scrypt/scrypt.1

.\" Copyright 2009 Colin Percival
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.Dd @DATE@
.Dt SCRYPT 1
.Os
.Sh NAME
.Nm scrypt
.Nd encrypt and decrypt files.
.Sh SYNOPSIS
.Nm
.Brq Cm enc | Cm dec | Cm info
.Op Fl f
.Op Fl -logN Ar value
.Op Fl M Ar maxmem
.Op Fl m Ar maxmemfrac
.Op Fl P
.Op Fl p Ar value
.Op Fl -passphrase Ar method:arg
.Op Fl r Ar value
.Op Fl t Ar maxtime
.Op Fl v
.Ar infile
.Op Ar outfile
.Nm
.Fl -version
.Sh DESCRIPTION
.Nm Cm enc
encrypts
.Ar infile
and writes the result to
.Ar 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.
.Pp
.Nm Cm dec
decrypts
.Ar infile
and writes the result to
.Ar 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.
.Pp
.Nm Cm info
provides information about the encryption parameters used for
.Ar infile .
.Pp
If
.Fl P
is not given,
.Nm
reads passphrases from its controlling terminal, or failing that,
from stdin.  Prompts are only printed when
.Nm
is reading passphrases from some terminal.  If
.Fl P
is given, then
.Nm
does not print any prompts, and reads a passphrase from stdin.
.Sh OPTIONS
.Bl -tag -width "-m maxmemfrac"
.It Fl f
Force the decryption to proceed even if it is anticipated to
require an excessive amount of memory or CPU time.
.It Fl -logN Ar value
Set the work parameter N to
.Pf 2^ Ar value .
If
.Fl -logN
is set,
.Fl r
and
.Fl p
must also be set.  If such explicit parameters are given, the
resource limits set by
.Fl M ,
.Fl m ,
and
.Fl t
are not enforced.
.It Fl M Ar maxmem
Use at most
.Ar maxmem
bytes of RAM to compute the derived encryption key.
.It Fl m Ar maxmemfrac
Use at most the fraction
.Ar maxmemfrac
of the available RAM to compute the derived encryption key.
.It Fl P
Always read passphrase from stdin, and do so only once even
when encrypting.  This cannot be used if
.Ar infile
is also stdin (aka '-').
.It Fl p Ar value
Set the work parameter p to
.Ar value .
If
.Fl p
is set,
.Fl -logN
and
.Fl r
must also be set.  If such explicit parameters are given, the
resource limits set by
.Fl M ,
.Fl m ,
and
.Fl t
are not enforced.
.It Fl -passphrase Ar method:arg
Read the passphrase using the specified method.
.Bl -tag -width aa
.It Ar dev:tty-stdin
Attempt to read the passphrase from /dev/tty; if that fails, read
it from stdin.  This is the default behaviour.
.It Ar dev:stdin-once
Attempt to read the passphrase from stdin, and do so only once
even when encrypting.  This cannot be used if
.Ar infile
is also stdin (aka '-').
.It Ar dev:tty-once
Attempt to read the passphrase from /dev/tty, and do so only once
even when encrypting.
.It Ar env:VAR
Read the passphrase from the environment variable specified by
.Ar VAR .
.Pp
.Bf Em
Storing a passphrase in an environment variable may be a security risk.
.Ef
Only use this option if you are certain that you know what you are doing.
.It Ar file:FILENAME
Read the passphrase from the file specified by
.Ar FILENAME .
.Pp
.Bf Em
Storing a passphrase in a file may be a security risk.
.Ef
Only use this option if you are certain that you know what you are doing.
.El
.It Fl r Ar value
Set the work parameter r to
.Ar value .
If
.Fl r
is set,
.Fl -logN
and
.Fl p
must also be set.  If such explicit parameters are given, the
resource limits set by
.Fl M ,
.Fl m ,
and
.Fl t
are not enforced.
.It Fl t Ar maxtime
Use at most
.Ar maxtime
seconds of CPU time to compute the derived encryption key.
.It Fl -v
Print encryption parameters (N, r, p) and memory/cpu limits.
.It Fl -version
Print version of scrypt, and exit.
.El
.Pp
In
.Nm Cm enc ,
the memory and CPU time limits are enforced by picking
appropriate parameters to the
.Nm
key derivation function.
In
.Nm Cm 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.
.Sh EXIT STATUS
The
.Nm
utility exits 0 on success, and >0 if an error occurs.
.Pp
Note that if the input encrypted file is corrupted,
.Nm Cm 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
.Nm
before using the decrypted data.
.Sh 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
.Fl M ,
.Fl m ,
and/or
.Fl t ) ,
unless you specify explicit parameters via
.Fl -logN ,
.Fl p ,
.Fl r .
.Sh SEE ALSO
.Rs
.%A "Colin Percival"
.%T "Stronger Key Derivation via Sequential Memory-Hard Functions"
.%R "BSDCan'09"
.%D "May 2009"
.Re
.Rs
.%A "Colin Percival"
.%A "Simon Josefsson"
.%T "The scrypt Password-Based Key Derivation Function"
.%R "IETF RFC 7914"
.%D "August 2016"
.Re
.Sh HISTORY
The
.Nm
utility was written in May 2009 by Colin Percival as a
demonstration of the
.Nm
key derivation function.
The
.Nm
key derivation function was invented in March 2009 by Colin
Percival in order to allow key files from the
.Nm Tarsnap
backup system to be passphrase protected.