSHA256:
+		d := sha256.Sum256(msg)
+		sig, err = signer.Sign(rand.Reader, d[:], &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash, Hash: crypto.SHA256})
+	case SigAlgEd25519:
+		sig, err = signer.Sign(rand.Reader, msg, crypto.Hash(0))
+	}
+	if err != nil {
+		return "", nil, fmt.Errorf("sign certificate proof: %w", err)
+	}
+	return sigAlg, sig, nil
+}
+
+// Verify checks that the proof's nonce was issued by this challenger to peerKey and is
+// still fresh, and that the signature validates against the leaf's public key. It
+// returns the parsed chain on success. Chain trust is deliberately not evaluated here.
+func (c *Challenger) Verify(proof Proof, peerKey []byte, now time.Time) ([]*x509.Certificate, error) {
+	if err := c.verifyNonce(proof.Nonce, peerKey, now); err != nil {
+		return nil, err
+	}
+	chain, err := parseChain(proof.Chain)
+	if err != nil {
+		return nil, err
+	}
+	leaf := chain[0]
+	sigAlg, err := sigAlgFor(leaf.PublicKey)
+	if err != nil {
+		return nil, err
+	}
+	if sigAlg != proof.SigAlg {
+		return nil, ErrSigAlgMismatch
+	}
+	if !verifySignature(leaf.PublicKey, sigAlg, proofMessage(proof.Nonce, peerKey), proof.Signature) {
+		return nil, ErrSignatureInvalid
+	}
+	return chain, nil
+}
+
+func proofMessage(nonce, peerKey []byte) []byte {
+	msg := make([]byte, 0, len(proofDomain)+len(nonce)+len(peerKey))
+	msg = append(msg, proofDomain...)
+	msg = append(msg, nonce...)
+	return append(msg, peerKey...)
+}
+
+func sigAlgFor(pub crypto.PublicKey) (string, error) {
+	switch k := pub.(type) {
+	case *ecdsa.PublicKey:
+		switch k.Curve {
+		case elliptic.P256():
+			return SigAlgECDSASHA256, nil
+		case elliptic.P384():
+			return SigAlgECDSASHA384, nil
+		}
+	case *rsa.PublicKey:
+		if k.N.BitLen() >= minRSABits {
+			return SigAlgRSAPSSSHA256, nil
+		}
+	case ed25519.PublicKey:
+		return SigAlgEd25519, nil
+	}
+	return "", ErrUnsupportedKey
+}
+
+func verifySignature(pub crypto.PublicKey, sigAlg string, msg, sig []byte) bool {
+	switch sigAlg {
+	case SigAlgECDSASHA256:
+		d := sha256.Sum256(msg)
+		return ecdsa.VerifyASN1(pub.(*ecdsa.PublicKey), d[:], sig)
+	case SigAlgECDSASHA384:
+		d := sha512.Sum384(msg)
+		return ecdsa.VerifyASN1(pub.(*ecdsa.PublicKey), d[:], sig)
+	case SigAlgRSAPSSSHA256:
+		d := sha256.Sum256(msg)
+		return rsa.VerifyPSS(pub.(*rsa.PublicKey), crypto.SHA256, d[:], sig, &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash}) == nil
+	case SigAlgEd25519:
+		return ed25519.Verify(pub.(ed25519.PublicKey), msg, sig)
+	}
+	return false
+}
+
+func parseChain(der [][]byte) ([]*x509.Certificate, error) {
+	if len(der) == 0 {
+		return nil, ErrEmptyChain
+	}
+	chain := make([]*x509.Certificate, 0, len(der))
+	for _, raw := range der {
+		cert, err := x509.ParseCertificate(raw)
+		if err != nil {
+			return nil, fmt.Errorf("parse certificate: %w", err)
+		}
+		chain = append(chain, cert)
+	}
+	return chain, nil
+}