Collisions in type_id · Issue #10389 · rust-lang/rust (original) (raw)

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Description

The implementation of type_id from #10182 uses SipHash on various parameters depending on the type. The output size of SipHash is only 64-bits, however, making it feasible to find collisions via a Birthday Attack. I believe the code below demonstrates a collision in the type_id value of two different ty_structs:

use std::hash; use std::hash::Streaming;

// I believe that this pretty much the same thing as hash_crate_independent() in ty.rs // for a ty_struct on a 64-bit platform fn hash_struct(local_hash: &str, node: u64) -> u64 { let mut state = hash::default_state(); state.input([18]); state.input(local_hash.as_bytes()); do node.iter_bytes(true) |bytes| { state.input(bytes); true }; state.result_u64() }

fn main() { // This represents two structures with different node values from a crate with a // local_hash value of "local" that end up getting the same hash and thus, // I think, the same type_id assert!(hash_struct("local", 0x9e02c8943c336302) == hash_struct("local", 0x366a806b1d5f1b2)); }