diff --git a/IPython/nbformat/sign.py b/IPython/nbformat/sign.py
new file mode 100644
index 0000000..3e15deb
--- /dev/null
+++ b/IPython/nbformat/sign.py
@@ -0,0 +1,141 @@
+"""Functions for signing notebooks"""
+#-----------------------------------------------------------------------------
+#  Copyright (C) 2014, The IPython Development Team
+#
+#  Distributed under the terms of the BSD License.  The full license is in
+#  the file COPYING, distributed as part of this software.
+#-----------------------------------------------------------------------------
+
+#-----------------------------------------------------------------------------
+# Imports
+#-----------------------------------------------------------------------------
+
+from contextlib import contextmanager
+import hashlib
+from hmac import HMAC
+
+from IPython.utils.py3compat import string_types, unicode_type, cast_bytes
+
+#-----------------------------------------------------------------------------
+# Code
+#-----------------------------------------------------------------------------
+
+
+def yield_everything(obj):
+    """Yield every item in a container as bytes
+    
+    Allows any JSONable object to be passed to an HMAC digester
+    without having to serialize the whole thing.
+    """
+    if isinstance(obj, dict):
+        for key in sorted(obj):
+            value = obj[key]
+            yield cast_bytes(key)
+            for b in yield_everything(value):
+                yield b
+    elif isinstance(obj, (list, tuple)):
+        for element in obj:
+            for b in yield_everything(element):
+                yield b
+    elif isinstance(obj, unicode_type):
+        yield obj.encode('utf8')
+    else:
+        yield unicode_type(obj).encode('utf8')
+
+
+@contextmanager
+def signature_removed(nb):
+    """Context manager for operating on a notebook with its signature removed
+    
+    Used for excluding the previous signature when computing a notebook's signature.
+    """
+    save_signature = nb['metadata'].pop('signature', None)
+    try:
+        yield
+    finally:
+        if save_signature is not None:
+            nb['metadata']['signature'] = save_signature
+
+
+def notebook_signature(nb, secret, scheme):
+    """Compute a notebook's signature
+    
+    by hashing the entire contents of the notebook via HMAC digest.
+    scheme is the hashing scheme, which must be an attribute of the hashlib module,
+    as listed in hashlib.algorithms.
+    """
+    hmac = HMAC(secret, digestmod=getattr(hashlib, scheme))
+    # don't include the previous hash in the content to hash
+    with signature_removed(nb):
+        # sign the whole thing
+        for b in yield_everything(nb):
+            hmac.update(b)
+        
+    return hmac.hexdigest()
+
+
+def check_notebook_signature(nb, secret):
+    """Check a notebook's stored signature
+    
+    If a signature is stored in the notebook's metadata,
+    a new signature is computed using the same hashing scheme,
+    and compared.
+    
+    If no signature can be found, or the scheme of the existing signature is unavailable,
+    it will return False.
+    """
+    stored_signature = nb['metadata'].get('signature', None)
+    if not stored_signature \
+        or not isinstance(stored_signature, string_types) \
+        or ':' not in stored_signature:
+        return False
+    scheme, sig = stored_signature.split(':', 1)
+    try:
+        my_signature = notebook_signature(nb, secret, scheme)
+    except AttributeError:
+        return False
+    return my_signature == sig
+
+
+def trust_notebook(nb, secret, scheme):
+    """Re-sign a notebook, indicating that its output is trusted
+    
+    stores 'scheme:hmac-hexdigest' in notebook.metadata.signature
+    
+    e.g. 'sha256:deadbeef123...'
+    """
+    signature = notebook_signature(nb, secret, scheme)
+    nb['metadata']['signature'] = "%s:%s" % (scheme, signature)
+
+
+def mark_trusted_cells(nb, secret):
+    """Mark cells as trusted if the notebook's signature can be verified
+    
+    Sets ``cell.trusted = True | False`` on all code cells,
+    depending on whether the stored signature can be verified.
+    """
+    if not nb['worksheets']:
+        # nothing to mark if there are no cells
+        return True
+    trusted = check_notebook_signature(nb, secret)
+    for cell in nb['worksheets'][0]['cells']:
+        if cell['cell_type'] == 'code':
+            cell['trusted'] = trusted
+    return trusted
+
+
+def check_trusted_cells(nb):
+    """Return whether all code cells are trusted
+    
+    If there are no code cells, return True.
+    """
+    if not nb['worksheets']:
+        return True
+    for cell in nb['worksheets'][0]['cells']:
+        if cell['cell_type'] != 'code':
+            continue
+        if not cell.get('trusted', False):
+            return False
+    return True
+
+        
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