crypt - encode/decode files
crypt reads from the standard input and writes on the standard output.
password is a key that selects a particular transformation. If no
password is given, crypt demands a key from the terminal and turns off
printing while the key is being typed in. crypt encrypts and decrypts
with the same key:
crypt key <<<<clear >>>>cypher
crypt key <<<<cypher|pr
The latter command decrypts the file and prints the clear version.
Files encrypted by crypt are compatible with those treated by the ed
editor in encryption mode (see ed(1)).
Security of encrypted files depends on three factors: the fundamental
method must be hard to solve; direct search of the key space must be
infeasible; ``sneak paths'' by which keys or clear text can become
visible must be minimized.
crypt implements a one-rotor machine designed along the lines of the
German Enigma, but with a 256-element rotor. Methods of attack on
such machines are widely known; thus crypt provides minimal security.
The transformation of a key into the internal settings of the machine
is deliberately designed to be expensive; i.e., to take a substantial
fraction of a second to compute. However, if keys are restricted to,
for example, three lowercase letters, then encrypted files can be read
by expending only a substantial fraction of five minutes of machine
Since the key is an argument to the crypt command, it is potentially
visible to users executing the ps or a derivative (see ps(1)). The
choice of keys and key security are the most vulnerable aspect of
The following example demonstrates the use of crypt to edit a file
that the user wants to keep strictly confidential:
$ crypt <<<<plans >>>>plans.x
$ rm plans
$ vi -x plans.x
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$ crypt <<<<plans.x | pr
Note that the -x option is the encryption mode of vi, and prompts the
user for the same key with which the file was encrypted.
If output is piped to nroff and the encryption key is not given on the
command line, crypt can leave terminal modes in a strange state (see
nroff(1) and stty(1)).
If two or more files encrypted with the same key are concatenated and
an attempt is made to decrypt the result, only the the first of the
original files is decrypted correctly.
/dev/tty for typed key
ed(1), makekey(1), stty(1).
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