diff --git a/apub-core/src/digest.rs b/apub-core/src/digest.rs
index 4709868..cbb6cc6 100644
--- a/apub-core/src/digest.rs
+++ b/apub-core/src/digest.rs
@@ -3,6 +3,8 @@ pub trait Digest {
 
     fn build() -> Self;
     fn digest(self, input: &[u8]) -> String;
+    fn update(&mut self, input: &[u8]);
+    fn verify(self, encoded: &str) -> bool;
 }
 
 pub trait DigestFactory {
diff --git a/apub-openssl/src/lib.rs b/apub-openssl/src/lib.rs
index e01f0b8..e6df060 100644
--- a/apub-openssl/src/lib.rs
+++ b/apub-openssl/src/lib.rs
@@ -51,6 +51,16 @@ impl<'a> apub_core::digest::Digest for OpenSslDigest {
 
         openssl::base64::encode_block(&bytes)
     }
+
+    fn update(&mut self, input: &[u8]) {
+        self.digest.update(input);
+    }
+
+    fn verify(self, encoded: &str) -> bool {
+        let bytes = self.digest.finish();
+
+        openssl::base64::encode_block(&bytes) == encoded
+    }
 }
 
 impl apub_core::signature::Sign for OpenSslSigner {
diff --git a/apub-rustcrypto/src/lib.rs b/apub-rustcrypto/src/lib.rs
index 0ba5196..d97496f 100644
--- a/apub-rustcrypto/src/lib.rs
+++ b/apub-rustcrypto/src/lib.rs
@@ -60,6 +60,16 @@ impl apub_core::digest::Digest for Sha256Digest {
 
         base64::encode(&bytes)
     }
+
+    fn update(&mut self, input: &[u8]) {
+        self.digest.update(input);
+    }
+
+    fn verify(self, encoded: &str) -> bool {
+        let bytes = self.digest.finalize();
+
+        base64::encode(&bytes) == encoded
+    }
 }
 
 impl apub_core::signature::Sign for RsaSigner {