sample hack session with sort command

$sort -t: -k3 --debug /etc/passwd
sort: using ‘en_US.UTF-8’ sorting rules
root:x:0:0:root:/root:/bin/bash
       ________________________
_______________________________
jeffrin:x:1000:1000:Jeffrin Jose T,,,:/home/jeffrin:/bin/bash
          ___________________________________________________
_____________________________________________________________
uuidd:x:100:101::/run/uuidd:/bin/false
        ______________________________
______________________________________
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
       ____________________________________________
___________________________________________________
Debian-exim:x:101:103::/var/spool/exim4:/bin/false
              ____________________________________
__________________________________________________
statd:x:102:65534::/var/lib/nfs:/bin/false
        __________________________________
__________________________________________
sshd:x:103:65534::/var/run/sshd:/usr/sbin/nologin
       __________________________________________
_________________________________________________
messagebus:x:104:106::/var/run/dbus:/bin/false
             _________________________________
______________________________________________
colord:x:105:109:colord colour management daemon,,,:/var/lib/colord:/bin/false
         _____________________________________________________________________
______________________________________________________________________________
usbmux:x:106:46:usbmux daemon,,,:/home/usbmux:/bin/false
         _______________________________________________
________________________________________________________
avahi:x:107:113:Avahi mDNS daemon,,,:/var/run/avahi-daemon:/bin/false
        _____________________________________________________________
_____________________________________________________________________
pulse:x:108:114:PulseAudio daemon,,,:/var/run/pulse:/bin/false
        ______________________________________________________
______________________________________________________________
speech-dispatcher:x:109:29:Speech Dispatcher,,,:/var/run/speech-dispatcher:/bin/sh
                    ______________________________________________________________
__________________________________________________________________________________
rtkit:x:110:116:RealtimeKit,,,:/proc:/bin/false
        _______________________________________
_______________________________________________
Debian-gdm:x:111:117:Gnome Display Manager:/var/lib/gdm3:/bin/false
             ______________________________________________________
___________________________________________________________________
saned:x:112:118::/var/lib/saned:/bin/false
        __________________________________
__________________________________________
ntp:x:113:120::/home/ntp:/bin/false
      _____________________________
___________________________________
geoclue:x:114:121::/var/lib/geoclue:/bin/false
          ____________________________________
______________________________________________
dnsmasq:x:115:65534:dnsmasq,,,:/var/lib/misc:/bin/false
          _____________________________________________
_______________________________________________________
fetchmail:x:116:65534::/var/lib/fetchmail:/bin/false
            ________________________________________
____________________________________________________
vde2-net:x:117:123::/var/run/vde2:/bin/false
           _________________________________
____________________________________________
timidity:x:118:125:TiMidity++ MIDI sequencer service:/etc/timidity:/bin/false
           __________________________________________________________________
_____________________________________________________________________________
epmd:x:119:128::/var/run/epmd:/bin/false
       _________________________________
________________________________________
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
         ______________________________________
_______________________________________________
systemd-timesync:x:120:129:systemd Time Synchronization,,,:/run/systemd:/bin/false
                   _______________________________________________________________
__________________________________________________________________________________
systemd-network:x:121:130:systemd Network Management,,,:/run/systemd/netif:/bin/false
                  ___________________________________________________________________
_____________________________________________________________________________________
systemd-resolve:x:122:131:systemd Resolver,,,:/run/systemd/resolve:/bin/false
                  ___________________________________________________________
_____________________________________________________________________________
systemd-bus-proxy:x:123:132:systemd Bus Proxy,,,:/run/systemd:/bin/false
                    ____________________________________________________
________________________________________________________________________
proftpd:x:124:65534::/run/proftpd:/bin/false
          __________________________________
____________________________________________
ftp:x:125:65534::/srv/ftp:/bin/false
      ______________________________
____________________________________
_apt:x:126:65534::/nonexistent:/bin/false
       __________________________________
_________________________________________
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
        __________________________________
__________________________________________
bin:x:2:2:bin:/bin:/usr/sbin/nologin
      ______________________________
____________________________________
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
           _________________________________________
____________________________________________________
sys:x:3:3:sys:/dev:/usr/sbin/nologin
      ______________________________
____________________________________
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
         ___________________________________________
____________________________________________________
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
       ______________________________________________________
_____________________________________________________________
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
      __________________________________________
________________________________________________
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
        _________________________________________________________________________
_________________________________________________________________________________
sync:x:4:65534:sync:/bin:/bin/sync
       ___________________________
__________________________________
games:x:5:60:games:/usr/games:/usr/sbin/nologin
        _______________________________________
_______________________________________________
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
      _________________________________________
_______________________________________________
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
         _________________________________________________
__________________________________________________________
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
     _______________________________________
____________________________________________
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
       ____________________________________
___________________________________________
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
       __________________________________________
_________________________________________________
$sort -t: -k3 --debug /etc/passwd
$sort -t: -k3 --debug /etc/passwd
sort: using ‘en_US.UTF-8’ sorting rules
root:x:0:0:root:/root:/bin/bash
       ________________________
_______________________________
jeffrin:x:1000:1000:Jeffrin Jose T,,,:/home/jeffrin:/bin/bash
          ___________________________________________________
_____________________________________________________________
uuidd:x:100:101::/run/uuidd:/bin/false
        ______________________________
______________________________________
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
       ____________________________________________
___________________________________________________
Debian-exim:x:101:103::/var/spool/exim4:/bin/false
              ____________________________________
__________________________________________________
statd:x:102:65534::/var/lib/nfs:/bin/false
        __________________________________
__________________________________________
sshd:x:103:65534::/var/run/sshd:/usr/sbin/nologin
       __________________________________________
_________________________________________________
messagebus:x:104:106::/var/run/dbus:/bin/false
             _________________________________
______________________________________________
colord:x:105:109:colord colour management daemon,,,:/var/lib/colord:/bin/false
         _____________________________________________________________________
______________________________________________________________________________
usbmux:x:106:46:usbmux daemon,,,:/home/usbmux:/bin/false
         _______________________________________________
________________________________________________________
avahi:x:107:113:Avahi mDNS daemon,,,:/var/run/avahi-daemon:/bin/false
        _____________________________________________________________
_____________________________________________________________________
pulse:x:108:114:PulseAudio daemon,,,:/var/run/pulse:/bin/false
        ______________________________________________________
______________________________________________________________
speech-dispatcher:x:109:29:Speech Dispatcher,,,:/var/run/speech-dispatcher:/bin/sh
                    ______________________________________________________________
__________________________________________________________________________________
rtkit:x:110:116:RealtimeKit,,,:/proc:/bin/false
        _______________________________________
_______________________________________________
Debian-gdm:x:111:117:Gnome Display Manager:/var/lib/gdm3:/bin/false
             ______________________________________________________
___________________________________________________________________
saned:x:112:118::/var/lib/saned:/bin/false
        __________________________________
__________________________________________
ntp:x:113:120::/home/ntp:/bin/false
      _____________________________
___________________________________
geoclue:x:114:121::/var/lib/geoclue:/bin/false
          ____________________________________
______________________________________________
dnsmasq:x:115:65534:dnsmasq,,,:/var/lib/misc:/bin/false
          _____________________________________________
_______________________________________________________
fetchmail:x:116:65534::/var/lib/fetchmail:/bin/false
            ________________________________________
____________________________________________________
vde2-net:x:117:123::/var/run/vde2:/bin/false
           _________________________________
____________________________________________
timidity:x:118:125:TiMidity++ MIDI sequencer service:/etc/timidity:/bin/false
           __________________________________________________________________
_____________________________________________________________________________
epmd:x:119:128::/var/run/epmd:/bin/false
       _________________________________
________________________________________
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
         ______________________________________
_______________________________________________
systemd-timesync:x:120:129:systemd Time Synchronization,,,:/run/systemd:/bin/false
                   _______________________________________________________________
__________________________________________________________________________________
systemd-network:x:121:130:systemd Network Management,,,:/run/systemd/netif:/bin/false
                  ___________________________________________________________________
_____________________________________________________________________________________
systemd-resolve:x:122:131:systemd Resolver,,,:/run/systemd/resolve:/bin/false
                  ___________________________________________________________
_____________________________________________________________________________
systemd-bus-proxy:x:123:132:systemd Bus Proxy,,,:/run/systemd:/bin/false
                    ____________________________________________________
________________________________________________________________________
proftpd:x:124:65534::/run/proftpd:/bin/false
          __________________________________
____________________________________________
ftp:x:125:65534::/srv/ftp:/bin/false
      ______________________________
____________________________________
_apt:x:126:65534::/nonexistent:/bin/false
       __________________________________
_________________________________________
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
        __________________________________
__________________________________________
bin:x:2:2:bin:/bin:/usr/sbin/nologin
      ______________________________
____________________________________
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
           _________________________________________
____________________________________________________
sys:x:3:3:sys:/dev:/usr/sbin/nologin
      ______________________________
____________________________________
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
         ___________________________________________
____________________________________________________
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
       ______________________________________________________
_____________________________________________________________
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
      __________________________________________
________________________________________________
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
        _________________________________________________________________________
_________________________________________________________________________________
sync:x:4:65534:sync:/bin:/bin/sync
       ___________________________
__________________________________
games:x:5:60:games:/usr/games:/usr/sbin/nologin
        _______________________________________
_______________________________________________
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
      _________________________________________
_______________________________________________
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
         _________________________________________________
__________________________________________________________
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
     _______________________________________
____________________________________________
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
       ____________________________________
___________________________________________
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
       __________________________________________
_________________________________________________
$

encryption related algorithms that are built into a typical Linux Kernel

ABOUT Cryptography

Cryptography or cryptology (from Ancient Greek: κρυπτός, translit. kryptós "hidden, secret"; and γράφειν graphein, "to write", or -λογία -logia, "study", respectively[1]) is the practice and study of techniques for secure communication in the presence of third parties called adversaries.[2] More generally, cryptography is about constructing and analyzing protocols that prevent third parties or the public from reading private messages;[3] various aspects in information security such as data confidentiality, data integrity, authentication, and non-repudiation[4] are central to modern cryptography. Modern cryptography exists at the intersection of the disciplines of mathematics, computer science, electrical engineering, communication science, and physics. Applications of cryptography include electronic commerce, chip-based payment cards, digital currencies, computer passwords, and military communications.

ABOUT CRC

A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data. Blocks of data entering these systems get a short check value attached, based on the remainder of a polynomial division of their contents. On retrieval, the calculation is repeated and, in the event the check values do not match, corrective action can be taken against data corruption. CRCs can be used for error correction (see bitfilters).[1]

CRC can't fix the mistakes in frames which change the value of DE("Discard eligibility"), for example if by a mistake DE value was set from 0 to 1, CRC won't be able to return it to 0, the frame will be discarded.

CRCs are so called because the check (data verification) value is a redundancy (it expands the message without adding information) and the algorithm is based on cyclic codes. CRCs are popular because they are simple to implement in binary hardware, easy to analyze mathematically, and particularly good at detecting common errors caused by noise in transmission channels. Because the check value has a fixed length, the function that generates it is occasionally used as a hash function.

[bash]
$sudo cat /proc/crypto
name : crct10dif
driver : crct10dif-pclmul
module : crct10dif_pclmul
priority : 200
refcnt : 1
selftest : passed
internal : no
type : shash
blocksize : 1
digestsize : 2

name : crc32
driver : crc32-pclmul
module : crc32_pclmul
priority : 200
refcnt : 1
selftest : passed
internal : no
type : shash
blocksize : 1
digestsize : 4

name : crc32c
driver : crc32c-intel
module : crc32c_intel
priority : 200
refcnt : 1
selftest : passed
internal : no
type : shash
blocksize : 1
digestsize : 4

name : lzo
driver : lzo-generic
module : kernel
priority : 0
refcnt : 5
selftest : passed
internal : no
type : compression

name : crct10dif
driver : crct10dif-generic
module : kernel
priority : 100
refcnt : 2
selftest : passed
internal : no
type : shash
blocksize : 1
digestsize : 2

name : aes
driver : aes-generic
module : kernel
priority : 100
refcnt : 1
selftest : passed
internal : no
type : cipher
blocksize : 16
min keysize : 16
max keysize : 32

name : sha1
driver : sha1-generic
module : kernel
priority : 0
refcnt : 1
selftest : passed
internal : no
type : shash
blocksize : 64
digestsize : 20

name : md5
driver : md5-generic
module : kernel
priority : 0
refcnt : 1
selftest : passed
internal : no
type : shash
blocksize : 64
digestsize : 16

name : digest_null
driver : digest_null-generic
module : kernel
priority : 0
refcnt : 1
selftest : passed
internal : no
type : shash
blocksize : 1
digestsize : 0

name : compress_null
driver : compress_null-generic
module : kernel
priority : 0
refcnt : 1
selftest : passed
internal : no
type : compression

name : ecb(cipher_null)
driver : ecb-cipher_null
module : kernel
priority : 100
refcnt : 1
selftest : passed
internal : no
type : blkcipher
blocksize : 1
min keysize : 0
max keysize : 0
ivsize : 0
geniv : <default>

name : cipher_null
driver : cipher_null-generic
module : kernel
priority : 0
refcnt : 1
selftest : passed
internal : no
type : cipher
blocksize : 1
min keysize : 0
max keysize : 0

$

[/bash]
LINK
https://en.wikipedia.org/wiki/Cryptography
https://en.wikipedia.org/wiki/Cyclic_redundancy_check
https://developer.ibm.com/linuxonpower/2017/08/11/adding-power-optimized-crc-32-existing-code/

sample session related to “command expansion” in Bash shell

$a=`time`

real	0m0.000s
user	0m0.000s
sys	0m0.000s
$a
bash: a: command not found
$a=`ls`
$a
bash: a: command not found
$echo a
a
$echo $a
a.txt b.txt trueangle
$echo `a`
bash: a: command not found

$a=`ls`
$b=time
$echo $b
time
$b=`time`

real	0m0.000s
user	0m0.000s
sys	0m0.000s
$echo $b

$b=`time`

real	0m0.000s
user	0m0.000s
sys	0m0.000s
$echo $b

$b=date
$echo $b
date
$b=`date`
$echo $b
Fri Apr 22 22:50:27 IST 2016
$


http://mywiki.wooledge.org/BashFAQ/032

using the dir() Built-in function in Python programming

>>> import struct
>>> dir()   # show the names in the module namespace
['__builtins__', '__doc__', '__name__', 'struct']
>>> dir(struct)   # show the names in the struct module
['Struct', '__builtins__', '__doc__', '__file__', '__name__',
 '__package__', '_clearcache', 'calcsize', 'error', 'pack', 'pack_into',
 'unpack', 'unpack_from']
>>> class Shape(object):
        def __dir__(self):
            return ['area', 'perimeter', 'location']
>>> s = Shape()
>>> dir(s)
['area', 'perimeter', 'location']

https://docs.python.org/2/library/functions.html#dir

sample session using “variable expansion” with Bash shell

$com=world
$echo com
com
$echo $com
world
$x=hello
$echo ${x}{com}
hello{com}
$echo ${x}${com}
helloworld
$echo $x$com
helloworld
$echo ${x}world
helloworld
$echo ${x} world
hello world
$echo $x world
hello world
$echo $xcom

$echo $x com
hello com
$echo $x $com
hello world
$echo $x$com
helloworld
$echo $xcom

$echo ${x}com
hellocom
$


$your_id=${USER}-on-${HOSTNAME}
$echo "$your_id"
jeffrin-on-debian
$your_id=$USER-on-$HOSTNAME
$echo "$your_id"
jeffrin-on-debian
$echo ${USER}5
jeffrin5
$echo $USER5

$echo $USER 5
jeffrin 5
$echo $USER 5
jeffrin 5
$your_id=hello
$echo $your_id5

$echo $your_id 5
hello 5
$echo ${your_id}5
hello5
$

http://www.tldp.org/LDP/abs/html/parameter-substitution.html
http://guide.bash.academy/03.variables.html