# crypto Module

The crypto modules provides various functions for working with cryptographic algorithms.

## crypto.encrypt()

Encrypts Lua strings.

#### Syntax
`crypto.encrypt(algo, key, plain [, iv])`

#### Parameters
  - `algo` the name of the encryption algorithm to use, one of
    - `"AES-ECB"` for 128-bit AES in ECB mode
    - `"AES-CBC"` for 128-bit AES in CBC mode
  - `key` the encryption key as a string; for AES encryption this *MUST* be 16 bytes long
  - `plain` the string to encrypt; it will be automatically zero-padded to a 16-byte boundary if necessary
  - `iv` the initilization vector, if using AES-CBC; defaults to all-zero if not given

#### Returns
The encrypted data as a binary string. For AES this is always a multiple of 16 bytes in length.

#### Example
```lua
print(crypto.toHex(crypto.encrypt("AES-ECB", "1234567890abcdef", "Hi, I'm secret!")))
```

#### See also
  - [`crypto.decrypt()`](#cryptodecrypt)


## crypto.decrypt()

Decrypts previously encrypted data.

#### Syntax
`crypto.decrypt(algo, key, cipher [, iv])`

#### Parameters
  - `algo` the name of the encryption algorithm to use, one of
    - `"AES-ECB"` for 128-bit AES in ECB mode
    - `"AES-CBC"` for 128-bit AES in CBC mode
  - `key` the encryption key as a string; for AES encryption this *MUST* be 16 bytes long
  - `cipher` the cipher text to decrypt (as obtained from `crypto.encrypt()`)
  - `iv` the initilization vector, if using AES-CBC; defaults to all-zero if not given

#### Returns
The decrypted string.

Note that the decrypted string may contain extra zero-bytes of padding at the end. One way of stripping such padding is to use `:match("(.-)%z*$")` on the decrypted string. Additional care needs to be taken if working on binary data, in which case the real length likely needs to be encoded with the data, and at which point `:sub(1, n)` can be used to strip the padding.

#### Example
```lua
key = "1234567890abcdef"
cipher = crypto.encrypt("AES-ECB", key, "Hi, I'm secret!")
print(crypto.toHex(cipher))
print(crypto.decrypt("AES-ECB", key, cipher))
```

#### See also
  - [`crypto.encrypt()`](#cryptoencrypt)


## crypto.hash()

Compute a cryptographic hash of a Lua string.

#### Syntax
`hash = crypto.hash(algo, str)`

#### Parameters
`algo` the hash algorithm to use, case insensitive string

Supported hash algorithms are:

- MD2 (not available by default, has to be explicitly enabled in `app/include/user_config.h`)
- MD5
- SHA1
- SHA256, SHA384, SHA512 (unless disabled in `app/include/user_config.h`)

#### Returns
A binary string containing the message digest. To obtain the textual version (ASCII hex characters), please use [`crypto.toHex()`](#cryptotohex	).

#### Example
```lua
print(crypto.toHex(crypto.hash("sha1","abc")))
```

## crypto.hmac()

Compute a [HMAC](https://en.wikipedia.org/wiki/Hash-based_message_authentication_code) (Hashed Message Authentication Code) signature for a Lua string.

#### Syntax
`signature = crypto.hmac(algo, str, key)`

#### Parameters
- `algo` hash algorithm to use, case insensitive string
- `str` data to calculate the hash for
- `key` key to use for signing, may be a binary string

Supported hash algorithms are:

- MD2 (not available by default, has to be explicitly enabled in `app/include/user_config.h`)
- MD5
- SHA1
- SHA256, SHA384, SHA512 (unless disabled in `app/include/user_config.h`)

#### Returns
A binary string containing the HMAC signature. Use [`crypto.toHex()`](#cryptotohex	) to obtain the textual version.

#### Example
```lua
print(crypto.toHex(crypto.hmac("sha1","abc","mysecret")))
```

## crypto.mask()

Applies an XOR mask to a Lua string. Note that this is not a proper cryptographic mechanism, but some protocols may use it nevertheless.

#### Syntax
`crypto.mask(message, mask)`

#### Parameters
- `message` message to mask
- `mask` the mask to apply, repeated if shorter than the message

#### Returns
The masked message, as a binary string. Use [`crypto.toHex()`](#cryptotohex) to get a textual representation of it.

#### Example
```lua
print(crypto.toHex(crypto.mask("some message to obscure","X0Y7")))
```

## crypto.toBase64()

Provides a Base64 representation of a (binary) Lua string.

#### Syntax
`b64 = crypto.toBase64(binary)`

#### Parameters
`binary` input string to Base64 encode

#### Return
A Base64 encoded string.

#### Example
```lua
print(crypto.toBase64(crypto.hash("sha1","abc")))
```

## crypto.toHex()

Provides an ASCII hex representation of a (binary) Lua string. Each byte in the input string is represented as two hex characters in the output.

#### Syntax
`hexstr = crypto.toHex(binary)`

#### Parameters
`binary` input string to get hex representation for

#### Returns
An ASCII hex string.

#### Example
```lua
print(crypto.toHex(crypto.hash("sha1","abc")))
```