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// Package hidden provides the ability to "hide" binary data in a string using
// a hex encoding with non-printing characters. Hidden data is demarcated with
// a leading and trailing NUL character.
package hidden
import (
"bytes"
"fmt"
"regexp"
"github.com/getlantern/hex"
)
// 16 non-printing characters
const hextable = "\x01\x02\x03\x04\x05\x06\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17"
var (
hexencoding = hex.NewEncoding(hextable)
re *regexp.Regexp
)
func init() {
var err error
re, err = regexp.Compile(fmt.Sprintf("\x00[%v]+\x00", hextable))
if err != nil {
panic(err)
}
}
// ToString encodes the given data as a hidden string, including leadnig and
// trailing NULs.
func ToString(data []byte) string {
buf := bytes.NewBuffer(make([]byte, 0, 2+hex.EncodedLen(len(data))))
// Leading NUL
buf.WriteByte(0)
buf.WriteString(hexencoding.EncodeToString(data))
// Trailing NUL
buf.WriteByte(0)
return buf.String()
}
// FromString extracts the hidden data from a string, which is expected to
// contain leading and trailing NULs.
func FromString(str string) ([]byte, error) {
return hexencoding.DecodeString(str[1 : len(str)-1])
}
// Extract extracts all hidden data from an arbitrary string.
func Extract(str string) ([][]byte, error) {
m := re.FindAllString(str, -1)
result := make([][]byte, 0, len(m))
for _, s := range m {
b, err := FromString(s)
if err != nil {
return nil, err
}
result = append(result, b)
}
return result, nil
}
// Clean removes any hidden data from an arbitrary string.
func Clean(str string) string {
return re.ReplaceAllString(str, "")
}
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