3 files changed, 94 insertions, 11 deletions
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))
+}