package cryptokit

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Module Cryptokit

The Cryptokit library provides a variety of cryptographic primitives that can be used to implement cryptographic protocols in security-sensitive applications. The primitives provided include:

  • Symmetric-key ciphers: AES, DES, Triple-DES, ARCfour, in ECB, CBC, CFB and OFB modes.
  • Public-key cryptography: RSA encryption, Diffie-Hellman key agreement.
  • Hash functions and MACs: SHA-1, SHA-256, SHA-512. SHA-3, RIPEMD-160, MD5, and MACs based on AES and DES.
  • Random number generation.
  • Encodings and compression: base 64, hexadecimal, Zlib compression.

To use this library, link with ocamlc unix.cma nums.cma cryptokit.cma or ocamlopt unix.cmxa nums.cmxa cryptokit.cmxa.

General-purpose abstract interfaces

class type transform = object ... end

A <I>transform</I> is an arbitrary mapping from sequences of characters to sequences of characters. Examples of transforms include ciphering, deciphering, compression, decompression, and encoding of binary data as text. Input data to a transform is provided by successive calls to the methods put_substring, put_string, put_char or put_byte. The result of transforming the input data is buffered internally, and can be obtained via the get_string, get_substring, get_char and get_byte methods.

val transform_string : transform -> string -> string

transform_string t s runs the string s through the transform t and returns the transformed string. The transform t is wiped before returning, hence can no longer be used for further transformations.

val transform_channel : transform -> ?len:int -> in_channel -> out_channel -> unit

transform_channel t ic oc reads characters from input channel ic, runs them through the transform t, and writes the transformed data to the output channel oc. If the optional len argument is provided, exactly len characters are read from ic and transformed; End_of_file is raised if ic does not contain at least len characters. If len is not provided, ic is read all the way to end of file. The transform t is wiped before returning, hence can no longer be used for further transformations.

val compose : transform -> transform -> transform

Compose two transforms, feeding the output of the first transform to the input of the second transform.

class type hash = object ... end

A <I>hash</I> is a function that maps arbitrarily-long character sequences to small, fixed-size strings.

val hash_string : hash -> string -> string

hash_string h s runs the string s through the hash function h and returns the hash value of s. The hash h is wiped before returning, hence can no longer be used for further hash computations.

val hash_channel : hash -> ?len:int -> in_channel -> string

hash_channel h ic reads characters from the input channel ic, computes their hash value and returns it. If the optional len argument is provided, exactly len characters are read from ic and hashed; End_of_file is raised if ic does not contain at least len characters. If len is not provided, ic is read all the way to end of file. The hash h is wiped before returning, hence can no longer be used for further hash computations.

Utilities: random numbers and padding schemes

module Random : sig ... end

The Random module provides random and pseudo-random number generators suitable for generating cryptographic keys, nonces, or challenges.

module Padding : sig ... end

The Padding module defines a generic interface for padding input data to an integral number of blocks, as well as two popular padding schemes.

Cryptographic primitives (simplified interface)

module Cipher : sig ... end

The Cipher module implements the AES, DES, Triple-DES, ARCfour and Blowfish symmetric ciphers. Symmetric ciphers are presented as transforms parameterized by a secret key and a ``direction'' indicating whether encryption or decryption is to be performed. The same secret key is used for encryption and for decryption.

module Hash : sig ... end

The Hash module implements unkeyed cryptographic hashes (SHA-1, SHA-256, SHA-512, SHA-3, RIPEMD-160 and MD5), also known as message digest functions. Hash functions used in cryptography are characterized as being <I>one-way</I> (given a hash value, it is computationally infeasible to find a text that hashes to this value) and <I>collision-resistant</I> (it is computationally infeasible to find two different texts that hash to the same value). Thus, the hash of a text can be used as a compact replacement for this text for the purposes of ensuring integrity of the text.

module MAC : sig ... end

The MAC module implements message authentication codes, also known as keyed hash functions. These are hash functions parameterized by a secret key. In addition to being one-way and collision-resistant, a MAC has the property that without knowing the secret key, it is computationally infeasible to find the hash for a known text, even if many pairs of (text, MAC) are known to the attacker. Thus, MAC can be used to authenticate the sender of a text: the receiver of a (text, MAC) pair can recompute the MAC from the text, and if it matches the transmitted MAC, be reasonably certain that the text was authentified by someone who possesses the secret key.

module RSA : sig ... end

The RSA module implements RSA public-key cryptography. Public-key cryptography is asymmetric: two distinct keys are used for encrypting a message, then decrypting it. Moreover, while one of the keys must remain secret, the other can be made public, since it is computationally very hard to reconstruct the private key from the public key. This feature supports both public-key encryption (anyone can encode with the public key, but only the owner of the private key can decrypt) and digital signature (only the owner of the private key can sign, but anyone can check the signature with the public key).

module DH : sig ... end

The DH module implements Diffie-Hellman key agreement. Key agreement is a protocol by which two parties can establish a shared secret (typically a key for a symmetric cipher or MAC) by exchanging messages, with the guarantee that even if an attacker eavesdrop on the messages, he cannot recover the shared secret. Diffie-Hellman is one such key agreement protocol, relying on the difficulty of computing discrete logarithms. Notice that the Diffie-Hellman protocol is vulnerable to active attacks (man-in-the-middle attacks).

Advanced, compositional interface to block ciphers and stream ciphers

module Block : sig ... end

The Block module provides classes that implements popular block ciphers, chaining modes, and wrapping of a block cipher as a general transform or as a hash function. The classes can be composed in a Lego-like fashion, facilitating the integration of new block ciphers, modes, etc.

module Stream : sig ... end

The Stream module provides classes that implement the ARCfour stream cipher, and the wrapping of a stream cipher as a general transform. The classes can be composed in a Lego-like fashion, facilitating the integration of new stream ciphers.

Encoding and compression of data

module Base64 : sig ... end

The Base64 module supports the encoding and decoding of binary data in base 64 format, using only alphanumeric characters that can safely be transmitted over e-mail or in URLs.

module Hexa : sig ... end

The Hexa module supports the encoding and decoding of binary data as hexadecimal strings. This is a popular format for transmitting keys in textual form.

module Zlib : sig ... end

The Zlib module supports the compression and decompression of data, using the zlib library. The algorithm used is Lempel-Ziv compression as in the gzip and zip compressors. While compression itself is not encryption, it is often used prior to encryption to hide regularities in the plaintext, and reduce the size of the ciphertext.

Error reporting

type error =
  1. | Wrong_key_size
    (*

    The key is too long or too short for the given cipher.

    *)
  2. | Wrong_IV_size
    (*

    The initialization vector does not have the same size as the block size.

    *)
  3. | Wrong_data_length
    (*

    The total length of the input data for a transform is not an integral multiple of the input block size.

    *)
  4. | Bad_padding
    (*

    Incorrect padding bytes were found after decryption.

    *)
  5. | Output_buffer_overflow
    (*

    The output buffer for a transform exceeds the maximal length of a Caml string.

    *)
  6. | Incompatible_block_size
    (*

    A combination of two block ciphers was attempted whereby the ciphers have different block sizes, while they must have the same.

    *)
  7. | Number_too_long
    (*

    Denotes an internal error in RSA key generation or encryption.

    *)
  8. | Seed_too_short
    (*

    The seed given to a pseudo random number generator is too short.

    *)
  9. | Message_too_long
    (*

    The message passed to RSA encryption or decryption is greater than the modulus of the RSA key

    *)
  10. | Bad_encoding
    (*

    Illegal characters were found in an encoding of binary data such as base 64 or hexadecimal.

    *)
  11. | Compression_error of string * string
    (*

    Error during compression or decompression.

    *)
  12. | No_entropy_source
    (*

    No entropy source (OS, /dev/random or EGD) was found for Cryptokit.Random.secure_rng.

    *)
  13. | Entropy_source_closed
    (*

    End of file on a device or EGD entropy source.

    *)
  14. | Compression_not_supported
    (*

    The data compression functions are not available.

    *)

Error codes for this library.

exception Error of error

Exception raised by functions in this library to report error conditions.

Miscellaneous utilities

val wipe_bytes : bytes -> unit

wipe_bytes s overwrites s with zeroes. Can be used to reduce the memory lifetime of sensitive data.

val wipe_string : string -> unit

wipe_string s overwrites s with zeroes. Can be used to reduce the memory lifetime of sensitive data.

val xor_bytes : bytes -> int -> bytes -> int -> int -> unit

xor_bytes src spos dst dpos len performs the xor (exclusive or) of characters spos, ..., spos + len - 1 of src with characters dpos, ..., dpos + len - 1 of dst, storing the result in dst starting at position dpos.

val xor_string : string -> int -> bytes -> int -> int -> unit

Same as xor_bytes, but the source is a string instead of a byte array.

val mod_power : string -> string -> string -> string

mod_power a b c computes a^b mod c, where the strings a, b, c and the result are viewed as arbitrary-precision integers in big-endian format. Requires a < c.

val mod_mult : string -> string -> string -> string

mod_mult a b c computes a*b mod c, where the strings a, b, c and the result are viewed as arbitrary-precision integers in big-endian format.