I am trying to do "SAML 2.0 Bearer Assertion for SalesForce"

I am getting {"error":"invalid_grant","error_description":"invalid assertion"}

Is there any way to validate bearer assertion at salesforce?

I did following in my code

String environment = "https://login.salesforce.com/services/oauth2/token?saml=MgoTx78aEPRbRaz0CkRqjaqrhP3sCa7w7.Y5wbrpGMNT07zKRYwcNWf0zs";
Map<String, String> map = new HashMap<String, String>();
HttpClient httpclient = new HttpClient();
PostMethod post = new PostMethod(environment);
post.addParameter("grant_type", "urn:ietf:params:oauth:grant-type:saml2-bearer");
post.addParameter("client_assertion", Base64.encode(samlResponse.getBytes()));
post.addParameter("client_assertion_type","urn:ietf:params:oauth:client_assertion_type:saml2-bearer");

post.addParameter("format", "json");
String accessToken= null;
String instanceUrl = null;
try {
httpclient.executeMethod(post);
JSONObject authResponse = new JSONObject(new JSONTokener(new InputStreamReader(post.getResponseBodyAsStream())));
System.out.println(authResponse.toString());

} catch (Exception e) {
e.printStackTrace();
} finally {
post.releaseConnection();
}

I have generated following assertion

<saml:Assertion xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
ID="s2d3a451cf30560ca819118cf5785e722ea6da7b64" IssueInstant="2012-03-06T12:34:13Z"
Version="2.0">
<saml:Issuer>http://localhost:8080/opensso
</saml:Issuer>
<ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
<ds:SignedInfo>
<ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
<ds:SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
<ds:Reference URI="#s2d3a451cf30560ca819118cf5785e722ea6da7b64">
<ds:Transforms>
<ds:Transform
Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
<ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
</ds:Transforms>
<ds:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
<ds:DigestValue>seHyxsFzsHCs0GaY7usF0DfMV58=
</ds:DigestValue>
</ds:Reference>
</ds:SignedInfo>
<ds:SignatureValue> signature.....</ds:SignatureValue>
<ds:KeyInfo>
<ds:X509Data>
<ds:X509Certificate> certificate.....</ds:X509Certificate>
</ds:X509Data>
</ds:KeyInfo>
</ds:Signature>
<saml:Subject>
<saml:NameID Format="urn:oasis:names:tc:SAML:2.0:nameid-format:transient"
NameQualifier="http://localhost:8080/opensso" SPNameQualifier="https://saml.salesforce.com">deepakmule</saml:NameID>
<saml:SubjectConfirmation Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
<saml:SubjectConfirmationData
NotOnOrAfter="2012-03-06T12:44:13Z"
Recipient="https://login.salesforce.com/?saml=MgoTx78aEPRbRaz0CkRqjaqrhP3sCa7w7.Y5wbrpGMNT07zKRYwcNWf0zs" />
</saml:SubjectConfirmation>
</saml:Subject>
<saml:Conditions NotBefore="2012-03-06T12:34:13Z"
NotOnOrAfter="2012-03-06T12:44:13Z">
<saml:AudienceRestriction>
<saml:Audience>https://saml.salesforce.com</saml:Audience>
</saml:AudienceRestriction>
</saml:Conditions>
<saml:AuthnStatement AuthnInstant="2012-03-06T12:34:13Z"
SessionIndex="s27fb03a2b73bd8dc6846851bed7885b85e1d9ed6f">
<saml:AuthnContext>
<saml:AuthnContextClassRef> urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport
</saml:AuthnContextClassRef>
</saml:AuthnContext>
</saml:AuthnStatement>
<saml:AttributeStatement>
<saml:Attribute Name="userid">
<saml:AttributeValue xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="xs:string">deepakmule</saml:AttributeValue>
</saml:Attribute>
</saml:AttributeStatement>
</saml:Assertion>