Plonky3 is a toolkit for polynomial IOPs (PIOPs). Prior to versions 0.4.3 and 0.5.3, an attacker controlling prover-side observations can craft distinct transcripts that produce identical challenges, breaking the binding property of Fiat-Shamir. This issue has been patched in versions 0.4.3 and 0.5.3.

Affected: crates.io: p3-challenger

### Impact

- **Key**: `challenger/src/multi_field_challenger.rs` | `MultiField32Challenger::duplexing` | `transcript_malleability` - **Affected files**: `challenger/src/multi_field_challenger.rs`, `field/src/helpers.rs` - **Violated invariant**: The Fiat-Shamir sponge must bind challenges to the exact sequence of observed field elements. Specifically: (1) absorption must be injective — distinct observation streams must produce distinct sponge states, (2) squeezing must be injective — distinct PF rate cells must yield distinct F challenge sequences, and (3) all bits of each absorbed PF element must influence the sponge state.

- **Exploit scenario**: An attacker controlling prover-side observations can craft distinct transcripts that produce identical challenges, breaking the binding property of Fiat-Shamir. Three independent attack vectors exist:

1. **Partial-chunk aliasing (absorb)**: `duplexing()` packs `input_buffer.chunks(num_f_elms)` via `reduce_32` (base 2^32) with no length marker and no zeroing of unused rate slots. Observing `[x]` followed by a sample yields the same sponge state as `[x, 0, ..., 0]` (padded to `num_f_elms`) followed by a sample, since `reduce_32` treats missing high limbs identically to explicit zeros. The attacker can extend or truncate the tail of any observation batch without changing future challenges.

2. **Non-injective squeeze (squeeze)**: `split_32` decomposes each PF rate cell into base-2^64 digits and maps each through `TF::from_u64`…