/**
 * Signature – HMAC-SHA256 cryptographic signature. Immutable value object.
 *
 * @remarks
 * Used by the envelope to sign payload + metadata; verification is responsibility of the SDK.
 */
import type { SignatureAlgorithm } from '../types.js';
export type SignatureVO = {
    readonly value: string;
    readonly algorithm: SignatureAlgorithm;
};
/**
 * Creates HMAC-SHA256 signature of the given content.
 *
 * @param content - String to sign (e.g. JSON.stringify(payload + metadata))
 * @param secretKey - Secret key for HMAC (must not be logged or serialized)
 * @returns Frozen Signature value object
 */
export declare function createSignature(content: string, secretKey: string): SignatureVO;
/**
 * Verifies that a signature matches the content (constant-time comparison should be used in production).
 */
export declare function verifySignature(content: string, signature: string, secretKey: string): boolean;
/**
 * Validates signature format (64 hex chars for HMAC-SHA256).
 */
export declare function isValidSignatureFormat(signature: string): boolean;
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