feat: backend service

backend/internal/auth/codes.go

@@ -0,0 +1,61 @@
+package auth
+
+import (
+	"crypto/rand"
+	"errors"
+	"fmt"
+	"strings"
+
+	"golang.org/x/crypto/bcrypt"
+)
+
+// CodeLength is the fixed length of the decimal code delivered by
+// SendEmailCode. The OpenAPI description ("six-digit") locks the value
+// at six; tests cannot lower it without breaking the contract test
+// against the schema.
+const CodeLength = 6
+
+// codeBcryptCost is the bcrypt cost used to store the hashed code in
+// auth_challenges.code_hash. Cost 10 matches the convention documented
+// for admin password storage in `backend/README.md` §12. Six-digit codes
+// have only ~1M entropy, so the bcrypt slowdown is what bounds online
+// attacks together with the per-challenge attempt ceiling.
+const codeBcryptCost = bcrypt.DefaultCost
+
+// generateCode returns a random CodeLength-character decimal string. The
+// modulo bias when mapping uniform bytes to ten digits is acceptable for
+// short-lived registration codes — the per-challenge attempt ceiling and
+// the TTL bound abuse far more tightly than the negligible bias.
+func generateCode() (string, error) {
+	digits := make([]byte, CodeLength)
+	if _, err := rand.Read(digits); err != nil {
+		return "", fmt.Errorf("auth: generate code: %w", err)
+	}
+	var sb strings.Builder
+	sb.Grow(CodeLength)
+	for _, b := range digits {
+		sb.WriteByte('0' + b%10)
+	}
+	return sb.String(), nil
+}
+
+// hashCode returns the bcrypt hash of code using the package-level cost.
+func hashCode(code string) ([]byte, error) {
+	return bcrypt.GenerateFromPassword([]byte(code), codeBcryptCost)
+}
+
+// verifyCode reports whether code matches hash. The function is a thin
+// wrapper around bcrypt.CompareHashAndPassword so the comparison is
+// constant-time on the matching path. Returns nil on match,
+// ErrCodeMismatch when the bcrypt mismatch error fires, and a wrapped
+// error for any other failure (e.g. malformed hash).
+func verifyCode(hash []byte, code string) error {
+	err := bcrypt.CompareHashAndPassword(hash, []byte(code))
+	if err == nil {
+		return nil
+	}
+	if errors.Is(err, bcrypt.ErrMismatchedHashAndPassword) {
+		return ErrCodeMismatch
+	}
+	return fmt.Errorf("auth: verify code: %w", err)
+}