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backend/internal/platform/security/jwt.go

@@ -0,0 +1,104 @@
+package security
+
+import (
+	"errors"
+	"fmt"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+	"github.com/google/uuid"
+)
+
+// Audience separates admin-space tokens from customer-space tokens so a
+// token issued for one space can never be accepted by the other, even
+// though both are signed with the same server secret.
+type Audience string
+
+const (
+	AudienceAdmin    Audience = "admin"
+	AudienceCustomer Audience = "customer"
+)
+
+// Claims are the JWT claims carried by access tokens issued by this platform.
+type Claims struct {
+	Role string `json:"role"`
+	jwt.RegisteredClaims
+}
+
+// IssueAccessToken creates a signed, short-lived access token for the given
+// user, role and audience (admin or customer space).
+func IssueAccessToken(secret string, ttl time.Duration, userID uuid.UUID, role string, aud Audience) (string, error) {
+	if role == "" {
+		return "", fmt.Errorf("role must not be empty")
+	}
+
+	now := time.Now()
+	claims := Claims{
+		Role: role,
+		RegisteredClaims: jwt.RegisteredClaims{
+			Subject:   userID.String(),
+			Audience:  jwt.ClaimStrings{string(aud)},
+			IssuedAt:  jwt.NewNumericDate(now),
+			NotBefore: jwt.NewNumericDate(now),
+			ExpiresAt: jwt.NewNumericDate(now.Add(ttl)),
+			ID:        uuid.NewString(),
+		},
+	}
+
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	signed, err := token.SignedString([]byte(secret))
+	if err != nil {
+		return "", fmt.Errorf("sign token: %w", err)
+	}
+	return signed, nil
+}
+
+// ParseAccessToken verifies signature, algorithm and every claim explicitly:
+// - signing method must be HMAC (rejects "alg":"none" and any asymmetric swap attempt)
+// - exp/iat/nbf must be present and internally consistent (no expired/not-yet-valid tokens)
+// - aud must match exactly the expected audience (admin token can never pass as a customer token, or vice versa)
+// - sub must be present and a well-formed UUID (matches our user ID format)
+// - role must be present and non-empty
+// - jti must be present (reserved for future revocation lookups)
+func ParseAccessToken(secret string, tokenString string, expectedAud Audience) (*Claims, error) {
+	claims := &Claims{}
+
+	token, err := jwt.ParseWithClaims(tokenString, claims, func(t *jwt.Token) (interface{}, error) {
+		if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, fmt.Errorf("unexpected signing method: %v", t.Header["alg"])
+		}
+		return []byte(secret), nil
+	}, jwt.WithValidMethods([]string{jwt.SigningMethodHS256.Name}))
+
+	if err != nil {
+		return nil, fmt.Errorf("parse token: %w", err)
+	}
+	if !token.Valid {
+		return nil, errors.New("invalid token")
+	}
+
+	if claims.ExpiresAt == nil {
+		return nil, errors.New("missing exp claim")
+	}
+	if claims.IssuedAt == nil {
+		return nil, errors.New("missing iat claim")
+	}
+	if claims.ID == "" {
+		return nil, errors.New("missing jti claim")
+	}
+	if claims.Subject == "" {
+		return nil, errors.New("missing sub claim")
+	}
+	if _, err := uuid.Parse(claims.Subject); err != nil {
+		return nil, fmt.Errorf("invalid sub claim: %w", err)
+	}
+	if claims.Role == "" {
+		return nil, errors.New("missing role claim")
+	}
+
+	if len(claims.Audience) != 1 || claims.Audience[0] != string(expectedAud) {
+		return nil, fmt.Errorf("token audience does not match expected space %q", expectedAud)
+	}
+
+	return claims, nil
+}