module OpenSSL::KDF

Provides functionality of various KDFs (key derivation function).

KDF is typically used for securely deriving arbitrary length symmetric keys to be used with an OpenSSL::Cipher from passwords. Another use case is for storing passwords: Due to the ability to tweak the effort of computation by increasing the iteration count, computation can be slowed down artificially in order to render possible attacks infeasible.

Currently, OpenSSL::KDF provides implementations for the following KDF:

  • PKCS #5 PBKDF2 (Password-Based Key Derivation Function 2) in combination with HMAC

  • scrypt

  • HKDF

Examples ↑

Generating a 128 bit key for a Cipher (e.g. AES) ↑

pass = "secret"
salt = OpenSSL::Random.random_bytes(16)
iter = 20_000
key_len = 16
key = OpenSSL::KDF.pbkdf2_hmac(pass, salt: salt, iterations: iter,
                               length: key_len, hash: "sha1")

Storing Passwords ↑

pass = "secret"
# store this with the generated value
salt = OpenSSL::Random.random_bytes(16)
iter = 20_000
hash = OpenSSL::Digest::SHA256.new
len = hash.digest_length
# the final value to be stored
value = OpenSSL::KDF.pbkdf2_hmac(pass, salt: salt, iterations: iter,
                                 length: len, hash: hash)

Important Note on Checking Passwords ↑

When comparing passwords provided by the user with previously stored values, a common mistake made is comparing the two values using “==”. Typically, “==” short-circuits on evaluation, and is therefore vulnerable to timing attacks. The proper way is to use a method that always takes the same amount of time when comparing two values, thus not leaking any information to potential attackers. To compare two values, the following could be used:

def eql_time_cmp(a, b)
  unless a.length == b.length
    return false
  end
  cmp = b.bytes
  result = 0
  a.bytes.each_with_index {|c,i|
    result |= c ^ cmp[i]
  }
  result == 0
end

Please note that the premature return in case of differing lengths typically does not leak valuable information - when using PBKDF2, the length of the values to be compared is of fixed size.

Public Class Methods

hkdf(ikm, salt:, info:, length:, hash:) → String click to toggle source

HMAC-based Extract-and-Expand Key Derivation Function (HKDF) as specified in RFC 5869.

New in OpenSSL 1.1.0.

Parameters ↑

ikm

The input keying material.

salt

The salt.

info

The context and application specific information.

length

The output length in octets. Must be <= 255 * HashLen, where HashLen is the length of the hash function output in octets.

hash

The hash function.

static VALUE
kdf_hkdf(int argc, VALUE *argv, VALUE self)
{
    VALUE ikm, salt, info, opts, kwargs[4], str;
    static ID kwargs_ids[4];
    int saltlen, ikmlen, infolen;
    size_t len;
    const EVP_MD *md;
    EVP_PKEY_CTX *pctx;

    if (!kwargs_ids[0]) {
        kwargs_ids[0] = rb_intern_const("salt");
        kwargs_ids[1] = rb_intern_const("info");
        kwargs_ids[2] = rb_intern_const("length");
        kwargs_ids[3] = rb_intern_const("hash");
    }
    rb_scan_args(argc, argv, "1:", &ikm, &opts);
    rb_get_kwargs(opts, kwargs_ids, 4, 0, kwargs);

    StringValue(ikm);
    ikmlen = RSTRING_LENINT(ikm);
    salt = StringValue(kwargs[0]);
    saltlen = RSTRING_LENINT(salt);
    info = StringValue(kwargs[1]);
    infolen = RSTRING_LENINT(info);
    len = (size_t)NUM2LONG(kwargs[2]);
    if (len > LONG_MAX)
        rb_raise(rb_eArgError, "length must be non-negative");
    md = ossl_evp_get_digestbyname(kwargs[3]);

    str = rb_str_new(NULL, (long)len);
    pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
    if (!pctx)
        ossl_raise(eKDF, "EVP_PKEY_CTX_new_id");
    if (EVP_PKEY_derive_init(pctx) <= 0) {
        EVP_PKEY_CTX_free(pctx);
        ossl_raise(eKDF, "EVP_PKEY_derive_init");
    }
    if (EVP_PKEY_CTX_set_hkdf_md(pctx, md) <= 0) {
        EVP_PKEY_CTX_free(pctx);
        ossl_raise(eKDF, "EVP_PKEY_CTX_set_hkdf_md");
    }
    if (EVP_PKEY_CTX_set1_hkdf_salt(pctx, (unsigned char *)RSTRING_PTR(salt),
                                    saltlen) <= 0) {
        EVP_PKEY_CTX_free(pctx);
        ossl_raise(eKDF, "EVP_PKEY_CTX_set_hkdf_salt");
    }
    if (EVP_PKEY_CTX_set1_hkdf_key(pctx, (unsigned char *)RSTRING_PTR(ikm),
                                   ikmlen) <= 0) {
        EVP_PKEY_CTX_free(pctx);
        ossl_raise(eKDF, "EVP_PKEY_CTX_set_hkdf_key");
    }
    if (EVP_PKEY_CTX_add1_hkdf_info(pctx, (unsigned char *)RSTRING_PTR(info),
                                    infolen) <= 0) {
        EVP_PKEY_CTX_free(pctx);
        ossl_raise(eKDF, "EVP_PKEY_CTX_set_hkdf_info");
    }
    if (EVP_PKEY_derive(pctx, (unsigned char *)RSTRING_PTR(str), &len) <= 0) {
        EVP_PKEY_CTX_free(pctx);
        ossl_raise(eKDF, "EVP_PKEY_derive");
    }
    rb_str_set_len(str, (long)len);
    EVP_PKEY_CTX_free(pctx);

    return str;
}
pbkdf2_hmac(pass, salt:, iterations:, length:, hash:) → aString click to toggle source

PKCS #5 PBKDF2 (Password-Based Key Derivation Function 2) in combination with HMAC. Takes pass, salt and iterations, and then derives a key of length bytes.

For more information about PBKDF2, see RFC 2898 Section 5.2 (tools.ietf.org/html/rfc2898#section-5.2).

Parameters ↑

pass

The passphrase.

salt

The salt. Salts prevent attacks based on dictionaries of common passwords and attacks based on rainbow tables. It is a public value that can be safely stored along with the password (e.g. if the derived value is used for password storage).

iterations

The iteration count. This provides the ability to tune the algorithm. It is better to use the highest count possible for the maximum resistance to brute-force attacks.

length

The desired length of the derived key in octets.

hash

The hash algorithm used with HMAC for the PRF. May be a String representing the algorithm name, or an instance of OpenSSL::Digest.

static VALUE
kdf_pbkdf2_hmac(int argc, VALUE *argv, VALUE self)
{
    VALUE pass, salt, opts, kwargs[4], str;
    static ID kwargs_ids[4];
    int iters, len;
    const EVP_MD *md;

    if (!kwargs_ids[0]) {
        kwargs_ids[0] = rb_intern_const("salt");
        kwargs_ids[1] = rb_intern_const("iterations");
        kwargs_ids[2] = rb_intern_const("length");
        kwargs_ids[3] = rb_intern_const("hash");
    }
    rb_scan_args(argc, argv, "1:", &pass, &opts);
    rb_get_kwargs(opts, kwargs_ids, 4, 0, kwargs);

    StringValue(pass);
    salt = StringValue(kwargs[0]);
    iters = NUM2INT(kwargs[1]);
    len = NUM2INT(kwargs[2]);
    md = ossl_evp_get_digestbyname(kwargs[3]);

    str = rb_str_new(0, len);
    if (!PKCS5_PBKDF2_HMAC(RSTRING_PTR(pass), RSTRING_LENINT(pass),
                           (unsigned char *)RSTRING_PTR(salt),
                           RSTRING_LENINT(salt), iters, md, len,
                           (unsigned char *)RSTRING_PTR(str)))
        ossl_raise(eKDF, "PKCS5_PBKDF2_HMAC");

    return str;
}
scrypt(pass, salt:, N:, r:, p:, length:) → aString click to toggle source

Derives a key from pass using given parameters with the scrypt password-based key derivation function. The result can be used for password storage.

scrypt is designed to be memory-hard and more secure against brute-force attacks using custom hardwares than alternative KDFs such as PBKDF2 or bcrypt.

The keyword arguments N, r and p can be used to tune scrypt. RFC 7914 (published on 2016-08, tools.ietf.org/html/rfc7914#section-2) states that using values r=8 and p=1 appears to yield good results.

See RFC 7914 (tools.ietf.org/html/rfc7914) for more information.

Parameters ↑

pass

Passphrase.

salt

Salt.

N

CPU/memory cost parameter. This must be a power of 2.

r

Block size parameter.

p

Parallelization parameter.

length

Length in octets of the derived key.

Example ↑

pass = "password"
salt = SecureRandom.random_bytes(16)
dk = OpenSSL::KDF.scrypt(pass, salt: salt, N: 2**14, r: 8, p: 1, length: 32)
p dk #=> "\xDA\xE4\xE2...\x7F\xA1\x01T"

static VALUE
kdf_scrypt(int argc, VALUE *argv, VALUE self)
{
    VALUE pass, salt, opts, kwargs[5], str;
    static ID kwargs_ids[5];
    size_t len;
    uint64_t N, r, p, maxmem;

    if (!kwargs_ids[0]) {
        kwargs_ids[0] = rb_intern_const("salt");
        kwargs_ids[1] = rb_intern_const("N");
        kwargs_ids[2] = rb_intern_const("r");
        kwargs_ids[3] = rb_intern_const("p");
        kwargs_ids[4] = rb_intern_const("length");
    }
    rb_scan_args(argc, argv, "1:", &pass, &opts);
    rb_get_kwargs(opts, kwargs_ids, 5, 0, kwargs);

    StringValue(pass);
    salt = StringValue(kwargs[0]);
    N = NUM2UINT64T(kwargs[1]);
    r = NUM2UINT64T(kwargs[2]);
    p = NUM2UINT64T(kwargs[3]);
    len = NUM2LONG(kwargs[4]);
    /*
     * OpenSSL uses 32MB by default (if zero is specified), which is too small.
     * Let's not limit memory consumption but just let malloc() fail inside
     * OpenSSL. The amount is controllable by other parameters.
     */
    maxmem = SIZE_MAX;

    str = rb_str_new(0, len);
    if (!EVP_PBE_scrypt(RSTRING_PTR(pass), RSTRING_LEN(pass),
                        (unsigned char *)RSTRING_PTR(salt), RSTRING_LEN(salt),
                        N, r, p, maxmem, (unsigned char *)RSTRING_PTR(str), len))
        ossl_raise(eKDF, "EVP_PBE_scrypt");

    return str;
}