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