diff --git a/examples/pwhash_selector.cr b/examples/pwhash_selector.cr
index 75b9fce..aad764d 100644
--- a/examples/pwhash_selector.cr
+++ b/examples/pwhash_selector.cr
@@ -4,7 +4,7 @@ if ARGV.empty?
   puts "Help select Pwhash ops/mem limits for your application."
   puts "Usage: #{PROGRAM_NAME} time_min [time_max] [mem_max]"
   puts "\ttime is in seconds"
-  puts "\tmem is in K"
+  puts "\tmem is in bytes"
   exit 1
 end
 
@@ -14,7 +14,7 @@ time_limit = if t = ARGV.shift?
              else
                time_min * 4
              end
-mem_limit = (ARGV.shift?.try &.to_i || (128*1024)).to_u64
+mem_limit = (ARGV.shift?.try &.to_i || (Cox::Pwhash::MEMLIMIT_MAX)).to_u64
 pwhash = Cox::Pwhash.new
 pass = "1234"
 
@@ -24,10 +24,10 @@ loop do
 
   loop do
     # p pwhash
-    t = Time.measure { pwhash.hash_str pass }.to_f
+    t = Time.measure { pwhash.store pass }.to_f
     s = String.build do |sb|
       sb << "mem_limit "
-      sb << "%7d" % pwhash.memlimit
+      sb << "%7d" % (pwhash.memlimit / 1024)
       sb << "K       ops_limit "
       sb << "%7d" % pwhash.opslimit
       sb << "       "
@@ -41,6 +41,7 @@ loop do
   puts ""
 
   break if pwhash.memlimit >= mem_limit
+  break if pwhash.opslimit == Cox::Pwhash::OPSLIMIT_MIN # Couldn't get past 1 iteration before going over time.
   pwhash.memlimit *= 2
 end
 
diff --git a/spec/cox/pwhash_spec.cr b/spec/cox/pwhash_spec.cr
index b7f61c9..28be1a7 100644
--- a/spec/cox/pwhash_spec.cr
+++ b/spec/cox/pwhash_spec.cr
@@ -1,15 +1,20 @@
 require "../spec_helper"
 
-describe Cox::Pwhash do
-  it "hashes and verifies a password" do
+private def pw_min
     pwhash = Cox::Pwhash.new
 
     # set to minimum to speed up tests
     pwhash.memlimit = Cox::Pwhash::MEMLIMIT_MIN
     pwhash.opslimit = Cox::Pwhash::OPSLIMIT_MIN
+    pwhash
+end
+
+describe Cox::Pwhash do
+  it "hashes and verifies a password" do
+    pwhash = pw_min
 
     pass = "1234"
-    hash = pwhash.hash_str pass
+    hash = pwhash.store pass
     pwhash.verify hash, pass
     expect_raises(Cox::Pwhash::PasswordVerifyError) do
       pwhash.verify hash, "5678"
@@ -19,4 +24,27 @@ describe Cox::Pwhash do
     pwhash.opslimit = Cox::Pwhash::OPSLIMIT_MAX
     pwhash.needs_rehash?(hash).should be_true
   end
+
+  it "key_derive fails without an algorithm" do
+    pwhash = pw_min
+    expect_raises(ArgumentError) do
+      pwhash.key_derive pwhash.salt, "foo", 16
+    end
+  end
+
+  it "derives a key from a password" do
+    pwhash = pw_min
+    pwhash.algorithm = Cox::Pwhash::Algorithm::Argon2id13
+    salt = pwhash.salt
+    key1 = pwhash.key_derive salt, "foo", 16
+    key2 = pwhash.key_derive salt, "foo", 16
+    key3 = pwhash.key_derive salt, "bar", 16
+    key4 = pwhash.key_derive pwhash.salt, "foo", 16
+
+    key1.bytesize.should eq 16
+    key1.should eq key2
+    key1.should_not eq key3
+    key1.should_not eq key4
+# BUG: validate against known passwords
+  end
 end
diff --git a/src/cox/lib_sodium.cr b/src/cox/lib_sodium.cr
index 97c3256..bd5d51c 100644
--- a/src/cox/lib_sodium.cr
+++ b/src/cox/lib_sodium.cr
@@ -21,6 +21,11 @@ module Cox
     fun crypto_pwhash_opslimit_sensitive()     : LibC::SizeT
     fun crypto_pwhash_opslimit_max()     : LibC::SizeT
     fun crypto_pwhash_strbytes()     : LibC::SizeT
+    fun crypto_pwhash_alg_argon2i13()     : LibC::Int
+    fun crypto_pwhash_alg_argon2id13()     : LibC::Int
+    fun crypto_pwhash_saltbytes : LibC::SizeT
+    fun crypto_pwhash_bytes_min() : LibC::SizeT
+    fun crypto_pwhash_bytes_max() : LibC::SizeT
     fun crypto_generichash_blake2b_statebytes : LibC::SizeT
     fun crypto_generichash_blake2b_bytes : LibC::SizeT
     fun crypto_generichash_blake2b_bytes_min : LibC::SizeT
@@ -109,6 +114,17 @@ module Cox
       key    : Pointer(LibC::UChar)
     ) : LibC::Int
 
+    fun crypto_pwhash(
+      key    : Pointer(LibC::UChar),
+      key_size    : LibC::ULongLong,
+      pass    : Pointer(LibC::UChar),
+      pass_size    : LibC::ULongLong,
+      salt    : Pointer(LibC::UChar),
+      optslimit    : LibC::ULongLong,
+      memlimit    : LibC::SizeT,
+      alg    : LibC::Int,
+    ) : LibC::Int
+
     fun crypto_pwhash_str(
       outstr    : Pointer(LibC::UChar),
       pass    : Pointer(LibC::UChar),
diff --git a/src/cox/pwhash.cr b/src/cox/pwhash.cr
index fb933c8..bd11be4 100644
--- a/src/cox/pwhash.cr
+++ b/src/cox/pwhash.cr
@@ -1,4 +1,7 @@
 module Cox
+  # [Argon2 Password Hashing](https://libsodium.gitbook.io/doc/password_hashing/the_argon2i_function)
+  # * #store #verify #needs_rehash? are used together for password verification.
+  # * #key_derive is used on it's own to generate password based keys.
   class Pwhash
     class PasswordVerifyError < Cox::Error
     end
@@ -13,10 +16,24 @@ module Cox
     MEMLIMIT_MAX         = LibSodium.crypto_pwhash_memlimit_max
     MEMLIMIT_INTERACTIVE = LibSodium.crypto_pwhash_memlimit_interactive
 
+    enum Algorithm
+      Argon2i13 = 1
+      Argon2id13 = 2
+    end
+
     property opslimit = OPSLIMIT_INTERACTIVE
+    # Specified in bytes.
     property memlimit = MEMLIMIT_INTERACTIVE
 
-    def hash_str(pass)
+    # Used by and must be set before calling #key_derive
+    property algorithm : Algorithm?
+
+    # Apply the most recent password hashing algorithm agains a password.
+    # Returns a opaque String which includes:
+    # * the result of a memory-hard, CPU-intensive hash function applied to the password
+    # * the automatically generated salt used for the previous computation
+    # * the other parameters required to verify the password, including the algorithm identifier, its version, opslimit and memlimit.
+    def store(pass)
       outstr = Bytes.new LibSodium::PWHASH_STR_SIZE
       if LibSodium.crypto_pwhash_str(outstr, pass, pass.bytesize, @opslimit, @memlimit) != 0
         raise Cox::Error.new("crypto_pwhash_str")
@@ -24,17 +41,20 @@ module Cox
       outstr
     end
 
+    # Verify a password against a stored String.
+    # raises PasswordVerifyError on failure.
     def verify(str, pass)
       # BUG: verify str length
       case LibSodium.crypto_pwhash_str_verify(str, pass, pass.bytesize)
       when 0
-        true
+        # Passed
       else
         raise PasswordVerifyError.new
       end
+      self
     end
 
-    def needs_rehash?(str)
+    def needs_rehash?(str) : Bool
       # BUG: verify str length
       case LibSodium.crypto_pwhash_str_needs_rehash(str, @opslimit, @memlimit)
       when 0
@@ -45,5 +65,29 @@ module Cox
         raise Cox::Error.new("crypto_pwhash_str_needs_rehash")
       end
     end
+
+    # Returns a consistent key based on [salt, pass, key_bytes, algorithm, ops_limit, mem_limit]
+    #
+    # Must set an algorithm before calling.
+    def key_derive(salt : Bytes, pass : Bytes, key_bytes) : Bytes
+      if alg = algorithm
+        key = Bytes.new key_bytes
+        if LibSodium.crypto_pwhash(key, key.bytesize, pass, pass.bytesize, salt, @opslimit, @memlimit, alg) != 0
+          raise Cox::Error.new("crypto_pwhash_str")
+        end
+        key
+      else
+        raise ArgumentError.new("algorithm not set")
+      end
+    end
+
+    def key_derive(salt, pass, key_bytes)
+      key_derive salt.to_slice, pass.to_slice, key_bytes
+    end
+
+    # Returns a random salt for use with #key_derive
+    def salt
+      Random::Secure.random_bytes LibSodium.crypto_pwhash_saltbytes
+    end
   end
 end