diff --git a/spec/sodium/digest/blake2b_spec.cr b/spec/sodium/digest/blake2b_spec.cr
index eee1d4d..56dcf01 100644
--- a/spec/sodium/digest/blake2b_spec.cr
+++ b/spec/sodium/digest/blake2b_spec.cr
@@ -89,8 +89,45 @@ describe Sodium::Digest::Blake2b do
     expect_raises ArgumentError do
       Sodium::Digest::Blake2b.new personal: Bytes.new(128)
     end
+
+    d = Sodium::Digest::Blake2b.new
+    expect_raises ArgumentError do
+      d.hexfinal Bytes.new(1)
+    end
+    expect_raises ArgumentError do
+      d.hexfinal Bytes.new(256)
+    end
   end
 
-  pending "dups" do
+  it "can't final twice or update after final" do
+    d = Sodium::Digest::Blake2b.new
+    d.hexfinal
+
+    expect_raises Digest::FinalizedError do
+      d.hexfinal
+    end
+
+    expect_raises Digest::FinalizedError do
+      d.update Bytes.new(0)
+    end
+  end
+
+  it "dups" do
+    data = "foo".to_slice
+    d1 = Sodium::Digest::Blake2b.new
+    d2 = d1.dup
+    d3 = d2.dup
+
+    d1.update data
+    h1 = d1.hexfinal
+
+    d2.update data
+    h2 = d2.hexfinal
+
+    d3.update data
+    h3 = d3.hexfinal
+
+    h1.should eq h2
+    h1.should eq h3
   end
 end
diff --git a/spec/spec_helper.cr b/spec/spec_helper.cr
index 3acb2a0..e3a8194 100644
--- a/spec/spec_helper.cr
+++ b/spec/spec_helper.cr
@@ -1,3 +1,4 @@
+require "log"
 require "spec"
 
 # require "../src/sodium"
diff --git a/src/sodium/digest/blake2b.cr b/src/sodium/digest/blake2b.cr
index fa3eeef..6dee1e6 100644
--- a/src/sodium/digest/blake2b.cr
+++ b/src/sodium/digest/blake2b.cr
@@ -19,6 +19,8 @@ module Sodium::Digest
   class Blake2b < ::Digest::Base
     include Wipe
 
+    Log = ::Log.for self
+
     # 32
     KEY_SIZE = LibSodium.crypto_generichash_blake2b_keybytes.to_i
     # 16
@@ -84,6 +86,18 @@ module Sodium::Digest
       def hexfinal : String
         final.hexstring
       end
+
+      def hexfinal(dst : Bytes) : Nil
+        dsize = digest_size
+        unless dst.bytesize == dsize * 2
+          raise ArgumentError.new("Incorrect dst size: #{dst.bytesize}, expected: #{dsize * 2}")
+        end
+
+        sary = uninitialized StaticArray(UInt8, 64)
+        tmp = sary.to_slice[0, dsize]
+        final tmp
+        tmp.hexstring dst
+      end
     {% end %}
 
     # Compatibility with Crystal <= 0.32
diff --git a/src/sodium/lib_sodium.cr b/src/sodium/lib_sodium.cr
index 4148b10..e846cea 100644
--- a/src/sodium/lib_sodium.cr
+++ b/src/sodium/lib_sodium.cr
@@ -1,3 +1,4 @@
+require "log"
 require "random/secure"
 require "./error"