Shamir seals now come in two varieties: legacy and new-style. Legacy
Shamir is automatically converted to new-style when a rekey operation
is performed. All new Vault initializations using Shamir are new-style.
New-style Shamir writes an encrypted master key to storage, just like
AutoUnseal. The stored master key is encrypted using the shared key that
is split via Shamir's algorithm. Thus when unsealing, we take the key
fragments given, combine them into a Key-Encryption-Key, and use that
to decrypt the master key on disk. Then the master key is used to read
the keyring that decrypts the barrier.
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