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;
}