Add ca-url based audience for AWS tokens

Fixes smallstep/step#156

authority/provisioner/aws.go

@@ -131,11 +131,12 @@ type AWS struct {
 	Claims                 *Claims  `json:"claims,omitempty"`
 	claimer                *Claimer
 	config                 *awsConfig
+	audiences              Audiences
 }
 
 // GetID returns the provisioner unique identifier.
 func (p *AWS) GetID() string {
-	return "aws:" + p.Name
+	return "aws/" + p.Name
 }
 
 // GetTokenID returns the identifier of the token.
@@ -170,7 +171,7 @@ func (p *AWS) GetEncryptedKey() (kid string, key string, ok bool) {
 
 // GetIdentityToken retrieves the identity document and it's signature and
 // generates a token with them.
-func (p *AWS) GetIdentityToken() (string, error) {
+func (p *AWS) GetIdentityToken(caURL string) (string, error) {
 	// Initialize the config if this method is used from the cli.
 	if err := p.assertConfig(); err != nil {
 		return "", err
@@ -196,6 +197,11 @@ func (p *AWS) GetIdentityToken() (string, error) {
 		return "", err
 	}
 
+	audience, err := generateSignAudience(caURL, p.GetID())
+	if err != nil {
+		return "", err
+	}
+
 	// Create unique ID for Trust On First Use (TOFU). Only the first instance
 	// per provisioner is allowed as we don't have a way to trust the given
 	// sans.
@@ -216,7 +222,7 @@ func (p *AWS) GetIdentityToken() (string, error) {
 		Claims: jose.Claims{
 			Issuer:    awsIssuer,
 			Subject:   idoc.InstanceID,
-			Audience:  []string{p.GetID()},
+			Audience:  []string{audience},
 			Expiry:    jose.NewNumericDate(now.Add(5 * time.Minute)),
 			NotBefore: jose.NewNumericDate(now),
 			IssuedAt:  jose.NewNumericDate(now),
@@ -254,6 +260,7 @@ func (p *AWS) Init(config Config) (err error) {
 	if p.config, err = newAWSConfig(); err != nil {
 		return err
 	}
+	p.audiences = config.Audiences.WithFragment(p.GetID())
 	return nil
 }
 
@@ -381,14 +388,18 @@ func (p *AWS) authorizeToken(token string) (*awsPayload, error) {
 	// more than a few minutes.
 	now := time.Now().UTC()
 	if err = payload.ValidateWithLeeway(jose.Expected{
-		Issuer:   awsIssuer,
-		Subject:  doc.InstanceID,
-		Audience: []string{p.GetID()},
-		Time:     now,
+		Issuer:  awsIssuer,
+		Subject: doc.InstanceID,
+		Time:    now,
 	}, time.Minute); err != nil {
 		return nil, errors.Wrapf(err, "invalid token")
 	}
 
+	// validate audiences with the defaults
+	if !matchesAudience(payload.Audience, p.audiences.Sign) {
+		return nil, errors.New("invalid token: invalid audience claim (aud)")
+	}
+
 	// validate accounts
 	if len(p.Accounts) > 0 {
 		var found bool