feat: view private key for addresses (closes #19)

- Add 'Show Private Key' button to address detail view
- Create dedicated password verification modal with warning text
- Derive private key from HD wallet mnemonic or use directly for imported keys
- Display key in read-only well with copy button
- Add getPrivateKeyForAddress() to shared/wallet.js
- Never log, cache, or auto-clear the private key
- Document clipboard non-clearing policy in README

README.md

@@ -835,6 +835,25 @@ Currently supported:
 - [ ] Multi-currency fiat display (EUR, GBP, etc.)
 - [ ] Security audit of key management
 
+## Private Key Export
+
+The address detail view includes a "Show Private Key" button. After entering the
+wallet password, the raw hex private key is displayed and can be copied to the
+clipboard.
+
+We intentionally **do not clear the clipboard** after copying a private key:
+
+1. **User expectations**: Clipboard clearing violates the principle of least
+   surprise. Users expect their clipboard to contain what they last copied until
+   they copy something else.
+2. **Data safety**: The user may copy something else important moments later. An
+   auto-clear timer could destroy that unrelated clipboard content, causing data
+   loss far worse than the theoretical risk it was meant to mitigate.
+
+If a user chooses to display their private key, they have already been warned
+that it controls all funds at the address. Managing sensitive data on their own
+clipboard is their responsibility.
+
 ## Policies
 
 - We don't mention "the other wallet" by name in code or documentation. We're

src/popup/index.html

@@ -306,6 +306,14 @@
                         + Token
                     </button>
                 </div>
+                <div class="flex gap-2 mb-3">
+                    <button
+                        id="btn-show-privkey"
+                        class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer flex-1 text-xs"
+                    >
+                        &#128273; Show Private Key
+                    </button>
+                </div>
 
                 <!-- transactions -->
                 <div class="mt-3">
@@ -878,6 +886,57 @@
                 </button>
             </div>
 
+            <!-- ============ SHOW PRIVATE KEY ============ -->
+            <div id="view-show-private-key" class="view hidden">
+                <button
+                    id="btn-privkey-back"
+                    class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer mb-2"
+                >
+                    &lt; Back
+                </button>
+                <h2 class="font-bold mb-3">Show Private Key</h2>
+
+                <div id="privkey-form">
+                    <div
+                        class="text-xs mb-3 border border-border border-dashed p-2"
+                    >
+                        The private key controls all funds at this address.
+                        Anyone who has it can spend your tokens. Do not share it
+                        with anyone.
+                    </div>
+                    <div class="mb-2">
+                        <label class="block mb-1">Password</label>
+                        <input
+                            type="password"
+                            id="privkey-password"
+                            class="border border-border p-1 w-full font-mono text-sm bg-bg text-fg"
+                            placeholder="Enter your password"
+                        />
+                    </div>
+                    <div id="privkey-error" class="text-xs mb-2 hidden"></div>
+                    <button
+                        id="btn-privkey-reveal"
+                        class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
+                    >
+                        &#128274;&#128176; Display Private Key
+                    </button>
+                </div>
+
+                <div id="privkey-result" class="hidden">
+                    <div class="text-xs text-muted mb-1">Private Key</div>
+                    <div
+                        id="privkey-result-key"
+                        class="bg-hover rounded-md p-3 text-xs break-all font-mono select-all mb-2"
+                    ></div>
+                    <button
+                        id="btn-privkey-copy"
+                        class="border border-border px-2 py-1 hover:bg-fg hover:text-bg cursor-pointer"
+                    >
+                        Copy
+                    </button>
+                </div>
+            </div>
+
             <!-- ============ SETTINGS: ADD TOKEN ============ -->
             <div id="view-settings-addtoken" class="view hidden">
                 <button

src/popup/index.js

@@ -21,6 +21,7 @@ const receive = require("./views/receive");
 const addToken = require("./views/addToken");
 const settings = require("./views/settings");
 const settingsAddToken = require("./views/settingsAddToken");
+const showPrivateKey = require("./views/showPrivateKey");
 const approval = require("./views/approval");
 
 function renderWalletList() {
@@ -63,6 +64,7 @@ const ctx = {
     showTransactionDetail: (tx) => transactionDetail.show(tx),
     showSettingsView: () => settings.show(),
     showSettingsAddTokenView: () => settingsAddToken.show(),
+    showPrivateKey: () => showPrivateKey.show(),
 };
 
 // Views that can be fully re-rendered from persisted state.
@@ -220,6 +222,7 @@ async function init() {
     addToken.init(ctx);
     settings.init(ctx);
     settingsAddToken.init(ctx);
+    showPrivateKey.init(ctx);
 
     if (!state.hasWallet) {
         showView("welcome");

src/popup/views/addressDetail.js

@@ -261,6 +261,10 @@ function init(_ctx) {
     });
 
     $("btn-add-token").addEventListener("click", ctx.showAddTokenView);
+
+    $("btn-show-privkey").addEventListener("click", () => {
+        ctx.showPrivateKey();
+    });
 }
 
 module.exports = { init, show };

src/popup/views/showPrivateKey.js

@@ -0,0 +1,71 @@
+const { $, showView, showFlash } = require("./helpers");
+const { state } = require("../../shared/state");
+const { decryptWithPassword } = require("../../shared/vault");
+const { getPrivateKeyForAddress } = require("../../shared/wallet");
+
+let ctx = null;
+
+function show() {
+    $("privkey-password").value = "";
+    $("privkey-error").textContent = "";
+    $("privkey-error").classList.add("hidden");
+    $("privkey-result").classList.add("hidden");
+    $("privkey-result-key").textContent = "";
+    $("privkey-form").classList.remove("hidden");
+    showView("show-private-key");
+}
+
+function init(_ctx) {
+    ctx = _ctx;
+
+    $("btn-privkey-back").addEventListener("click", () => {
+        // Clear displayed key on navigation away
+        $("privkey-result-key").textContent = "";
+        ctx.showAddressDetail();
+    });
+
+    $("btn-privkey-reveal").addEventListener("click", async () => {
+        const pw = $("privkey-password").value;
+        if (!pw) {
+            $("privkey-error").textContent = "Please enter your password.";
+            $("privkey-error").classList.remove("hidden");
+            return;
+        }
+
+        const wallet = state.wallets[state.selectedWallet];
+        const addrIndex = state.selectedAddress;
+
+        let decryptedSecret;
+        try {
+            decryptedSecret = await decryptWithPassword(
+                wallet.encryptedSecret,
+                pw,
+            );
+        } catch (_e) {
+            $("privkey-error").textContent = "Wrong password.";
+            $("privkey-error").classList.remove("hidden");
+            return;
+        }
+
+        const privateKey = getPrivateKeyForAddress(
+            wallet,
+            addrIndex,
+            decryptedSecret,
+        );
+
+        // Do NOT log the private key
+        $("privkey-form").classList.add("hidden");
+        $("privkey-result-key").textContent = privateKey;
+        $("privkey-result").classList.remove("hidden");
+    });
+
+    $("btn-privkey-copy").addEventListener("click", () => {
+        const key = $("privkey-result-key").textContent;
+        if (key) {
+            navigator.clipboard.writeText(key);
+            showFlash("Copied!");
+        }
+    });
+}
+
+module.exports = { init, show };

src/shared/wallet.js

@@ -45,11 +45,25 @@ function isValidMnemonic(mnemonic) {
     return Mnemonic.isValidMnemonic(mnemonic);
 }
 
+function getPrivateKeyForAddress(walletData, addrIndex, decryptedSecret) {
+    if (walletData.type === "hd") {
+        const node = HDNodeWallet.fromPhrase(
+            decryptedSecret,
+            "",
+            BIP44_ETH_PATH,
+        );
+        return node.deriveChild(addrIndex).privateKey;
+    }
+    // For imported wallets, the decrypted secret IS the private key
+    return decryptedSecret;
+}
+
 module.exports = {
     generateMnemonic,
     deriveAddressFromXpub,
     hdWalletFromMnemonic,
     addressFromPrivateKey,
     getSignerForAddress,
+    getPrivateKeyForAddress,
     isValidMnemonic,
 };