diff --git a/crypto_box.js b/crypto_box.js
index 1f2af82..2eb8d44 100644
--- a/crypto_box.js
+++ b/crypto_box.js
@@ -4,6 +4,7 @@ const { randombytes } = require('./randombytes')
 const { crypto_generichash_batch } = require('./crypto_generichash')
 const { crypto_secretbox_open_easy, crypto_secretbox_easy } = require('./crypto_secretbox')
 const xsalsa20 = require('xsalsa20')
+const assert = require('nanoassert')
 
 var crypto_box_PUBLICKEYBYTES = 32,
     crypto_box_SECRETKEYBYTES = 32,
@@ -39,13 +40,13 @@ function crypto_box_keypair(pk, sk) {
 }
 
 function crypto_box_seed_keypair(pk, sk, seed) {
-  check(pk, crypto_box_PUBLICKEYBYTES)
-  check(sk, crypto_box_SECRETKEYBYTES)
-  check(sk, crypto_box_SEEDBYTES)
+  assert(pk.byteLength === crypto_box_PUBLICKEYBYTES, "pk should be 'crypto_box_PUBLICKEYBYTES' bytes")
+  assert(sk.byteLength === crypto_box_SECRETKEYBYTES, "sk should be 'crypto_box_SECRETKEYBYTES' bytes")
+  assert(sk.byteLength === crypto_box_SEEDBYTES, "sk should be 'crypto_box_SEEDBYTES' bytes")
 
-  const hash = Buffer.alloc(64)
+  const hash = new Uint8Array(64)
   crypto_hash_sha512(hash, seed, 32)
-  hash.copy(sk, 0, 0, 32)
+  sk.set(hash.subarray(0, 32))
   hash.fill(0)
 
   return crypto_scalarmult_base(pk, sk)
diff --git a/crypto_hash.js b/crypto_hash.js
index 73599ec..53c2c2b 100644
--- a/crypto_hash.js
+++ b/crypto_hash.js
@@ -1,19 +1,20 @@
 const sha256 = require('sha256-wasm')
 const sha512 = require('sha512-wasm')
+const assert = require('nanoassert')
 
 var crypto_hash_sha256_BYTES = 32
 var crypto_hash_sha512_BYTES = 64
 var crypto_hash_BYTES = crypto_hash_sha512_BYTES
 
 function crypto_hash_sha256 (out, m, n) {
-  check(out, crypto_hash_sha256_BYTES)
+  assert(out.byteLength === crypto_hash_sha256_BYTES, "out must be 'crypto_hash_sha256_BYTES' bytes long")
 
   sha256().update(m.subarray(0, n)).digest(out)
   return 0
 }
 
 function crypto_hash_sha512 (out, m, n) {
-  check(out, crypto_hash_sha512_BYTES)
+  assert(out.byteLength === crypto_hash_sha512_BYTES, "out must be 'crypto_hash_sha512_BYTES' bytes long")
 
   sha512().update(m.subarray(0, n)).digest(out)
   return 0
@@ -30,7 +31,3 @@ module.exports = {
   crypto_hash_BYTES,
   crypto_hash_sha256_BYTES
 }
-
-function check (buf, len) {
-  if (!buf || (len && buf.length < len)) throw new Error('Argument must be a buffer' + (len ? ' of length ' + len : ''))
-}
diff --git a/crypto_kx.js b/crypto_kx.js
index e77c6fb..3477c16 100644
--- a/crypto_kx.js
+++ b/crypto_kx.js
@@ -1,32 +1,29 @@
 const { crypto_scalarmult_base } = require('./crypto_scalarmult')
 const { crypto_generichash } = require('./crypto_generichash')
 const { randombytes_buf } = require('./randombytes')
+const assert = require('nanoassert')
 
 var crypto_kx_SEEDBYTES = 32
 var crypto_kx_PUBLICKEYBYTES = 32
 var crypto_kx_SECRETKEYBYTES = 32
 
 function crypto_kx_keypair (pk, sk) {
-  check(pk, crypto_kx_PUBLICKEYBYTES)
-  check(sk, crypto_kx_SECRETKEYBYTES)
+  assert(pk.byteLength === crypto_kx_PUBLICKEYBYTES, "pk must be 'crypto_kx_PUBLICKEYBYTES' bytes")
+  assert(sk.byteLength === crypto_kx_SECRETKEYBYTES, "sk must be 'crypto_kx_SECRETKEYBYTES' bytes")
 
   randombytes_buf(sk, crypto_kx_SECRETKEYBYTES)
   return crypto_scalarmult_base(pk, sk)
 }
 
 function crypto_kx_seed_keypair (pk, sk, seed) {
-  check(pk, crypto_kx_PUBLICKEYBYTES)
-  check(sk, crypto_kx_SECRETKEYBYTES)
-  check(seed, crypto_kx_SEEDBYTES)
+  assert(pk.byteLength === crypto_kx_PUBLICKEYBYTES, "pk must be 'crypto_kx_PUBLICKEYBYTES' bytes")
+  assert(sk.byteLength === crypto_kx_SECRETKEYBYTES, "sk must be 'crypto_kx_SECRETKEYBYTES' bytes")
+  assert(seed.byteLength === crypto_kx_SEEDBYTES, "seed must be 'crypto_kx_SEEDBYTES' bytes")
 
   crypto_generichash(sk, seed)
   return crypto_scalarmult_base(pk, sk)
 }
 
-function check (buf, len) {
-  if (!buf || (len && buf.length < len)) throw new Error('Argument must be a buffer' + (len ? ' of length ' + len : ''))
-}
-
 module.exports = {
   crypto_kx_keypair,
   crypto_kx_seed_keypair,