cipher: `StreamCipherCoreWrapper::get_pos()` with a block size of 0 can reach `unreachable_unchecked()`

[LegionMammal978]

The safety comment in the function says "pos is set only to values smaller than block size", but if the block size of the StreamCipherCore is equal to 0 (in a contrived scenario), then initializing pos to 0 breaks this invariant. When debug assertions are disabled, this can cause get_pos() to reach the unreachable_unchecked(). This can be seen with Miri:

// RUSTFLAGS=-Cdebug-assertions=off cargo +nightly miri run
// cipher = "=0.4.3"
use cipher::{
    consts::U0, BlockSizeUser, StreamCipher, StreamCipherCore, StreamCipherCoreWrapper,
    StreamClosure,
};

struct Dummy;

impl BlockSizeUser for Dummy {
    type BlockSize = U0;
}

impl StreamCipherCore for Dummy {
    fn remaining_blocks(&self) -> Option<usize> {
        None
    }

    fn process_with_backend(&mut self, _: impl StreamClosure<BlockSize = Self::BlockSize>) {
        unimplemented!()
    }
}

fn main() {
    let mut wrapper = StreamCipherCoreWrapper::from_core(Dummy);
    wrapper.apply_keystream(&mut []);
}

error: Undefined Behavior: entering unreachable code
  --> /home/lm978/.cargo/registry/src/github.com-1ecc6299db9ec823/cipher-0.4.3/src/stream_wrapper.rs:53:22
   |
53 |             unsafe { core::hint::unreachable_unchecked() }
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ entering unreachable code
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
   = note: BACKTRACE:
   = note: inside `cipher::StreamCipherCoreWrapper::<Dummy>::get_pos` at /home/lm978/.cargo/registry/src/github.com-1ecc6299db9ec823/cipher-0.4.3/src/stream_wrapper.rs:53:22: 53:57
   = note: inside `<cipher::StreamCipherCoreWrapper<Dummy> as cipher::StreamCipher>::try_apply_keystream_inout` at /home/lm978/.cargo/registry/src/github.com-1ecc6299db9ec823/cipher-0.4.3/src/stream_wrapper.rs:118:19: 118:33
   = note: inside `<cipher::StreamCipherCoreWrapper<Dummy> as cipher::StreamCipher>::try_apply_keystream` at /home/lm978/.cargo/registry/src/github.com-1ecc6299db9ec823/cipher-0.4.3/src/stream.rs:94:9: 94:51
   = note: inside `<cipher::StreamCipherCoreWrapper<Dummy> as cipher::StreamCipher>::apply_keystream` at /home/lm978/.cargo/registry/src/github.com-1ecc6299db9ec823/cipher-0.4.3/src/stream.rs:120:9: 120:38
note: inside `main`
  --> src/main.rs:26:5
   |
26 |     wrapper.apply_keystream(&mut []);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to previous error

Perhaps a defensive guard against the BlockSize = U0 case could be added somewhere.

[newpavlov]

Thank you for reporting this! In cipher v0.5 it will be fixed by the BlockSizes trait, but for now I will add a panic for zero block sizes before the branch.