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.new('SHA256') 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 do this, use OpenSSL.fixed_length_secure_compare
.
Public Class Methods
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.
Example¶ ↑
# The values from https://datatracker.ietf.org/doc/html/rfc5869#appendix-A.1 ikm = ["0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b"].pack("H*") salt = ["000102030405060708090a0b0c"].pack("H*") info = ["f0f1f2f3f4f5f6f7f8f9"].pack("H*") p OpenSSL::KDF.hkdf(ikm, salt: salt, info: info, length: 42, hash: "SHA256").unpack1("H*") # => "3cb25f25faacd57a90434f64d0362f2a2d2d0a90cf1a5a4c5db02d56ecc4c5bf34007208d5b887185865"
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; }
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 password.
- 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 ofOpenSSL::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; }
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; }