package symm

import (
	"crypto/cipher"
	"errors"
)

var (
	ErrSegmentNotFullBlocks = errors.New("ciphertext segment is not a full block size")
)

func decryptECBBlocks(block cipher.Block, src []byte) ([]byte, error) {
	if len(src) == 0 {
		return []byte{}, nil
	}
	if len(src)%block.BlockSize() != 0 {
		return nil, ErrSegmentNotFullBlocks
	}
	out := make([]byte, len(src))
	ecbDecryptBlocks(block, out, src)
	return out, nil
}

// decryptCBCFromSecondBlock decrypts a CBC ciphertext segment that starts from the second block (or later).
// prevCipherBlock must be the previous ciphertext block (C[i-1]); for i=1 this is the original IV.
func decryptCBCFromSecondBlock(block cipher.Block, src, prevCipherBlock []byte) ([]byte, error) {
	if len(src) == 0 {
		return []byte{}, nil
	}
	if len(prevCipherBlock) != block.BlockSize() {
		return nil, errors.New("prev cipher block length must match block size")
	}
	if len(src)%block.BlockSize() != 0 {
		return nil, ErrSegmentNotFullBlocks
	}
	out := make([]byte, len(src))
	cipher.NewCBCDecrypter(block, prevCipherBlock).CryptBlocks(out, src)
	return out, nil
}

// decryptCFBFromSecondBlock decrypts a CFB ciphertext segment that starts from the second block (or later).
// prevCipherBlock must be the previous ciphertext block (C[i-1]); for i=1 this is the original IV.
func decryptCFBFromSecondBlock(block cipher.Block, src, prevCipherBlock []byte) ([]byte, error) {
	if len(src) == 0 {
		return []byte{}, nil
	}
	if len(prevCipherBlock) != block.BlockSize() {
		return nil, errors.New("prev cipher block length must match block size")
	}
	stream := cipher.NewCFBDecrypter(block, prevCipherBlock)
	out := make([]byte, len(src))
	stream.XORKeyStream(out, src)
	return out, nil
}