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Fix CVE-2020-13757: detect cyphertext modifications by prepending zero bytes

Reject cyphertexts that have been modified by prepending zero bytes, by
checking the cyphertext length against the expected size (given the
decryption key). This resolves CVE-2020-13757.

The same approach is used when verifying a signature.

Thanks Carnil for pointing this out on https://github.com/sybrenstuvel/python-rsa/issues/146

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