diff --git a/spec/sodium/crypto_box/secret_key_spec.cr b/spec/sodium/crypto_box/secret_key_spec.cr
index f577d2a..b2fe2f8 100644
--- a/spec/sodium/crypto_box/secret_key_spec.cr
+++ b/spec/sodium/crypto_box/secret_key_spec.cr
@@ -27,7 +27,7 @@ describe Sodium::CryptoBox::SecretKey do
     key1.public_key.bytes.should eq key2.public_key.bytes
   end
 
-  it "easy encrypt/decrypt" do
+  it "authenticated easy encrypt/decrypt" do
     data = "Hello World!"
 
     # Alice is the sender
@@ -50,6 +50,21 @@ describe Sodium::CryptoBox::SecretKey do
     end
   end
 
+  it "unauthenticated seal encrypt/decrypt" do
+    data = "foo bar"
+
+    # Bob is the recipient
+    bob = Sodium::CryptoBox::SecretKey.new
+
+    # Encrypt a message for Bob using his public key.  No signature.
+    encrypted = bob.public_key.encrypt data
+
+    # Decrypt the message using Bob's secret key.
+    decrypted = bob.decrypt encrypted
+
+    String.new(decrypted).should eq(data)
+  end
+
   it "wipes keys" do
     check_wiped new_key_bytes
   end
diff --git a/src/sodium/crypto_box/box.cr b/src/sodium/crypto_box/box.cr
index 19e8dd5..a991e17 100644
--- a/src/sodium/crypto_box/box.cr
+++ b/src/sodium/crypto_box/box.cr
@@ -4,6 +4,8 @@ module Sodium::CryptoBox
   class Box
     include Wipe
 
+    MAC_SIZE = LibSodium.crypto_box_macbytes
+
     # BUG: precompute size
     @[Wipe::Var]
     @bytes = Bytes.new(1)
@@ -16,14 +18,14 @@ module Sodium::CryptoBox
       encrypt_easy src.to_slice
     end
 
-    def encrypt_easy(src : Bytes, dst = Bytes.new(src.bytesize + LibSodium::MAC_SIZE), nonce = Nonce.new)
+    def encrypt_easy(src : Bytes, dst = Bytes.new(src.bytesize + MAC_SIZE), nonce = Nonce.new)
       if LibSodium.crypto_box_easy(dst, src, src.bytesize, nonce.to_slice, @public_key.to_slice, @secret_key.to_slice) != 0
         raise Error.new("crypto_box_easy")
       end
       {nonce, dst}
     end
 
-    def decrypt_easy(src : Bytes, dst = Bytes.new(src.bytesize - LibSodium::MAC_SIZE), nonce = Nonce.new) : Bytes
+    def decrypt_easy(src : Bytes, dst = Bytes.new(src.bytesize - MAC_SIZE), nonce = Nonce.new) : Bytes
       if LibSodium.crypto_box_open_easy(dst, src, src.bytesize, nonce.to_slice, @public_key.to_slice, @secret_key.to_slice) != 0
         raise Error::DecryptionFailed.new("crypto_box_open_easy")
       end
diff --git a/src/sodium/crypto_box/public_key.cr b/src/sodium/crypto_box/public_key.cr
index 9670038..be01474 100644
--- a/src/sodium/crypto_box/public_key.cr
+++ b/src/sodium/crypto_box/public_key.cr
@@ -3,7 +3,8 @@ require "../lib_sodium"
 module Sodium::CryptoBox
   class PublicKey < Key
     include Wipe
-    KEY_SIZE = LibSodium.crypto_box_publickeybytes
+    KEY_SIZE  = LibSodium.crypto_box_publickeybytes
+    SEAL_SIZE = LibSodium.crypto_box_sealbytes
 
     getter bytes : Bytes
 
@@ -18,5 +19,18 @@ module Sodium::CryptoBox
         raise ArgumentError.new("Public key must be #{KEY_SIZE} bytes, got #{bytes.bytesize}")
       end
     end
+
+    # Anonymously send messages to a recipient given its public key.
+    # For authenticated message use `secret_key.box(recipient_public_key).encrypt`.
+    def encrypt(src)
+      encrypt src.to_slice
+    end
+
+    def encrypt(src : Bytes, dst : Bytes = Bytes.new(src.bytesize + SEAL_SIZE)) : Bytes
+      if LibSodium.crypto_box_seal(dst, src, src.bytesize, @bytes) != 0
+        raise Sodium::Error.new("crypto_box_seal")
+      end
+      dst
+    end
   end
 end
diff --git a/src/sodium/crypto_box/secret_key.cr b/src/sodium/crypto_box/secret_key.cr
index 85c2355..f16c687 100644
--- a/src/sodium/crypto_box/secret_key.cr
+++ b/src/sodium/crypto_box/secret_key.cr
@@ -7,7 +7,7 @@ module Sodium::CryptoBox
     include Wipe
     KEY_SIZE  = LibSodium.crypto_box_secretkeybytes
     SEED_SIZE = LibSodium.crypto_box_seedbytes
-    MAC_SIZE  = LibSodium::MAC_SIZE
+    SEAL_SIZE = LibSodium.crypto_box_sealbytes
 
     getter public_key : PublicKey
 
@@ -51,7 +51,7 @@ module Sodium::CryptoBox
       end
     end
 
-    # Return a Box containing a precomputed shared secret for use with encryption/decryption.
+    # Return a Box containing a precomputed shared secret for use with authenticated encryption/decryption.
     def box(public_key) : Box
       Box.new self, public_key
     end
@@ -65,5 +65,18 @@ module Sodium::CryptoBox
         b.close
       end
     end
+
+    # Anonymously receive messages without signatures.
+    # For authenticated messages use `secret_key.box(recipient_public_key).decrypt`.
+    def decrypt(src)
+      encrypt src.to_slice
+    end
+
+    def decrypt(src : Bytes, dst : Bytes = Bytes.new(src.bytesize - SEAL_SIZE)) : Bytes
+      if LibSodium.crypto_box_seal_open(dst, src, src.bytesize, @public_key.bytes, @bytes) != 0
+        raise Sodium::Error.new("crypto_box_seal_open")
+      end
+      dst
+    end
   end
 end
diff --git a/src/sodium/lib_sodium.cr b/src/sodium/lib_sodium.cr
index 8797456..9d24234 100644
--- a/src/sodium/lib_sodium.cr
+++ b/src/sodium/lib_sodium.cr
@@ -7,6 +7,7 @@ module Sodium
     fun crypto_box_secretkeybytes : LibC::SizeT
     fun crypto_box_seedbytes : LibC::SizeT
     fun crypto_box_noncebytes : LibC::SizeT
+    fun crypto_box_sealbytes : LibC::SizeT
     fun crypto_box_macbytes : LibC::SizeT
     fun crypto_sign_publickeybytes : LibC::SizeT
     fun crypto_sign_secretkeybytes : LibC::SizeT
@@ -42,9 +43,7 @@ module Sodium
     fun crypto_generichash_blake2b_personalbytes : LibC::SizeT
     fun sodium_memzero(Pointer(LibC::UChar), LibC::SizeT) : Nil
 
-    NONCE_SIZE     = crypto_box_noncebytes()
-    MAC_SIZE       = crypto_box_macbytes()
-    SIGNATURE_SIZE = crypto_sign_bytes()
+    NONCE_SIZE = crypto_box_noncebytes()
 
     fun crypto_secretbox_easy(
       output : Pointer(LibC::UChar),
@@ -225,8 +224,4 @@ module Sodium
   if LibSodium.crypto_secretbox_noncebytes != LibSodium.crypto_box_noncebytes
     raise "Assumptions in this library regarding nonce sizes may not be valid"
   end
-
-  if LibSodium.crypto_secretbox_macbytes != LibSodium.crypto_box_macbytes
-    raise "Assumptions in this library regarding mac sizes may not be valid"
-  end
 end
diff --git a/src/sodium/secret_box.cr b/src/sodium/secret_box.cr
index f8935fd..4e329bc 100644
--- a/src/sodium/secret_box.cr
+++ b/src/sodium/secret_box.cr
@@ -15,7 +15,7 @@ module Sodium
   class SecretBox < Key
     KEY_SIZE   = LibSodium.crypto_secretbox_keybytes
     NONCE_SIZE = LibSodium.crypto_secretbox_noncebytes
-    MAC_SIZE   = LibSodium::MAC_SIZE
+    MAC_SIZE   = LibSodium.crypto_secretbox_macbytes
 
     @[Wipe::Var]
     property bytes : Bytes
diff --git a/src/sodium/sign/public_key.cr b/src/sodium/sign/public_key.cr
index b311bf2..e9cd4dd 100644
--- a/src/sodium/sign/public_key.cr
+++ b/src/sodium/sign/public_key.cr
@@ -4,6 +4,7 @@ module Sodium
   class Sign::PublicKey < Key
     include Wipe
     KEY_SIZE = LibSodium.crypto_sign_publickeybytes
+    SIG_SIZE = LibSodium.crypto_sign_bytes
 
     getter bytes : Bytes
 
@@ -26,7 +27,7 @@ module Sodium
     end
 
     def verify_detached(message : Bytes, sig : Bytes)
-      raise ArgumentError.new("Signature must be #{LibSodium::SIGNATURE_SIZE} bytes, got #{sig.bytesize}") if sig.bytesize != LibSodium::SIGNATURE_SIZE
+      raise ArgumentError.new("Signature must be #{SIG_SIZE} bytes, got #{sig.bytesize}") if sig.bytesize != SIG_SIZE
 
       v = LibSodium.crypto_sign_verify_detached sig, message, message.bytesize, @bytes
       if v != 0
diff --git a/src/sodium/sign/secret_key.cr b/src/sodium/sign/secret_key.cr
index 005dff2..3222481 100644
--- a/src/sodium/sign/secret_key.cr
+++ b/src/sodium/sign/secret_key.cr
@@ -9,6 +9,7 @@ module Sodium
   # ```
   class Sign::SecretKey < Sodium::Key
     KEY_SIZE  = LibSodium.crypto_sign_secretkeybytes
+    SIG_SIZE  = LibSodium.crypto_sign_bytes
     SEED_SIZE = LibSodium.crypto_sign_seedbytes
 
     getter public_key : PublicKey
@@ -61,7 +62,7 @@ module Sodium
     end
 
     def sign_detached(message : Bytes)
-      sig = Bytes.new(LibSodium::SIGNATURE_SIZE)
+      sig = Bytes.new(SIG_SIZE)
       if LibSodium.crypto_sign_detached(sig, out sig_len, message, message.bytesize, @bytes) != 0
         raise Error.new("crypto_sign_detached")
       end