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path: root/zsig/header.go
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package zsig

import (
	"bufio"
	"bytes"
	"crypto/sha512"
	"encoding/hex"
	"errors"
	"fmt"
	"io"
	"strconv"
	"strings"
	"time"
)

const (
	Alg       = "SHA512/256"
	BlockSize = 65536
)

type ZHeader struct {
	Date      time.Time
	KeyFile   string
	Alg       string
	BlockSize int64
	Sums      [][]byte
}

func (h ZHeader) Print(w io.Writer) error {
	fmt.Fprintf(w, "date=%v\n", h.Date.Format(time.RFC3339))
	fmt.Fprintf(w, "key=%v\n", h.KeyFile)
	fmt.Fprintf(w, "algorithm=%v\n", h.Alg)
	fmt.Fprintf(w, "blocksize=%v\n\n", h.BlockSize)
	for _, sum := range h.Sums {
		fmt.Fprintf(w, "%x\n", sum)
	}
	return nil
}

func ParseBytes(data []byte) (ZHeader, error) {
	r := bytes.NewReader(data)
	return Parse(r)
}

func Parse(r io.Reader) (ZHeader, error) {
	var h ZHeader
	s := bufio.NewScanner(r)
	for s.Scan() {
		line := s.Text()
		switch {
		case strings.HasPrefix(line, "date="):
			t, err := time.Parse(time.RFC3339, line[5:])
			if err != nil {
				return ZHeader{}, err
			}
			h.Date = t
		case strings.HasPrefix(line, "key="):
			h.KeyFile = line[4:]
		case strings.HasPrefix(line, "algorithm="):
			h.Alg = line[10:]
		case strings.HasPrefix(line, "blocksize="):
			i, err := strconv.ParseInt(line[10:], 10, 64)
			if err != nil {
				return ZHeader{}, err
			}
			h.BlockSize = i
		case line == "":
			for s.Scan() {
				line = s.Text()
				sum, err := hex.DecodeString(line)
				if err != nil {
					return ZHeader{}, err
				}
				h.Sums = append(h.Sums, sum)
			}
		}
	}
	return h, s.Err()
}

func (z ZHeader) Verify(r io.Reader) error {
	if z.Alg != Alg {
		return errors.New("invalid hash algorithm")
	}
	h := sha512.New512_256() // from z.Alg
	blockCount := len(z.Sums)
	for _, sum := range z.Sums {
		h.Reset()
		n, err := io.CopyN(h, r, z.BlockSize)
		if n == 0 && err == io.EOF {
			break
		}
		if err != nil && err != io.EOF {
			return err
		}
		if !bytes.Equal(sum, h.Sum(nil)) {
			return errors.New("sum mismatch")
		}
		blockCount--
	}
	if blockCount != 0 {
		return errors.New("count mismatch")
	}
	return nil
}

func NewHeader(r io.Reader) (ZHeader, error) {
	hdr := ZHeader{
		Date:      time.Now(),
		Alg:       Alg,
		BlockSize: BlockSize,
	}
	h := sha512.New512_256()
	for {
		h.Reset()
		n, err := io.CopyN(h, r, hdr.BlockSize)
		if n == 0 && err == io.EOF {
			break
		}
		if err != nil && err != io.EOF {
			return ZHeader{}, err
		}
		hdr.Sums = append(hdr.Sums, h.Sum(nil))
	}
	return hdr, nil
}