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- implement compression for multi-user ciphertexts, - update multi-user encapsulation tests (also include compression).
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The previous version of multi-user encapsulation had malleable parts in the ciphertext. This was not in line with our CCA claims. The updated version uses a CCA secure KEM and DEM to construct a CCA secure multi-user KEM. It does so by generating a random session key and encrypting this session key with AES-GCM using the IBKEM shared secret as symmetric key for each user, respectively. The downsides are the extra dependency on AES, the overhead (encrypted session key, nonce and tag for each user).
Since ciphertexts produced by earlier versions are not compatible, this is a breaking change.