/**
 * Encoding of OAuth client credentials for the HTTP Basic `Authorization` header.
 *
 * @module auth/client-credentials
 */
/**
 * Builds the base64 payload for an `Authorization: Basic` header carrying OAuth
 * client credentials, per RFC 6749 §2.3.1.
 *
 * The client identifier and client password are each form-url-encoded (RFC 6749
 * Appendix B) *before* being joined with a colon and Base64-encoded (RFC 7617
 * §2, the HTTP Basic scheme). Skipping the per-component encoding corrupts any
 * credential containing characters the server form-url-decodes: a raw `+` is
 * read as a space and a valid `%xx` escape is decoded to another byte (both
 * yield `invalid_client` -- a wrong-but-valid secret), while an invalid escape
 * such as `%zz` makes a strict decoder error out (`unauthorized_client` /
 * "Unexpected error when authenticating client"). The same credential succeeds
 * when sent in the request body, which is form-url-encoded by construction --
 * the "body works, Basic fails" symptom.
 *
 * @param clientId - The OAuth client identifier
 * @param clientSecret - The OAuth client password/secret
 * @returns The Base64 string to place after `Basic ` in the `Authorization` header
 * @see https://datatracker.ietf.org/doc/html/rfc6749#section-2.3.1
 * @see https://datatracker.ietf.org/doc/html/rfc6749#appendix-B
 */
export declare function encodeBasicClientCredentials(clientId: string, clientSecret: string): string;