diff --git a/lib/signed-xml.js b/lib/signed-xml.js
index b02ff078..8db07f21 100644
--- a/lib/signed-xml.js
+++ b/lib/signed-xml.js
@@ -85,10 +85,11 @@ function RSASHA1() {
   * Sign the given string using the given key
   *
   */
-  this.getSignature = function(signedInfo, signingKey) {
+  this.getSignature = function(signedInfo, signingKey, callback) {
     var signer = crypto.createSign("RSA-SHA1")
     signer.update(signedInfo)
     var res = signer.sign(signingKey, 'base64')
+    if (callback) callback(null, res)
     return res
   }
 
@@ -96,10 +97,11 @@ function RSASHA1() {
   * Verify the given signature of the given string using key
   *
   */
-  this.verifySignature = function(str, key, signatureValue) {
+  this.verifySignature = function(str, key, signatureValue, callback) {
     var verifier = crypto.createVerify("RSA-SHA1")
     verifier.update(str)
     var res = verifier.verify(key, signatureValue, 'base64')
+    if (callback) callback(null, res)
     return res
   }
 
@@ -120,10 +122,11 @@ function RSASHA256() {
   * Sign the given string using the given key
   *
   */
-  this.getSignature = function(signedInfo, signingKey) {
+  this.getSignature = function(signedInfo, signingKey, callback) {
     var signer = crypto.createSign("RSA-SHA256")
     signer.update(signedInfo)
     var res = signer.sign(signingKey, 'base64')
+    if (callback) callback(null, res)
     return res
   }
 
@@ -131,10 +134,11 @@ function RSASHA256() {
   * Verify the given signature of the given string using key
   *
   */
-  this.verifySignature = function(str, key, signatureValue) {
+  this.verifySignature = function(str, key, signatureValue, callback) {
     var verifier = crypto.createVerify("RSA-SHA256")
     verifier.update(str)
     var res = verifier.verify(key, signatureValue, 'base64')
+    if (callback) callback(null, res)
     return res
   }
 
@@ -154,10 +158,11 @@ function RSASHA512() {
   * Sign the given string using the given key
   *
   */
-  this.getSignature = function(signedInfo, signingKey) {
+  this.getSignature = function(signedInfo, signingKey, callback) {
     var signer = crypto.createSign("RSA-SHA512")
     signer.update(signedInfo)
     var res = signer.sign(signingKey, 'base64')
+    if (callback) callback(null, res)
     return res
   }
 
@@ -165,10 +170,11 @@ function RSASHA512() {
   * Verify the given signature of the given string using key
   *
   */
-  this.verifySignature = function(str, key, signatureValue) {
+  this.verifySignature = function(str, key, signatureValue, callback) {
     var verifier = crypto.createVerify("RSA-SHA512")
     verifier.update(str)
     var res = verifier.verify(key, signatureValue, 'base64')
+    if (callback) callback(null, res)
     return res
   }