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Customizable Erlang native ECIES public-key cryptography library

An Erlang native library implementing the ECIES (Elliptic Curve Integrated Encryption Scheme) public-key cryptography, providing elliptic curve encryption as an alternative to the deprecated crypto public_encrypt, private_decrypt functions.

Motivation

The Erlang OTP team decided to deprecate several crypto module functions in OTP 27 (See functions deprecated in OTP 27). Notably, no alternatives were provided for two of them crypto:public_encrypt/4 and crypto:private_decrypt/4.

Some information about potential background of these deprecations can be found here.

It is worth mentioning that the above mentioned functions are RSA specific and cannot be used with Elliptic Curve cryptography anyway. In our projects we mainly use Elliptic Curve cryptography and we decided to share this small library for ECIES with the Erlang developers community.

We will be happy if you find it useful and use in your project. If you find any ideas for improvements or notice any missing functionality, please open an issue here; even better propose a Pull Request.

Usage

The API of library is simple:

Example: 

% Bob generates keys, and publish his public key
{BobPublicKey, BobPrivateKey} = ecies:generate_key(),
% Alice knowing Bob's public key encrypts a message for him
Data = ecies:public_encrypt(BobPublicKey, <<"top secret message">>),
% Bob is able to decrypt the message using his private key
<<"top secret message">> = ecies:private_decrypt(BobPrivateKey, Data).

In the above example the default params are used (as returned by ecies:default_params/0):

  #{
    curve    => secp256k1,
    cipher   => aes_256_cbc,
    kdf      => {kdf, sha256},      % ANSI-X9.63 key derivation
    mac      => {hmac, sha256, 256} % HMAC SHA256 with 256 bits (32 bytes) output
  }.

Openssl interoperability

You can read or write keys in PEM format using function from ecies_pem module:

Example:

openssl ecparam -noout -genkey -conv_form compressed -name secp256k1 | tee private.pem

{ok, Pem} = file:read_file("private.pem"),
{Pub, Priv} = ecies_pem:decode_keypair(Pem),
Pub = ecies_pem:decode_public(Pem),
Priv = ecies_pem:decode_private(Pem),
file:write_file("public.pem", ecies_pem:encode_public(Pub)).
openssl ec -pubin -in public.pem -noout -text

Customisation

Using ecies:generate_key/1, ecies:public_encrypt/3, ecies:private_decrypt/3 functions which accepts extra Params argument you can customize elliptic curve and algorithms used in all steps of encryption/decryption process.

There are a few library API functions that helps with customisation:

We also provide default params compatible with existing ECIES variants used in some other libraries.

Example 1:

% Alice and Bob agrees on the following params
Params = #{
  curve  => x25519,         % Edwards curve 25519
  kdf    => {hkdf, sha256}, % HMAC-based Extract-and-Expand KDF with SHA256 hash
  cipher => aes_256_ctr,
  mac    => {hmac, sha256, 96} % HMAC with SHA256 and 96 bits output
},
  % Bob generates keys
{BobPublicKey, BobPrivateKey} = ecies:generate_key(Params),
% Alice knowing Bob's public key encrypts a message for him
Data = ecies:public_encrypt(BobPublicKey, <<"top secret message">>, Params),
% Bob is able to decrypt the message using his private key
<<"top secret message">> = ecies:private_decrypt(BobPrivateKey, Data, Params).

Example 2:

% Decrypting electrum compatible message
Params = ecies_electrum:default_params(),
PrivateKey = binary:decode_hex(<<"ee3231b5deea48b619814d72a6e1aa04a9f521df281afad5ada89f5393941b1c">>),
MessageBase64 = <<"QklFMQJdmY+9Ys1WjqANreLwXaau62N01r9lebJ9Rp7Az+XRMdNAVgg3J8EEVhni5gn2v+WOD59uDMDp0zY/xPT3IElReQo6XUCSMmgRgRtYl+TUEw==">>,
<<"hello world">> = ecies:private_decrypt(PrivateKey, MessageBase64, Params).

The list of all supported elliptic curves (curve param), ciphers (cipher param) and hash functions used by KDF and MAC algorithms can be obtained using ecies:supports/1 function:

-spec supports(hashs)  -> [digest_type()];
              (curves) -> [named_curve()];
              (ciphers) -> [cipher()];
              (cmac_ciphers) -> [cmac_cipher()];
              (aead_ciphers) -> [aead_cipher()].

[!NOTE] For Edwards Curves 25519 and 448 use x25519, x448, and not ed25519, ed448

For key derivation (kdf param) you can use:

-type kdf_type() :: {hkdf, digest_type()} | % HMAC-based Extract-and-Expand Key Derivation Function (HKDF)
                    {kdf, digest_type()}  | % ANSI-X9.63 KDF
                    {concat_kdf, digest_type()}  | % NIST SP 800-56 Concatenation Key Derivation Function (see section 5.8.1).
                    kdf_fun().
% Custom KDF function
-type kdf_fun()  :: fun((SharedKey :: binary(), Info :: binary(), Length :: pos_integer()) -> Result :: binary()).

For message authentication (mac param):

-type mac_type()    :: {hmac, digest_type(), mac_bits()} | % HMAC for given digest function with specified output bits
                       {cmac, cmac_cipher(), mac_bits()} | % CMAC with AES-*-CBC cipher and given output bits
                       {aead, mac_bits()}. % Special case when AES CCM/GCM ciphers are used to just specify tag output bits 
-type mac_bits()    :: pos_integer() | default. % default atom means output size equal to given mac key length

License

ecies library is MIT-licensed, as per LICENSE.md.