diff --git a/internal/apikey/apikey.go b/internal/apikey/apikey.go
new file mode 100644
index 0000000..da497dd
--- /dev/null
+++ b/internal/apikey/apikey.go
@@ -0,0 +1,143 @@
+// Key structure: hmy_live_<43 secret><6 checksum>
+
+package apikey
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"hash/crc32"
+	"strings"
+)
+
+const (
+	Issuer      = "hmy"
+	secretBytes = 32 // 256 bit
+	secretLen   = 43
+	checksumLen = 6
+	bodyLen     = secretLen + checksumLen
+
+	// Enough to identify, not enough to brute-force
+	displayLen = 16
+)
+
+var (
+	ErrMalformed   = errors.New("apikey: malformed key")
+	ErrBadChecksum = errors.New("apikey: checksum mismatch")
+	ErrEnvironment = errors.New("apikey: unknown environment")
+)
+
+// Environment for live and test keys
+type Environment string
+
+const (
+	Live Environment = "live"
+	Test Environment = "test"
+)
+
+func (e Environment) valid() bool {
+	return e == Live || e == Test
+}
+
+// Generated is the result of issuing a key.
+// Plaintext shown to owner and never stored.
+type Generated struct {
+	Plaintext string
+	Hash      []byte
+	Display   string
+	Env       Environment
+}
+
+// Castagnoli instead of IEEE table
+var crcTable = crc32.MakeTable(crc32.Castagnoli)
+
+func checksum(secret string) string {
+	sum := crc32.Checksum([]byte(secret), crcTable)
+
+	b := []byte{
+		byte(sum >> 24),
+		byte(sum >> 16),
+		byte(sum >> 8),
+		byte(sum),
+	}
+
+	return base64.RawURLEncoding.EncodeToString(b)
+}
+
+// Generate issues a ney key. Store Hash and Display, show Plaintext once
+func Generate(env Environment) (Generated, error) {
+	if !env.valid() {
+		return Generated{}, ErrEnvironment
+	}
+
+	buf := make([]byte, secretBytes)
+	if _, err := rand.Read(buf); err != nil {
+		return Generated{}, fmt.Errorf("apikey: reading entropy: %w", err)
+	}
+
+	secret := base64.RawURLEncoding.EncodeToString(buf)
+	plaintext := Issuer + "_" + string(env) + "_" + secret + checksum(secret)
+
+	display := plaintext
+	if len(display) > displayLen {
+		display = display[:displayLen]
+	}
+
+	return Generated{
+		Plaintext: plaintext,
+		Hash:      Hash(plaintext),
+		Display:   display,
+		Env:       env,
+	}, nil
+}
+
+// Hash is what is stored and looked up
+func Hash(key string) []byte {
+	sum := sha256.Sum256([]byte(key))
+	return sum[:]
+}
+
+// Validate checks the structure and checksum. Call before hashing.
+func Validate(key string) (Environment, error) {
+	parts := strings.SplitN(key, "_", 3)
+	if len(parts) != 3 {
+		return "", ErrMalformed
+	}
+
+	if parts[0] != Issuer {
+		return "", ErrMalformed
+	}
+
+	env := Environment(parts[1])
+	if !env.valid() {
+		return "", ErrEnvironment
+	}
+
+	body := parts[2]
+	if len(body) != bodyLen {
+		return "", ErrMalformed
+	}
+
+	secret, want := body[:secretLen], body[secretLen:]
+
+	if _, err := base64.RawURLEncoding.DecodeString(secret); err != nil {
+		return "", ErrMalformed
+	}
+
+	if checksum(secret) != want {
+		return "", ErrBadChecksum
+	}
+
+	return env, nil
+}
+
+// Redact makes the key safe for logging
+func Redact(key string) string {
+	if len(key) <= displayLen {
+		return Issuer + "_***"
+	}
+
+	return key[:displayLen] + "_***"
+}
diff --git a/internal/apikey/apikey_test.go b/internal/apikey/apikey_test.go
new file mode 100644
index 0000000..fce3b5b
--- /dev/null
+++ b/internal/apikey/apikey_test.go
@@ -0,0 +1,178 @@
+package apikey
+
+import (
+	"bytes"
+	"errors"
+	"strings"
+	"testing"
+)
+
+func TestGenerateRoundTrip(t *testing.T) {
+	for _, env := range []Environment{Live, Test} {
+		g, err := Generate(env)
+		if err != nil {
+			t.Fatalf("Generate(%s): %v", env, err)
+		}
+
+		got, err := Validate(g.Plaintext)
+		if err != nil {
+			t.Fatalf("Validate(%q): %v", g.Plaintext, err)
+		}
+
+		if got != env {
+			t.Errorf("environment round trip: got %q want %q", got, env)
+		}
+
+		if !strings.HasPrefix(g.Plaintext, Issuer+"_"+string(env)+"_") {
+			t.Errorf("prefix wrong: %q", g.Plaintext)
+		}
+
+		if len(g.Hash) != 32 {
+			t.Errorf("hash length %d, want 32 (the api.keys CHECK requires it)", len(g.Hash))
+		}
+
+		if !bytes.Equal(g.Hash, Hash(g.Plaintext)) {
+			t.Error("Generate's hash differs from Hash of its own plaintext")
+		}
+
+		if len(g.Display) != displayLen {
+			t.Errorf("display length %d, want %d", len(g.Display), displayLen)
+		}
+
+		if strings.Contains(g.Plaintext[len(g.Display):], "") && g.Display == g.Plaintext {
+			t.Error("display prefix is the whole key")
+		}
+	}
+}
+
+func TestGenerateIsUnique(t *testing.T) {
+	const n = 2000
+
+	seen := make(map[string]bool, n)
+
+	for i := 0; i < n; i++ {
+		g, err := Generate(Live)
+		if err != nil {
+			t.Fatalf("Generate: %v", err)
+		}
+
+		if seen[g.Plaintext] {
+			t.Fatalf("duplicate key after %d generations", i)
+		}
+
+		seen[g.Plaintext] = true
+	}
+}
+
+func TestGenerateRejectsUnknownEnvironment(t *testing.T) {
+	if _, err := Generate(Environment("prod")); !errors.Is(err, ErrEnvironment) {
+		t.Errorf("got %v, want ErrEnvironment", err)
+	}
+}
+
+func TestValidateRejectsMalformed(t *testing.T) {
+	good, err := Generate(Live)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cases := []struct {
+		name string
+		key  string
+		want error
+	}{
+		{"empty", "", ErrMalformed},
+		{"no separators", "notakey", ErrMalformed},
+		{"one separator", "hmy_live", ErrMalformed},
+		{"wrong issuer", "xxx_live_" + good.Plaintext[9:], ErrMalformed},
+		{"unknown env", "hmy_prod_" + good.Plaintext[9:], ErrEnvironment},
+		{"body too short", "hmy_live_abc", ErrMalformed},
+		{"body too long", good.Plaintext + "X", ErrMalformed},
+		{"body truncated by one", good.Plaintext[:len(good.Plaintext)-1], ErrMalformed},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			if _, err := Validate(c.key); !errors.Is(err, c.want) {
+				t.Errorf("Validate(%q) = %v, want %v", c.key, err, c.want)
+			}
+		})
+	}
+}
+
+// The point of the checksum: a key with a typo is rejected without a database
+// round trip.
+func TestValidateCatchesTampering(t *testing.T) {
+	g, err := Generate(Live)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	body := []byte(g.Plaintext)
+	caught := 0
+
+	// Flip one character at each position in the secret and confirm the
+	// checksum notices.
+	for i := len("hmy_live_"); i < len(body)-checksumLen; i++ {
+		orig := body[i]
+
+		if orig == 'A' {
+			body[i] = 'B'
+		} else {
+			body[i] = 'A'
+		}
+
+		if _, err := Validate(string(body)); errors.Is(err, ErrBadChecksum) {
+			caught++
+		}
+
+		body[i] = orig
+	}
+
+	total := len(body) - checksumLen - len("hmy_live_")
+
+	if caught != total {
+		t.Errorf("checksum caught %d/%d single-character corruptions", caught, total)
+	}
+}
+
+func TestHashIsStable(t *testing.T) {
+	const key = "hmy_live_0123456789abcdefghijklmnopqrstuvwxyzABCDEFGabcdef"
+
+	a, b := Hash(key), Hash(key)
+
+	if !bytes.Equal(a, b) {
+		t.Error("Hash is not deterministic")
+	}
+
+	if bytes.Equal(a, Hash(key+"x")) {
+		t.Error("Hash collided on different input")
+	}
+}
+
+func TestRedact(t *testing.T) {
+	g, err := Generate(Live)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	r := Redact(g.Plaintext)
+
+	if strings.Contains(g.Plaintext, r) && len(r) >= len(g.Plaintext) {
+		t.Error("Redact returned the whole key")
+	}
+
+	if len(r) > displayLen+3 {
+		t.Errorf("Redact returned %d chars, too much", len(r))
+	}
+
+	// A secret must never survive redaction.
+	secret := g.Plaintext[len("hmy_live_"):]
+	if strings.Contains(r, secret) {
+		t.Error("Redact leaked the secret")
+	}
+
+	if got := Redact("short"); got != "hmy_***" {
+		t.Errorf("Redact(short) = %q", got)
+	}
+}