/**
 * Beam Shield — E2E Encryption (S5)
 * X25519 Key Agreement + ChaCha20-Poly1305 AEAD
 *
 * Flow:
 * 1. Sender generates ephemeral X25519 keypair
 * 2. ECDH(ephemeral.private, recipient.dhPublicKey) → sharedSecret
 * 3. HKDF(sharedSecret) → encryptionKey
 * 4. ChaCha20-Poly1305(key, nonce, payload) → ciphertext
 * 5. Send: { encrypted: true, ephemeralPublicKey, ciphertext, nonce }
 */
export interface X25519KeyPair {
    publicKey: string;
    privateKey: string;
}
export interface EncryptedPayload {
    encrypted: true;
    ephemeralPublicKey: string;
    ciphertext: string;
    nonce: string;
    algorithm: 'chacha20-poly1305';
}
/** Generate a new X25519 keypair for E2E encryption */
export declare function generateX25519KeyPair(): X25519KeyPair;
/** Encrypt a payload for a recipient with their X25519 DH public key */
export declare function encryptPayload(payload: Record<string, unknown>, recipientDhPublicKeyBase64: string): EncryptedPayload;
/** Decrypt a payload using own X25519 DH private key */
export declare function decryptPayload(encryptedPayload: EncryptedPayload, myDhPrivateKeyBase64: string): Record<string, unknown>;
/** Check if a payload is encrypted */
export declare function isEncryptedPayload(payload: unknown): payload is EncryptedPayload;