frost_core: add 'static bound to Ciphersuite trait

frost-core/CHANGELOG.md

@@ -4,6 +4,11 @@ Entries are listed in reverse chronological order.
 
 ## Unreleased
 
+* Added a `'static` bound to the `Ciphersuite` trait. This is a breaking change,
+  but it's likely to not require any code changes since most ciphersuite
+  implementations are probably just empty structs. The bound makes it possible
+  to use `frost_core::Error<C>` in `Box<dyn std::error::Error>`.
+
 ## 2.0.0-rc.0
 
 * Changed the `deserialize()` function of Elements and structs containing

frost-core/src/traits.rs

@@ -138,7 +138,8 @@ pub type Element<C> = <<C as Ciphersuite>::Group as Group>::Element;
 /// function.
 ///
 /// [FROST ciphersuite]: https://www.ietf.org/archive/id/draft-irtf-cfrg-frost-14.html#name-ciphersuites
-pub trait Ciphersuite: Copy + Clone + PartialEq + Debug {
+// See https://github.com/ZcashFoundation/frost/issues/693 for reasoning about the 'static bound.
+pub trait Ciphersuite: Copy + Clone + PartialEq + Debug + 'static {
     /// The ciphersuite ID string. It should be equal to the contextString in
     /// the spec. For new ciphersuites, this should be a string that identifies
     /// the ciphersuite; it's recommended to use a similar format to the