1 files changed, 81 insertions, 0 deletions
diff --git a/crypto.go b/crypto.go
new file mode 100644
index 0000000..47c16bb
--- /dev/null
+++ b/crypto.go
@@ -0,0 +1,81 @@
+package stfe
+
+import (
+	"fmt"
+	"time"
+
+	"crypto"
+	"crypto/rand"
+	"crypto/tls"
+	"crypto/x509"
+
+	"github.com/system-transparency/stfe/x509util"
+)
+
+// buildChainFromDerList builds an X.509 certificate chain from a list of
+// DER-encoded certificates using the log's configured trust anchors, extended
+// key-usages, and maximum chain length (which includes the trust anchor).
+func (lp *LogParameters) buildChainFromDerList(derChain [][]byte) ([]*x509.Certificate, error) {
+	certificate, intermediatePool, err := x509util.ParseDerChain(derChain)
+	if err != nil {
+		return nil, err
+	}
+	opts := x509.VerifyOptions{
+		Roots:         lp.AnchorPool,
+		Intermediates: intermediatePool,
+		KeyUsages:     lp.KeyUsage, // no extended key usage passes by default
+	}
+
+	chains, err := certificate.Verify(opts)
+	if err != nil {
+		return nil, fmt.Errorf("chain verification failed: %v", err)
+	}
+	if len(chains) == 0 {
+		return nil, fmt.Errorf("bad certificate chain length: empty")
+	}
+
+	// there might be several valid chains
+	for _, chain := range chains {
+		if int64(len(chain)) <= lp.MaxChain {
+			return chain, nil // just pick the first valid chain
+		}
+	}
+	return nil, fmt.Errorf("bad certificate chain length: too large")
+}
+
+// verifySignature checks if signature is valid for some serialized data.  The
+// only supported signature scheme is ed25519(0x0807), see §4.2.3 in RFC 8446.
+func (lp *LogParameters) verifySignature(certificate *x509.Certificate, scheme tls.SignatureScheme, serialized, signature []byte) error {
+	if scheme != tls.Ed25519 {
+		return fmt.Errorf("unsupported signature scheme: %v", scheme)
+	}
+	if err := certificate.CheckSignature(x509.PureEd25519, serialized, signature); err != nil {
+		return fmt.Errorf("invalid signature: %v", err)
+	}
+	return nil
+}
+
+// genV1Sdi issues a new SignedDebugInfoV1 StItem from a serialized leaf value
+func (lp *LogParameters) genV1Sdi(serialized []byte) (*StItem, error) {
+	sig, err := lp.Signer.Sign(rand.Reader, serialized, crypto.Hash(0)) // ed25519
+	if err != nil {
+		return nil, fmt.Errorf("ed25519 signature failed: %v", err)
+	}
+	lastSdiTimestamp.Set(float64(time.Now().Unix()), lp.id())
+	return NewSignedDebugInfoV1(lp.LogId, []byte("reserved"), sig), nil
+}
+
+// genV1Sth issues a new SignedTreeHeadV1 StItem from a TreeHeadV1 structure
+func (lp *LogParameters) genV1Sth(th *TreeHeadV1) (*StItem, error) {
+	serialized, err := th.Marshal()
+	if err != nil {
+		return nil, fmt.Errorf("failed tls marshaling tree head: %v", err)
+	}
+	sig, err := lp.Signer.Sign(rand.Reader, serialized, crypto.Hash(0)) // ed25519
+	if err != nil {
+		return nil, fmt.Errorf("ed25519 signature failed: %v", err)
+	}
+	lastSthTimestamp.Set(float64(time.Now().Unix()), lp.id())
+	lastSthSize.Set(float64(th.TreeSize), lp.id())
+	return NewSignedTreeHeadV1(th, lp.LogId, sig), nil
+}