diff --git a/PrivaPub.Tests/Statistics/InteractionsTests.cs b/PrivaPub.Tests/Statistics/InteractionsTests.cs
index 877d69b..d75f6f5 100644
--- a/PrivaPub.Tests/Statistics/InteractionsTests.cs
+++ b/PrivaPub.Tests/Statistics/InteractionsTests.cs
@@ -49,6 +49,8 @@ namespace PrivaPub.Tests.Statistics
 			Assert.Equal("-", Interactions.Host("evil host/with path"));
 			Assert.Equal("-", Interactions.Host(default));
 			Assert.Equal("social.example", Interactions.HostOf("https://Social.Example:8443/users/x"));
+			// RFC 9421's algorithm names carry an underscore
+			Assert.Equal("rfc9421:rsa-v1_5-sha256", Interactions.Sanitize(new InteractionEvent { Signature = "rfc9421:rsa-v1_5-sha256" }).Signature);
 		}
 
 		[Fact]
diff --git a/PrivaPub/Infrastructure/Statistics/Interactions.cs b/PrivaPub/Infrastructure/Statistics/Interactions.cs
index c2d31db..2345c20 100644
--- a/PrivaPub/Infrastructure/Statistics/Interactions.cs
+++ b/PrivaPub/Infrastructure/Statistics/Interactions.cs
@@ -95,7 +95,7 @@ namespace PrivaPub.Infrastructure.Statistics
 		[GeneratedRegex("^[a-z0-9][a-z0-9-]{0,39}$")]
 		private static partial Regex KebabWord();
 
-		[GeneratedRegex("^[a-z0-9][a-z0-9:-]{0,39}$")]
+		[GeneratedRegex("^[a-z0-9][a-z0-9:_-]{0,39}$")]
 		private static partial Regex SignatureName();
 
 		[GeneratedRegex("^[a-z0-9][a-z0-9:/+.-]{0,63}$")]