Merge transit engine branch, resolve conflicts in shared files

internal/engine/helpers.go

@@ -0,0 +1,28 @@
+package engine
+
+import (
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/rsa"
+)
+
+// ZeroizeKey overwrites an asymmetric private key's sensitive material.
+func ZeroizeKey(key crypto.PrivateKey) {
+	if key == nil {
+		return
+	}
+	switch k := key.(type) {
+	case *ecdsa.PrivateKey:
+		k.D.SetInt64(0)
+	case *rsa.PrivateKey:
+		k.D.SetInt64(0)
+		for _, p := range k.Primes {
+			p.SetInt64(0)
+		}
+	case ed25519.PrivateKey:
+		for i := range k {
+			k[i] = 0
+		}
+	}
+}