VDAF-06+ ping-pong topology (#683)

Implements the ping-pong topology introduced in VDAF-06 and refined
in VDAF-07.

We add a `topology` module, on the premise that we might someday add new
topologies and their implementations there, and `topology::ping_pong`.

This also adds an implementation of a dummy VDAF, brought over from
Janus ([1]). `vdaf::dummy` is only compiled if the `test-util` Cargo
feature is enabled. The dummy VDAF implements the `vdaf::{Vdaf,
Aggregator, Client, Collector}` and provides associated types for output
shares, prepare shares, etc., but it doesn't do anything except return
success, making it useful for testing higher-level constructions like
ping-pong.

Finally, we replace the derived `std::fmt::Debug` implementations on a
few `prio3` and `poplar1` associated types so that they redact fields
that are either sensitive secrets or just too big to be worth printing
when debugging. This is so that we can provide `Debug` impls on new
types in `topology::ping_pong` without pulling in crate `derivative`,
which would require us to do 9,000+ lines of audits.

[1]: https://github.com/divviup/janus

Cargo.toml

@@ -46,7 +46,7 @@ itertools = "0.11.0"
 modinverse = "0.1.0"
 num-bigint = "0.4.4"
 once_cell = "1.18.0"
-prio = { path = ".", features = ["crypto-dependencies"] }
+prio = { path = ".", features = ["crypto-dependencies", "test-util"] }
 rand = "0.8"
 serde_json = "1.0"
 statrs = "0.16.0"
@@ -58,6 +58,7 @@ experimental = ["bitvec", "fiat-crypto", "fixed", "num-bigint", "num-rational",
 multithreaded = ["rayon"]
 prio2 = ["crypto-dependencies", "hmac", "sha2"]
 crypto-dependencies = ["aes", "ctr"]
+test-util = ["rand"]
 
 [workspace]
 members = [".", "binaries"]

src/lib.rs

@@ -30,4 +30,5 @@ mod fp;
 pub mod idpf;
 mod polynomial;
 mod prng;
+pub mod topology;
 pub mod vdaf;

src/topology/mod.rs

@@ -0,0 +1,7 @@
+// SPDX-License-Identifier: MPL-2.0
+
+//! Implementations of some aggregator communication topologies specified in [VDAF].
+//!
+//! [VDAF]: https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-vdaf-06#section-5.7
+
+pub mod ping_pong;