diff --git a/srv/api/api.go b/srv/api/api.go
index 15627f8..39d73d9 100644
--- a/srv/api/api.go
+++ b/srv/api/api.go
@@ -22,10 +22,11 @@ import (
 // Params are used to instantiate a new API instance. All fields are required
 // unless otherwise noted.
 type Params struct {
-	Logger      *mlog.Logger
-	PowManager  pow.Manager
-	MailingList mailinglist.MailingList
-	GlobalRoom  chat.Room
+	Logger           *mlog.Logger
+	PowManager       pow.Manager
+	MailingList      mailinglist.MailingList
+	GlobalRoom       chat.Room
+	UserIDCalculator chat.UserIDCalculator
 
 	// ListenProto and ListenAddr are passed into net.Listen to create the
 	// API's listener. Both "tcp" and "unix" protocols are explicitly
diff --git a/srv/api/chat.go b/srv/api/chat.go
index 84fffde..55d9d02 100644
--- a/srv/api/chat.go
+++ b/srv/api/chat.go
@@ -4,6 +4,8 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"strings"
+	"unicode"
 
 	"github.com/mediocregopher/blog.mediocregopher.com/srv/chat"
 )
@@ -39,3 +41,48 @@ func (a *api) chatHistoryHandler() http.Handler {
 		})
 	})
 }
+
+func (a *api) getUserID(r *http.Request) (chat.UserID, error) {
+	name := r.PostFormValue("name")
+	if l := len(name); l == 0 {
+		return chat.UserID{}, errors.New("name is required")
+	} else if l > 16 {
+		return chat.UserID{}, errors.New("name too long")
+	}
+
+	nameClean := strings.Map(func(r rune) rune {
+		if !unicode.IsPrint(r) {
+			return -1
+		}
+		return r
+	}, name)
+
+	if nameClean != name {
+		return chat.UserID{}, errors.New("name contains invalid characters")
+	}
+
+	password := r.PostFormValue("password")
+	if l := len(password); l == 0 {
+		return chat.UserID{}, errors.New("password is required")
+	} else if l > 128 {
+		return chat.UserID{}, errors.New("password too long")
+	}
+
+	return a.params.UserIDCalculator.Calculate(name, password), nil
+}
+
+func (a *api) getUserIDHandler() http.Handler {
+	return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+		userID, err := a.getUserID(r)
+		if err != nil {
+			badRequest(rw, r, err)
+			return
+		}
+
+		jsonResult(rw, r, struct {
+			UserID chat.UserID `json:"userID"`
+		}{
+			UserID: userID,
+		})
+	})
+}
diff --git a/srv/chat/chat.go b/srv/chat/chat.go
index 44449cd..ae305ac 100644
--- a/srv/chat/chat.go
+++ b/srv/chat/chat.go
@@ -29,17 +29,6 @@ var (
 	errInvalidMessageID = ErrInvalidArg{Err: errors.New("invalid Message ID")}
 )
 
-// UserID uniquely identifies an individual user who has posted a message in a
-// Room.
-type UserID struct {
-
-	// Name will be the user's chosen display name.
-	Name string `json:"name"`
-
-	// Hash will be a hex string generated from a secret only the user knows.
-	Hash string `json:"id"`
-}
-
 // Message describes a message which has been posted to a Room.
 type Message struct {
 	ID     string `json:"id"`
diff --git a/srv/chat/user.go b/srv/chat/user.go
new file mode 100644
index 0000000..1279a45
--- /dev/null
+++ b/srv/chat/user.go
@@ -0,0 +1,68 @@
+package chat
+
+import (
+	"encoding/hex"
+	"fmt"
+	"sync"
+
+	"golang.org/x/crypto/argon2"
+)
+
+// UserID uniquely identifies an individual user who has posted a message in a
+// Room.
+type UserID struct {
+
+	// Name will be the user's chosen display name.
+	Name string `json:"name"`
+
+	// Hash will be a hex string generated from a secret only the user knows.
+	Hash string `json:"id"`
+}
+
+// UserIDCalculator is used to calculate UserIDs.
+type UserIDCalculator struct {
+
+	// Secret is used when calculating UserID Hash salts.
+	Secret []byte
+
+	// TimeCost, MemoryCost, and Threads are used as inputs to the Argon2id
+	// algorithm which is used to generate the Hash.
+	TimeCost, MemoryCost uint32
+	Threads              uint8
+
+	// HashLen specifies the number of bytes the Hash should be.
+	HashLen uint32
+
+	// Lock, if set, forces concurrent Calculate calls to occur sequentially.
+	Lock *sync.Mutex
+}
+
+// NewUserIDCalculator returns a UserIDCalculator with sane defaults.
+func NewUserIDCalculator(secret []byte) UserIDCalculator {
+	return UserIDCalculator{
+		Secret:     secret,
+		TimeCost:   15,
+		MemoryCost: 128 * 1024,
+		Threads:    2,
+		HashLen:    16,
+		Lock:       new(sync.Mutex),
+	}
+}
+
+// Calculate accepts a name and password and returns the calculated UserID.
+func (c UserIDCalculator) Calculate(name, password string) UserID {
+
+	input := fmt.Sprintf("%q:%q", name, password)
+
+	hashB := argon2.IDKey(
+		[]byte(input),
+		c.Secret, // salt
+		c.TimeCost, c.MemoryCost, c.Threads,
+		c.HashLen,
+	)
+
+	return UserID{
+		Name: name,
+		Hash: hex.EncodeToString(hashB),
+	}
+}
diff --git a/srv/chat/user_test.go b/srv/chat/user_test.go
new file mode 100644
index 0000000..2169cde
--- /dev/null
+++ b/srv/chat/user_test.go
@@ -0,0 +1,26 @@
+package chat
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestUserIDCalculator(t *testing.T) {
+
+	const name, password = "name", "password"
+
+	c := NewUserIDCalculator([]byte("foo"))
+
+	// calculating with same params twice should result in same UserID
+	userID := c.Calculate(name, password)
+	assert.Equal(t, userID, c.Calculate(name, password))
+
+	// changing either name or password should result in a different Hash
+	assert.NotEqual(t, userID.Hash, c.Calculate(name+"!", password).Hash)
+	assert.NotEqual(t, userID.Hash, c.Calculate(name, password+"!").Hash)
+
+	// changing the secret should change the UserID
+	c.Secret = []byte("bar")
+	assert.NotEqual(t, userID, c.Calculate(name, password))
+}
diff --git a/srv/cmd/userid-calc-cli/main.go b/srv/cmd/userid-calc-cli/main.go
new file mode 100644
index 0000000..c86ae5d
--- /dev/null
+++ b/srv/cmd/userid-calc-cli/main.go
@@ -0,0 +1,27 @@
+package main
+
+import (
+	"encoding/json"
+	"flag"
+	"fmt"
+
+	"github.com/mediocregopher/blog.mediocregopher.com/srv/chat"
+)
+
+func main() {
+
+	secret := flag.String("secret", "", "Secret to use when calculating UserIDs")
+	name := flag.String("name", "", "")
+	password := flag.String("password", "", "")
+	flag.Parse()
+
+	calc := chat.NewUserIDCalculator([]byte(*secret))
+	userID := calc.Calculate(*name, *password)
+
+	b, err := json.Marshal(userID)
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Println(string(b))
+}
diff --git a/srv/cmd/userid-calc-cli/userid-calc-cli b/srv/cmd/userid-calc-cli/userid-calc-cli
new file mode 100755
index 0000000..ca18cc3
Binary files /dev/null and b/srv/cmd/userid-calc-cli/userid-calc-cli differ
diff --git a/srv/go.mod b/srv/go.mod
index e2e43f0..8587186 100644
--- a/srv/go.mod
+++ b/srv/go.mod
@@ -13,4 +13,5 @@ require (
 	github.com/stretchr/testify v1.7.0
 	github.com/tilinna/clock v1.1.0
 	github.com/ziutek/mymysql v1.5.4 // indirect
+	golang.org/x/crypto v0.0.0-20210817164053-32db794688a5 // indirect
 )
diff --git a/srv/go.sum b/srv/go.sum
index b5b3455..6c96538 100644
--- a/srv/go.sum
+++ b/srv/go.sum
@@ -176,6 +176,8 @@ golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8U
 golang.org/x/crypto v0.0.0-20191122220453-ac88ee75c92c/go.mod h1:LzIPMQfyMNhhGPhUkYOs5KpL4U8rLKemX1yGLhDgUto=
 golang.org/x/crypto v0.0.0-20200820211705-5c72a883971a h1:vclmkQCjlDX5OydZ9wv8rBCcS0QyQY66Mpf/7BZbInM=
 golang.org/x/crypto v0.0.0-20200820211705-5c72a883971a/go.mod h1:LzIPMQfyMNhhGPhUkYOs5KpL4U8rLKemX1yGLhDgUto=
+golang.org/x/crypto v0.0.0-20210817164053-32db794688a5 h1:HWj/xjIHfjYU5nVXpTM0s39J9CbLn7Cc5a7IC5rwsMQ=
+golang.org/x/crypto v0.0.0-20210817164053-32db794688a5/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=
 golang.org/x/lint v0.0.0-20181026193005-c67002cb31c3/go.mod h1:UVdnD1Gm6xHRNCYTkRU2/jEulfH38KcIWyp/GAMgvoE=
 golang.org/x/lint v0.0.0-20190313153728-d0100b6bd8b3/go.mod h1:6SW0HCj/g11FgYtHlgUYUwCkIfeOF89ocIRzGO/8vkc=
 golang.org/x/mod v0.2.0/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
@@ -187,6 +189,7 @@ golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn
 golang.org/x/net v0.0.0-20190522155817-f3200d17e092/go.mod h1:HSz+uSET+XFnRR8LxR5pz3Of3rY3CfYBVs4xY44aLks=
 golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
 golang.org/x/net v0.0.0-20200226121028-0de0cce0169b/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
+golang.org/x/net v0.0.0-20210226172049-e18ecbb05110/go.mod h1:m0MpNAwzfU5UDzcl9v0D8zg8gWTRqZa9RBIspLL5mdg=
 golang.org/x/oauth2 v0.0.0-20180821212333-d2e6202438be/go.mod h1:N/0e6XlmueqKjAGxoOufVs8QHGRruUQn6yWY3a++T0U=
 golang.org/x/sync v0.0.0-20180314180146-1d60e4601c6f/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20181108010431-42b317875d0f/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
@@ -201,9 +204,15 @@ golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5h
 golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20190422165155-953cdadca894 h1:Cz4ceDQGXuKRnVBDTS23GTn/pU5OE2C0WrNTOYK1Uuc=
 golang.org/x/sys v0.0.0-20190422165155-953cdadca894/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1 h1:SrN+KX8Art/Sf4HNj6Zcz06G7VEz+7w9tdXTPOZ7+l4=
+golang.org/x/sys v0.0.0-20210615035016-665e8c7367d1/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
 golang.org/x/time v0.0.0-20190308202827-9d24e82272b4/go.mod h1:tRJNPiyCQ0inRvYxbN9jk5I+vvW/OXSQhTDSoE431IQ=
 golang.org/x/tools v0.0.0-20180221164845-07fd8470d635/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
+golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 golang.org/x/tools v0.0.0-20190114222345-bf090417da8b/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 golang.org/x/tools v0.0.0-20190311212946-11955173bddd/go.mod h1:LCzVGOaR6xXOjkQ3onu1FJEFr0SW1gC7cKk1uF8kGRs=
 golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=