# Crypto

# Cryptography

OCamlnet-4 does not include any implementations of cryptographic ciphers or hashes. It does, however, include a binding to GnuTLS and GNU Nettle, providing cryptography, and it defines a number of helper functions to use cryptography efficiently.

Before OCamlnet-4, some modules used Xavier Leroy's Cryptokit. This dependency is gone now.

## Cryptography providers

Like for TLS (see `Tls`), first-class modules are used to modularize the provider of the crypto functions:

- `Netsys_crypto_types.SYMMETRIC_CRYPTO`: Defines symmetric ciphers
- `Netsys_crypto_types.DIGESTS`: Defines digests (hashes)

There are default providers:

- `Netsys_crypto.default_symmetric_crypto`
- `Netsys_crypto.default_digests`

If not initialized, these providers are empty, i.e. the lists of available ciphers and digests are empty. This can be changed by initializing a provider. So far, there is only GnuTLS/Nettle, and you can enable this by

- linking with the package nettls-gnutls
- calling `Nettls_gnutls.init()`

## Using cryptography

It is not advised to call any functions of the providers directly: the API is not yet stable, and may change, and there are some inconveniences in the buffer management. Instead, use the following functionality:

- `Netsys_ciphers` is the main user-oriented API for encrypting or decrypting a message with symmetric ciphers
- Also see the channels described here: Symmetric Cryptography
- `Netsys_digests` is the main API for digesting messages.

## Examples

Encrypt a string `s` with AES-128 in CBC mode and length-style padding:

    let key = "0123456789abcdef"
    let iv = "0123456789abcdef"
    let cipher = Netsys_ciphers.find ("AES-128", "CBC")
    let ctx = cipher # create key `Length
    let () = ctx # set_iv iv
    let s_enc = ctx # encrypt_string s

Compute the SHA1 digest of a string `s`:

    let digest = Netsys_digests.find "SHA1-160"
    let ctx = digest # create()
    let () = ctx # add_substring s 0 (String.length s)
    let result = ctx # finish()

## Supported ciphers

If using nettls-gnutls as provider, you can normally expect:

- AES (key sizes: 128, 192, 256)
- ARCFOUR (key size: 128) . RC2 (key sizes: 40, 64, 128)
- BLOWFISH (key size variable)
- CAMELLIA (key sizes: 128, 192, 256)
- CAST (key size: 128)
- DES (key size: 56)
- 3DES (key size: 112)
- SERPENT (key sizes: 128, 192, 256)
- TWOFISH (key sizes: 128, 192, 256)

All ciphers are supported in ECB, CBC, CTR, and OFB modes.

If using a recent version of GnuTLS/Nettls:

- AES in CBC mode is accelerated if the CPU supports it (Intel AES-NI, or VIA Padlock)
- AES in GCM mode, also with an acceleration option
- CAMELLIA in GCM mode

## Supported digests

If using nettls-gnutls as provider, you can normally expect:

- MD2
- MD4
- MD5
- SHA1
- SHA2-256

If using a recent version of GnuTLS/Nettls:

- SHA2 with block sizes 224, 256, 384 and 512 bits
- SHA3 with block sizes 224, 256, 384 and 512 bits
- RIPEMD (160 bits)
- GOSTHASH94 (256 bits)