diff --git a/src/main/java/org/scribe/builder/api/YammerApi.java b/src/main/java/org/scribe/builder/api/YammerApi.java
index 85477a5..68bcda5 100644
--- a/src/main/java/org/scribe/builder/api/YammerApi.java
+++ b/src/main/java/org/scribe/builder/api/YammerApi.java
@@ -1,26 +1,33 @@
 package org.scribe.builder.api;
 
-import org.scribe.model.Token;
+import org.scribe.model.*;
+import org.scribe.services.*;
 
 public class YammerApi extends DefaultApi10a
 {
   private static final String AUTHORIZATION_URL = "'https://www.yammer.com/oauth/authorize?oauth_token=%s'";
 
-	@Override
-	public String getRequestTokenEndpoint()
+  @Override
+  public String getRequestTokenEndpoint()
   {
-		return "https://www.yammer.com/oauth/request_token";
-	}
+    return "https://www.yammer.com/oauth/request_token";
+  }
 
-	@Override
-	public String getAccessTokenEndpoint()
+  @Override
+  public String getAccessTokenEndpoint()
   {
-		return "https://www.yammer.com/oauth/access_token";
-	}
-	
-	@Override
-	public String getAuthorizationUrl(Token requestToken)
-	{
-	  return String.format(AUTHORIZATION_URL, requestToken.getToken());
-	}
+    return "https://www.yammer.com/oauth/access_token";
+  }
+
+  @Override
+  public String getAuthorizationUrl(Token requestToken)
+  {
+    return String.format(AUTHORIZATION_URL, requestToken.getToken());
+  }
+
+  @Override
+  public SignatureService getSignatureService()
+  {
+    return new PlaintextSignatureService();
+  }
 }
diff --git a/src/main/java/org/scribe/services/PlaintextSignatureService.java b/src/main/java/org/scribe/services/PlaintextSignatureService.java
new file mode 100644
index 0000000..57fb70d
--- /dev/null
+++ b/src/main/java/org/scribe/services/PlaintextSignatureService.java
@@ -0,0 +1,43 @@
+package org.scribe.services;
+
+import org.scribe.exceptions.*;
+import org.scribe.utils.*;
+
+/**
+ * plaintext implementation of {@SignatureService}
+ *
+ * @author Pablo Fernandez
+ *
+ */
+public class PlaintextSignatureService implements SignatureService
+{
+  private static final String EMPTY_STRING = "";
+  private static final String CARRIAGE_RETURN = "\r\n";
+  private static final String UTF8 = "UTF-8";
+  private static final String METHOD = "plaintext";
+
+  /**
+   * {@inheritDoc}
+   */
+  public String getSignature(String baseString, String apiSecret, String tokenSecret)
+  {
+    try
+    {
+      Preconditions.checkEmptyString(apiSecret, "Api secret cant be null or empty string");
+      return URLUtils.percentEncode(apiSecret) + '&' + URLUtils.percentEncode(tokenSecret);
+    }
+    catch (Exception e)
+    {
+      throw new OAuthSignatureException(baseString, e);
+    }
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  public String getSignatureMethod()
+  {
+    return METHOD;
+  }
+}
+