An old but still solid digital signature scheme based on RSA.
It is more formally called RSASSA-PKCS1-v1_5
in Section 8.2 of RFC8017.
The following example shows how a private RSA key (loaded from a file) can be used to compute the signature of a message:
>>> from Crypto.Signature import pkcs1_15
>>> from Crypto.Hash import SHA256
>>> from Crypto.PublicKey import RSA
>>>
>>> message = b'To be signed'
>>> key = RSA.import_key(open('private_key.der').read())
>>> h = SHA256.new(message)
>>> signature = pkcs1_15.new(key).sign(h)
At the other end, the receiver can verify the signature (and therefore the authenticity of the message) using the matching public RSA key:
>>> key = RSA.import_key(open('public_key.der').read())
>>> h = SHA256.new(message)
>>> try:
>>> pkcs1_15.new(key).verify(h, signature)
>>> print "The signature is valid."
>>> except (ValueError, TypeError):
>>> print "The signature is not valid."
.. automodule:: Crypto.Signature.pkcs1_15
:members: