stegtext 0.1.1

//! Text [steganography][] tool.
//!
//! Hide secret value in [UTF-8][] text using [homoglyphs][].
//!
//! See also: [IDN homograph attack][].
//!
//! # Examples
//!
//! Hide secret in text:
//!
//! ```
//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
//! // secret
//! const SECRET: [u8; 5] = *b"hello";
//!
//! // input text
//! const TEXT: &str = "\
//! Тhe Нitсhhikеr's Guіdе to thе Galaxy is a 1979 ѕcіеncе fiсtion соmedy
//! novеl bу Englіѕh author Douglаѕ Adаmѕ, adарtеd frоm the fіrѕt four parts
//! of hiѕ radiо соmedу ѕеriеѕ оf thе sаmе name.";
//!
//! // expected output
//! const EXP: &str = "\
//! Тhe Нitсhhikеr's Guide to the Galaxy is a 1979 ѕcіеncе fiсtion соmedy
//! novеl bу Englіѕh author Douglаѕ Adаmѕ, adарtеd frоm the fіrѕt four parts
//! of hiѕ radiо соmedу ѕеriеѕ оf thе sаmе name.";
//!
//! // encode secret, check result
//! assert_eq!(stegtext::encode(&SECRET, TEXT)?, EXP);
//! # Ok(())
//! # }
//! ```
//!
//! Get secret from text:
//!
//! ```
//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
//! // text with embedded secret
//! const TEXT: &str = "\
//! Тhe Нitсhhikеr's Guide to the Galaxy is a 1979 ѕcіеncе fiсtion соmedy
//! novеl bу Englіѕh author Douglаѕ Adаmѕ, adарtеd frоm the fіrѕt four parts
//! of hiѕ radiо соmedу ѕеriеѕ оf thе sаmе name.";
//!
//! // decode secret, check result
//! assert_eq!(stegtext::decode(TEXT)?, "hello");
//! # Ok(())
//! # }
//! ```
//!
//! Get text embedding capacity:
//!
//! ```
//! # fn main() {
//! // input text
//! const TEXT: &str = "\
//! Тhe Нitсhhikеr's Guіdе to thе Galaxy is a 1979 ѕcіеncе fiсtion соmedy
//! novеl bу Englіѕh author Douglаѕ Adаmѕ, adарtеd frоm the fіrѕt four parts
//! of hiѕ radiо соmedу ѕеriеѕ оf thе sаmе name. Іt centrеѕ оn the
//! misаdvеntureѕ оf Arthur Dent, the оnly man tо ѕurvive the destruction of
//! Earth, as he roams the cosmos and learns the truth behind his very
//! existence. The novel's namesake is an in-universe electronic travel
//! guide written in the form of an encyclopaedia, through which the story
//! is framed.";
//!
//! // calculate capacity, check result
//! assert_eq!(stegtext::capacity(TEXT), 23);
//! # }
//! ```
//!
//! # Technical Details
//!
//! Encoding process:
//!
//! 1. The first prefix byte is constructed.  The lower 4 bits of the
//!    first prefix byte are the are a constant tag (`0x5`) and the
//!    upper 4 bits of the first prefix byte are bits `0..4` of the
//!    length of the secret.
//! 2. The second prefix byte is constructed from bits `4..12` of the
//!    length of the secret.
//! 3. The secret bytes are appended to the prefix bytes to create a
//!    payload.
//! 4. The total number of payload bits are calculated.
//! 5. The input text is scanned for characters that can be replaced
//!    by [homoglyphs][].
//! 6. Non-matching characters are passed through to the output
//!    unaltered.
//! 7. When a matching character is encountered it is modified
//!    based on the value of the current payload bit:
//!    - if the current payload bit is **set**, then the character
//!      is replaced with a non-Latin [homoglyph][] character.  For
//!      example, [`'A'` (`U+0041`)][a-l] is replaced with [`'А'` (`U+0410`)][a-c],
//!      [`'B'` (`U+0042`)][b-l] is replaced by [`'В'` (`U+0412`)][b-c],
//!      and so on.
//!    - if the current payload bit is **not set**, then the character
//!      is replaced with a Latin character.
//! 8. The current payload bit position is incremented.
//! 9. Steps 5-8 are repeated until the current payload bit position
//!    equals the total number of payload bits.
//! 10. If payload bits remain when the end of the input text is
//!     reached, then [`Err::Capacity`] is returned.
//! 11. Once all the payload bits have been encoded, the remaining
//!     characters from the input text are passed through to the output
//!     unaltered.
//!
//! [steganography]: https://en.wikipedia.org/wiki/Steganography
//!   "Steganography (Wikipedia)"
//! [utf-8]: https://en.wikipedia.org/wiki/UTF-8
//!   "UTF-8 (Wikipedia)"
//! [homoglyph]: https://en.wikipedia.org/wiki/Homoglyph
//!   "Homoglyph (Wikipedia)"
//! [homoglyphs]: https://en.wikipedia.org/wiki/Homoglyph
//!   "Homoglyph (Wikipedia)"
//! [idn homograph attack]: https://en.wikipedia.org/wiki/IDN_homograph_attack#Cyrillic
//!   "IDN homograph attack (Wikipedia)"
//! [a-l]: https://en.wikipedia.org/wiki/A
//!   "A (Latin)"
//! [a-c]: https://en.wikipedia.org/wiki/A_(Cyrillic)
//!   "A (Cyrillic)"
//! [b-l]: https://en.wikipedia.org/wiki/B
//!   "B (Latin)"
//! [b-c]: https://en.wikipedia.org/wiki/B_(Cyrillic)
//!   "B (Cyrillic)"

#![deny(missing_docs)]
#![deny(unsafe_code)]

// TODO:
// - ci/cd
// - doc: tests
// - doc: linting
// - doc: coverage
// - doc: fuzzing
// - api doc example: errors
// - api doc example: encode_vec
// - api doc example: decode_vec

use std::io::Write;

/// Prefix tag.
///
/// Lower 4 bits of first decoded byte.  Used to identify text with
/// embedded steganographic data.
const TAG: u8 = 0x5;

/// Maximum secret length.
///
/// Note: encoded in prefix bytes as a 12-bit integer.
const MAX_SECRET_LEN: usize = 4095;

/// Encode/decode error.
#[derive(Clone, Debug, PartialEq)]
pub enum Err {
  /// Secret length is greater than 4095 bytes.
  SecretLen(usize),

  /// Not enough text capacity to encode secret.
  Capacity,

  /// Write to output buffer failed.
  Write,

  /// UTF-8 encoding error.
  Encoding(std::string::FromUtf8Error),

  /// Invalid tag in encoded text.
  BadTag(u8),

  /// Truncated secret.
  TruncatedSecret(usize, usize),
}

impl std::error::Error for Err {}

impl std::fmt::Display for Err {
  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
    match self {
      Err::SecretLen(len) => write!(f, "secret length greater than {MAX_SECRET_LEN} bytes: {len}"),
      Err::Capacity => write!(f, "not enough text capacity to encode secret"),
      Err::Write => write!(f, "buffer write failed"),
      Err::Encoding(err) => write!(f, "encode error: {err}"),
      Err::BadTag(val) => write!(f, "invalid tag: got 0x{val:x}, expected 0x{TAG:x}"),
      Err::TruncatedSecret(got, exp) => write!(f, "truncated secret: got {got} bytes, expected {exp} bytes"),
    }
  }
}

/// Does this character have a homoglyph?
fn is_encodable(c: char) -> bool {
  matches!(c, 'A' | 'B' | 'C' | 'E' | 'H' | 'I' | 'J' | 'K' | 'M' | 'O' | 'P' | 'S' | 'T' | 'X' | 'a' | 'c' | 'e' | 'i' | 'j' | 'o' | 'p' | 'q' | 's' | 'w' | 'x' | 'y' | 'А' | 'В' | 'С' | 'Е' | 'Н' | 'І' | 'Ј' | 'К' | 'М' | 'О' | 'Р' | 'Ѕ' | 'Т' | 'Х' | 'а' | 'с' | 'е' | 'і' | 'ј' | 'о' | 'р' | 'ԛ' | 'ѕ' | 'ԝ' | 'х' | 'у')
}

/// Encode bit value in character.
fn encode_char(c: char, bit: bool) -> char {
  match (c, bit) {
    ('A' | 'А', true) => 'А',
    ('A' | 'А', false) => 'A',
    ('B' | 'В', true) => 'В',
    ('B' | 'В', false) => 'B',
    ('C' | 'С', true) => 'С',
    ('C' | 'С', false) => 'C',
    ('E' | 'Е', true) => 'Е',
    ('E' | 'Е', false) => 'E',
    ('H' | 'Н', true) => 'Н',
    ('H' | 'Н', false) => 'H',
    ('I' | 'І', true) => 'І',
    ('I' | 'І', false) => 'I',
    ('J' | 'Ј', true) => 'Ј',
    ('J' | 'Ј', false) => 'J',
    ('K' | 'К', true) => 'К',
    ('K' | 'К', false) => 'K',
    ('M' | 'М', true) => 'М',
    ('M' | 'М', false) => 'M',
    ('O' | 'О', true) => 'О',
    ('O' | 'О', false) => 'O',
    ('P' | 'Р', true) => 'Р',
    ('P' | 'Р', false) => 'P',
    ('S' | 'Ѕ', true) => 'Ѕ',
    ('S' | 'Ѕ', false) => 'S',
    ('T' | 'Т', true) => 'Т',
    ('T' | 'Т', false) => 'T',
    ('X' | 'Х', true) => 'Х',
    ('X' | 'Х', false) => 'X',
    ('a' | 'а', true) => 'а',
    ('a' | 'а', false) => 'a',
    ('c' | 'с', true) => 'с',
    ('c' | 'с', false) => 'c',
    ('e' | 'е', true) => 'е',
    ('e' | 'е', false) => 'e',
    ('i' | 'і', true) => 'і',
    ('i' | 'і', false) => 'i',
    ('j' | 'ј', true) => 'ј',
    ('j' | 'ј', false) => 'j',
    ('o' | 'о', true) => 'о',
    ('o' | 'о', false) => 'o',
    ('p' | 'р', true) => 'р',
    ('p' | 'р', false) => 'p',
    ('q' | 'ԛ', true) => 'ԛ',
    ('q' | 'ԛ', false) => 'q',
    ('s' | 'ѕ', true) => 'ѕ',
    ('s' | 'ѕ', false) => 's',
    ('w' | 'ԝ', true) => 'ԝ',
    ('w' | 'ԝ', false) => 'w',
    ('x' | 'х', true) => 'х',
    ('x' | 'х', false) => 'x',
    ('y' | 'у', true) => 'у',
    ('y' | 'у', false) => 'y',
    _ => c,
  }
}

/// Hide secret in text and return result as byte vector.
pub fn encode_vec(secret: &[u8], text: &str) -> Result<Vec<u8>, Err> {
  // check secret length
  if secret.len() > MAX_SECRET_LEN {
    return Err(Err::SecretLen(secret.len()));
  }

  // build tag byte and encoded secret
  let tag_byte = (((secret.len() & 0xf) << 4) as u8) | TAG;
  let mut src = vec![tag_byte, (secret.len() >> 4) as u8];
  src.extend(secret);

  let mut src_pos: usize = 0; // bit position
  let src_bit_len = src.len() << 3; // src len, in bits

  // build output
  let mut dst = Vec::new();
  for c in text.chars() {
    let c = if src_pos < src_bit_len && is_encodable(c) {
      let bit = ((src[src_pos >> 3] >> (src_pos & 7)) & 1) == 1;
      src_pos += 1;
      encode_char(c, bit)
    } else {
      c // pass character through
    };

    // write character to output buffer
    match write!(dst, "{c}") {
      Ok(_) => {},
      Err(_) => return Err(Err::Write),
    }
  }

  // were all secret bits encoded?
  if src_pos != src_bit_len {
    return Err(Err::Capacity)
  }

  Ok(dst)
}

/// Hide secret in text and return result as string.
///
/// # Example
///
/// ```
/// # fn main() -> Result<(), Box<dyn std::error::Error>> {
/// // secret
/// const SECRET: [u8; 5] = *b"hello";
///
/// // input text
/// const TEXT: &str = "\
/// Тhe Нitсhhikеr's Guіdе to thе Galaxy is a 1979 ѕcіеncе fiсtion соmedy
/// novеl bу Englіѕh author Douglаѕ Adаmѕ, adарtеd frоm the fіrѕt four parts
/// of hiѕ radiо соmedу ѕеriеѕ оf thе sаmе name.";
///
/// // expected output
/// const EXP: &str = "\
/// Тhe Нitсhhikеr's Guide to the Galaxy is a 1979 ѕcіеncе fiсtion соmedy
/// novеl bу Englіѕh author Douglаѕ Adаmѕ, adарtеd frоm the fіrѕt four parts
/// of hiѕ radiо соmedу ѕеriеѕ оf thе sаmе name.";
///
/// // encode secret, check result
/// assert_eq!(stegtext::encode(&SECRET, TEXT)?, EXP);
/// # Ok(())
/// # }
/// ```
pub fn encode(secret: &[u8], text: &str) -> Result<String, Err> {
  match String::from_utf8(encode_vec(secret, text)?) {
    Ok(s) => Ok(s),
    Err(err) => Err(Err::Encoding(err)),
  }
}

/// Get embedding capacity, in bytes.
///
/// # Example
///
/// ```
/// # fn main() {
/// // input text
/// const TEXT: &str = "\
/// Тhe Нitсhhikеr's Guіdе to thе Galaxy is a 1979 ѕcіеncе fiсtion соmedy
/// novеl bу Englіѕh author Douglаѕ Adаmѕ, adарtеd frоm the fіrѕt four parts
/// of hiѕ radiо соmedу ѕеriеѕ оf thе sаmе name. Іt centrеѕ оn the
/// misаdvеntureѕ оf Arthur Dent, the оnly man tо ѕurvive the destruction of
/// Earth, as he roams the cosmos and learns the truth behind his very
/// existence. The novel's namesake is an in-universe electronic travel
/// guide written in the form of an encyclopaedia, through which the story
/// is framed.";
///
/// // get capacity, check result
/// assert_eq!(stegtext::capacity(TEXT), 23);
/// # }
/// ```
pub fn capacity(text: &str) -> usize {
  let len = text.chars().filter(|c| is_encodable(*c)).count();
  if len > 16 {
    // truncate to multiple of 8, subtract two bytes for tag and length
    (len / 8) - 2
  } else {
    0
  }
}

/// Maybe decode bit from character.
fn decode_char(c: char) -> Option<bool> {
  match c {
    'A' | 'B' | 'C' | 'E' | 'H' | 'I' | 'J' | 'K' | 'M' | 'O' | 'P' | 'S' | 'T' | 'X' | 'a' | 'c' | 'e' | 'i' | 'j' | 'o' | 'p' | 'q' | 's' | 'w' | 'x' | 'y' => Some(false), // latin
    'А' | 'В' | 'С' | 'Е' | 'Н' | 'І' | 'Ј' | 'К' | 'М' | 'О' | 'Р' | 'Ѕ' | 'Т' | 'Х' | 'а' | 'с' | 'е' | 'і' | 'ј' | 'о' | 'р' | 'ԛ' | 'ѕ' | 'ԝ' | 'х' | 'у' => Some(true), // cyrillic
    _ => None,
  }
}

/// Extract secret from text and return result as byte vector.
pub fn decode_vec(text: &str) -> Result<Vec<u8>, Err> {
  let mut pos: usize = 0; // current bit
  let mut byte: u8 = 0; // current byte
  let mut exp_len: usize = 0; // expected output length

  let mut dst = Vec::new(); // output vector
  for c in text.chars() {
    if let Some(bit) = decode_char(c) {
      byte |= u8::from(bit) << pos;
      pos += 1; // advance

      if pos == 8 {
        dst.push(byte); // add byte to output
        byte = 0; // clear byte
        pos = 0; // clear position

        match (exp_len, dst.len()) {
          (0, 2) => {
            // get tag and expected length
            let tag = dst[0] & 0xf;
            exp_len = ((dst[0] >> 4) as usize) | ((dst[1] as usize) << 8);

            // check tag and length
            if tag != TAG {
              return Err(Err::BadTag(tag));
            }

            dst.clear(); // clear vector
            if exp_len == 0 {
              return Ok(dst);
            }
          },
          (_, dst_len) if dst_len == exp_len => break,
          (_, _) => {},
        };
      }
    }
  }

  // check decoded secret length
  if dst.len() == exp_len {
    Ok(dst)
  } else {
    Err(Err::TruncatedSecret(dst.len(), exp_len))
  }
}

/// Extract secret from text and return result as string.
///
/// ```
/// // Text with hidden secret.
/// const TEXT: &str = "\
/// Тhe Нitсhhikеr's Guide to the Galaxy is a 1979 ѕcіеncе fiсtion соmedy
/// novеl bу Englіѕh author Douglаѕ Adаmѕ, adарtеd frоm the fіrѕt four parts
/// of hiѕ radiо соmedу ѕеriеѕ оf thе sаmе name.";
///
/// # fn main() -> Result<(), Box<dyn std::error::Error>> {
/// // decode secret, check result
/// assert_eq!(stegtext::decode(TEXT)?, "hello");
/// # Ok(())
/// # }
/// ```
pub fn decode(text: &str) -> Result<String, Err> {
  match String::from_utf8(decode_vec(text)?) {
    Ok(s) => Ok(s),
    Err(err) => Err(Err::Encoding(err)),
  }
}

#[cfg(test)]
mod tests {
  use super::*;

  #[test]
  fn test_err_fmt() {
    let bad_utf8 = vec![0x80u8];
    let from_err = String::from_utf8(bad_utf8).unwrap_err();
    let tests = vec![
      (Err::SecretLen(4096), "secret length greater than 4095 bytes: 4096"),
      (Err::Capacity, "not enough text capacity to encode secret"),
      (Err::Write, "buffer write failed"),
      (Err::Encoding(from_err), "encode error: invalid utf-8 sequence of 1 bytes from index 0"),
      (Err::BadTag(0), "invalid tag: got 0x0, expected 0x5"),
      (Err::TruncatedSecret(0, 1), "truncated secret: got 0 bytes, expected 1 bytes"),
    ];

    for (err, exp) in tests {
      assert_eq!(err.to_string(), exp, "{err:?}");
    }
  }

  #[test]
  fn test_encode_char() {
    let tests = vec![
      ('A', true, 'А'),
      ('А', true, 'А'),
      ('A', false, 'A'),
      ('А', false, 'A'),
      ('B', true, 'В'),
      ('В', true, 'В'),
      ('B', false, 'B'),
      ('В', false, 'B'),
      ('C', true, 'С'),
      ('С', true, 'С'),
      ('C', false, 'C'),
      ('С', false, 'C'),
      ('E', true, 'Е'),
      ('Е', true, 'Е'),
      ('E', false, 'E'),
      ('Е', false, 'E'),
      ('H', true, 'Н'),
      ('Н', true, 'Н'),
      ('H', false, 'H'),
      ('Н', false, 'H'),
      ('I', true, 'І'),
      ('І', true, 'І'),
      ('I', false, 'I'),
      ('І', false, 'I'),
      ('J', true, 'Ј'),
      ('Ј', true, 'Ј'),
      ('J', false, 'J'),
      ('Ј', false, 'J'),
      ('K', true, 'К'),
      ('К', true, 'К'),
      ('K', false, 'K'),
      ('К', false, 'K'),
      ('M', true, 'М'),
      ('М', true, 'М'),
      ('M', false, 'M'),
      ('М', false, 'M'),
      ('O', true, 'О'),
      ('О', true, 'О'),
      ('O', false, 'O'),
      ('О', false, 'O'),
      ('P', true, 'Р'),
      ('Р', true, 'Р'),
      ('P', false, 'P'),
      ('Р', false, 'P'),
      ('S', true, 'Ѕ'),
      ('Ѕ', true, 'Ѕ'),
      ('S', false, 'S'),
      ('Ѕ', false, 'S'),
      ('T', true, 'Т'),
      ('Т', true, 'Т'),
      ('T', false, 'T'),
      ('Т', false, 'T'),
      ('X', true, 'Х'),
      ('Х', true, 'Х'),
      ('X', false, 'X'),
      ('Х', false, 'X'),
      ('a', true, 'а'),
      ('а', true, 'а'),
      ('a', false, 'a'),
      ('а', false, 'a'),
      ('c', true, 'с'),
      ('с', true, 'с'),
      ('c', false, 'c'),
      ('с', false, 'c'),
      ('e', true, 'е'),
      ('е', true, 'е'),
      ('e', false, 'e'),
      ('е', false, 'e'),
      ('i', true, 'і'),
      ('і', true, 'і'),
      ('i', false, 'i'),
      ('і', false, 'i'),
      ('j', true, 'ј'),
      ('ј', true, 'ј'),
      ('j', false, 'j'),
      ('ј', false, 'j'),
      ('o', true, 'о'),
      ('о', true, 'о'),
      ('o', false, 'o'),
      ('о', false, 'o'),
      ('p', true, 'р'),
      ('р', true, 'р'),
      ('p', false, 'p'),
      ('р', false, 'p'),
      ('q', true, 'ԛ'),
      ('ԛ', true, 'ԛ'),
      ('q', false, 'q'),
      ('ԛ', false, 'q'),
      ('s', true, 'ѕ'),
      ('ѕ', true, 'ѕ'),
      ('s', false, 's'),
      ('ѕ', false, 's'),
      ('w', true, 'ԝ'),
      ('ԝ', true, 'ԝ'),
      ('w', false, 'w'),
      ('ԝ', false, 'w'),
      ('x', true, 'х'),
      ('х', true, 'х'),
      ('x', false, 'x'),
      ('х', false, 'x'),
      ('y', true, 'у'),
      ('у', true, 'у'),
      ('y', false, 'y'),
      ('у', false, 'y'),

      ('z', false, 'z'),
    ];

    for (c, bit, exp) in tests {
      assert_eq!(encode_char(c, bit), exp, "{} (U+{:04x})", c, u32::from(c));
    }
  }

  #[test]
  fn test_encode_vec_fail() {
    let tests = vec![(
      "long secret", // name
      [0u8; MAX_SECRET_LEN+1].to_vec(), // secret
      "aaaaaaaa aaaaaaaa aaaaaaaa", // text
      Err(Err::SecretLen(MAX_SECRET_LEN+1)),
    ), (
      "short text", // name
      vec![0u8], // secret
      "aaaaaaaa aaaaaaaa", // text
      Err(Err::Capacity),
    )];

    for (name, msg, text, exp) in tests {
      assert_eq!(encode_vec(&msg, text), exp, "{name}");
    }
  }

  #[test]
  fn test_encode_vec_pass() {
    let tests = vec![(
      "aaa", // name
      vec![1], // secret
      "aaaaaaaa aaaaaaaa aaaaaaaa", // text
      vec![ // exp
        0xd0, 0xb0, 0x61, 0xd0, 0xb0, 0x61, 0xd0, 0xb0, 0x61, 0x61, 0x61,
        0x20,
        0x61, 0x61, 0x61, 0x61, 0x61, 0x61, 0x61, 0x61,
        0x20,
        0xd0, 0xb0, 0x61, 0x61, 0x61, 0x61, 0x61, 0x61, 0x61,
      ],
    ), (
      "acc", // name
      vec![1], // secret
      "aaaaaaaa cccccccc cccccccc", // text
      vec![ // exp
        0xd0, 0xb0, 0x61, 0xd0, 0xb0, 0x61, 0xd0, 0xb0, 0x61, 0x61, 0x61,
        0x20,
        0x63, 0x63, 0x63, 0x63, 0x63, 0x63, 0x63, 0x63,
        0x20,
        0xd1, 0x81, 0x63, 0x63, 0x63, 0x63, 0x63, 0x63, 0x63,
      ],
    ), (
      "ace", // name
      vec![1], // secret
      "aaaaaaaa cccccccc eeeeeeee", // text
      vec![ // exp
        0xd0, 0xb0, 0x61, 0xd0, 0xb0, 0x61, 0xd0, 0xb0, 0x61, 0x61, 0x61,
        0x20,
        0x63, 0x63, 0x63, 0x63, 0x63, 0x63, 0x63, 0x63,
        0x20,
        0xd0, 0xb5, 0x65, 0x65, 0x65, 0x65, 0x65, 0x65, 0x65,
      ],
    ), (
      "aop", // name
      vec![1], // secret
      "aaaaaaaa oooooooo pppppppp", // text
      vec![ // exp
        0xd0, 0xb0, 0x61, 0xd0, 0xb0, 0x61, 0xd0, 0xb0, 0x61, 0x61, 0x61,
        0x20,
        0x6f, 0x6f, 0x6f, 0x6f, 0x6f, 0x6f, 0x6f, 0x6f,
        0x20,
        0xd1, 0x80, 0x70, 0x70, 0x70, 0x70, 0x70, 0x70, 0x70,
      ],
    ), (
      "pxy", // name
      vec![1], // secret
      "pppppppp xxxxxxxx yyyyyyyy", // text
      vec![ // exp
        0xd1, 0x80, 0x70, 0xd1, 0x80, 0x70, 0xd1, 0x80, 0x70, 0x70, 0x70,
        0x20,
        0x78, 0x78, 0x78, 0x78, 0x78, 0x78, 0x78, 0x78,
        0x20,
        0xd1, 0x83, 0x79, 0x79, 0x79, 0x79, 0x79, 0x79, 0x79,
      ],
    ), (
      "ijqs", // name
      vec![1, 1], // secret
      "iiiiiiii jjjjjjjj qqqqqqqq ssssssss", // text
      vec![ // exp
        0xd1, 0x96, 0x69, 0xd1, 0x96, 0x69, 0x69, 0xd1, 0x96, 0x69, 0x69,
        0x20,
        0x6a, 0x6a, 0x6a, 0x6a, 0x6a, 0x6a, 0x6a, 0x6a,
        0x20,
        0xd4, 0x9b, 0x71, 0x71, 0x71, 0x71, 0x71, 0x71, 0x71,
        0x20,
        0xd1, 0x95, 0x73, 0x73, 0x73, 0x73, 0x73, 0x73, 0x73,
      ],
    ), (
      "wwww", // name
      vec![1, 1], // secret
      "wwwwwwww wwwwwwww wwwwwwww wwwwwwww", // text
      vec![ // exp
        0xd4, 0x9d, 0x77, 0xd4, 0x9d, 0x77, 0x77, 0xd4, 0x9d, 0x77, 0x77,
        0x20,
        0x77, 0x77, 0x77, 0x77, 0x77, 0x77, 0x77, 0x77,
        0x20,
        0xd4, 0x9d, 0x77, 0x77, 0x77, 0x77, 0x77, 0x77, 0x77,
        0x20,
        0xd4, 0x9d, 0x77, 0x77, 0x77, 0x77, 0x77, 0x77, 0x77,
      ],
    ), (
      "ABCE", // name
      vec![1, 1], // secret
      "AAAAAAAA BBBBBBBB CCCCCCCC EEEEEEEE", // text
      vec![ // exp
        0xd0, 0x90, 0x41, 0xd0, 0x90, 0x41, 0x41, 0xd0, 0x90, 0x41, 0x41,
        0x20,
        0x42, 0x42, 0x42, 0x42, 0x42, 0x42, 0x42, 0x42,
        0x20,
        0xd0, 0xa1, 0x43, 0x43, 0x43, 0x43, 0x43, 0x43, 0x43,
        0x20,
        0xd0, 0x95, 0x45, 0x45, 0x45, 0x45, 0x45, 0x45, 0x45,
      ],
    ), (
      "HIJKM", // name
      vec![1, 1, 1], // secret
      "HHHHHHHH IIIIIIII JJJJJJJJ KKKKKKKK MMMMMMMM", // text
      vec![ // exp
        0xd0, 0x9d, 0x48, 0xd0, 0x9d, 0x48, 0xd0, 0x9d, 0xd0, 0x9d, 0x48, 0x48,
        0x20,
        0x49, 0x49, 0x49, 0x49, 0x49, 0x49, 0x49, 0x49,
        0x20,
        0xd0, 0x88, 0x4a, 0x4a, 0x4a, 0x4a, 0x4a, 0x4a, 0x4a,
        0x20,
        0xd0, 0x9a, 0x4b, 0x4b, 0x4b, 0x4b, 0x4b, 0x4b, 0x4b,
        0x20,
        0xd0, 0x9c, 0x4d, 0x4d, 0x4d, 0x4d, 0x4d, 0x4d, 0x4d,
      ],
    ), (
      "OPSTX", // name
      vec![1, 1, 1], // secret
      "OOOOOOOO PPPPPPPP SSSSSSSS TTTTTTTT XXXXXXXX", // text
      vec![ // exp
        0xd0, 0x9e, 0x4f, 0xd0, 0x9e, 0x4f, 0xd0, 0x9e, 0xd0, 0x9e, 0x4f, 0x4f,
        0x20,
        0x50, 0x50, 0x50, 0x50, 0x50, 0x50, 0x50, 0x50,
        0x20,
        0xd0, 0x85, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53,
        0x20,
        0xd0, 0xa2, 0x54, 0x54, 0x54, 0x54, 0x54, 0x54, 0x54,
        0x20,
        0xd0, 0xa5, 0x58, 0x58, 0x58, 0x58, 0x58, 0x58, 0x58,
      ],
    )];

    for (name, secret, text, exp) in tests {
      assert_eq!(encode_vec(&secret, text), Ok(exp), "{name}");
    }
  }

  #[test]
  fn test_encode_fail() {
    let tests = vec![(
      "long secret", // name
      [0u8; MAX_SECRET_LEN+1].to_vec(), // secret
      "aaaaaaaa aaaaaaaa aaaaaaaa", // text
      Err(Err::SecretLen(MAX_SECRET_LEN+1)),
    ), (
      "short text", // name
      vec![0u8], // secret
      "aaaaaaaa aaaaaaaa", // text
      Err(Err::Capacity),
    )];

    for (name, msg, text, exp) in tests {
      assert_eq!(encode(&msg, text), exp, "{name}");
    }
  }

  #[test]
  fn test_encode_pass() {
    let tests = vec![(
      "aaa", // name
      b"A".to_vec(), // secret
      "aaaaaaaa aaaaaaaa aaaaaaaa", // text
      "аaаaаaaa aaaaaaaa аaaaaaаa".to_string(), // expected value
    )];

    for (name, secret, text, exp) in tests {
      assert_eq!(encode(&secret, text), Ok(exp), "{name}");
    }
  }

  #[test]
  fn test_capacity() {
    let tests = vec![
      ("", 0),
      ("aaaa", 0),
      ("aaaaaaaa aaaaaaaa", 0),
      ("aaaaaaaa aaaaaaaa aaaaaaa", 0),
      ("aaaaaaaa aaaaaaaa aaaaaaaa", 1),
      ("aaaaaaaa aaaaaaaa aaaaaaaa aa", 1),
      ("aaaaaaaa aaaaaaaa aaaaaaaa a", 1),
      ("aaaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa", 2),
      ("aaaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa ", 2),
    ];

    for (s, exp) in tests {
      assert_eq!(capacity(s), exp, "s = {s}");
    }
  }

  #[test]
  fn test_decode_vec_fail() {
    let invalid_len_str = String::from_utf8(vec![ // text
      0xd0, 0x9e, 0x4f, 0xd0, 0x9e, 0x4f, 0xd0, 0x9e, 0xd0, 0x9e, 0x4f, 0x4f,
      0x20,
      0x50, 0x50, 0x50, 0x50, 0x50, 0x50, 0x50, 0x50,
      0x20,
      0xd0, 0x85, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53,
      0x20,
      0xd0, 0xa2, 0x54, 0x54, 0x54, 0x54, 0x54, 0x54, 0x54,
      0x20,
    ]).unwrap();

    let tests = vec![(
      "invalid tag", // name
      "aaaaaaaa aaaaaaaa aaaaaaaa", // text
      Err(Err::BadTag(0)),
    ), (
      "invalid length", // name
      invalid_len_str.as_str(),
      Err(Err::TruncatedSecret(2, 3)),
    )];

    for (name, text, exp) in tests {
      assert_eq!(decode_vec(text), exp, "{name}");
    }
  }

  #[test]
  fn test_decode_vec_pass() {
    let trips_str = String::from_utf8(vec![
      0xd0, 0x9e, 0x4f, 0xd0, 0x9e, 0x4f, 0xd0, 0x9e, 0xd0, 0x9e, 0x4f, 0x4f,
      0x20,
      0x50, 0x50, 0x50, 0x50, 0x50, 0x50, 0x50, 0x50,
      0x20,
      0xd0, 0x85, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53,
      0x20,
      0xd0, 0xa2, 0x54, 0x54, 0x54, 0x54, 0x54, 0x54, 0x54,
      0x20,
      0xd0, 0xa5, 0x58, 0x58, 0x58, 0x58, 0x58, 0x58, 0x58,
    ]).unwrap();

    let tests = vec![(
      "none", // name
      "", // text
      Ok(Vec::<u8>::new()), // exp
    ), (
      "trips", // name
      trips_str.as_str(), // text
      Ok(vec![1, 1, 1]),
    )];

    for (name, text, exp) in tests {
      assert_eq!(decode_vec(text), exp, "{name}");
    }
  }

  #[test]
  fn test_decode_fail() {
    let invalid_len_str = String::from_utf8(vec![ // text
      0xd0, 0x9e, 0x4f, 0xd0, 0x9e, 0x4f, 0xd0, 0x9e, 0xd0, 0x9e, 0x4f, 0x4f,
      0x20,
      0x50, 0x50, 0x50, 0x50, 0x50, 0x50, 0x50, 0x50,
      0x20,
      0xd0, 0x85, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53, 0x53,
      0x20,
      0xd0, 0xa2, 0x54, 0x54, 0x54, 0x54, 0x54, 0x54, 0x54,
      0x20,
    ]).unwrap();

    let tests = vec![(
      "invalid tag", // name
      "aaaaaaaa aaaaaaaa aaaaaaaa", // text
      Err(Err::BadTag(0)),
    ), (
      "invalid length", // name
      invalid_len_str.as_str(),
      Err(Err::TruncatedSecret(2, 3)),
    )];

    for (name, text, exp) in tests {
      assert_eq!(decode(text), exp, "{name}");
    }
  }


  #[test]
  fn test_decode_pass() {
    let tests = vec![(
      "empty", // name
      "аaаaaaaa aaaaaaaa".to_string(), // text
      "".to_string(), // exp
    ), (
      "A", // name
      "аaаaаaaa aaaaaaaa аaaaaaаa".to_string(), // text
      "A".to_string(), // exp
    )];

    for (name, text, exp) in tests {
      assert_eq!(decode(&text), Ok(exp), "{name}");
    }
  }

}