---
title: "![SlunkCrypt](etc/img/SlunkCrypt-Logo.png)"
---


Introduction
============

SlunkCrypt is an experimental cryptography library and command-line tool. See [*encryption algorithm*](#encryption-algorithm) for details.


Legal Warning
=============

Use of SlunkCrypt may be illegal in countries where encryption is outlawed. We believe it is legal to use SlunkCrypt in many countries all around the world, but we are not lawyers, and so if in doubt you should seek legal advice before downloading it. You may find useful information at [cryptolaw.org](http://www.cryptolaw.org/), which collects information on cryptography laws in many countries.


Command-line Usage
==================

This section describes the SlunkCypt command-line application.

Synopsis
--------

The SlunkCypt command-line program is invoked as follows:

    slunkcrypt --encrypt [[@][:]<passphrase>] <input> <output>
    slunkcrypt --decrypt [[@][:]<passphrase>] <input> <output>
    slunkcrypt --make-pw [<length>]

Commands
--------

One of the following commands must be chosen:

- **`--encrypt` (`-e`):**  
  Run application in ***encrypt*** mode. Reads the given *plaintext* and generates *ciphertext*.
- **`--decrypt` (`-d`):**  
  Run application in ***decrypt*** mode. Reads the given *ciphertext* and restores *plaintext*.
- **`--make-pw` (`-p`):**  
  Generate and output a "strong" random passphrase suitable for use with SlunkCrypt.
- **`--self-test` (`-t`):**  
  Run application in ***self-test*** mode. Program will exit when test is completed.

Options
-------

The following options are available:

- **`<passphrase>`**:  
  * The passphrase used to "protect" the message. The same passphrase must be used for both, ***encrypt*** and ***decrypt*** mode.
  * It will only be possible decrypt the ciphertext, if the "correct" passphrase is known.
  * Use **`--make-pw`** to generate a random passphrase. The passphrase must be kept confidential under all circumstances!
  * **Syntax:**
    - If the passphrase is prefixed with an **`@`** character, then it specifies the file to read the passphrase from.
    - If the passphrase is set to **`@-`**, then the passphrase is read from the standard input stream.
    - If the passphrase is prefixed with an **`:`** character, then the leading character is ignored; use if passphrase contains **`@`** character.
    - If the parameter is *omitted*, then the passphrase is read from the `SLUNK_PASSPHRASE` environment variable.
  * *Note:* In order to thwart brute force attacks, it is recommended to choose a "random" password that is at least 12 characters in length and that consists of upper-case characters, lower-case characters, digits as well as other "special" characters.
- **`<input>`**:  
  * In ***encrypt*** mode, specifies the *plaintext* (unencrypted information) file that is to be encrypted.
  * In ***decrypt*** mode, specifies the *ciphertext* (result of encryption) file that is to be decrypted.
- **`<output>`**:  
  * In ***encrypt*** mode, specifies the file where the *ciphertext* (result of encryption) will be stored.
  * In ***decrypt*** mode, specifies the file where the *plaintext* (unencrypted information) will be stored.
- **`<length>`**:  
  * Speicifes the length of the passphrase to be generated, in characters. If *not* specified, defaults to 24.

Examples
--------

Examples on how to use the SlunkCrypt command-line application:

1. Let's generate a new secure password first:

       slunkcrypt --make-pw

   Example output:

       cdG2=fh<C=3[SSCzf[)iDjIV

2. Now, encrypt the plaintext message, using the generated password:

       slunkcrypt --encrypt "cdG2=fh<C=3[SSCzf[)iDjIV" plaintext.txt ciphertext.enc

   Optionally, let's have a look at the ciphertext:

       hexdump -C ciphertext.enc

3. Finally, decrypt the ciphertext, using the same password as before:

       slunkcrypt --decrypt "cdG2=fh<C=3[SSCzf[)iDjIV" ciphertext.enc plaintext.out

   Optionally, verify that files are actually the same:

       cmp --silent plaintext.txt plaintext.out && echo "files are the same"


Encryption algorithm
====================

The SlunkCrypt algorithm is based on concepts of the well-known [**Enigma**](https://en.wikipedia.org/wiki/Enigma_machine) machine, but with various improvements:

- The original Enigma machine had only *three* (or somtimes *four*) rotors, plus a static "reflector" wheel. In SlunkCrypt, we uses **256** simulated rotors for an improved security.

- The original Enigma machine supported only 26 distinct symbols, i.e. the letters `A` to `Z`. In SlunkCrypt, we use **256** distinct symbols, i.e. the byte values `0x00` to `0xFF`, which allows the encryption (and decryption) of arbitrary streams of bytes, rather than just plain text. Of course, SlunkCrypt can encrypt (and decrypt) text files as well.

- In the original Enigma machine, the signal passes through the rotors *twice*, once in forward direction and then again in backwards direction &ndash; thus the "reflector" wheel. This way, the Enigma's encryption was made *involutory*, i.e. encryption and decryption were the same operation. While this was highly convenient, it also severely weakened the cryptographic strength of the Enigma machine, because the number of possible permutations was reduced drastically! This is one of the main reasons why the Enigma machine eventually was defeated. In SlunkCrypt, the signal passes through the simulated rotors just *once*, in order to maximize the number of possible permutations. This eliminates the most important known weakness of the Enigma machine. Obviously, in SlunkCrypt, separate modes for encryption and decryption need to be provided, because encryption and decryption *no* longer are the same.

- In the original Enigma machine, *only* the rightmost rotor was stepped after every symbol. The other rotors were stepped infrequently. Specifically, if one rotor had completed a full turn, it caused the next rotor to move by one step &ndash; much like the odometer in a car. The fact that most of the rotors remained in the same "static" position for a long time was another important weakness of the Enigma machine that ultimately lead to its demise. Furthermore, the positions of the Enigma's rotors would start to repeat after "only" 16,900 characters. In SlunkCrypt, an improved stepping algorithm is used, which ensures that *all* rotors are stepped often. At the same time, the rotor positions in SlunkCrypt *never* repeat (practically).

- The internal wiring of each of the original Enigma machine's rotors was *fixed*. Each rotor "type" came with a different internal wiring (i.e. permutation). Some models had up to eight rotor "types" to choose from, but only three or four rotors were used at a time. Nonetheless, the internal wiring (i.e. permutation) of each of the supplied rotors was **not** modifiable. This severely restricted the key space of the Enigma machine, as far as the rotors are concerned, because *only* the order of the rotors and the initial position of each rotor could be varied. In SlunkCrypt, a fully *randomized* wiring (i.e. permutation) is generated from the password for each of the 256 simulated rotors. The initial rotor positions are *randomized* as well.

- SlunkCrypt does **not** currently implement the *plugboard* of the original Enigma machine. That is because, even though the plugboard has a large key space, it is just a *fixed* substitution cipher that does *not* contribute too much to the cryptographic strength of the Enigma machine. In fact, the plugboard could be "erased" by Welchman's [diagonal board](https://en.wikipedia.org/wiki/Bombe#Stecker_values).


Programming Interface (API)
===========================

C99 API
-------

This section describes the "low-level" C99 API of the SlunkCypt library.

### slunkcrypt_alloc()

Allocate and initialize a new SlunkCrypt encryption/decryption context.

    slunkcrypt_t slunkcrypt_alloc(
        const uint64_t nonce,
        const uint8_t *const passwd,
        const size_t passwd_len
    );

***Parameters:***

  * `nonce`  
    The *nonce* (number used once) to be used for the encryption/decryption process. The purpose of the nonce is to ensure that each message will be encrypted differently, even when the same password is used to encrypt *multiple* (possibly identical) messages. Therefore, a new *random* nonce **must** be chosen for each message to be encrypted! It is *not* necessary to keep the nonce confidential, but the same nonce **must** be used for both, encryption *and* decryption. Typically, the nonce is stored/transmitted alongside the ciphertext.
    
    *Note:* It is recommended to generate a random nonce via the `slunkcrypt_generate_nonce()` function for each message!
  
  * `passwd`  
    The password to "protect" the message. The password is given as a byte array (`uint8_t`), e.g. UTF-8 encoded characters; a terminating NULL character is *not* required, as the length of the password is specified explicitly. The same password **may** be used to encrypt *multiple* messages. Also, the same password **must** be used for both, encryption *and* decryption; it will *only* be possible decrypt the ciphertext, if the "correct" password is known. The password must be kept confidential under all circumstances!
    
    *Note:* In order to thwart *brute force* attacks, it is recommended to choose a "random" password that is at least 12 characters in length and that consists of upper-case characters, lower-case characters, digits as well as other "special" characters.
  
  * `passwd_len`  
    The length of password given by the `passwd` parameter, in bytes, **not** counting a terminating NULL character. The minimum/maximum length of the password are given by the `SLUNKCRYPT_PWDLEN_MIN` and `SLUNKCRYPT_PWDLEN_MAX` constants, respectively.

***Return value:***

  * If successful, a handle to the new SlunkCrypt context is return; otherwise `SLUNKCRYPT_NULL` is returned.
    
    *Note:* Applications **should** treat `slunkcrypt_t` as an *opaque* handle type. Also, as soon as the SlunkCrypt context is *not* needed anymore, the application **shall** call `slunkcrypt_free()` in order to "clear" and de-allocate that context.

### slunkcrypt_reset()
  
Re-initialize an existing SlunkCrypt encryption/decryption context.

    int slunkcrypt_reset(
        const slunkcrypt_t context,
        const uint64_t nonce,
        const uint8_t *const passwd,
        const size_t passwd_len
    );

***Parameters:***

  * `context`  
    The existing SlunkCrypt context to be re-initialized. This must be a valid handle that was returned by a previous invocation of the `slunkcrypt_alloc()` function.

  * `nonce`  
    Please refer to the  `slunkcrypt_alloc()` function for details!

  * `passwd`  
    Please refer to the  `slunkcrypt_alloc()` function for details!

  * `passwd_len`  
    Please refer to the  `slunkcrypt_alloc()` function for details!

***Return value:***

  * If successful, `SLUNKCRYPT_SUCCESS` is returned; otherwise `SLUNKCRYPT_FAILURE` or `SLUNKCRYPT_ABORTED` is returned.

### slunkcrypt_free()

De-allocate an existing SlunkCrypt encryption/decryption context. This will "clear" and release any memory occupied by the context.

    void slunkcrypt_free(
        const slunkcrypt_t context
    );

***Parameters:***

 * `context`  
    The existing SlunkCrypt context to be de-allocated. This must be a valid handle that was returned by a previous invocation of the `slunkcrypt_alloc()` function.
    
    *Note:* Once a handle has been passed to this function, that handle is *invalidated* and **must not** be used again!

### slunkcrypt_generate_nonce()

Generate a new random *nonce* (number used once), using the system's "cryptographically secure" entropy source.

    int slunkcrypt_generate_nonce(
      int64_t* const nonce
    );

***Parameters:***

 * `nonce`  
   A pointer to a variable of type `int64_t` that receives the new random nonce.

***Return value:***

  * If successful, `SLUNKCRYPT_SUCCESS` is returned; otherwise `SLUNKCRYPT_FAILURE` or `SLUNKCRYPT_ABORTED` is returned.

### slunkcrypt_encrypt()

Encrypt the next message chunk, using separate input/output buffers.

    int slunkcrypt_encrypt(
        const slunkcrypt_t context,
        const uint8_t *const input,
        uint8_t* const output,
        size_t length
    );

***Parameters:***

 * `context`  
    The existing SlunkCrypt context to be used for encrypting the message chunk. This context will be updated.

 * `input`  
    A pointer to the *input* buffer containing the next chunk of the plaintext to be encrypted. The plaintext is given as a byte array (`uint8_t`). This can be arbitrary binary data, e.g. UTF-8 encoded text. NULL bytes are **not** treated specially.
    
    The *input* buffer must contain *at least* `length` bytes of data. If the buffer is longer than `length` bytes, then only the first `length` bytes will be processed and the remainder is ignored!

 * `output`  
    A pointer to the *output* buffer where the ciphertext chunk that corresponds to the given plaintext chunk will be stored. The ciphertext is stored as a byte array (`uint8_t`); it has the same length as the plaintext data.
    
    The *output* buffer must provide sufficient space for storing *at least* `length` bytes of encrypted data. If the buffer is longer than `length` bytes, then only the first `length` bytes of the buffer will be filled with encrypted data!

 * `length`  
   The length of the plaintext chunk contained in the *input* buffer given by the `input` parameter, in bytes. At the same time, this determines the minimum required size of the *output* buffer given by the `output` parameters, in bytes.

***Return value:***

  * If successful, `SLUNKCRYPT_SUCCESS` is returned; otherwise `SLUNKCRYPT_FAILURE` or `SLUNKCRYPT_ABORTED` is returned.

### slunkcrypt_encrypt_inplace()

Encrypt the next message chunk, using a single buffer.

    int slunkcrypt_encrypt_inplace(
        const slunkcrypt_t context,
        uint8_t* const buffer,
        size_t length
    );

***Parameters:***

 * `context`  
    The existing SlunkCrypt context to be used for encrypting the message chunk. This context will be updated.

 * `buffer`  
    A pointer to the buffer initially containing the next chunk of the plaintext to be encrypted. The plaintext is given as a byte array (`uint8_t`). This can be arbitrary binary data, e.g. UTF-8 encoded text. NULL bytes are **not** treated specially. The ciphertext chunk that corresponds to the given plaintext chunk will be stored to the *same* buffer, thus replacing the plaintext data.
    
    The buffer must initially contain *at least* `length` bytes of input data; the first `length` bytes of the buffer will be overwritten with the encrypted data. If the buffer is longer than `length` bytes, then only the first `length` bytes will be processed and overwritten.

 * `length`  
   The length of the plaintext chunk initially contained in the input/output buffer given by the `buffer` parameter, in bytes. At the same time, this determines the portion of the input/output buffer that will be overwritten with encrypted data, in bytes.

***Return value:***

  * If successful, `SLUNKCRYPT_SUCCESS` is returned; otherwise `SLUNKCRYPT_FAILURE` or `SLUNKCRYPT_ABORTED` is returned.

### slunkcrypt_decrypt()

Decrypt the next ciphertext chunk, using separate input/output buffers.

    int slunkcrypt_decrypt(
        const slunkcrypt_t context,
        const uint8_t *const input,
        uint8_t* const output,
        size_t length
    );

***Parameters:***

 * `context`  
    The existing SlunkCrypt context to be used for decrypting the ciphertext chunk. This context will be updated.

 * `input`  
    A pointer to the *input* buffer containing the next chunk of the ciphertext to be decrypted. The ciphertext is given as a byte array (`uint8_t`).
    
    The *input* buffer must contain *at least* `length` bytes of data. If the buffer is longer than `length` bytes, then only the first `length` bytes will be processed and the remainder is ignored!

 * `output`  
    A pointer to the *output* buffer where the plaintext chunk that corresponds to the given ciphertext chunk will be stored. The plaintext is stored as a byte array (`uint8_t`); it has the same length as the plaintext data.
    
    The *output* buffer must provide sufficient space for storing *at least* `length` bytes of decrypted data. If the buffer is longer than `length` bytes, then only the first `length` bytes of the buffer will be filled with decrypted data!

 * `length`  
   The length of the ciphertext chunk contained in the *input* buffer given by the `input` parameter, in bytes. At the same time, this determines the minimum required size of the *output* buffer given by the `output` parameters, in bytes.

***Return value:***

  * If successful, `SLUNKCRYPT_SUCCESS` is returned; otherwise `SLUNKCRYPT_FAILURE` or `SLUNKCRYPT_ABORTED` is returned.

### slunkcrypt_decrypt_inplace()

Decrypt the next ciphertext chunk, using a single buffer.

    int slunkcrypt_decrypt_inplace(
        const slunkcrypt_t context,
        uint8_t* const buffer,
        size_t length
    );

***Parameters:***

 * `context`  
    The existing SlunkCrypt context to be used for decrypting the ciphertext chunk. This context will be updated.

 * `buffer`  
    A pointer to the buffer initially containing the next chunk of the ciphertext to be decrypted. The ciphertext is given as a byte array (`uint8_t`). The plaintext that corresponds to the given ciphertext will be stored to the *same* buffer, thus replacing the plaintext data.
    
    The buffer must initially contain *at least* `length` bytes of input data; the first `length` bytes of the buffer will be overwritten with the decrypted data. If the buffer is longer than `length` bytes, then only the first `length` bytes will be processed and overwritten.

 * `length`  
   The length of the ciphertext chunk initially contained in the input/output buffer given by the `buffer` parameter, in bytes. At the same time, this determines the portion of the input/output buffer that will be overwritten with decrypted data, in bytes.

***Return value:***

  * If successful, `SLUNKCRYPT_SUCCESS` is returned; otherwise `SLUNKCRYPT_FAILURE` or `SLUNKCRYPT_ABORTED` is returned.

### slunkcrypt_random_bytes()

Generate a sequence of random bytes, using the system's "cryptographically secure" entropy source.

    size_t slunkcrypt_random_bytes(
      uint8_t* const buffer,
      const size_t length
    );

***Parameters:***

 * `buffer`  
   A pointer to the *output* buffer where the random bytes will be stored.

   The *output* buffer must provide sufficient space for storing *at least* `length` bytes of random data. If the buffer is longer than `length` bytes, then *at most* the first `length` bytes of the buffer will be filled with random data!

 * `length`  
   The number of random bytes to be generated. At the same time, this determines the minimum required size of the *output* buffer given by the `output` parameters, in bytes.

***Return value:***

  * If successful, the number of random bytes that have been generated and stored to the *output* buffer is returned; otherwise `0` is returned.
    
    The number of generated bytes can be *at most* `length`. Less than `length` bytes *may* be generated, if the entropy source could **not** provide the requested number of bytes at this time. In that case, you can try again.


### slunkcrypt_bzero()

Erase the contents of a byte array, by overwriting it with *zero* bytes. Compiler optimizations will **not** remove the erase operation.

    void slunkcrypt_bzero(
      void* const buffer,
      const size_t length
    );

***Parameters:***

 * `buffer`  
   A pointer to the buffer whose content is to be erased.

   The buffer must be *at least* `length` bytes in size. If the buffer is longer than `length` bytes, then *only* the first `length` bytes of the buffer will be erased; the remainder of the buffer will *not* be modified!

 * `length`  
   The size of the buffer to be erased, in bytes.


C++ API
-------

This section describes the "high-level" C++ API of the SlunkCypt library.

### SlunkCryptEncr

Class for *encrypting* data using the SlunkCrypt library.

#### Constructor

Create and initialize a new **``SlunkCryptEncr``** instance. Also generated a new, random nonce.

    SlunkCryptEncr::SlunkCryptEncr(
      const std::string &passwd
    );

***Parameters:***

  * `passwd`  
    The password to "protect" the message. The password is given as an `std::string`, e.g. UTF-8 encoded characters. The same password **may** be used to encrypt *multiple* messages. Also, the same password **must** be used for both, encryption *and* decryption; it will *only* be possible decrypt the ciphertext, if the "correct" password is known. The password must be kept confidential under all circumstances!
    
    *Note:* In order to thwart *brute force* attacks, it is recommended to choose a "random" password that is at least 12 characters in length and that consists of upper-case characters, lower-case characters, digits as well as other "special" characters.

***Exceptions:***

  * Throws `std::runtime_error`, if the nonce could not be generated, or if the SlunkCrypt context could not be allocated.

#### SlunkCryptEncr::encrypt() [1]

Encrypt the next message chunk, using separate input/output buffers.

    bool encrypt(
      const uint8_t* const input,
      uint8_t* const output,
      size_t length
    );

***Parameters:***

 * `input`  
    A pointer to the *input* buffer containing the next chunk of the plaintext to be encrypted. The plaintext is given as a byte array (`uint8_t`). This can be arbitrary binary data, e.g. UTF-8 encoded text. NULL bytes are **not** treated specially.
    
    The *input* buffer must contain *at least* `length` bytes of data. If the buffer is longer than `length` bytes, then only the first `length` bytes will be processed and the remainder is ignored!

 * `output`  
    A pointer to the *output* buffer where the ciphertext chunk that corresponds to the given plaintext chunk will be stored. The ciphertext is stored as a byte array (`uint8_t`); it has the same length as the plaintext data.
    
    The *output* buffer must provide sufficient space for storing *at least* `length` bytes of encrypted data. If the buffer is longer than `length` bytes, then only the first `length` bytes of the buffer will be filled with encrypted data!

 * `length`  
   The length of the plaintext chunk contained in the *input* buffer given by the `input` parameter, in bytes. At the same time, this determines the minimum required size of the *output* buffer given by the `output` parameters, in bytes.

***Return value:***

  * If successful, `true` is returned; otherwise `false` is returned.

#### SlunkCryptEncr::encrypt() [2]

Encrypt the next message chunk, using separate input/output containers (`std::vector`).

    bool encrypt(
      const std::vector<uint8_t> &input,
      std::vector<uint8_t> &output
    );

***Parameters:***

 * `input`  
    A reference to the `std::vector<uint8_t>` instance containing the next chunk of the plaintext to be encrypted. This can be arbitrary binary data, e.g. UTF-8 encoded text. NULL bytes are **not** treated specially.

 * `output`  
    A reference to the `std::vector<uint8_t>` instance where the ciphertext that corresponds to the given plaintext will be stored.
    
    The `output.size()` must be *greater than or equal* to `input.size()`. If the `output.size()` is larger than the `input.size()`, then only the first `input.size()` elements of `output` will be filled with encrypted data!

***Return value:***

  * If successful, `true` is returned; otherwise `false` is returned. The function fails, if the *output* `std::vector` is too small.

#### SlunkCryptEncr::encrypt_inplace() [1]

Encrypt the next message chunk, using a single buffer.

    bool encrypt_inplace(
      uint8_t* const buffer,
      size_t length
    );

***Parameters:***

 * `buffer`  
    A pointer to the buffer initially containing the next chunk of the plaintext to be encrypted. The plaintext is given as a byte array (`uint8_t`). This can be arbitrary binary data, e.g. UTF-8 encoded text. NULL bytes are **not** treated specially. The ciphertext chunk that corresponds to the given plaintext chunk will be stored to the *same* buffer, thus replacing the plaintext data.
    
    The buffer must initially contain *at least* `length` bytes of input data; the first `length` bytes of the buffer will be overwritten with the encrypted data. If the buffer is longer than `length` bytes, then only the first `length` bytes will be processed and overwritten.

 * `length`  
   The length of the plaintext chunk initially contained in the input/output buffer given by the `buffer` parameter, in bytes. At the same time, this determines the portion of the input/output buffer that will be overwritten with encrypted data, in bytes.

***Return value:***

  * If successful, `true` is returned; otherwise `false` is returned.

#### SlunkCryptEncr::encrypt_inplace() [2]

Encrypt the next message chunk, using a single container (`std::vector`).

    bool encrypt_inplace(
      std::vector<uint8_t> &buffer
    );

***Parameters:***

 * `buffer`  
    A reference to the `std::vector<uint8_t>` initially containing the next chunk of the plaintext to be encrypted. This can be arbitrary binary data, e.g. UTF-8 encoded text. NULL bytes are **not** treated specially. The ciphertext chunk that corresponds to the given plaintext chunk will be stored to the *same* `std::vector<uint8_t>`, thus replacing all the plaintext data.

***Return value:***

  * If successful, `true` is returned; otherwise `false` is returned.

#### SlunkCryptEncr::get_nonce()

Retrieve the random nonce that is used to encrypt the message.

    uint64_t get_nonce();

***Return value:***

  * Returns the nonce that is used to encrypt the message. The purpose of the nonce is to ensure that each message will be encrypted differently, even when the same password is used to encrypt multiple (possibly identical) messages. Therefore, a new random nonce must be chosen for each message! It is not necessary to keep the nonce confidential, but the same nonce must be used for both, encryption and decryption. Typically, the nonce is stored/transmitted alongside the ciphertext.
    
    *Note:* The `SlunkCryptEncr` class automatically generates a new, random nonce for each message to be encrypted. Use *this* function to retrieve that nonce, so that it can be passed to `SlunkCryptDecr` for decryption later.

### SlunkCryptDecr

Class for *decrypting* data using the SlunkCrypt library.

#### Constructor

Create and initialize a new **``SlunkCryptDecr``** instance.

    SlunkCryptDecr::SlunkCryptDecr(
      const uint64_t nonce,
      const std::string& passwd
    );

***Parameters:***

  * `nonce`  
    The *nonce* (number used once) to be used for the decryption process. The purpose of the nonce is to ensure that each message will be encrypted differently, even when the same password is used to encrypt *multiple* (possibly identical) messages. Therefore, a new *random* nonce **must** be chosen for each message! It is *not* necessary to keep the nonce confidential, but the same nonce **must** be used for both, encryption *and* decryption. Typically, the nonce is stored/transmitted alongside the ciphertext.
    
    *Note:* The `SlunkCryptEncr` class automatically generates a new, random nonce for each message to be encrypted. Use `SlunkCryptEncr::get_nonce()` to retrieve that nonce, so that it can be passed to `SlunkCryptDecr` for decryption later.
  
  * `passwd`  
    The password to "protect" the message. The password is given as an `std::string`, e.g. UTF-8 encoded characters. The same password **may** be used to encrypt *multiple* messages. Also, the same password **must** be used for both, encryption *and* decryption; it will *only* be possible decrypt the ciphertext, if the "correct" password is known. The password must be kept confidential under all circumstances!
    
    *Note:* In order to thwart *brute force* attacks, it is recommended to choose a "random" password that is at least 12 characters in length and that consists of upper-case characters, lower-case characters, digits as well as other "special" characters.

***Exceptions:***

  * Throws `std::runtime_error`, if the SlunkCrypt context could not be allocated.

#### SlunkCryptDecr::decrypt() [1]

Decrypt the next message chunk, using separate input/output buffers.

    bool decrypt(
      const uint8_t* const input,
      uint8_t* const output,
      size_t length
    );

***Parameters:***

 * `input`  
    A pointer to the *input* buffer containing the next chunk of the ciphertext to be decrypted. The ciphertext is given as a byte array (`uint8_t`).
    
    The *input* buffer must contain *at least* `length` bytes of data. If the buffer is longer than `length` bytes, then only the first `length` bytes will be processed and the remainder is ignored!

 * `output`  
    A pointer to the *output* buffer where the plaintext chunk that corresponds to the given ciphertext chunk will be stored. The plaintext is stored as a byte array (`uint8_t`); it has the same length as the ciphertext data.
    
    The *output* buffer must provide sufficient space for storing *at least* `length` bytes of decrypted data. If the buffer is longer than `length` bytes, then only the first `length` bytes of the buffer will be filled with decrypted data!

 * `length`  
   The length of the ciphertext chunk contained in the *input* buffer given by the `input` parameter, in bytes. At the same time, this determines the minimum required size of the *output* buffer given by the `output` parameters, in bytes.

***Return value:***

  * If successful, `true` is returned; otherwise `false` is returned.

#### SlunkCryptDecr::decrypt() [2]

Decrypt the next message chunk, using separate input/output containers (`std::vector`).

    bool decrypt(
      const std::vector<uint8_t> &input,
      std::vector<uint8_t> &output
    );

***Parameters:***

 * `input`  
    A reference to the `std::vector<uint8_t>` instance containing the next chunk of the ciphertext to be decrypted.

 * `output`  
    A reference to the `std::vector<uint8_t>` instance where the plaintext that corresponds to the given ciphertext will be stored.
    
    The `output.size()` must be *greater than or equal* to `input.size()`. If the `output.size()` is greater than the `input.size()`, then only the first `input.size()` elements of `output` will be filled with decrypted data!

***Return value:***

  * If successful, `true` is returned; otherwise `false` is returned. The function fails, if the *output* `std::vector` is too small.

#### SlunkCryptDecr::decrypt_inplace() [1]

Decrypt the next message chunk, using a single buffer.

    bool decrypt_inplace(
      uint8_t* const buffer,
      size_t length
    );

***Parameters:***

 * `buffer`  
    A pointer to the buffer initially containing the next chunk of the ciphertext to be decrypted. The ciphertext is given as a byte array (`uint8_t`). The plaintext that corresponds to the given ciphertext will be stored to the *same* buffer, replacing the plaintext data.
    
    The buffer must initially contain *at least* `length` bytes of input data; the first `length` bytes of the buffer will be overwritten with the encrypted data. If the buffer is longer than `length` bytes, then only the first `length` bytes will be processed and overwritten.

 * `length`  
   The length of the ciphertext chunk initially contained in the input/output buffer given by the `buffer` parameter, in bytes. At the same time, this determines the portion of the input/output buffer that will be overwritten with decrypted data, in bytes.

***Return value:***

  * If successful, `true` is returned; otherwise `false` is returned.

#### SlunkCryptDecr::decrypt_inplace() [2]

Decrypt the next message chunk, using a single container (`std::vector`).

    bool decrypt_inplace(
      std::vector<uint8_t> &buffer
    );

***Parameters:***

 * `buffer`  
    A reference to the `std::vector<uint8_t>` initially containing the next chunk of the ciphertext to be decrypted. The plaintext that corresponds to the given ciphertext will be stored to the *same* `std::vector<uint8_t>`, replacing all the ciphertext data.

***Return value:***

  * If successful, `true` is returned; otherwise `false` is returned.

Thread safety
-------------

The following functions are fully "thread-safe" and thus may safely be called by *any* thread at *any* time ***without*** the need for synchronization:

* `slunkcrypt_alloc()`
* `slunkcrypt_random_bytes()`
* `slunkcrypt_bzero()`
* `SlunkCryptEncr::SlunkCryptEncr()`
* `SlunkCryptDecr::SlunkCryptDecr()`

The following functions are "reentrant" and thus may safely be called by *any* thread at *any* time, ***without*** the need for synchronization, provided that each `slunkcrypt_t`, `SlunkCryptEncr` or `SlunkCryptDecr` instance is "owned" (i.e. accessed *exclusively*) and by a *single* thread:

* `slunkcrypt_reset()`
* `slunkcrypt_free()`
* `slunkcrypt_encrypt()`
* `slunkcrypt_encrypt_inplace()`
* `slunkcrypt_decrypt()`
* `slunkcrypt_decrypt_inplace()`
* `SlunkCryptEncr::encrypt()`
* `SlunkCryptEncr::encrypt_inplace()`
* `SlunkCryptEncr::get_nonce()`
* `SlunkCryptDecr::decrypt()`
* `SlunkCryptDecr::decrypt_inplace()`

***Note:*** Iff the same `slunkcrypt_t`, `SlunkCryptEncr` or `SlunkCryptDecr` instance needs to be shared across *multiple* threads, then the application **must** *serialize* any invocations of the above functions (on the shared instance), by an *explicit* "mutex" synchronization!


License
=======

This work has been released under the **CC0 1.0 Universal** license.

For details, please refer to:  
<https://creativecommons.org/publicdomain/zero/1.0/legalcode>