/**
 * Best-effort runtime identification used for SDK metadata/user-agent strings.
 *
 * The function prefers browser-style detection via `navigator.userAgent` and
 * falls back to Node.js `process.version` when available.
 *
 * @returns A runtime identifier string, or `"<unknown-runtime>"` when not detectable.
 */
export declare function getRuntime(): string;
/**
 * Derives JWT `kid` from a private key using an RFC 7638 JWK thumbprint.
 *
 * Expected key format:
 * - PKCS#8 PEM-encoded private key
 * - EC P-521 curve (matches ES512 signing)
 *
 * Steps:
 * 1. Decode PEM to PKCS#8 DER bytes.
 * 2. Import key through Web Crypto.
 * 3. Export as JWK and validate required fields.
 * 4. Hash the canonical RFC 7638 thumbprint input with SHA-256.
 * 5. Return base64url encoded digest.
 *
 * @param privateKey PEM-encoded PKCS#8 EC private key.
 * @returns The base64url-encoded JWK thumbprint used as JWT `kid`.
 * @throws If key parsing/import/export/validation/digest fails.
 */
export declare function getKeyId(privateKey: string): Promise<string>;
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