/**
 * RFC5054 SRP group parameters.
 * These must match the server-side implementation exactly.
 */

import { SRPGroup } from './types';

/**
 * Get the SRP group parameters for the specified group ID.
 *
 * @param groupID - Group ID (3, 4, or 5)
 * @returns SRP group parameters
 * @throws Error if group ID is invalid
 *
 * @risk Spoofing: Weak group parameters allow offline attacks.
 * Always use RFC5054 standard groups.
 */
export function getGroup(groupID: number): SRPGroup {
  switch (groupID) {
    case 3: // RFC5054 2048-bit group (recommended minimum).
      return {
        N: BigInt(
          '0x' +
          'AC6BDB41324A9A9BF166DE5E1389582FAF72B6651987EE07FC3192943DB56050' +
          'A37329CBB4A099ED8193E0757767A13DD52312AB4B03310DCD7F48A9DA04FD50' +
          'E8083969EDB767B0CF6095179A163AB3661A05FBD5FAAAE82918A9962F0B93B8' +
          '55F97993EC975EEAA80D740ADBF4FF747359D041D5C33EA71D281E446B14773B' +
          'CA97B43A23FB801676BD207A436C6481F1D2B9078717461A5B9D32E688F87748' +
          '544523B524B0D57D5EA77A2775D2ECFA032CFBDBF52FB3786160279004E57AE6' +
          'AF874E7303CE53299CCC041C7BC308D82A5698F3A8D0C38271AE35F8E9DBFBB6' +
          '94B5C803D89F7AE435DE236D525F54759B65E372FCD68EF20FA7111F9E4AFF73'
        ),
        g: BigInt(2),
        bitLength: 2048
      }
    case 4: //RFC5054 3072-bit group (stronger security, higher CPU cost).
      return {
        N: BigInt(
          '0x' +
          'FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74' +
          '020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F1437' +
          '4FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED' +
          'EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3DC2007CB8A163BF05' +
          '98DA48361C55D39A69163FA8FD24CF5F83655D23DCA3AD961C62F356208552BB' +
          '9ED529077096966D670C354E4ABC9804F1746C08CA18217C32905E462E36CE3B' +
          'E39E772C180E86039B2783A2EC07A28FB5C55DF06F4C52C9DE2BCBF695581718' +
          '3995497CEA956AE515D2261898FA051015728E5A8AAAC42DAD33170D04507A33' +
          'A85521ABDF1CBA64ECFB850458DBEF0A8AEA71575D060C7DB3970F85A6E1E4C7' +
          'ABF5AE8CDB0933D71E8C94E04A25619DCEE3D2261AD2EE6BF12FFA06D98A0864' +
          'D87602733EC86A64521F2B18177B200CBBE117577A615D6C770988C0BAD946E2' +
          '08E24FA074E5AB3143DB5BFCE0FD108E4B82D120A93AD2CAFFFFFFFFFFFFFFFF'
        ),
        g: BigInt(5),
        bitLength: 3072
      }
    case 5: // RFC5054 4096-bit group (strongest security, significant CPU cost).
      return {
        N: BigInt(
          '0x' +
          'FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74' +
          '020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F1437' +
          '4FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED' +
          'EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3DC2007CB8A163BF05' +
          '98DA48361C55D39A69163FA8FD24CF5F83655D23DCA3AD961C62F356208552BB' +
          '9ED529077096966D670C354E4ABC9804F1746C08CA18217C32905E462E36CE3B' +
          'E39E772C180E86039B2783A2EC07A28FB5C55DF06F4C52C9DE2BCBF695581718' +
          '3995497CEA956AE515D2261898FA051015728E5A8AAAC42DAD33170D04507A33' +
          'A85521ABDF1CBA64ECFB850458DBEF0A8AEA71575D060C7DB3970F85A6E1E4C7' +
          'ABF5AE8CDB0933D71E8C94E04A25619DCEE3D2261AD2EE6BF12FFA06D98A0864' +
          'D87602733EC86A64521F2B18177B200CBBE117577A615D6C770988C0BAD946E2' +
          '08E24FA074E5AB3143DB5BFCE0FD108E4B82D120A92108011A723C12A787E6D7' +
          '88719A10BDBA5B2699C327186AF4E23C1A946834B6150BDA2583E9CA2AD44CE8' +
          'DBBBC2DB04DE8EF92E8EFC141FBECAA6287C59474E6BC05D99B2964FA090C3A2' +
          '233BA186515BE7ED1F612970CEE2D7AFB81BDD762170481CD0069127D5B05AA9' +
          '93B4EA988D8FDDC186FFB7DC90A6C08F4DF435C934063199FFFFFFFFFFFFFFFF'
        ),
        g: BigInt(5),
        bitLength: 4096
      }
    default:
      throw new Error(`Invalid group ID: ${groupID} (valid: 3, 4, 5)`);
  }
}


/**
 * Compute the SRP multiplier parameter k = H(N | g).
 *
 * @param group - SRP group parameters
 * @returns Multiplier k
 *
 * @mitigation Tampering: Correct k computation per RFC5054/SRP-6a.
 */
export async function computeK(group: SRPGroup): Promise<bigint> {
  // k = H(N | PAD(g))
  const NBytes = bigIntToBytes(group.N);
  const gBytes = bigIntToBytes(group.g);

  // Pad g to the same length as N
  const paddedG = new Uint8Array(NBytes.length);
  paddedG.set(gBytes, paddedG.length - gBytes.length);

  // Concatenate N and paddedG
  const combined = new Uint8Array(NBytes.length + paddedG.length);
  combined.set(NBytes, 0);
  combined.set(paddedG, NBytes.length);

  // Hash with SHA-256
  const hashBuffer = await crypto.subtle.digest('SHA-256', combined);
  const hashBytes = new Uint8Array(hashBuffer);

  return bytesToBigInt(hashBytes);
}

/**
 * Convert a bigint to a Uint8Array (big-endian).
 */
export function bigIntToBytes(value: bigint): Uint8Array {
  const hex = value.toString(16);
  const paddedHex = hex.length % 2 === 0 ? hex : '0' + hex;
  const bytes = new Uint8Array(paddedHex.length / 2);

  for (let i = 0; i < bytes.length; i++) {
    bytes[i] = parseInt(paddedHex.substr(i * 2, 2), 16);
  }

  return bytes;
}

/**
 * Convert a Uint8Array to a bigint (big-endian).
 */
export function bytesToBigInt(bytes: Uint8Array): bigint {
  let hex = '';
  for (let i = 0; i < bytes.length; i++) {
    const byte = bytes[i].toString(16).padStart(2, '0');
    hex += byte;
  }
  return BigInt('0x' + hex);
}

/**
 * Pad bytes to a specific length with leading zeros.
 */
export function padBytes(bytes: Uint8Array, length: number): Uint8Array {
  if (bytes.length >= length) {
    return bytes;
  }

  const padded = new Uint8Array(length);
  padded.set(bytes, length - bytes.length);
  return padded;
}