/**
 * Encrypts a plaintext payload using an Azure Key Vault key (A256CBC) and returns
 * the result as a base64url-encoded string.
 * The initialization vector is read from a Key Vault secret.
 *
 * @param keyURL - The URL of the Azure Key Vault instance
 * @param keyName - The name of the key to use for encryption
 * @param payload - The plaintext string to encrypt
 * @param ivSecretName - The name of the secret containing the initialization vector
 * @returns A promise resolving to the base64url-encoded ciphertext
 */
export declare function encryptData(keyURL: string, keyName: string, payload: string, ivSecretName: string): Promise<string>;