diff --git a/crates/ruma-signatures/Cargo.toml b/crates/ruma-signatures/Cargo.toml
index 695223fd..8d98c5ed 100644
--- a/crates/ruma-signatures/Cargo.toml
+++ b/crates/ruma-signatures/Cargo.toml
@@ -14,6 +14,7 @@ edition = "2021"
 all-features = true
 
 [features]
+compat = []
 ring-compat = ["dep:subslice"]
 unstable-exhaustive-types = []
 
diff --git a/crates/ruma-signatures/src/lib.rs b/crates/ruma-signatures/src/lib.rs
index 518ea88c..a271b847 100644
--- a/crates/ruma-signatures/src/lib.rs
+++ b/crates/ruma-signatures/src/lib.rs
@@ -85,7 +85,11 @@ fn split_id(id: &str) -> Result<(Algorithm, String), Error> {
 
     let version = signature_id[1];
 
-    if !version.bytes().all(|ch| ch.is_ascii_alphanumeric() || ch == b'_') {
+    #[cfg(feature = "compat")]
+    const EXTRA_ALLOWED: [u8; 3] = [b'_', b'+', b'/'];
+    #[cfg(not(feature = "compat"))]
+    const EXTRA_ALLOWED: [u8; 1] = [b'_'];
+    if !version.bytes().all(|ch| ch.is_ascii_alphanumeric() || EXTRA_ALLOWED.contains(&ch)) {
         return Err(Error::InvalidVersion(version.into()));
     }
 
diff --git a/crates/ruma/Cargo.toml b/crates/ruma/Cargo.toml
index 045e0217..3821297b 100644
--- a/crates/ruma/Cargo.toml
+++ b/crates/ruma/Cargo.toml
@@ -97,6 +97,7 @@ compat = [
     "ruma-common/compat",
     "ruma-client-api?/compat",
     "ruma-federation-api?/compat",
+    "ruma-signatures?/compat",
 ]
 
 # Specific compatibility for past ring public/private key documents.