Zeroize potentially sensitive bytes

[lvkv]

Some points in this codebase handle potential secrets. These need debug impls that don't leak and, ideally, bytes that are zeroized on drop or before being libc free'd (context-dependent). This PR:

• Adds a "secret bytes" type that wraps potentially sensitive bytes and zeroizes on drop
• Returns this type whenever the value is likely to contain a potential secret
• Internally, zeroizes PAM/libc-allocated buffers before freeing them
• Redacts debug implementations where appropriate

This required writing a new secret_cstr_item macro, modeled after the existing cstr_item macro, to handle the AuthTok and OldAuthTok types.

[Copilot]

Pull request overview

This PR introduces a dedicated “secret bytes” container and threads it through PAM conversation/item APIs to reduce accidental leakage (notably via Debug) and to zeroize sensitive buffers on drop / before free().

Changes:

• Add SecretBytes (owned, redacted Debug, zeroize-on-drop) and export it via a new secret module.
• Update PAM conversation (Conv::send) to return SecretBytes and explicitly zero PAM/libc buffers before freeing.
• Replace AuthTok / OldAuthTok item wrappers with secret-aware wrappers that redact Debug and can produce owned secret bytes.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
pam/src/secret.rs	Adds SecretBytes zeroize-on-drop wrapper + redacted Debug.
pam/src/lib.rs	Exports the new secret module.
pam/src/items.rs	Adds secret_cstr_item! and uses it for AuthTok / OldAuthTok.
pam/src/conv.rs	Returns SecretBytes from send() and zeroizes response buffers before free().
pam/examples/quiz.rs	Updates example to use the new SecretBytes API (as_str).
pam/Cargo.toml	Adds the zeroize dependency.

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.