diff --git a/lib/expmt/cbc.js b/lib/expmt/cbc.js
new file mode 100644
index 0000000..bc4f060
--- /dev/null
+++ b/lib/expmt/cbc.js
@@ -0,0 +1,14 @@
+var Random = require('../random');
+
+var CBC = function CBC() {
+};
+
+CBC.pkcs7pad = function(buf, blocksize) {
+  var bytesize = blocksize / 8;
+  var padbytesize = bytesize - buf.length;
+  var pad = new Buffer(padbytesize);
+  pad.fill(padbytesize);
+  return Buffer.concat([buf, pad]);
+};
+
+module.exports = CBC;
diff --git a/test/test.cbc.js b/test/test.cbc.js
new file mode 100644
index 0000000..e662feb
--- /dev/null
+++ b/test/test.cbc.js
@@ -0,0 +1,21 @@
+var should = require('chai').should();
+var CBC = require('../lib/expmt/cbc');
+
+describe('CBC', function() {
+  
+  describe('@pkcs7pad', function() {
+    
+    it('should pad this 32 bit buffer to 128 bits with the number 128/8 - 32/8', function() {
+      var buf = new Buffer(32 / 8);
+      buf.fill(0);
+      var padbuf = CBC.pkcs7pad(buf, 128);
+      padbuf.length.should.equal(128 / 8);
+      padbuf[32 / 8].should.equal(128 / 8 - 32 / 8);
+      padbuf[32 / 8 + 1].should.equal(128 / 8 - 32 / 8);
+      // ...
+      padbuf[32 / 8 + 128 / 8 - 32 / 8 - 1].should.equal(128 / 8 - 32 / 8);
+    });
+
+  });
+
+});