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)

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