feat: add S/MIME store for managing key records and public certificates

- Implemented Zustand store for S/MIME functionality, including state management for key records and public certificates.
- Added methods for importing PKCS#12 files and public certificates, binding identities to keys, and managing unlocked keys.
- Introduced session storage for remembering unlocked keys across sessions.
- Enhanced error handling and loading states during data operations.

lib/smime/pkcs12-export.ts

@@ -0,0 +1,157 @@
+import * as asn1js from 'asn1js';
+import * as pkijs from 'pkijs';
+import { decryptPrivateKeyBytes } from './pkcs12-import';
+import type { SmimeKeyRecord } from './types';
+
+function stringToArrayBuffer(str: string): ArrayBuffer {
+  const buf = new ArrayBuffer(str.length);
+  const view = new Uint8Array(buf);
+  for (let i = 0; i < str.length; i++) {
+    view[i] = str.charCodeAt(i);
+  }
+  return buf;
+}
+
+/**
+ * Export an S/MIME key record as a PKCS#12 (.p12) file.
+ *
+ * Flow:
+ *  1. Decrypt the stored PKCS#8 private key bytes using the storage passphrase.
+ *  2. Build a PKCS#12 container with the private key, leaf cert, and chain.
+ *  3. Protect the PKCS#12 with the export passphrase.
+ *  4. Return the resulting bytes for browser download.
+ */
+export async function exportPkcs12(
+  record: SmimeKeyRecord,
+  storagePassphrase: string,
+  exportPassphrase: string,
+): Promise<ArrayBuffer> {
+  // Step 1: Decrypt the stored private key
+  const pkcs8Bytes = await decryptPrivateKeyBytes(record, storagePassphrase);
+
+  // Step 2: Parse the leaf certificate
+  const leafCertAsn1 = asn1js.fromBER(record.certificate);
+  if (leafCertAsn1.offset === -1) {
+    throw new Error('Failed to parse leaf certificate');
+  }
+  const leafCert = new pkijs.Certificate({ schema: leafCertAsn1.result });
+
+  // Parse chain certificates
+  const chainCerts = record.certificateChain.map((chainDer) => {
+    const chainAsn1 = asn1js.fromBER(chainDer);
+    if (chainAsn1.offset === -1) {
+      throw new Error('Failed to parse chain certificate');
+    }
+    return new pkijs.Certificate({ schema: chainAsn1.result });
+  });
+
+  const passwordBuf = stringToArrayBuffer(exportPassphrase);
+
+  // Step 3: Build the PKCS#12 structure
+  // Create key bag
+  const keyBag = new pkijs.PKCS8ShroudedKeyBag({
+    parsedValue: pkijs.PrivateKeyInfo.fromBER(pkcs8Bytes),
+  });
+
+  await keyBag.makeInternalValues({
+    password: passwordBuf,
+    contentEncryptionAlgorithm: {
+      name: 'AES-CBC',
+      length: 256,
+    } as unknown as Parameters<typeof keyBag.makeInternalValues>[0]['contentEncryptionAlgorithm'],
+    hmacHashAlgorithm: 'SHA-256',
+    iterationCount: 100_000,
+  });
+
+  const keyBagSafe = new pkijs.SafeBag({
+    bagId: '1.2.840.113549.1.12.10.1.2', // pkcs8ShroudedKeyBag
+    bagValue: keyBag,
+    bagAttributes: [
+      new pkijs.Attribute({
+        type: '1.2.840.113549.1.9.20', // friendlyName
+        values: [new asn1js.BmpString({ value: record.email })],
+      }),
+    ],
+  });
+
+  // Create cert bags
+  const certBags = [
+    new pkijs.SafeBag({
+      bagId: '1.2.840.113549.1.12.10.1.3', // certBag
+      bagValue: new pkijs.CertBag({
+        parsedValue: leafCert,
+      }),
+      bagAttributes: [
+        new pkijs.Attribute({
+          type: '1.2.840.113549.1.9.20',
+          values: [new asn1js.BmpString({ value: record.email })],
+        }),
+      ],
+    }),
+    ...chainCerts.map(
+      (cert) =>
+        new pkijs.SafeBag({
+          bagId: '1.2.840.113549.1.12.10.1.3',
+          bagValue: new pkijs.CertBag({
+            parsedValue: cert,
+          }),
+        }),
+    ),
+  ];
+
+  // Build authenticated safe with two SafeContents:
+  // 1. Key bag (password-encrypted)
+  // 2. Cert bags (unencrypted)
+  const authenticatedSafe = new pkijs.AuthenticatedSafe({
+    parsedValue: {
+      safeContents: [
+        {
+          privacyMode: 0, // no extra encryption — key bag is already shrouded
+          value: new pkijs.SafeContents({
+            safeBags: [keyBagSafe],
+          }),
+        },
+        {
+          privacyMode: 0,
+          value: new pkijs.SafeContents({
+            safeBags: certBags,
+          }),
+        },
+      ],
+    },
+  });
+
+  await authenticatedSafe.makeInternalValues({
+    safeContents: [{}, {}],
+  });
+
+  const pfx = new pkijs.PFX({
+    parsedValue: {
+      integrityMode: 0,
+      authenticatedSafe,
+    },
+  });
+
+  await pfx.makeInternalValues({
+    password: passwordBuf,
+    iterations: 100_000,
+    pbkdf2HashAlgorithm: 'SHA-256',
+    hmacHashAlgorithm: 'SHA-256',
+  });
+
+  // Step 4: Serialize to DER
+  return pfx.toSchema().toBER(false);
+}
+
+/** Trigger a browser download of the PKCS#12 file. */
+export function downloadPkcs12(p12Bytes: ArrayBuffer, filename: string): void {
+  const blob = new Blob([p12Bytes], { type: 'application/x-pkcs12' });
+  const url = URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = url;
+  a.download = filename;
+  document.body.appendChild(a);
+  a.click();
+  document.body.removeChild(a);
+  URL.revokeObjectURL(url);
+}