EVP_KDF_SCRYPT.7ssl - Man Page

The scrypt EVP_KDF implementation


Support for computing the scrypt password-based KDF through the EVP_KDF API.

The EVP_KDF_SCRYPT algorithm implements the scrypt password-based key derivation function, as described in RFC 7914.  It is memory-hard in the sense that it deliberately requires a significant amount of RAM for efficient computation. The intention of this is to render brute forcing of passwords on systems that lack large amounts of main memory (such as GPUs or ASICs) computationally infeasible.

scrypt provides three work factors that can be customized: N, r and p. N, which has to be a positive power of two, is the general work factor and scales CPU time in an approximately linear fashion. r is the block size of the internally used hash function and p is the parallelization factor. Both r and p need to be greater than zero. The amount of RAM that scrypt requires for its computation is roughly (128 * N * r * p) bytes.

In the original paper of Colin Percival (“Stronger Key Derivation via Sequential Memory-Hard Functions”, 2009), the suggested values that give a computation time of less than 5 seconds on a 2.5 GHz Intel Core 2 Duo are N = 2^20 = 1048576, r = 8, p = 1. Consequently, the required amount of memory for this computation is roughly 1 GiB. On a more recent CPU (Intel i7-5930K at 3.5 GHz), this computation takes about 3 seconds. When N, r or p are not specified, they default to 1048576, 8, and 1, respectively. The maximum amount of RAM that may be used by scrypt defaults to 1025 MiB.

Numeric identity

EVP_KDF_SCRYPT is the numeric identity for this implementation; it can be used with the EVP_KDF_CTX_new_id() function.

Supported controls

The supported controls are:


These controls work as described in “CONTROLS” in EVP_KDF_CTX(3).


EVP_KDF_CTRL_SET_SCRYPT_N expects one argument: uint64_t N

EVP_KDF_CTRL_SET_SCRYPT_R expects one argument: uint32_t r

EVP_KDF_CTRL_SET_SCRYPT_P expects one argument: uint32_t p

These controls configure the scrypt work factors N, r and p.

EVP_KDF_ctrl_str() type strings: “N”, “r” and “p”, respectively.

The corresponding value strings are expected to be decimal numbers.


A context for scrypt can be obtained by calling:


The output length of an scrypt key derivation is specified via the keylen parameter to the EVP_KDF_derive(3) function.


This example derives a 64-byte long test vector using scrypt with the password “password”, salt “NaCl” and N = 1024, r = 8, p = 16.

 EVP_KDF_CTX *kctx;
 unsigned char out[64];

 kctx = EVP_KDF_CTX_new_id(EVP_KDF_SCRYPT);

 if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_PASS, "password", (size_t)8) <= 0) {
 if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_SALT, "NaCl", (size_t)4) <= 0) {
 if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_SCRYPT_N, (uint64_t)1024) <= 0) {
 if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_SCRYPT_R, (uint32_t)8) <= 0) {
 if (EVP_KDF_ctrl(kctx, EVP_KDF_CTRL_SET_SCRYPT_P, (uint32_t)16) <= 0) {
 if (EVP_KDF_derive(kctx, out, sizeof(out)) <= 0) {

     const unsigned char expected[sizeof(out)] = {
         0xfd, 0xba, 0xbe, 0x1c, 0x9d, 0x34, 0x72, 0x00,
         0x78, 0x56, 0xe7, 0x19, 0x0d, 0x01, 0xe9, 0xfe,
         0x7c, 0x6a, 0xd7, 0xcb, 0xc8, 0x23, 0x78, 0x30,
         0xe7, 0x73, 0x76, 0x63, 0x4b, 0x37, 0x31, 0x62,
         0x2e, 0xaf, 0x30, 0xd9, 0x2e, 0x22, 0xa3, 0x88,
         0x6f, 0xf1, 0x09, 0x27, 0x9d, 0x98, 0x30, 0xda,
         0xc7, 0x27, 0xaf, 0xb9, 0x4a, 0x83, 0xee, 0x6d,
         0x83, 0x60, 0xcb, 0xdf, 0xa2, 0xcc, 0x06, 0x40

     assert(!memcmp(out, expected, sizeof(out)));


Conforming to

RFC 7914

See Also

EVP_KDF_CTX, EVP_KDF_CTX_new_id(3), EVP_KDF_CTX_free(3), EVP_KDF_ctrl(3), EVP_KDF_derive(3), “CONTROLS” in EVP_KDF_CTX(3)

Referenced By


2021-03-26 1.1.1k OpenSSL