[release/11.0] X25519: Handle zero peer keys on downlevel Windows platforms - #134607
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An all-zero peer (public) key should always produce a zero shared secret, which should get rejected during key agreement. Windows normally rejects this during importation time, but that is not enabled because Windows would also eagerly reject off-twist public keys which should work. With this change, when a "zero" public key is imported (either by naturally being zero or reduced to zero) we skip importing it into bcrypt and retain "This was a zero key". During derivation we throw since that would produce a zero shared secret. This keeps Windows consistent with other platforms, where import is not the protected path, but derivation is. For X25519DHCng (ncrypt) its not possible to really gate this when the peer key is zero. However that responsibility falls to the person creating the public key handle, and the handle is external in this case. --------- Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Backport of #134535 to release/11.0
/cc @vcsjones
Customer Impact
The X25519DiffieHellman class that was introduced in .NET 11 did not handle certain invalid public keys in an expected manner. These public keys should allow being imported, but should reject being used while deriving secret agreements.
On older, but supported, builds of Windows 10, the Windows implementation would either reject too early, or too late, making its behavior inconsistent with other platforms. This was uncovered when
Hpkehad test cases added that exercised error paths.Regression
Testing
New test were added.
Risk
Low. Small, well understood, and isolated change to X25519DiffieHellman's Windows implementation.