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for an rsa key,
if one has the session key, ciphertext and plaintext,
and, at some point,
md5 is 'fully' broken,
would it not be possible to retrieve the secret key and passphrase?
I think you may have two different uses of symmetric keys mixed up.
A symmetric key protects the private part of your public/private keypair;
it is the one generated from your passphrase (using a hash, such as MD5).
Another "session key" protects the contents of a message; for messages
encrypted to a public key, it is random, and involves no hashing or
An "attacker" can already generate any number of session keys and
ciphertexts to go with your plaintext and signature (once you're
willing to reveal that). They can encrypt those session keys with
your public key. Giving out the one session key for a particular
ciphertext does no harm, unless that session key is (improperly)
related to others.
If you were considering encrypting directly to a passphrase, and the
hash were *badly* broken (such that you could generate pre-images of a
constrained pattern from an end hash), then indeed, this could be
a concern. From the session key and salt, you might be able to
back-compute the passphrase. If it were used elsewhere, then you
could be in trouble. Still a little far-fetched. Moreover,
you said "RSA", so I don't think this is what you meant.
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