This page describes pyHanko’s signing API.
Before continuing, you may want to take a look at the background on PDF signatures in the CLI documentation.
General API design¶
The value entry (
/V) of a signature field in a PDF file is given by a PDF
dictionary: the “signature object”.
This signature object in turn contains a
/Contents key (a byte string)
with a DER-encoded rendition of the CMS object (see RFC 5652) containing the
actual cryptographic signature.
To avoid confusion, the latter will be referred to as the “signature CMS object”,
and we’ll reserve the term “signature object” for the PDF dictionary that is the
value of the signature field.
The signature object contains a
/ByteRange key outlining the bytes of the
document that should be hashed to validate the signature.
As a general rule, the hash of the PDF file used in the signature is computed
over all bytes in the file, except those under the
In particular, the
/ByteRange key of the signature object is actually
part of the signed data, which implies that the size of the signature
CMS object needs to be estimated ahead of time. As we’ll see soon, this has
some minor implications for the API design (see
this subsection in particular).
The pyHanko signing API is spread across sevaral modules in the
pyhanko.sign package. Broadly speaking, it has three aspects:
PdfSignatureMetadataspecifies high-level metadata & structural requirements for the signature object and (to a lesser degree) the signature CMS object.
Signerand its subclasses are responsible for the construction of the signature CMS object, but are in principle “PDF-agnostic”.
PdfSigneris the “steering” class that invokes the
IncrementalPdfFileWriterand takes care of formatting the resulting signature object according to the specifications of a
This summary, while a bit of an oversimplification, provides a decent enough picture of the separation of concerns in the signing API. In particular, the fact that construction of the CMS object is delegated to another class that doesn’t need to bother with any of the PDF-specific minutiae makes it relatively easy to support other signing technology (e.g. particular HSMs).
A simple example¶
Virtually all parameters of
PdfSignatureMetadata have sane defaults.
The only exception is the one specifying the signature field to contain the
signature—this parameter is always mandatory if the number of empty
signature fields in the document isn’t exactly one.
In simple cases, signing a document can therefore be as easy as this:
from pyhanko.sign import signers from pyhanko.pdf_utils.incremental_writer import IncrementalPdfFileWriter cms_signer = signers.SimpleSigner.load( 'path/to/signer/key.pem', 'path/to/signer/cert.pem', ca_chain_files=('path/to/relevant/certs.pem',), key_passphrase=b'secret' ) with open('document.pdf', 'rb') as doc: w = IncrementalPdfFileWriter(doc) out = signers.sign_pdf( w, signers.PdfSignatureMetadata(field_name='Signature1'), signer=cms_signer, ) # do stuff with 'out' # ...
from pyhanko.sign import signers from pyhanko.pdf_utils.incremental_writer import IncrementalPdfFileWriter cms_signer = signers.SimpleSigner.load( 'path/to/signer/key.pem', 'path/to/signer/cert.pem', ca_chain_files=('path/to/relevant/certs.pem',), key_passphrase=b'secret' ) with open('document.pdf', 'rb') as doc: w = IncrementalPdfFileWriter(doc) out = signers.PdfSigner( signers.PdfSignatureMetadata(field_name='Signature1'), signer=cms_signer, ).sign_pdf(w) # do stuff with 'out' # ...
The advantages of instantiating the
PdfSigner object yourself include
reusability and more granular control over the signature’s appearance.
In the above examples,
out ends up containing a byte buffer
io.BytesIO object) with the signed output.
You can control the output stream using the
parameters; see the documentation for
used in the creation of a signature should be discarded afterwards.
Further modifications would simply invalidate the signature anyway.
If there is no signature field with the name specified in the
PdfSignatureMetadata, pyHanko will (by default) create an invisible
signature field to contain the signature.
This behaviour can be turned off using the
sign_pdf(), or you can supply
a custom field spec when initialising the
For more details on signature fields and how to create them, take a look at Signature fields.
Cryptographic timestamps (specified by RFC 3161) play a role in PDF signatures in two different ways.
They can be used as part of a PDF signature (embedded into the signature CMS object) to establish a (verifiable) record of the time of signing.
They can also be used in a stand-alone way to provide document timestamps (PDF 2.0).
From a PDF syntax point of view, standalone document timestamps are formally
very similar to PDF signatures.
PyHanko implements these using the
timestamp_pdf() method of
(which is actually a superclass of
Timestamp tokens (TST) embedded into PDF signatures are arguably the more common occurrence. These function as countersignatures to the signer’s signature, proving that a signature existed at a certain point in time. This is a necessary condition for (most) long-term verifiability schemes.
Typically, such timestamp tokens are provided over HTTP, from a trusted time
stamping authority (TSA), using the protocol specified in RFC 3161.
PyHanko provides a client for this protocol; see
The example from the previous section doesn’t need to be modified by a lot to include a trusted timestamp in the signature.
from pyhanko.sign import signers from pyhanko.pdf_utils.incremental_writer import IncrementalPdfFileWriter cms_signer = signers.SimpleSigner.load( 'path/to/signer/key.pem', 'path/to/signer/cert.pem', ca_chain_files=('path/to/relevant/certs.pem',), key_passphrase=b'secret' ) tst_client = timestamps.HTTPTimeStamper('http://example.com/tsa') with open('document.pdf', 'rb') as doc: w = IncrementalPdfFileWriter(doc) out = signers.sign_pdf( w, signers.PdfSignatureMetadata(field_name='Signature1'), signer=cms_signer, timestamper=tst_client ) # do stuff with 'out' # ...
As a general rule, pyHanko will attempt to obtain a timestamp token whenever
TimeStamper is available, but you may sometimes see more TST requests
go over the wire than the number of signatures you’re creating.
This is normal: since the timestamps are to be embedded into the signature CMS
object of the signature, pyHanko needs a sample token to estimate the CMS
These “dummy tokens” are cached on the
TimeStamper, so you
can cut down on the number of such unnecessary requests by reusing the same
TimeStamper for many signatures.
Providing detailed guidance on how to implement your own
is beyond the scope of this guide—the implementations
PKCS11Signer should help.
This subsection merely highlights some of the issues you should keep in mind.
First, if all you want to do is implement a signing device or technique that’s
not supported by pyHanko1, it should be sufficient to implement
This method computes the raw cryptographic signature of some data (typically
a document hash) with the appropriate key material.
It also takes a
dry_run flag, signifying that the returned object should
merely have the correct size, but the content doesn’t matter2.
If your requirements necessitate further modifications to the structure of the
CMS object, you’ll most likely have to override
sign(), which is responsible for the
construction of the CMS object itself.
… and doesn’t require any cryptographic parameters. Signature mechanisms with parameters (such as RSA-PSS) are a bit more tricky. RSA-PSS is on the roadmap for pyHanko; once that has been implemented, using custom signature mechanisms with parameters should also become a bit more straightforward.
dry_runflag is used in the estimation of the CMS object’s size. With key material held in memory it doesn’t really matter all that much, but if the signature is provided by a HSM, or requires additional input on the user’s end (such as a PIN), you typically don’t want to use the “real” signing method in dry-run mode.
The size of a timestamp token is difficult to predict ahead of time, since it depends on many unknown factors, including the number & form of the various certificates that might come embedded within them.