diff --git a/spec/cox/blake2b_spec.cr b/spec/cox/blake2b_spec.cr
new file mode 100644
index 0000000..f858174
--- /dev/null
+++ b/spec/cox/blake2b_spec.cr
@@ -0,0 +1,89 @@
+require "../spec_helper"
+
+libsodium_comparisons = [
+  {
+    key: nil,
+    input: "",
+    output: "0e5751c026e543b2e8ab2eb06099daa1d1e5df47778f7787faab45cdf12fe3a8",
+    out_size: 32,
+  },
+]
+
+# from https://github.com/BLAKE2/BLAKE2/tree/master/testvectors
+test_vectors = [
+  {
+    key: "000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f303132333435363738393a3b3c3d3e3f",
+    input: "",
+    output: "10ebb67700b1868efb4417987acf4690ae9d972fb7a590c2f02871799aaa4786b5e996e8f0f4eb981fc214b005f42d2ff4233499391653df7aefcbc13fc51568",
+  },
+  {
+    key: "000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f303132333435363738393a3b3c3d3e3f",
+    input: "00",
+    output: "961f6dd1e4dd30f63901690c512e78e4b45e4742ed197c3c5e45c549fd25f2e4187b0bc9fe30492b16b0d0bc4ef9b0f34c7003fac09a5ef1532e69430234cebd",
+  },
+]
+
+
+describe Cox::Blake2b do
+  it "libsodium comparisons" do
+    libsodium_comparisons.each do |vec|
+      d = Cox::Blake2b.new vec[:out_size], key: vec[:key].try(&.hexbytes)
+      d.update vec[:input].hexbytes
+      d.hexdigest.should eq vec[:output]
+    end
+  end
+
+  it "test vectors" do
+    test_vectors.each do |vec|
+      d = Cox::Blake2b.new 64, key: vec[:key].hexbytes
+      d.update vec[:input].hexbytes
+      d.hexdigest.should eq vec[:output]
+    end
+  end
+
+  it "produces different output with different salt or personal params" do
+    key = Bytes.new Cox::Blake2b::KEY_SIZE
+    salt = Bytes.new Cox::Blake2b::SALT_SIZE
+    salt2 = Bytes.new Cox::Blake2b::SALT_SIZE
+    salt2 = salt.dup
+    salt2[0] = 1
+    personal = Bytes.new Cox::Blake2b::PERSONAL_SIZE
+    personal2 = personal.dup
+    personal2[0] = 1
+
+
+    d = Cox::Blake2b.new key: key, salt: salt, personal: personal
+    d.update "foo".to_slice
+    output = d.hexdigest
+
+    d = Cox::Blake2b.new key: key, salt: salt2, personal: personal
+    d.update "foo".to_slice
+    saltout = d.hexdigest
+
+    d = Cox::Blake2b.new key: key, salt: salt, personal: personal2
+    d.update "foo".to_slice
+    personalout = d.hexdigest
+
+    output.should_not eq saltout
+    output.should_not eq personalout
+    saltout.should_not eq personalout
+  end
+
+  it "raises on invalid " do
+    expect_raises ArgumentError do
+      Cox::Blake2b.new key: Bytes.new(128)
+    end
+
+    expect_raises ArgumentError do
+      Cox::Blake2b.new salt: Bytes.new(1)
+    end
+
+    expect_raises ArgumentError do
+      Cox::Blake2b.new salt: Bytes.new(128)
+    end
+
+    expect_raises ArgumentError do
+      Cox::Blake2b.new personal: Bytes.new(128)
+    end
+  end
+end
diff --git a/src/cox/blake2b.cr b/src/cox/blake2b.cr
new file mode 100644
index 0000000..f290d14
--- /dev/null
+++ b/src/cox/blake2b.cr
@@ -0,0 +1,95 @@
+require "openssl/digest/digest_base"
+
+module Cox
+  class Blake2b
+    # provides copying digest/hexdigest methods
+    include OpenSSL::DigestBase
+
+    KEY_SIZE = LibSodium.crypto_generichash_blake2b_keybytes
+    KEY_SIZE_MIN = LibSodium.crypto_generichash_blake2b_keybytes_min
+    KEY_SIZE_MAX = LibSodium.crypto_generichash_blake2b_keybytes_max
+
+    SALT_SIZE = LibSodium.crypto_generichash_blake2b_saltbytes
+
+    PERSONAL_SIZE = LibSodium.crypto_generichash_blake2b_personalbytes
+
+    OUT_SIZE = LibSodium.crypto_generichash_blake2b_bytes.to_i32
+    OUT_SIZE_MIN = LibSodium.crypto_generichash_blake2b_bytes_min.to_i32
+    OUT_SIZE_MAX = LibSodium.crypto_generichash_blake2b_bytes_max.to_i32
+
+    @state = StaticArray(UInt8, 384).new 0
+    @key_size = 0
+    @have_salt = false
+    @have_personal = false
+
+    # implemented as static array's so clone works without jumping through hoops.
+    @key = StaticArray(UInt8, 64).new 0
+    @salt = StaticArray(UInt8, 16).new 0
+    @personal = StaticArray(UInt8, 16).new 0
+
+    def initialize(@out_size : Int32 = OUT_SIZE, key : Bytes? = nil, salt : Bytes? = nil, personal : Bytes? = nil)
+      if k = key
+        raise ArgumentError.new("key larger than KEY_SIZE_MAX, got #{k.bytesize}") if k.bytesize > KEY_SIZE_MAX
+        @key_size = k.bytesize
+        k.copy_to @key.to_slice
+      end
+
+      if sa = salt
+        raise ArgumentError.new("salt must be SALT_SIZE bytes, got #{sa.bytesize}") if sa.bytesize != SALT_SIZE
+        sa.copy_to @salt.to_slice
+        @have_salt = true
+      end
+
+      if pe = personal
+        raise ArgumentError.new("personal must be PERSONAL_SIZE bytes, got #{pe.bytesize}") if pe.bytesize != PERSONAL_SIZE
+        pe.copy_to @personal.to_slice
+        @have_personal = true
+      end
+
+      reset
+    end
+
+    def reset
+      key = @key_size > 0 ? @key.to_unsafe : nil
+      salt = @have_salt ? @salt.to_unsafe : nil
+      personal = @have_personal ? @personal.to_unsafe : nil
+
+      if LibSodium.crypto_generichash_blake2b_init_salt_personal(@state, key, @key_size, @out_size, salt, personal) != 0
+        raise Cox::Error.new("blake2b_init_key_salt_personal")
+      end
+    end
+
+    def update(data : Bytes)
+      if LibSodium.crypto_generichash_blake2b_update(@state, data, data.bytesize) != 0
+        raise Cox::Error.new("crypto_generichash_blake2b_update")
+      end
+
+      self
+    end
+
+    def finish
+      data = Bytes.new @out_size
+      if LibSodium.crypto_generichash_blake2b_final(@state, data, data.bytesize) != 0
+        raise Cox::Error.new("crypto_generichash_blake2b_final")
+      end
+      data
+    end
+
+    def clone
+      dup
+    end
+
+    # :nodoc:
+    def __validate_sizes__
+      state_size = LibSodium.crypto_generichash_blake2b_statebytes
+      abort "@state.bytesize doesn't match library version #{@state.to_slice.bytesize} #{state_size}" if @state.to_slice.bytesize < state_size
+      abort "@key.bytesize doesn't match library version" if @key.to_slice.bytesize != KEY_SIZE_MAX
+      abort "@salt.bytesize doesn't match library version #{@salt.to_slice.bytesize} #{SALT_SIZE}" if @salt.to_slice.bytesize != SALT_SIZE
+      abort "@personal.bytesize doesn't match library version #{@personal.to_slice.bytesize} #{PERSONAL_SIZE}" if @personal.to_slice.bytesize != SALT_SIZE
+    end
+  end
+
+  Blake2b.new.__validate_sizes__
+end
+
+
diff --git a/src/cox/lib_sodium.cr b/src/cox/lib_sodium.cr
index 33c78a4..9303fbe 100644
--- a/src/cox/lib_sodium.cr
+++ b/src/cox/lib_sodium.cr
@@ -21,6 +21,15 @@ module Cox
     fun crypto_pwhash_opslimit_sensitive()     : LibC::SizeT
     fun crypto_pwhash_opslimit_max()     : LibC::SizeT
     fun crypto_pwhash_strbytes()     : LibC::SizeT
+    fun crypto_generichash_blake2b_statebytes : LibC::SizeT
+    fun crypto_generichash_blake2b_bytes : LibC::SizeT
+    fun crypto_generichash_blake2b_bytes_min : LibC::SizeT
+    fun crypto_generichash_blake2b_bytes_max : LibC::SizeT
+    fun crypto_generichash_blake2b_keybytes : LibC::SizeT
+    fun crypto_generichash_blake2b_keybytes_min : LibC::SizeT
+    fun crypto_generichash_blake2b_keybytes_max : LibC::SizeT
+    fun crypto_generichash_blake2b_saltbytes : LibC::SizeT
+    fun crypto_generichash_blake2b_personalbytes : LibC::SizeT
 
     PUBLIC_KEY_BYTES  = crypto_box_publickeybytes()
     SECRET_KEY_BYTES  = crypto_box_secretkeybytes()
@@ -119,5 +128,26 @@ module Cox
       optslimit    : LibC::ULongLong,
       memlimit    : LibC::SizeT,
     ) : LibC::Int
+
+    fun crypto_generichash_blake2b_init_salt_personal(
+      state : Pointer(LibC::UChar),
+      key : Pointer(LibC::UChar),
+      key_len : UInt8,
+      out_len : UInt8,
+      salt :  Pointer(LibC::UChar),
+      personal : Pointer(LibC::UChar),
+    ) : LibC::Int
+
+    fun crypto_generichash_blake2b_update(
+      state : Pointer(LibC::UChar),
+      in : Pointer(LibC::UChar),
+      in_len : UInt64,
+    ) : LibC::Int
+
+    fun crypto_generichash_blake2b_final(
+      state : Pointer(LibC::UChar),
+      output : Pointer(LibC::UChar),
+      output_len : UInt64,
+    ) : LibC::Int
   end
 end