zeroize: implement Zeroize for CString

[robinkrahl]

I have to store a password that will be passed to a C API using the Rust FFI. To avoid unnecessary copies, I store it as a std::ffi::CString. But as Zeroize is not implemented for CString, I can’t wrap this field in a Secret.

Could you please add a Zeroize implementation for CString?

[tony-iqlusion]

I think this would be a good addition, yes.

[cyberia-ng]

Since CString is only in std (and not in alloc), I think this would have to be gated behind another crate feature?

[tony-iqlusion]

Yep, it would require a new std feature, which would need to be off-by-default.

[cyberia-ng]

Taking a look at this today, I think it might not be possible.

Rust's CString type "is a static guarantee that the underlying bytes contain no interior 0 bytes ("nul characters") and that the final byte is 0 ("nul terminator")" (docs). Therefore overwriting interior bytes with 0s is explicitly breaking this guarantee.

This might not prevent doing some kind of SecretCString type over in secrecy though, with a Drop implementation that does the zeroing immediately before free.

[cyberia-ng]

We could do something slightly wacky and overwrite internal bytes with 0xFF or something, so as to obliterate the data while preserving the "no internal NUL bytes" guarantee.

[tony-iqlusion]

Moved to RustCrypto/utils#650