Function determineAlgorithm

src/security/encryption-dict.ts:127–163

* Determine the encryption algorithm from version and revision.

(
  version: EncryptionVersion,
  revision: EncryptionRevision,
  cryptFilters?: Map<string, CryptFilter>,
  streamFilter?: string,
)

 * Determine the encryption algorithm from version and revision.
 */
function determineAlgorithm(
  version: EncryptionVersion,
  revision: EncryptionRevision,
  cryptFilters?: Map<string, CryptFilter>,
  streamFilter?: string,
): EncryptionAlgorithm {
  // R5-R6 always use AES-256
  if (revision >= 5) {
    return "AES-256";
  }

  // V4 with crypt filters - check the stream filter
  if (version === 4 && cryptFilters && streamFilter) {
    const filter = cryptFilters.get(streamFilter);

    if (filter) {
      switch (filter.cfm) {
        case "AESV3":
          return "AES-256";
        case "AESV2":
          return "AES-128";
        case "V2":
          return "RC4";
        case "None":
          return "RC4"; // Identity, but we still need a default
      }
    }
  }

  // V4 without explicit filter typically means AES-128
  if (version === 4) {
    return "AES-128";
  }

  // V1-V3 use RC4
  return "RC4";
}

/**
 * Parse and validate the encryption version.