NAME

     scrypt — encrypt and decrypt files.

SYNOPSIS

     scrypt {enc | dec | info} [-f] [--logN value] [-M maxmem] [-m maxmemfrac] [-P] [-p value]
            [--passphrase method:arg] [-r value] [-t maxtime] [-v] infile [outfile]
     scrypt --version

DESCRIPTION

     scrypt enc encrypts infile and writes the result to outfile if specified, or the standard
     output otherwise.  The user will be prompted to enter a passphrase (twice) to be used to
     generate a derived encryption key.

     scrypt dec decrypts infile and writes the result to outfile if specified, or the standard
     output otherwise.  The user will be prompted to enter the passphrase used at encryption time
     to generate the derived encryption key.

     scrypt info provides information about the encryption parameters used for infile.

     If -P is not given, scrypt reads passphrases from its controlling terminal, or failing that,
     from stdin.  Prompts are only printed when scrypt is reading passphrases from some terminal.
     If -P is given, then scrypt does not print any prompts, and reads a passphrase from stdin.

OPTIONS

     -f             Force the decryption to proceed even if it is anticipated to require an
                    excessive amount of memory or CPU time.

     --logN value   Set the work parameter N to 2^value.  If --logN is set, -r and -p must also
                    be set.  If such explicit parameters are given, the resource limits set by
                    -M, -m, and -t are not enforced.

     -M maxmem      Use at most maxmem bytes of RAM to compute the derived encryption key.

     -m maxmemfrac  Use at most the fraction maxmemfrac of the available RAM to compute the
                    derived encryption key.

     -P             Always read passphrase from stdin, and do so only once even when encrypting.
                    This cannot be used if infile is also stdin (aka '-').

     -p value       Set the work parameter p to value.  If -p is set, --logN and -r must also be
                    set.  If such explicit parameters are given, the resource limits set by -M,
                    -m, and -t are not enforced.

     --passphrase method:arg
                    Read the passphrase using the specified method.

                    dev:tty-stdin
                        Attempt to read the passphrase from /dev/tty; if that fails, read it from
                        stdin.  This is the default behaviour.

                    dev:stdin-once
                        Attempt to read the passphrase from stdin, and do so only once even when
                        encrypting.  This cannot be used if infile is also stdin (aka '-').

                    dev:tty-once
                        Attempt to read the passphrase from /dev/tty, and do so only once even
                        when encrypting.

                    env:VAR
                        Read the passphrase from the environment variable specified by VAR.

                        Storing a passphrase in an environment variable may be a security risk.
                        Only use this option if you are certain that you know what you are doing.

                    file:FILENAME
                        Read the passphrase from the file specified by FILENAME.

                        Storing a passphrase in a file may be a security risk.  Only use this
                        option if you are certain that you know what you are doing.

     -r value       Set the work parameter r to value.  If -r is set, --logN and -p must also be
                    set.  If such explicit parameters are given, the resource limits set by -M,
                    -m, and -t are not enforced.

     -t maxtime     Use at most maxtime seconds of CPU time to compute the derived encryption
                    key.

     --v            Print encryption parameters (N, r, p) and memory/cpu limits.

     --version      Print version of scrypt, and exit.

     In scrypt enc, the memory and CPU time limits are enforced by picking appropriate parameters
     to the scrypt key derivation function.  In scrypt dec, the memory and CPU time limits are
     enforced by exiting with an error if decrypting the file would require too much memory or
     CPU time.

EXIT STATUS

     The scrypt utility exits 0 on success, and >0 if an error occurs.

     Note that if the input encrypted file is corrupted, scrypt dec may produce output prior to
     determining that the input was corrupt and exiting with a non-zero status; so users should
     direct the output to a safe location and check the exit status of scrypt before using the
     decrypted data.

ALGORITHM PARAMETERS

     The scrypt algorithm has three tuneable work parameters: N, r, p.  When decrypting, scrypt
     will always use the values specified by the encryption header.  When encrypting, scrypt will
     choose appropriate values based on your system's speed and memory (influenced by -M, -m,
     and/or -t), unless you specify explicit parameters via --logN, -p, -r.

SEE ALSO

     Colin Percival, Stronger Key Derivation via Sequential Memory-Hard Functions, BSDCan'09, May
     2009.

     Colin Percival and Simon Josefsson, The scrypt Password-Based Key Derivation Function, IETF
     RFC 7914, August 2016.

HISTORY

     The scrypt utility was written in May 2009 by Colin Percival as a demonstration of the
     scrypt key derivation function.  The scrypt key derivation function was invented in March
     2009 by Colin Percival in order to allow key files from the Tarsnap backup system to be
     passphrase protected.