@noble/curves/src/utils.ts

Audited & minimal JS implementation of elliptic curve cryptography

/**
 * Hex, bytes and number utilities.
 * @module
 */
/*! noble-curves - MIT License (c) 2022 Paul Miller (paulmillr.com) */
import {
  abytes as abytes_,
  anumber as anumber_,
  bytesToHex as bytesToHex_,
  concatBytes as concatBytes_,
  hexToBytes as hexToBytes_,
  isBytes as isBytes_,
  randomBytes as randomBytes_,
} from '@noble/hashes/utils.js';
/**
 * Bytes API type helpers for old + new TypeScript.
 *
 * TS 5.6 has `Uint8Array`, while TS 5.9+ made it generic `Uint8Array<ArrayBuffer>`.
 * We can't use specific return type, because TS 5.6 will error.
 * We can't use generic return type, because most TS 5.9 software will expect specific type.
 *
 * Maps typed-array input leaves to broad forms.
 * These are compatibility adapters, not ownership guarantees.
 *
 * - `TArg` keeps byte inputs broad.
 * - `TRet` marks byte outputs for TS 5.6 and TS 5.9+ compatibility.
 */
export type TypedArg<T> = T extends BigInt64Array
  ? BigInt64Array
  : T extends BigUint64Array
    ? BigUint64Array
    : T extends Float32Array
      ? Float32Array
      : T extends Float64Array
        ? Float64Array
        : T extends Int16Array
          ? Int16Array
          : T extends Int32Array
            ? Int32Array
            : T extends Int8Array
              ? Int8Array
              : T extends Uint16Array
                ? Uint16Array
                : T extends Uint32Array
                  ? Uint32Array
                  : T extends Uint8ClampedArray
                    ? Uint8ClampedArray
                    : T extends Uint8Array
                      ? Uint8Array
                      : never;
/** Maps typed-array output leaves to narrow TS-compatible forms. */
export type TypedRet<T> = T extends BigInt64Array
  ? ReturnType<typeof BigInt64Array.of>
  : T extends BigUint64Array
    ? ReturnType<typeof BigUint64Array.of>
    : T extends Float32Array
      ? ReturnType<typeof Float32Array.of>
      : T extends Float64Array
        ? ReturnType<typeof Float64Array.of>
        : T extends Int16Array
          ? ReturnType<typeof Int16Array.of>
          : T extends Int32Array
            ? ReturnType<typeof Int32Array.of>
            : T extends Int8Array
              ? ReturnType<typeof Int8Array.of>
              : T extends Uint16Array
                ? ReturnType<typeof Uint16Array.of>
                : T extends Uint32Array
                  ? ReturnType<typeof Uint32Array.of>
                  : T extends Uint8ClampedArray
                    ? ReturnType<typeof Uint8ClampedArray.of>
                    : T extends Uint8Array
                      ? ReturnType<typeof Uint8Array.of>
                      : never;
/** Recursively adapts byte-carrying API input types. See {@link TypedArg}. */
export type TArg<T> =
  | T
  | ([TypedArg<T>] extends [never]
      ? T extends (...args: infer A) => infer R
        ? ((...args: { [K in keyof A]: TRet<A[K]> }) => TArg<R>) & {
            [K in keyof T]: T[K] extends (...args: any) => any ? T[K] : TArg<T[K]>;
          }
        : T extends [infer A, ...infer R]
          ? [TArg<A>, ...{ [K in keyof R]: TArg<R[K]> }]
          : T extends readonly [infer A, ...infer R]
            ? readonly [TArg<A>, ...{ [K in keyof R]: TArg<R[K]> }]
            : T extends (infer A)[]
              ? TArg<A>[]
              : T extends readonly (infer A)[]
                ? readonly TArg<A>[]
                : T extends Promise<infer A>
                  ? Promise<TArg<A>>
                  : T extends object
                    ? { [K in keyof T]: TArg<T[K]> }
                    : T
      : TypedArg<T>);
/** Recursively adapts byte-carrying API output types. See {@link TypedArg}. */
export type TRet<T> = T extends unknown
  ? T &
      ([TypedRet<T>] extends [never]
        ? T extends (...args: infer A) => infer R
          ? ((...args: { [K in keyof A]: TArg<A[K]> }) => TRet<R>) & {
              [K in keyof T]: T[K] extends (...args: any) => any ? T[K] : TRet<T[K]>;
            }
          : T extends [infer A, ...infer R]
            ? [TRet<A>, ...{ [K in keyof R]: TRet<R[K]> }]
            : T extends readonly [infer A, ...infer R]
              ? readonly [TRet<A>, ...{ [K in keyof R]: TRet<R[K]> }]
              : T extends (infer A)[]
                ? TRet<A>[]
                : T extends readonly (infer A)[]
                  ? readonly TRet<A>[]
                  : T extends Promise<infer A>
                    ? Promise<TRet<A>>
                    : T extends object
                      ? { [K in keyof T]: TRet<T[K]> }
                      : T
        : TypedRet<T>)
  : never;
/**
 * Validates that a value is a byte array.
 * @param value - Value to validate.
 * @param length - Optional exact byte length.
 * @param title - Optional field name.
 * @returns Original byte array.
 * @example
 * Reject non-byte input before passing data into curve code.
 *
 * ```ts
 * abytes(new Uint8Array(1));
 * ```
 */
export const abytes = <T extends TArg<Uint8Array>>(value: T, length?: number, title?: string): T =>
  abytes_(value, length, title) as T;
/**
 * Validates that a value is a non-negative safe integer.
 * @param n - Value to validate.
 * @param title - Optional field name.
 * @example
 * Validate a numeric length before allocating buffers.
 *
 * ```ts
 * anumber(1);
 * ```
 */
export const anumber: typeof anumber_ = anumber_;
/**
 * Encodes bytes as lowercase hex.
 * @param bytes - Bytes to encode.
 * @returns Lowercase hex string.
 * @example
 * Serialize bytes as hex for logging or fixtures.
 *
 * ```ts
 * bytesToHex(Uint8Array.of(1, 2, 3));
 * ```
 */
export const bytesToHex: typeof bytesToHex_ = bytesToHex_;
/**
 * Concatenates byte arrays.
 * @param arrays - Byte arrays to join.
 * @returns Concatenated bytes.
 * @example
 * Join domain-separated chunks into one buffer.
 *
 * ```ts
 * concatBytes(Uint8Array.of(1), Uint8Array.of(2));
 * ```
 */
export const concatBytes = (...arrays: TArg<Uint8Array[]>): TRet<Uint8Array> =>
  concatBytes_(...arrays) as TRet<Uint8Array>;
/**
 * Decodes lowercase or uppercase hex into bytes.
 * @param hex - Hex string to decode.
 * @returns Decoded bytes.
 * @example
 * Parse fixture hex into bytes before hashing.
 *
 * ```ts
 * hexToBytes('0102');
 * ```
 */
export const hexToBytes = (hex: string): TRet<Uint8Array> => hexToBytes_(hex) as TRet<Uint8Array>;
/**
 * Checks whether a value is a Uint8Array.
 * @param a - Value to inspect.
 * @returns `true` when `a` is a Uint8Array.
 * @example
 * Branch on byte input before decoding it.
 *
 * ```ts
 * isBytes(new Uint8Array(1));
 * ```
 */
export const isBytes: typeof isBytes_ = isBytes_;
/**
 * Reads random bytes from the platform CSPRNG.
 * @param bytesLength - Number of random bytes to read.
 * @returns Fresh random bytes.
 * @example
 * Generate a random seed for a keypair.
 *
 * ```ts
 * randomBytes(2);
 * ```
 */
export const randomBytes = (bytesLength?: number): TRet<Uint8Array> =>
  randomBytes_(bytesLength) as TRet<Uint8Array>;
const _0n = /* @__PURE__ */ BigInt(0);
const _1n = /* @__PURE__ */ BigInt(1);

/** Callable hash interface with metadata and optional extendable output support. */
export type CHash = {
  /**
   * Hash one message.
   * @param message - Message bytes to hash.
   * @returns Digest bytes.
   */
  (message: TArg<Uint8Array>): TRet<Uint8Array>;
  /** Hash block length in bytes. */
  blockLen: number;
  /** Default output length in bytes. */
  outputLen: number;
  /** Whether `.create()` can be used as an XOF stream. */
  canXOF: boolean;
  /**
   * Create one stateful hash or XOF instance, for example SHAKE with a custom output length.
   * @param opts - Optional extendable-output configuration:
   *   - `dkLen` (optional): Optional output length for XOF-style hashes.
   * @returns Hash instance.
   */
  create(opts?: { dkLen?: number }): any;
};
/** Plain callable hash interface. */
export type FHash = (message: TArg<Uint8Array>) => TRet<Uint8Array>;
/** HMAC callback signature. */
export type HmacFn = (key: TArg<Uint8Array>, message: TArg<Uint8Array>) => TRet<Uint8Array>;
/**
 * Validates that a flag is boolean.
 * @param value - Value to validate.
 * @param title - Optional field name.
 * @returns Original value.
 * @throws On wrong argument types. {@link TypeError}
 * @example
 * Reject non-boolean option flags early.
 *
 * ```ts
 * abool(true);
 * ```
 */
export function abool(value: boolean, title: string = ''): boolean {
  if (typeof value !== 'boolean') {
    const prefix = title && `"${title}" `;
    throw new TypeError(prefix + 'expected boolean, got type=' + typeof value);
  }
  return value;
}

/**
 * Validates that a value is a non-negative bigint or safe integer.
 * @param n - Value to validate.
 * @returns The same validated value.
 * @throws On wrong argument ranges or values. {@link RangeError}
 * @example
 * Validate one integer-like value before serializing it.
 *
 * ```ts
 * abignumber(1n);
 * ```
 */
export function abignumber<T extends number | bigint>(n: T): T {
  if (typeof n === 'bigint') {
    if (!isPosBig(n)) throw new RangeError('positive bigint expected, got ' + n);
  } else anumber(n);
  return n;
}

/**
 * Validates that a value is a safe integer.
 * @param value - Integer to validate.
 * @param title - Optional field name.
 * @throws On wrong argument types. {@link TypeError}
 * @throws On wrong argument ranges or values. {@link RangeError}
 * @example
 * Validate a window size before scalar arithmetic uses it.
 *
 * ```ts
 * asafenumber(1);
 * ```
 */
export function asafenumber(value: number, title: string = ''): void {
  if (typeof value !== 'number') {
    const prefix = title && `"${title}" `;
    throw new TypeError(prefix + 'expected number, got type=' + typeof value);
  }
  if (!Number.isSafeInteger(value)) {
    const prefix = title && `"${title}" `;
    throw new RangeError(prefix + 'expected safe integer, got ' + value);
  }
}

/**
 * Encodes a bigint into even-length big-endian hex.
 * The historical "unpadded" name only means "no fixed-width field padding"; odd-length hex still
 * gets one leading zero nibble so the result always represents whole bytes.
 * @param num - Number to encode.
 * @returns Big-endian hex string.
 * @throws On wrong argument ranges or values. {@link RangeError}
 * @example
 * Encode a scalar into hex without a `0x` prefix.
 *
 * ```ts
 * numberToHexUnpadded(255n);
 * ```
 */
export function numberToHexUnpadded(num: number | bigint): string {
  const hex = abignumber(num).toString(16);
  return hex.length & 1 ? '0' + hex : hex;
}

/**
 * Parses a big-endian hex string into bigint.
 * Accepts odd-length hex through the native `BigInt('0x' + hex)` parser and currently surfaces the
 * same native `SyntaxError` for malformed hex instead of wrapping it in a library-specific error.
 * @param hex - Hex string without `0x`.
 * @returns Parsed bigint value.
 * @throws On wrong argument types. {@link TypeError}
 * @example
 * Parse a scalar from fixture hex.
 *
 * ```ts
 * hexToNumber('ff');
 * ```
 */
export function hexToNumber(hex: string): bigint {
  if (typeof hex !== 'string') throw new TypeError('hex string expected, got ' + typeof hex);
  return hex === '' ? _0n : BigInt('0x' + hex); // Big Endian
}

// BE: Big Endian, LE: Little Endian
/**
 * Parses big-endian bytes into bigint.
 * @param bytes - Bytes in big-endian order.
 * @returns Parsed bigint value.
 * @throws On wrong argument types. {@link TypeError}
 * @example
 * Read a scalar encoded in network byte order.
 *
 * ```ts
 * bytesToNumberBE(Uint8Array.of(1, 0));
 * ```
 */
export function bytesToNumberBE(bytes: TArg<Uint8Array>): bigint {
  return hexToNumber(bytesToHex_(bytes));
}
/**
 * Parses little-endian bytes into bigint.
 * @param bytes - Bytes in little-endian order.
 * @returns Parsed bigint value.
 * @throws On wrong argument types. {@link TypeError}
 * @example
 * Read a scalar encoded in little-endian form.
 *
 * ```ts
 * bytesToNumberLE(Uint8Array.of(1, 0));
 * ```
 */
export function bytesToNumberLE(bytes: TArg<Uint8Array>): bigint {
  return hexToNumber(bytesToHex_(copyBytes(abytes_(bytes)).reverse()));
}

/**
 * Encodes a bigint into fixed-length big-endian bytes.
 * @param n - Number to encode.
 * @param len - Output length in bytes. Must be greater than zero.
 * @returns Big-endian byte array.
 * @throws On wrong argument ranges or values. {@link RangeError}
 * @example
 * Serialize a scalar into a 32-byte field element.
 *
 * ```ts
 * numberToBytesBE(255n, 2);
 * ```
 */
export function numberToBytesBE(n: number | bigint, len: number): TRet<Uint8Array> {
  anumber_(len);
  if (len === 0) throw new RangeError('zero length');
  n = abignumber(n);
  const hex = n.toString(16);
  // Detect overflow before hex parsing so oversized values don't leak the shared odd-hex error.
  if (hex.length > len * 2) throw new RangeError('number too large');
  return hexToBytes_(hex.padStart(len * 2, '0')) as TRet<Uint8Array>;
}
/**
 * Encodes a bigint into fixed-length little-endian bytes.
 * @param n - Number to encode.
 * @param len - Output length in bytes.
 * @returns Little-endian byte array.
 * @throws On wrong argument ranges or values. {@link RangeError}
 * @example
 * Serialize a scalar for little-endian protocols.
 *
 * ```ts
 * numberToBytesLE(255n, 2);
 * ```
 */
export function numberToBytesLE(n: number | bigint, len: number): TRet<Uint8Array> {
  return numberToBytesBE(n, len).reverse() as TRet<Uint8Array>;
}
// Unpadded, rarely used
/**
 * Encodes a bigint into variable-length big-endian bytes.
 * @param n - Number to encode.
 * @returns Variable-length big-endian bytes.
 * @throws On wrong argument ranges or values. {@link RangeError}
 * @example
 * Serialize a bigint without fixed-width padding.
 *
 * ```ts
 * numberToVarBytesBE(255n);
 * ```
 */
export function numberToVarBytesBE(n: number | bigint): TRet<Uint8Array> {
  return hexToBytes_(numberToHexUnpadded(abignumber(n))) as TRet<Uint8Array>;
}

// Compares 2 u8a-s in kinda constant time
/**
 * Compares two byte arrays in constant-ish time.
 * @param a - Left byte array.
 * @param b - Right byte array.
 * @returns `true` when bytes match.
 * @example
 * Compare two encoded points without early exit.
 *
 * ```ts
 * equalBytes(Uint8Array.of(1), Uint8Array.of(1));
 * ```
 */
export function equalBytes(a: TArg<Uint8Array>, b: TArg<Uint8Array>): boolean {
  a = abytes(a);
  b = abytes(b);
  if (a.length !== b.length) return false;
  let diff = 0;
  for (let i = 0; i < a.length; i++) diff |= a[i] ^ b[i];
  return diff === 0;
}

/**
 * Copies Uint8Array. We can't use u8a.slice(), because u8a can be Buffer,
 * and Buffer#slice creates mutable copy. Never use Buffers!
 * @param bytes - Bytes to copy.
 * @returns Detached copy.
 * @example
 * Make an isolated copy before mutating serialized bytes.
 *
 * ```ts
 * copyBytes(Uint8Array.of(1, 2, 3));
 * ```
 */
export function copyBytes(bytes: TArg<Uint8Array>): TRet<Uint8Array> {
  // `Uint8Array.from(...)` would also accept arrays / other typed arrays. Keep this helper strict
  // because callers use it at byte-validation boundaries before mutating the detached copy.
  return Uint8Array.from(abytes(bytes)) as TRet<Uint8Array>;
}

/**
 * Decodes 7-bit ASCII string to Uint8Array, throws on non-ascii symbols
 * Should be safe to use for things expected to be ASCII.
 * Returns exact same result as `TextEncoder` for ASCII or throws.
 * @param ascii - ASCII input text.
 * @returns Encoded bytes.
 * @throws On wrong argument types. {@link TypeError}
 * @example
 * Encode an ASCII domain-separation tag.
 *
 * ```ts
 * asciiToBytes('ABC');
 * ```
 */
export function asciiToBytes(ascii: string): TRet<Uint8Array> {
  if (typeof ascii !== 'string') throw new TypeError('ascii string expected, got ' + typeof ascii);
  return Uint8Array.from(ascii, (c, i) => {
    const charCode = c.charCodeAt(0);
    if (c.length !== 1 || charCode > 127) {
      throw new RangeError(
        `string contains non-ASCII character "${ascii[i]}" with code ${charCode} at position ${i}`
      );
    }
    return charCode;
  }) as TRet<Uint8Array>;
}

// Historical name: this accepts non-negative bigints, including zero.
const isPosBig = (n: bigint) => typeof n === 'bigint' && _0n <= n;

/**
 * Checks whether a bigint lies inside a half-open range.
 * @param n - Candidate value.
 * @param min - Inclusive lower bound.
 * @param max - Exclusive upper bound.
 * @returns `true` when the value is inside the range.
 * @example
 * Check whether a candidate scalar fits the field order.
 *
 * ```ts
 * inRange(2n, 1n, 3n);
 * ```
 */
export function inRange(n: bigint, min: bigint, max: bigint): boolean {
  return isPosBig(n) && isPosBig(min) && isPosBig(max) && min <= n && n < max;
}

/**
 * Asserts `min <= n < max`. NOTE: upper bound is exclusive.
 * @param title - Value label for error messages.
 * @param n - Candidate value.
 * @param min - Inclusive lower bound.
 * @param max - Exclusive upper bound.
 * Wrong-type inputs are not separated from out-of-range values here: they still flow through the
 * shared `RangeError` path because this is only a throwing wrapper around `inRange(...)`.
 * @throws On wrong argument ranges or values. {@link RangeError}
 * @example
 * Assert that a bigint stays within one half-open range.
 *
 * ```ts
 * aInRange('x', 2n, 1n, 256n);
 * ```
 */
export function aInRange(title: string, n: bigint, min: bigint, max: bigint): void {
  // Why min <= n < max and not a (min < n < max) OR b (min <= n <= max)?
  // consider P=256n, min=0n, max=P
  // - a for min=0 would require -1:          `inRange('x', x, -1n, P)`
  // - b would commonly require subtraction:  `inRange('x', x, 0n, P - 1n)`
  // - our way is the cleanest:               `inRange('x', x, 0n, P)
  if (!inRange(n, min, max))
    throw new RangeError('expected valid ' + title + ': ' + min + ' <= n < ' + max + ', got ' + n);
}

// Bit operations

/**
 * Calculates amount of bits in a bigint.
 * Same as `n.toString(2).length`
 * TODO: merge with nLength in modular
 * @param n - Value to inspect.
 * @returns Bit length.
 * @throws If the value is negative. {@link Error}
 * @example
 * Measure the bit length of a scalar before serialization.
 *
 * ```ts
 * bitLen(8n);
 * ```
 */
export function bitLen(n: bigint): number {
  // Size callers in this repo only use non-negative orders / scalars, so negative inputs are a
  // contract bug and must not silently collapse to zero bits.
  if (n < _0n) throw new Error('expected non-negative bigint, got ' + n);
  let len;
  for (len = 0; n > _0n; n >>= _1n, len += 1);
  return len;
}

/**
 * Gets single bit at position.
 * NOTE: first bit position is 0 (same as arrays)
 * Same as `!!+Array.from(n.toString(2)).reverse()[pos]`
 * @param n - Source value.
 * @param pos - Bit position. Negative positions are passed through to raw
 *   bigint shift semantics; because the mask is built as `1n << pos`,
 *   they currently collapse to `0n` and make the helper a no-op.
 * @returns Bit as bigint.
 * @example
 * Gets single bit at position.
 *
 * ```ts
 * bitGet(5n, 0);
 * ```
 */
export function bitGet(n: bigint, pos: number): bigint {
  return (n >> BigInt(pos)) & _1n;
}

/**
 * Sets single bit at position.
 * @param n - Source value.
 * @param pos - Bit position. Negative positions are passed through to raw bigint shift semantics,
 *   so they currently behave like left shifts.
 * @param value - Whether the bit should be set.
 * @returns Updated bigint.
 * @example
 * Sets single bit at position.
 *
 * ```ts
 * bitSet(0n, 1, true);
 * ```
 */
export function bitSet(n: bigint, pos: number, value: boolean): bigint {
  const mask = _1n << BigInt(pos);
  // Clearing needs AND-not here; OR with zero leaves an already-set bit untouched.
  return value ? n | mask : n & ~mask;
}

/**
 * Calculate mask for N bits. Not using ** operator with bigints because of old engines.
 * Same as BigInt(`0b${Array(i).fill('1').join('')}`)
 * @param n - Number of bits. Negative widths are currently passed through to raw bigint shift
 *   semantics and therefore produce `-1n`.
 * @returns Bitmask value.
 * @example
 * Calculate mask for N bits.
 *
 * ```ts
 * bitMask(4);
 * ```
 */
export const bitMask = (n: number): bigint => (_1n << BigInt(n)) - _1n;

// DRBG

type Pred<T> = (v: TArg<Uint8Array>) => T | undefined;
/**
 * Minimal HMAC-DRBG from NIST 800-90 for RFC6979 sigs.
 * @param hashLen - Hash output size in bytes. Callers are expected to pass a positive length; `0`
 *   is not rejected here and would make the internal generate loop non-progressing.
 * @param qByteLen - Requested output size in bytes. Callers are expected to pass a positive length.
 * @param hmacFn - HMAC implementation.
 * @returns Function that will call DRBG until the predicate returns anything
 *   other than `undefined`.
 * @throws On wrong argument types. {@link TypeError}
 * @example
 * Build a deterministic nonce generator for RFC6979-style signing.
 *
 * ```ts
 * import { createHmacDrbg } from '@noble/curves/utils.js';
 * import { hmac } from '@noble/hashes/hmac.js';
 * import { sha256 } from '@noble/hashes/sha2.js';
 * const drbg = createHmacDrbg(32, 32, (key, msg) => hmac(sha256, key, msg));
 * const seed = new Uint8Array(32);
 * drbg(seed, (bytes) => bytes);
 * ```
 */
export function createHmacDrbg<T>(
  hashLen: number,
  qByteLen: number,
  hmacFn: TArg<HmacFn>
): TRet<(seed: Uint8Array, predicate: Pred<T>) => T> {
  anumber_(hashLen, 'hashLen');
  anumber_(qByteLen, 'qByteLen');
  if (typeof hmacFn !== 'function') throw new TypeError('hmacFn must be a function');
  // creates Uint8Array
  const u8n = (len: number): TRet<Uint8Array> => new Uint8Array(len) as TRet<Uint8Array>;
  const NULL = Uint8Array.of();
  const byte0 = Uint8Array.of(0x00);
  const byte1 = Uint8Array.of(0x01);
  const _maxDrbgIters = 1000;

  // Step B, Step C: set hashLen to 8*ceil(hlen/8).
  // Minimal non-full-spec HMAC-DRBG from NIST 800-90 for RFC6979 signatures.
  let v: Uint8Array = u8n(hashLen);
  // Steps B and C of RFC6979 3.2.
  let k: Uint8Array = u8n(hashLen);
  let i = 0; // Iterations counter, will throw when over 1000
  const reset = () => {
    v.fill(1);
    k.fill(0);
    i = 0;
  };
  // hmac(k)(v, ...values)
  const h = (...msgs: TArg<Uint8Array[]>) => (hmacFn as HmacFn)(k, concatBytes(v, ...msgs));
  const reseed = (seed: TArg<Uint8Array> = NULL) => {
    // HMAC-DRBG reseed() function. Steps D-G
    k = h(byte0, seed); // k = hmac(k || v || 0x00 || seed)
    v = h(); // v = hmac(k || v)
    if (seed.length === 0) return;
    k = h(byte1, seed); // k = hmac(k || v || 0x01 || seed)
    v = h(); // v = hmac(k || v)
  };
  const gen = () => {
    // HMAC-DRBG generate() function
    if (i++ >= _maxDrbgIters) throw new Error('drbg: tried max amount of iterations');
    let len = 0;
    const out: Uint8Array[] = [];
    while (len < qByteLen) {
      v = h();
      const sl = v.slice();
      out.push(sl);
      len += v.length;
    }
    return concatBytes(...out);
  };
  const genUntil = (seed: TArg<Uint8Array>, pred: TArg<Pred<T>>): T => {
    reset();
    reseed(seed); // Steps D-G
    let res: T | undefined = undefined; // Step H: grind until the predicate accepts a candidate.
    // Falsy values like 0 are valid outputs.
    while ((res = (pred as Pred<T>)(gen())) === undefined) reseed();
    reset();
    return res;
  };
  return genUntil as TRet<(seed: Uint8Array, predicate: Pred<T>) => T>;
}

/**
 * Validates declared required and optional field types on a plain object.
 * Extra keys are intentionally ignored because many callers validate only the subset they use from
 * richer option bags or runtime objects.
 * @param object - Object to validate.
 * @param fields - Required field types.
 * @param optFields - Optional field types.
 * @throws On wrong argument types. {@link TypeError}
 * @example
 * Check user options before building a curve helper.
 *
 * ```ts
 * validateObject({ flag: true }, { flag: 'boolean' });
 * ```
 */
export function validateObject(
  object: Record<string, any>,
  fields: Record<string, string> = {},
  optFields: Record<string, string> = {}
): void {
  if (Object.prototype.toString.call(object) !== '[object Object]')
    throw new TypeError('expected valid options object');
  type Item = keyof typeof object;
  function checkField(fieldName: Item, expectedType: string, isOpt: boolean) {
    // Config/data fields must be explicit own properties, but runtime objects such as Field
    // instances intentionally satisfy required method slots via their shared prototype.
    if (!isOpt && expectedType !== 'function' && !Object.hasOwn(object, fieldName))
      throw new TypeError(`param "${fieldName}" is invalid: expected own property`);
    const val = object[fieldName];
    if (isOpt && val === undefined) return;
    const current = typeof val;
    if (current !== expectedType || val === null)
      throw new TypeError(
        `param "${fieldName}" is invalid: expected ${expectedType}, got ${current}`
      );
  }
  const iter = (f: typeof fields, isOpt: boolean) =>
    Object.entries(f).forEach(([k, v]) => checkField(k, v, isOpt));
  iter(fields, false);
  iter(optFields, true);
}

/**
 * Throws not implemented error.
 * @returns Never returns.
 * @throws If the unfinished code path is reached. {@link Error}
 * @example
 * Surface the placeholder error from an unfinished code path.
 *
 * ```ts
 * try {
 *   notImplemented();
 * } catch {}
 * ```
 */
export const notImplemented = (): never => {
  throw new Error('not implemented');
};

/** Generic keygen/getPublicKey interface shared by curve helpers. */
export interface CryptoKeys {
  /** Public byte lengths for keys and optional seeds. */
  lengths: { seed?: number; public?: number; secret?: number };
  /**
   * Generate one secret/public keypair.
   * @param seed - Optional seed bytes for deterministic key generation.
   * @returns Fresh secret/public keypair.
   */
  keygen: (seed?: Uint8Array) => { secretKey: Uint8Array; publicKey: Uint8Array };
  /**
   * Derive one public key from a secret key.
   * @param secretKey - Secret key bytes.
   * @returns Public key bytes.
   */
  getPublicKey: (secretKey: Uint8Array) => Uint8Array;
}

/** Generic interface for signatures. Has keygen, sign and verify. */
export interface Signer extends CryptoKeys {
  // Interfaces are fun. We cannot just add new fields without copying old ones.
  /** Public byte lengths for keys, signatures, and optional signing randomness. */
  lengths: {
    seed?: number;
    public?: number;
    secret?: number;
    signRand?: number;
    signature?: number;
  };
  /**
   * Sign one message.
   * @param msg - Message bytes to sign.
   * @param secretKey - Secret key bytes.
   * @returns Signature bytes.
   */
  sign: (msg: Uint8Array, secretKey: Uint8Array) => Uint8Array;
  /**
   * Verify one signature.
   * @param sig - Signature bytes.
   * @param msg - Signed message bytes.
   * @param publicKey - Public key bytes.
   * @returns `true` when the signature is valid.
   */
  verify: (sig: Uint8Array, msg: Uint8Array, publicKey: Uint8Array) => boolean;
}