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pyUmbral is the reference implementation of the Umbral threshold proxy re-encryption scheme. It is open-source, built with Python, and uses OpenSSL and

Using Umbral, Alice (the data owner) can delegate decryption rights to Bob for any ciphertext intended to her, through a re-encryption process performed by a set of semi-trusted proxies or Ursulas. When a threshold of these proxies participate by performing re-encryption, Bob is able to combine these independent re-encryptions and decrypt the original message using his private key.

pyUmbral is the cryptographic engine behind nucypher, a proxy re-encryption network to empower privacy in decentralized systems.

Academic Whitepaper

The Umbral scheme academic whitepaper and cryptographic specifications are available on GitHub.

“Umbral: A Threshold Proxy Re-Encryption Scheme” by David Nuñez.


If you identify vulnerabilities with _any_ nucypher code, please email with relevant information to your findings. We will work with researchers to coordinate vulnerability disclosure between our partners and users to ensure successful mitigation of vulnerabilities.

Throughout the reporting process, we expect researchers to honor an embargo period that may vary depending on the severity of the disclosure. This ensures that we have the opportunity to fix any issues, identify further issues (if any), and inform our users.

Sometimes vulnerabilities are of a more sensitive nature and require extra precautions. We are happy to work together to use a more secure medium, such as Signal. Email and we will coordinate a communication channel that we’re both comfortable with.

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