The Scrypt memory-hard function

As a conservative alternative to Argon2, Sodium provides an implementation of the Scrypt password hashing function.

Unless you have specific reasons to use scrypt, you should instead consider the default function, Argon2.

Example 1: key derivation

#define PASSWORD "Correct Horse Battery Staple"
#define KEY_LEN crypto_box_SEEDBYTES

unsigned char salt[crypto_pwhash_scryptsalsa208sha256_SALTBYTES];
unsigned char key[KEY_LEN];

randombytes_buf(salt, sizeof salt);

if (crypto_pwhash_scryptsalsa208sha256
    (key, sizeof key, PASSWORD, strlen(PASSWORD), salt,
     crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_INTERACTIVE,
     crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_INTERACTIVE) != 0) {
    /* out of memory */
}

Example 2: password storage

#define PASSWORD "Correct Horse Battery Staple"

char hashed_password[crypto_pwhash_scryptsalsa208sha256_STRBYTES];

if (crypto_pwhash_scryptsalsa208sha256_str
    (hashed_password, PASSWORD, strlen(PASSWORD),
     crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_SENSITIVE,
     crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_SENSITIVE) != 0) {
    /* out of memory */
}

if (crypto_pwhash_scryptsalsa208sha256_str_verify
    (hashed_password, PASSWORD, strlen(PASSWORD)) != 0) {
    /* wrong password */
}

Key derivation

int crypto_pwhash_scryptsalsa208sha256(unsigned char * const out,
                                       unsigned long long outlen,
                                       const char * const passwd,
                                       unsigned long long passwdlen,
                                       const unsigned char * const salt,
                                       unsigned long long opslimit,
                                       size_t memlimit);

The crypto_pwhash_scryptsalsa208sha256() function derives an outlen bytes long key from a password passwd whose length is passwdlen and a salt salt whose fixed length is crypto_pwhash_scryptsalsa208sha256_SALTBYTES bytes.

The computed key is stored into out. out (and hence outlen) should be at least crypto_pwhash_scryptsalsa208sha256_BYTES_MIN and at most crypto_pwhash_scryptsalsa208sha256_BYTES_MAX (~ 127 GB).

passwd (and hence passwdlen) should be at least crypto_pwhash_scryptsalsa208sha256_PASSWD_MIN and at most crypto_pwhash_scryptsalsa208sha256_PASSWD_MAX.

opslimit represents a maximum amount of computations to perform. Raising this number will make the function require more CPU cycles to compute a key. This number must be between crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_MIN and crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_MAX

memlimit is the maximum amount of RAM that the function will use, in bytes. It is highly recommended to allow the function to use at least 16 megabytes. This number must be between crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_MIN and crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_MAX

For interactive, online operations, crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_INTERACTIVE and crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_INTERACTIVE provide a safe base line for these two parameters. However, using higher values may improve security.

For highly sensitive data, crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_SENSITIVE and crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_SENSITIVE can be used as an alternative. But with these parameters, deriving a key takes about 2 seconds on a 2.8 Ghz Core i7 CPU and requires up to 1 gigabyte of dedicated RAM.

The salt should be unpredictable. randombytes_buf() is the easiest way to fill the crypto_pwhash_scryptsalsa208sha256_SALTBYTES bytes of the salt.

Keep in mind that in order to produce the same key from the same password, the same salt, and the same values for opslimit and memlimit have to be used. Therefore, these parameters have to be stored for each user.

The function returns 0 on success, and -1 if the computation didn't complete, usually because the operating system refused to allocate the amount of requested memory.

Password storage

int crypto_pwhash_scryptsalsa208sha256_str(char out[crypto_pwhash_scryptsalsa208sha256_STRBYTES],
                                           const char * const passwd,
                                           unsigned long long passwdlen,
                                           unsigned long long opslimit,
                                           size_t memlimit);

The crypto_pwhash_scryptsalsa208sha256_str() function puts an ASCII encoded string into out, which includes:

  • the result of a memory-hard, CPU-intensive hash function applied to the password passwd of length passwdlen
  • the automatically generated salt used for the previous computation
  • the other parameters required to verify the password: opslimit and memlimit.

crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_INTERACTIVE and crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_INTERACTIVE are safe baseline values to use for opslimit and memlimit.

The output string is zero-terminated, includes only ASCII characters and can be safely stored into SQL databases and other data stores. No extra information has to be stored in order to verify the password.

The function returns 0 on success and -1 if it didn't complete successfully.

int crypto_pwhash_scryptsalsa208sha256_str_verify(const char str[crypto_pwhash_scryptsalsa208sha256_STRBYTES],
                                                  const char * const passwd,
                                                  unsigned long long passwdlen);

This function verifies that the password str is a valid password verification string (as generated by crypto_pwhash_scryptsalsa208sha256_str()) for passwd whose length is passwdlen.

str has to be zero-terminated.

It returns 0 if the verification succeeds, and -1 on error.

Guidelines for choosing scrypt parameters

Start by determining how much memory the scrypt function can use. What will be the highest number of threads/processes evaluating the function simultaneously (ideally, no more than 1 per CPU core)? How much physical memory is guaranteed to be available?

memlimit should be a power of 2. Do not use anything less than 16 Mb, even for interactive use.

Then, a reasonable starting point for opslimit is memlimit / 32.

Measure how long the scrypt function needs in order to hash a password. If this it is way too long for your application, reduce memlimit and adjust opslimit using the above formula.

If the function is so fast that you can afford it to be more computationally intensive without any usability issues, increase opslimit.

For online use (e.g. login in on a website), a 1 second computation is likely to be the acceptable maximum.

For interactive use (e.g. a desktop application), a 5 second pause after having entered a password is acceptable if the password doesn't need to be entered more than once per session.

For non-interactive use and infrequent use (e.g. restoring an encrypted backup), an even slower computation can be an option.

But the best defense against brute-force password cracking remains using strong passwords. Libraries such as passwdqc can help enforce this.

Low-level scrypt API

The traditional, low-level scrypt API is also available:

int crypto_pwhash_scryptsalsa208sha256_ll(const uint8_t * passwd, size_t passwdlen,
                                          const uint8_t * salt, size_t saltlen,
                                          uint64_t N, uint32_t r, uint32_t p,
                                          uint8_t * buf, size_t buflen);

Please note that r is specified in kilobytes, and not in bytes as in the Sodium API.

Constants

  • crypto_pwhash_scryptsalsa208sha256_BYTES_MIN
  • crypto_pwhash_scryptsalsa208sha256_BYTES_MAX
  • crypto_pwhash_scryptsalsa208sha256_PASSWD_MIN
  • crypto_pwhash_scryptsalsa208sha256_PASSWD_MAX
  • crypto_pwhash_scryptsalsa208sha256_SALTBYTES
  • crypto_pwhash_scryptsalsa208sha256_STRBYTES
  • crypto_pwhash_scryptsalsa208sha256_STRPREFIX
  • crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_MIN
  • crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_MAX
  • crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_MIN
  • crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_MAX
  • crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_INTERACTIVE
  • crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_INTERACTIVE
  • crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_SENSITIVE
  • crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_SENSITIVE

Notes

Do not forget to initialize the library with sodium_init(). crypto_pwhash_scryptsalsa208sha256_* will still work without doing so, but possibly way slower.

Do not use constants (including crypto_pwhash_cryptsalsa208sha256_OPSLIMIT_* and crypto_pwhash_cryptsalsa208sha256_MEMLIMIT_*) in order to verify a password or produce a deterministic output. Save the parameters along with the hash instead.

For password verification, the recommended interface is crypto_pwhash_cryptsalsa208sha256_str() and crypto_pwhash_cryptsalsa208sha256_str_verify(). The string produced by crypto_pwhash_cryptsalsa208sha256_str() already includes an algorithm identifier, as well as all the parameters (including the automatically generated salt) that have been used to hash the password. Subsequently, crypto_pwhash_cryptsalsa208sha256_str_verify() automatically decodes these parameters.

By doing so, passwords can be rehashed using different parameters if required later on.

Cleartext passwords should not stay in memory longer than needed.

It is highly recommended to use sodium_mlock() to lock memory regions storing cleartext passwords, and to call sodium_munlock() right after crypto_pwhash_scryptsalsa208sha256_str() and crypto_pwhash_scryptsalsa208sha256_str_verify() return.

sodium_munlock() overwrites the region with zeros before unlocking it, so it doesn't have to be done before calling this function.

By design, a password whose length is 65 bytes or more is reduced to SHA-256(password). This can have security implications if the password is present in another password database using raw, unsalted SHA-256. Or when upgrading passwords previously hashed with unsalted SHA-256 to scrypt.

If this is a concern, passwords should be pre-hashed before being hashed using scrypt:

char prehashed_password[56];
crypto_generichash((unsigned char *) prehashed_password, 56,
    (const unsigned char *) password, strlen(password), NULL, 0);
crypto_pwhash_scryptsalsa208sha256_str(out, prehashed_password, 56, ...);
...
crypto_pwhash_scryptsalsa208sha256_str_verify(str, prehashed_password, 56);

Algorithm details

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