Initial commit.

internal/authproxy/crypto.go

@@ -0,0 +1,71 @@
+package authproxy
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"io"
+)
+
+// box seals and opens small JSON payloads with AES-256-GCM. The purpose
+// string is bound in as additional authenticated data so a token minted for
+// one use (e.g. OAuth state) can never be replayed as another (e.g. a
+// session cookie).
+type box struct {
+	aead cipher.AEAD
+}
+
+func newBox(secret string) (*box, error) {
+	key := sha256.Sum256([]byte(secret))
+	block, err := aes.NewCipher(key[:])
+	if err != nil {
+		return nil, err
+	}
+	aead, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, err
+	}
+	return &box{aead: aead}, nil
+}
+
+func (b *box) seal(purpose string, v any) (string, error) {
+	plain, err := json.Marshal(v)
+	if err != nil {
+		return "", err
+	}
+	nonce := make([]byte, b.aead.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return "", err
+	}
+	out := b.aead.Seal(nonce, nonce, plain, []byte(purpose))
+	return base64.RawURLEncoding.EncodeToString(out), nil
+}
+
+func (b *box) open(purpose, token string, v any) error {
+	raw, err := base64.RawURLEncoding.DecodeString(token)
+	if err != nil {
+		return fmt.Errorf("malformed token: %w", err)
+	}
+	ns := b.aead.NonceSize()
+	if len(raw) <= ns {
+		return fmt.Errorf("malformed token: too short")
+	}
+	plain, err := b.aead.Open(nil, raw[:ns], raw[ns:], []byte(purpose))
+	if err != nil {
+		return fmt.Errorf("token failed authentication: %w", err)
+	}
+	return json.Unmarshal(plain, v)
+}
+
+func randToken() string {
+	buf := make([]byte, 16)
+	if _, err := io.ReadFull(rand.Reader, buf); err != nil {
+		panic(err) // crypto/rand failure is unrecoverable
+	}
+	return hex.EncodeToString(buf)
+}