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Change password
Older Messenger clients
Windows Messenger and other older clients sign in with a challenge/response scheme: the client proves it knows your password by sending MD5(challenge + password). To check that, the server must be able to recompute it, so it has to keep a copy of your password in a form that is not one-way hashed.
Security trade-off. Your normal password is stored as a salted, slow hash. Turning this on additionally stores a recoverable copy used only for Messenger sign-in. It is weaker protection than the default. Use a password you do not use anywhere else. Changing your password updates the copy.
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