authkit: initial shared auth module

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extract.go

@@ -0,0 +1,42 @@
+package authkit
+
+import (
+	"crypto/sha256"
+	"crypto/subtle"
+	"encoding/hex"
+	"net/http"
+	"strings"
+)
+
+// ExtractAPIKey pulls the API key from r.
+// Checks X-API-Key first, then "Authorization: Bearer <k>".
+// Returns "" if neither is present.
+func ExtractAPIKey(r *http.Request) string {
+	if v := strings.TrimSpace(r.Header.Get("X-API-Key")); v != "" {
+		return v
+	}
+	auth := strings.TrimSpace(r.Header.Get("Authorization"))
+	if strings.HasPrefix(auth, "Bearer ") {
+		return strings.TrimSpace(auth[len("Bearer "):])
+	}
+	return ""
+}
+
+// ConstantTimeEqualString compares a and b in constant time.
+func ConstantTimeEqualString(a, b string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	return subtle.ConstantTimeCompare([]byte(a), []byte(b)) == 1
+}
+
+// ConstantTimeEqualHashedKey hashes candidatePlain with sha256 and compares
+// against storedHashHex in constant time. Returns false on bad hex; never panics.
+func ConstantTimeEqualHashedKey(candidatePlain, storedHashHex string) bool {
+	stored, err := hex.DecodeString(storedHashHex)
+	if err != nil || len(stored) != sha256.Size {
+		return false
+	}
+	sum := sha256.Sum256([]byte(candidatePlain))
+	return subtle.ConstantTimeCompare(sum[:], stored) == 1
+}