Updated the README

Added new security layers

internal/auth/gc2.go

@@ -0,0 +1,78 @@
+package auth
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"strings"
+	"time"
+)
+
+type Claims struct {
+	Sub string `json:"sub"`           // account ID (acc_…)
+	Exp int64  `json:"exp"`           // unix seconds
+	Nbf int64  `json:"nbf,omitempty"` // not before
+	Iss string `json:"iss,omitempty"` // greencoast
+	Aud string `json:"aud,omitempty"` // api
+	Jti string `json:"jti,omitempty"` // token id (optional)
+	CNF string `json:"cnf,omitempty"` // key binding: "p256:<b64raw>" or "ed25519:<b64raw>"
+}
+
+func MintGC2(signKey []byte, c Claims) (string, error) {
+	if len(signKey) == 0 {
+		return "", errors.New("sign key missing")
+	}
+	if c.Sub == "" || c.Exp == 0 {
+		return "", errors.New("claims incomplete")
+	}
+	body, _ := json.Marshal(c)
+	mac := hmac.New(sha256.New, signKey)
+	mac.Write(body)
+	sig := mac.Sum(nil)
+	return "gc2." + base64.RawURLEncoding.EncodeToString(body) + "." + base64.RawURLEncoding.EncodeToString(sig), nil
+}
+
+func VerifyGC2(signKey []byte, tok string, now time.Time) (Claims, error) {
+	var zero Claims
+	if !strings.HasPrefix(tok, "gc2.") {
+		return zero, errors.New("bad prefix")
+	}
+	parts := strings.Split(tok, ".")
+	if len(parts) != 3 {
+		return zero, errors.New("bad parts")
+	}
+	body, err := base64.RawURLEncoding.DecodeString(parts[1])
+	if err != nil {
+		return zero, err
+	}
+	want, err := base64.RawURLEncoding.DecodeString(parts[2])
+	if err != nil {
+		return zero, err
+	}
+	mac := hmac.New(sha256.New, signKey)
+	mac.Write(body)
+	if !hmac.Equal(want, mac.Sum(nil)) {
+		return zero, errors.New("bad sig")
+	}
+	var c Claims
+	if err := json.Unmarshal(body, &c); err != nil {
+		return zero, err
+	}
+	t := now.Unix()
+	if c.Nbf != 0 && t < c.Nbf {
+		return zero, errors.New("nbf")
+	}
+	if t > c.Exp {
+		return zero, errors.New("expired")
+	}
+	return c, nil
+}
+
+func AccountIDFromPub(raw []byte) string {
+	// acc_<first32 hex of sha256(pub)>
+	sum := sha256.Sum256(raw)
+	return "acc_" + hex.EncodeToString(sum[:16])
+}