seekctr

cipher.Stream does not implement io.Seeker despite XOR stream ciphers being seekable.

Usage

package main

import (
	"fmt"
	"io"
	"log"
	"os"

	"github.com/uhthomas/seekctr"
)

func main() {
	// your key and initialization vector
	var key, iv []byte
	// open the encrypted file or stream
	f, err := os.Open("encrypted file")
	if err != nil {
		log.Fatal(err)
	}
	r, err := seekctr.NewReader(f, key, iv)
	if err != nil {
		log.Fatal(err)
	}
	// Seek past the first 1Kb
	if _, err := r.Seek(1 << 10); err != nil {
		log.Fatal(err)
	}
	// copy the remaining contents to stdout
	if _, err := io.Copy(os.Stdout, r); err != nil {
		log.Fatal(err)
	}
}

Note

Recreating the original stream cipher with a new initialization vector (where iv += offset / block size) and discarding the remaining bytes (offset % block size) may be preferrable.

var key, iv [16]byte

b, err := aes.NewCipher(key[:])
if err != nil { ... }

offset := uint64(4 << 10)

// offset in chunks
chunks := uint64(int(offset) / b.BlockSize())

// iv += offset
var c uint16
for i := len(iv[:]) - 1; i >= 0; i-- {
	c = uint16(iv[i]) + uint16(chunks & 0xFF) + c
	iv[i], c, chunks = byte(c), c >> 8, chunks >> 8
}

// Reinitialize cipher
s := cipher.NewCTR(b, iv[:])

// Discard n bytes
d := make([]byte, int(offset) % b.BlockSize())
s.XORKeyStream(d, d)