I am getting the below exception when I am calling the SecureStockQuoteProxy service with a proxy service. Here I have exposed this secured service as a REST API (which is unsecured). When I am calling the REST API, ESB throws the below error.

[2013-11-24 12:41:46,386] ERROR - AxisEngine InvalidSecurity
org.apache.axis2.AxisFault: InvalidSecurity
    at org.apache.rampart.handler.PostDispatchVerificationHandler.invoke(PostDispatchVerificationHandler.java:164)
    at org.apache.axis2.engine.Phase.invokeHandler(Phase.java:340)
    at org.apache.axis2.engine.Phase.invoke(Phase.java:313)
    at org.apache.axis2.engine.AxisEngine.invoke(AxisEngine.java:261)
    at org.apache.axis2.engine.AxisEngine.receive(AxisEngine.java:167)
    at org.apache.synapse.transport.passthru.ClientWorker.run(ClientWorker.java:225)
    at org.apache.axis2.transport.base.threads.NativeWorkerPool$1.run(NativeWorkerPool.java:172)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
    at java.lang.Thread.run(Thread.java:662)
[2013-11-24 12:41:46,387] ERROR - ClientWorker Fault processing response message through Axis2
org.apache.axis2.AxisFault: InvalidSecurity
    at org.apache.rampart.handler.PostDispatchVerificationHandler.invoke(PostDispatchVerificationHandler.java:164)
    at org.apache.axis2.engine.Phase.invokeHandler(Phase.java:340)
    at org.apache.axis2.engine.Phase.invoke(Phase.java:313)
    at org.apache.axis2.engine.AxisEngine.invoke(AxisEngine.java:261)
    at org.apache.axis2.engine.AxisEngine.receive(AxisEngine.java:167)
    at org.apache.synapse.transport.passthru.ClientWorker.run(ClientWorker.java:225)
    at org.apache.axis2.transport.base.threads.NativeWorkerPool$1.run(NativeWorkerPool.java:172)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
    at java.lang.Thread.run(Thread.java:662)

In the Sample axis2Server backend, following error can be seen

13/11/24 12:41:46 ERROR engine.AxisEngine: An unsupported signature or encryption algorithm was used (unsupported key transport encryption algorithm: No such algorithm: http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p); nested exception is:
    java.security.NoSuchAlgorithmException: Cannot find any provider supporting RSA/ECB/OAEPPadding
org.apache.axis2.AxisFault: An unsupported signature or encryption algorithm was used (unsupported key transport encryption algorithm: No such algorithm: http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p); nested exception is:
    java.security.NoSuchAlgorithmException: Cannot find any provider supporting RSA/ECB/OAEPPadding
    at org.apache.rampart.handler.RampartReceiver.setFaultCodeAndThrowAxisFault(RampartReceiver.java:180)
    at org.apache.rampart.handler.RampartReceiver.invoke(RampartReceiver.java:95)
    at org.apache.axis2.engine.Phase.invokeHandler(Phase.java:340)
    at org.apache.axis2.engine.Phase.invoke(Phase.java:313)
    at org.apache.axis2.engine.AxisEngine.invoke(AxisEngine.java:261)
    at org.apache.axis2.engine.AxisEngine.receive(AxisEngine.java:167)
    at org.apache.axis2.transport.http.HTTPTransportUtils.processHTTPPostRequest(HTTPTransportUtils.java:172)
    at org.apache.synapse.transport.nhttp.ServerWorker.processEntityEnclosingMethod(ServerWorker.java:459)
    at org.apache.synapse.transport.nhttp.ServerWorker.run(ServerWorker.java:279)
    at org.apache.axis2.transport.base.threads.NativeWorkerPool$1.run(NativeWorkerPool.java:172)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
    at java.lang.Thread.run(Thread.java:662)
Caused by: org.apache.ws.security.WSSecurityException: An unsupported signature or encryption algorithm was used (unsupported key transport encryption algorithm: No such algorithm: http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p); nested exception is:
    java.security.NoSuchAlgorithmException: Cannot find any provider supporting RSA/ECB/OAEPPadding
    at org.apache.ws.security.util.WSSecurityUtil.getCipherInstance(WSSecurityUtil.java:785)
    at org.apache.ws.security.processor.EncryptedKeyProcessor.handleEncryptedKey(EncryptedKeyProcessor.java:154)
    at org.apache.ws.security.processor.EncryptedKeyProcessor.handleEncryptedKey(EncryptedKeyProcessor.java:117)
    at org.apache.ws.security.processor.EncryptedKeyProcessor.handleToken(EncryptedKeyProcessor.java:93)
    at org.apache.ws.security.WSSecurityEngine.processSecurityHeader(WSSecurityEngine.java:332)
    at org.apache.ws.security.WSSecurityEngine.processSecurityHeader(WSSecurityEngine.java:249)
    at org.apache.rampart.RampartEngine.process(RampartEngine.java:214)
    at org.apache.rampart.handler.RampartReceiver.invoke(RampartReceiver.java:92)
    ... 11 more
Caused by: java.security.NoSuchAlgorithmException: Cannot find any provider supporting RSA/ECB/OAEPPadding
    at javax.crypto.Cipher.getInstance(DashoA13*..)
    at org.apache.ws.security.util.WSSecurityUtil.getCipherInstance(WSSecurityUtil.java:777)
    ... 18 more
13/11/24 12:41:46 ERROR nhttp.ServerWorker: Error processing POST request
org.apache.axis2.AxisFault: An unsupported signature or encryption algorithm was used (unsupported key transport encryption algorithm: No such algorithm: http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p); nested exception is:
    java.security.NoSuchAlgorithmException: Cannot find any provider supporting RSA/ECB/OAEPPadding
    at org.apache.rampart.handler.RampartReceiver.setFaultCodeAndThrowAxisFault(RampartReceiver.java:180)
    at org.apache.rampart.handler.RampartReceiver.invoke(RampartReceiver.java:95)
    at org.apache.axis2.engine.Phase.invokeHandler(Phase.java:340)
    at org.apache.axis2.engine.Phase.invoke(Phase.java:313)
    at org.apache.axis2.engine.AxisEngine.invoke(AxisEngine.java:261)
    at org.apache.axis2.engine.AxisEngine.receive(AxisEngine.java:167)
    at org.apache.axis2.transport.http.HTTPTransportUtils.processHTTPPostRequest(HTTPTransportUtils.java:172)
    at org.apache.synapse.transport.nhttp.ServerWorker.processEntityEnclosingMethod(ServerWorker.java:459)
    at org.apache.synapse.transport.nhttp.ServerWorker.run(ServerWorker.java:279)
    at org.apache.axis2.transport.base.threads.NativeWorkerPool$1.run(NativeWorkerPool.java:172)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
    at java.lang.Thread.run(Thread.java:662)
Caused by: org.apache.ws.security.WSSecurityException: An unsupported signature or encryption algorithm was used (unsupported key transport encryption algorithm: No such algorithm: http://www.w3.org/2001/04/xmlenc#rsa-oaep-mgf1p); nested exception is:
    java.security.NoSuchAlgorithmException: Cannot find any provider supporting RSA/ECB/OAEPPadding
    at org.apache.ws.security.util.WSSecurityUtil.getCipherInstance(WSSecurityUtil.java:785)
    at org.apache.ws.security.processor.EncryptedKeyProcessor.handleEncryptedKey(EncryptedKeyProcessor.java:154)
    at org.apache.ws.security.processor.EncryptedKeyProcessor.handleEncryptedKey(EncryptedKeyProcessor.java:117)
    at org.apache.ws.security.processor.EncryptedKeyProcessor.handleToken(EncryptedKeyProcessor.java:93)
    at org.apache.ws.security.WSSecurityEngine.processSecurityHeader(WSSecurityEngine.java:332)
    at org.apache.ws.security.WSSecurityEngine.processSecurityHeader(WSSecurityEngine.java:249)
    at org.apache.rampart.RampartEngine.process(RampartEngine.java:214)
    at org.apache.rampart.handler.RampartReceiver.invoke(RampartReceiver.java:92)
    ... 11 more
Caused by: java.security.NoSuchAlgorithmException: Cannot find any provider supporting RSA/ECB/OAEPPadding
    at javax.crypto.Cipher.getInstance(DashoA13*..)
    at org.apache.ws.security.util.WSSecurityUtil.getCipherInstance(WSSecurityUtil.java:777)
    ... 18 more

This exception can be solved by enabling the "rampart" module in axis2.xml file of the sample axis2server which is located under ESB_HOME/samples/axis2Server/repository/conf folder. We need to enable the rampart module as below.

<!-- ================================================= -->
<!-- Global Modules  -->
<!-- ================================================= -->
<!-- Comment this to disable Addressing -->
<module ref="addressing"/>
<module ref="rampart" />

Hope this helps.