This fix eliminates a scenario where a user misuses the `ExpandedSecretKey` API
in a way that leaks the user's secret key. In short, if a user sends
`ExpandedSecretKey::sign(sk, msg, pk1)` followed by
`ExpandedSecretKey::sign(sk, msg, pk2)`, where `pk1 != pk2`, a passive
adversary [can easily][0] derive `sk`. To mitigate this, we remove the API
entirely.
[0]: https://github.com/MystenLabs/ed25519-unsafe-libs
The `signature` crate provides `Signer` and `Verifier` traits generic
over signature types:
https://github.com/RustCrypto/traits/tree/master/signature
There's presently an open call to stabilize the parts of its API needed
by Ed25519 signatures and release a 1.0 version:
https://github.com/RustCrypto/traits/issues/78
The `ed25519` crate, based on the `signature` crate, provides an
`ed25519::Signature` type which can be shared across multiple Ed25519
crates (e.g. it is also used by the `yubihsm` crate):
https://github.com/RustCrypto/signatures/tree/master/ed25519
This commit integrates the `ed25519::Signature` type, and changes the
existing `sign` and `verify` methods (where applicable) to use the
`Signer` and `Verifier` traits from the `signature` crate. Additionally,
it replaces `SignatureError` with the `signature` crate's error type.
This has the drawback of requiring the `Signer` and/or `Verifier` traits
are in scope in order to create and/or verify signatures, but with the
benefit of supporting interoperability with other Ed25519 crates which
also make use of these traits.
* ADD new "batch" feature for feature-gating ed25519 batch verification; off by
default. The "batch" feature is the only thing which depends on all of the
`rand` crate, since it requires the functionality of `rand::thread_rng()`.
Without batch verification, the rest of ed25519-dalek only depends on
`rand_os` and `rand_core`.
This implements https://github.com/dalek-cryptography/ed25519-dalek/issues/64
You can still choose the "prehash" algorithm, as long as it has 64 bytes of
output. Otherwise, everything is hardcoded to use sha2::Sha512. To use a
different implementation you'll need a [patch.crates-io] section in cargo
config.
This also simplifies the verification logic. Because the verification check
happens in variable time, we don't need to do a constant-time eq check at the
end, so we can drop the `subtle` dependency entirely.
The `DecodingError` type becomes `SignatureError` and is also used to
signal failing verifications.
This code was significantly based off without boats' error types in
commit 6c1acaca7c, and also upon
conversation with them. Please target them with praise, and blame me
for whatever mistakes I might have made.
Advantages of a custom error type:
- It can be more easily integrated into other error types by clients;
they can implement From<FromBytesError> for their error types, or
they can use a library like failure.
- It is a zero-sized type, which can enable some representational
optimizations.
- It can be easier and more stable to test for.
