diff --git a/README.md b/README.md
index 73d06fa..1dcc99a 100644
--- a/README.md
+++ b/README.md
@@ -24,6 +24,7 @@ Crystal bindings for the [libsodium API](https://libsodium.gitbook.io/doc/)
   - Secret Box
     - [x] [Combined mode](https://libsodium.gitbook.io/doc/secret-key_cryptography/authenticated_encryption)
     - [ ] [Detached mode](https://libsodium.gitbook.io/doc/secret-key_cryptography/authenticated_encryption)
+  - [x] [Secret Stream](https://libsodium.gitbook.io/doc/secret-key_cryptography/secretstream)
   - [AEAD](https://libsodium.gitbook.io/doc/secret-key_cryptography/aead)
    - [ ] AES256-GCM (Requires hardware acceleration)
    - [ ] XChaCha20-Poly1305-IETF
@@ -34,7 +35,7 @@ Crystal bindings for the [libsodium API](https://libsodium.gitbook.io/doc/)
     - [x] Complete libsodium implementation including `key`, `salt`, `personal` and fully selectable output sizes.
   - [ ] [SipHash](https://libsodium.gitbook.io/doc/hashing/short-input_hashing)
 - [Password Hashing](https://libsodium.gitbook.io/doc/password_hashing)
-  - [x] [x] [Argon2](https://libsodium.gitbook.io/doc/password_hashing/the_argon2i_function) (Use for new applications)
+  - [x] ☑ [Argon2](https://libsodium.gitbook.io/doc/password_hashing/the_argon2i_function) (Use for new applications)
   - [ ] [Scrypt](https://libsodium.gitbook.io/doc/advanced/scrypt) (For compatibility with older applications)
 - Other
   - [x] [Key Derivation](https://libsodium.gitbook.io/doc/key_derivation)
diff --git a/spec/sodium/cipher/secret_stream_spec.cr b/spec/sodium/cipher/secret_stream_spec.cr
new file mode 100644
index 0000000..0982004
--- /dev/null
+++ b/spec/sodium/cipher/secret_stream_spec.cr
@@ -0,0 +1,82 @@
+require "../../spec_helper"
+require "../../../src/sodium/cipher/secret_stream"
+
+private def new_ciphers
+  cipher1 = Sodium::Cipher::SecretStream::XChaCha20Poly1305.new
+  cipher2 = Sodium::Cipher::SecretStream::XChaCha20Poly1305.new
+
+  cipher1.encrypt
+  cipher2.decrypt
+
+  {cipher1, cipher2}
+end
+
+private def new_ciphers_with_data
+  data = Bytes.new(100)
+  data.bytesize.times do |i|
+    data[i] = (i % 256).to_u8
+  end
+
+  cipher1, cipher2 = new_ciphers
+
+  key = cipher1.random_key
+  cipher2.key = key
+
+  header = cipher1.header
+  cipher2.header = header
+
+  {cipher1, cipher2, data}
+end
+
+# TODO: verify against test vectors.
+describe Sodium::Cipher::SecretStream do
+  it "encrypts/decrypts" do
+    cipher1, cipher2, data = new_ciphers_with_data
+
+    3.times do
+      output = cipher1.update data
+      output.should_not eq data
+
+      cipher2.update(output).should eq data
+    end
+
+    cipher1.final.should eq Bytes.new(0)
+    cipher2.final.should eq Bytes.new(0)
+  end
+
+  it "encrypts/decrypts with additional" do
+    cipher1, cipher2, data = new_ciphers_with_data
+
+    ["foo", "bar", nil, "baz"].each do |additional|
+      additional = additional.try &.to_slice
+      cipher1.additional = additional
+      output = cipher1.update data
+      cipher1.additional.should eq nil # Additional reset after encrypt.
+      output.should_not eq data
+
+      cipher2.additional = additional
+      cipher2.update(output).should eq data
+      cipher2.additional.should eq nil # Additional reset after encrypt.
+    end
+
+    cipher1.final.should eq Bytes.new(0)
+    cipher2.final.should eq Bytes.new(0)
+  end
+
+  it "encrypts/decrypts with tags" do
+    cipher1, cipher2, data = new_ciphers_with_data
+
+    [cipher1.tag_push, cipher1.tag_rekey, cipher1.tag_final].each do |tag|
+      cipher1.tag = tag
+      output = cipher1.update data
+      cipher1.tag.should eq 0_u8 # Tag reset after encrypt.
+      output.should_not eq data
+
+      cipher2.update(output).should eq data
+      cipher2.tag.should eq tag # Tag set on decrypt.
+    end
+
+    cipher1.final.should eq Bytes.new(0)
+    cipher2.final.should eq Bytes.new(0)
+  end
+end
diff --git a/src/sodium/cipher/secret_stream.cr b/src/sodium/cipher/secret_stream.cr
new file mode 100644
index 0000000..085e408
--- /dev/null
+++ b/src/sodium/cipher/secret_stream.cr
@@ -0,0 +1,176 @@
+require "../lib_sodium"
+require "../secure_buffer"
+
+module Sodium::Cipher
+  abstract class SecretStream
+    @state : SecureBuffer
+    @encrypt_decrypt = 0
+    @initialized = false
+
+    # * Set tag before encrypting
+    # * Tag is set after decrypting
+    property tag = 0_u8
+
+    # Used to authentication but not encrypt additional data.
+    #
+    # * Set this before encrypting **and** decrypting.
+    # * This property is set to nil after calling .update.
+    property additional : Bytes? = nil
+
+    @key : Bytes | SecureBuffer | Nil = nil
+
+    def initialize
+      @state = SecureBuffer.new state_size
+    end
+
+    def encrypt
+      @encrypt_decrypt = 1
+    end
+
+    def decrypt
+      @encrypt_decrypt = -1
+    end
+
+    def key=(key : Bytes | SecureBuffer)
+      raise ArgumentError.new("key must be #{key_size} bytes, got #{key.bytesize}") if key.bytesize != key_size
+      @key = key
+      key
+    end
+
+    # Returns a random key in a SecureBuffer.
+    def random_key
+      self.key = SecureBuffer.random key_size
+    end
+
+    # Only used for encryption.
+    def header
+      raise "only call when encrypting" if @encrypt_decrypt != 1
+      buf = Bytes.new header_size
+      init_state buf
+      buf
+    end
+
+    # Only used for decryption.
+    def header=(buf : Bytes)
+      raise "only call when decrypting" if @encrypt_decrypt != -1
+      init_state buf
+      buf
+    end
+
+    def update(src : Bytes) : Bytes
+      update src, Bytes.new(src.bytesize + (auth_tag_size * @encrypt_decrypt))
+    end
+
+    # Provided for compatibility with block ciphers.
+    # Stream ciphers don't have additional data.
+    def final
+      Bytes.new(0)
+    end
+
+    abstract def update(src : Bytes, dst : Bytes)
+    abstract def init_state(header_buf : Bytes) : Nil
+    protected abstract def state_size : Int32
+    abstract def key_size : Int32
+    abstract def header_size : Int32
+    abstract def auth_tag_size : Int32
+  end
+
+  {% for key, val in {"XChaCha20Poly1305" => "xchacha20poly1305"} %}
+    # [Libsodium Secret Stream API](https://libsodium.gitbook.io/doc/secret-key_cryptography/secretstream)
+    #
+    # This class mimicks the OpenSSL::Cipher interface with minor differences.
+    # * .header must be called for encryption before calling .update
+    # * .header= must be called for decryption with the data returned from .header before calling .update
+    # * every .update is it's own authenticated message.  Unlike OpenSSL this class doesn't buffer data.  You must handle the framing yourself.
+    # * A tag may be set before encrypting and is set after calling .update when decrypting.
+    # * .additional may be set before encrypting and must be set before decrypting.
+    #
+    # See `spec/sodium/cipher/secret_stream_spec.cr` for examples on how to use this class.
+    #
+    # WARNING: Not verified against test vectors.
+    class SecretStream::{{ key.id }} < SecretStream
+      def update(src : Bytes, dst : Bytes) : Bytes
+        raise Sodium::Error.new("must call .header or .header= first") unless @initialized
+        min_dst_size = src.bytesize + (auth_tag_size * @encrypt_decrypt)
+        raise ArgumentError.new("dst bytesize must at least #{min_dst_size}, got #{dst.bytesize}") if dst.bytesize < min_dst_size
+
+	ad, ad_size = if a = @additional
+	  {a.to_unsafe, a.bytesize}
+	else
+	  {Pointer(UInt8).null, 0}
+	end
+
+        case @encrypt_decrypt
+        when 1
+          if LibSodium.crypto_secretstream_{{ val.id }}_push(@state.to_slice, dst.to_slice, out dst_size, src, src.bytesize, ad, ad_size, @tag) != 0
+            raise Sodium::Error.new("crypto_streamsecret_{{ val.id }}_xor_ic")
+          end
+          @tag = 0
+          @additional = nil
+          dst[0, dst_size]
+        when -1
+          if LibSodium.crypto_secretstream_{{ val.id }}_pull(@state.to_slice, dst.to_slice, out dst_size2, out @tag, src, src.bytesize, ad, ad_size) != 0
+            raise Sodium::Error.new("crypto_streamsecret_{{ val.id }}_xor_ic")
+          end
+          @additional = nil
+          dst[0, dst_size2]
+        else
+            abort "invalid encrypt_decrypt state #{@encrypt_decrypt}"
+        end
+      end
+
+      protected def init_state(header_buf : Bytes) : Nil
+        raise Sodium::Error.new("can't initialize more than once") if @initialized
+
+        if k = @key
+          case @encrypt_decrypt
+          when 1
+            if LibSodium.crypto_secretstream_xchacha20poly1305_init_push(@state.to_slice, header_buf.to_slice, k.to_slice) != 0
+              raise Sodium::Error.new("crypto_secretstream_xchacha20poly1305_init_push")
+            end
+          when -1
+            if LibSodium.crypto_secretstream_xchacha20poly1305_init_pull(@state.to_slice, header_buf.to_slice, k.to_slice) != 0
+              raise Sodium::Error.new("crypto_secretstream_xchacha20poly1305_init_push")
+            end
+          when 0
+            raise Sodium::Error.new("must call .encrypt or .decrypt first")
+          else
+            abort "invalid encrypt_decrypt state #{@encrypt_decrypt}"
+          end
+        else
+            raise Sodium::Error.new("must set an encryption/decryption key")
+        end
+
+        @initialized = true
+      end
+
+      protected def state_size : Int32
+        LibSodium.crypto_secretstream_{{ val.id }}_statebytes.to_i
+      end
+
+      def key_size : Int32
+        LibSodium.crypto_secretstream_{{ val.id }}_keybytes.to_i
+      end
+
+      def header_size : Int32
+        LibSodium.crypto_secretstream_{{ val.id }}_headerbytes.to_i
+      end
+
+      def auth_tag_size : Int32
+        LibSodium.crypto_secretstream_{{ val.id }}_abytes.to_i
+      end
+
+      def tag_push
+        LibSodium.crypto_secretstream_{{ val.id }}_tag_push
+      end
+
+      def tag_rekey
+        LibSodium.crypto_secretstream_{{ val.id }}_tag_rekey
+      end
+
+      def tag_final
+        LibSodium.crypto_secretstream_{{ val.id }}_tag_final
+      end
+    end
+  {% end %}
+end
diff --git a/src/sodium/lib_sodium.cr b/src/sodium/lib_sodium.cr
index a474d4e..23cb917 100644
--- a/src/sodium/lib_sodium.cr
+++ b/src/sodium/lib_sodium.cr
@@ -72,6 +72,50 @@ module Sodium
       key : Pointer(LibC::UChar)
     ) : LibC::Int
 
+    {% for name in ["_xchacha20poly1305"] %}
+      {% for name2 in %w(keybytes headerbytes statebytes abytes) %}
+        fun crypto_secretstream{{ name.id }}_{{ name2.id }} : LibC::SizeT
+      {% end %}
+
+      {% for name2 in %w(tag_rekey tag_push tag_final) %}
+        fun crypto_secretstream{{ name.id }}_{{ name2.id }} : LibC::UChar
+      {% end %}
+
+      fun crypto_secretstream{{ name.id }}_init_push(
+        state : Pointer(LibC::UChar),
+        header : Pointer(LibC::UChar),
+        key : Pointer(LibC::UChar),
+      ) : LibC::Int
+
+      fun crypto_secretstream{{ name.id }}_init_pull(
+        state : Pointer(LibC::UChar),
+        header : Pointer(LibC::UChar),
+        key : Pointer(LibC::UChar),
+      ) : LibC::Int
+
+      fun crypto_secretstream{{ name.id }}_push(
+        state : Pointer(LibC::UChar),
+        c : Pointer(LibC::UChar),
+        clen : Pointer(LibC::ULongLong),
+        m : Pointer(LibC::UChar),
+        mlen : LibC::ULongLong,
+        ad : Pointer(LibC::UChar),
+        adlen : LibC::ULongLong,
+        tag : LibC::UChar,
+      ) : LibC::Int
+
+      fun crypto_secretstream{{ name.id }}_pull(
+        state : Pointer(LibC::UChar),
+        m : Pointer(LibC::UChar),
+        mlen : Pointer(LibC::ULongLong),
+        tag : Pointer(LibC::UChar),
+        c : Pointer(LibC::UChar),
+        clen : LibC::ULongLong,
+        ad : Pointer(LibC::UChar),
+        adlen : LibC::ULongLong,
+      ) : LibC::Int
+    {% end %}
+
     # TODO: Add reduced round variants.
     {% for name in ["_chacha20", "_chacha20_ietf", "_xchacha20", "_salsa20", "_xsalsa20"] %}
       fun crypto_stream{{ name.id }}_keybytes() : LibC::SizeT