package crypto

import (
    "encoding/binary"
    "crypto/sha256"
)

func buildOneTimeKey(secretKey [32]byte, counter uint32) [32]byte {
    buf := make([]byte, 36)
    copy(buf[0:], secretKey[:])
    binary.BigEndian.PutUint32(buf[32:], counter)
    return sha256.Sum256(buf)
}

func xor32(input [32]byte, key [32]byte) [32]byte {
    var out [32]byte
    for i := 0; i < 32; i++ {
        out[i] = input[i] ^ key[i]
    }
    return out
}

func XorWithOneTimeKey(ciphertext [32]byte, secretKey [32]byte, counter uint32) [32]byte {
    ephemeralKey := buildOneTimeKey(secretKey, counter)
    return xor32(ciphertext, ephemeralKey)
}

func Sha256(data []byte) [32]byte {
    return sha256.Sum256(data)
}