Function computeURc4_40

computeURc4_40(
    userPw: ByteArray,
    oValue: ByteArray,
    permissions: number,
    fileId: ByteArray,
): Promise<ByteArray>
Computes the /U value for RC4-40 PDF encryption. The /U value is used to verify the user password.
Parameters
- userPw: ByteArray
  The user password.
- oValue: ByteArray
  The /O value.
- permissions: number
  The /P value (permissions flags).
- fileId: ByteArray
  The first element of the /ID array.
Returns Promise<ByteArray>
A promise that resolves to the 32-byte /U value.
Example
```
const U = await computeURc4_40(userPassword, O, permissions, fileId)
```
- Defined in packages/pdf-lite/src/crypto/key-gen/key-gen-rc4-40.ts:116