/**
 * PKCE (Proof Key for Code Exchange) utilities for secure authorization flows.
 * Implements RFC 7636 using the Web Crypto API for cryptographic operations.
 */
/**
 * Generates a cryptographically random code verifier for PKCE.
 * The verifier is a Base64URL-encoded string of 32 random bytes (43 characters).
 *
 * @returns A random code verifier string.
 * @throws `Error` - If cryptographic random generation fails (requires secure context/HTTPS).
 *
 * @example
 * ```typescript
 * const verifier = generateCodeVerifier();
 * // e.g., "dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk"
 * ```
 */
export declare function generateCodeVerifier(): string;
/**
 * Generates a code challenge from a code verifier using SHA-256.
 * The challenge is the Base64URL-encoded SHA-256 hash of the verifier.
 *
 * @param verifier - The code verifier to hash.
 * @returns A promise that resolves with the code challenge string.
 *
 * @example
 * ```typescript
 * const verifier = generateCodeVerifier();
 * const challenge = await generateCodeChallenge(verifier);
 * // Send challenge to server, keep verifier secret
 * ```
 */
export declare function generateCodeChallenge(verifier: string): Promise<string>;
/**
 * Generates a cryptographically random state parameter for OAuth flows.
 * The state is a Base64URL-encoded string of 16 random bytes (22 characters).
 *
 * Used to prevent CSRF attacks and to correlate requests with responses.
 *
 * @returns A random state string.
 * @throws `Error` - If cryptographic random generation fails (requires secure context/HTTPS).
 *
 * @example
 * ```typescript
 * const state = generateState();
 * // e.g., "xyzzy123ABC_def-GHI"
 * ```
 */
export declare function generateState(): string;
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