Petr Zak

Finance and IT enthusiast, problem solver

Must-Know Ports



 20, 21 FTP
 22 SSH
 23 Telnet
 25 SMTP
 42 WINS
 53 DNS
 80, 8080 HTTP
 88 Kerberos
 110 POP3
 111 Portmapper Linux
123 NTP
139 SMB
143 IMAP
161, 162 SNMP
389 LDAP
445 CIFS
514 Syslog
636 Secure LDAP


Network topologies

  • Bus – simple but vulnerable
  • Ring – simple, vulnerability can be handled by concentric design
  • Star – most common, resistance to outages, node failure does not cause the whole network to go down
  • Mesh – for mission critical services, high level of redundancy; Internet
  • Hybrid – mix of the above

Hacker’s workflow

Everyone has its own, but it usually consists of:

  • Footprinting (passive)
  • Scanning (active)
  • Enumeration – is there a value behind the door?
  • Hacking – attack execution
  • Covering – hiding the activity from previous steps
  • Backdoors – wanna come back?

DNA Nucleotides

DNA uses a code made up of four letters (actually molecules, ATCG) – we call it Nucleobasis.

  • Adenine
  • Thymine
  • Cytosine
  • Guanine

If you imagine the ladder they form up the rungs (pairs). Watson-Crick base pair rule: A pairs with T, C pairs with G.

Nucleotide is nucleobase + phosphate + d-sugar.

Block vs Stream Cipher

Block encryption means that a whole block of data is processed, for instance block of 4 bits. If we have only 2 bits of plaintext, we need to add the so called pad bits (agreed before communication or using such way that  would not confuse our partner) to fill in the whole block. Stream cipher encrypts a bit/byte at a time.

Ideal block cipher maps all input blocks of bits of plain text to cipher text blocks. The key is then this mapping but we can agree to omit the plaintext blocks. Nevertheless, the key length would be n . 2n (we have 2n possible variations of blocks and each block is of length n).

Product cipher

Product cipher is a combination of substitution ciphers and transposition ciphers. It is quite common nowadays to combine these two approaches, see DES, AES, 3DES, etc.

Permutation Cipher

Permutation cipher is another member of transposition ciphers family. Encryption is realized through the following steps:

  • choose the length of the key N; it specifies number of columns we will use for encryption
  • create a permutation of numbers 1 to N; for instance for N = 4, it could be [2,4,3,1]
  • write the plain text to columns (from left to right, from top to bottom)
  • shuffle it based on the key and write to rows; in our case key [2,4,3,1] says column one will be second part of the cipher text, column two will be fourth, etc.

Continue reading

Rail Fence Cipher

In last few articles we discussed substitution ciphers. Ie. the plain text was substituted with a different alphabet and that gave us cipher text. Rail Fence Cipher is a member of transposition ciphers family. Plain text is shuffled based on some rule a that gives us cipher text. The alphabet remains the same for both plain text and cipher text. Continue reading

Hack Aging

Biohacking is not just using the technology to hack your body (improve your skills, enhance your abilities). You can hack your body simply by exercising – if you exercise you will hack your body to loose fat and gain muscles.

A good book to read about aging and how to prevent it is Fantastic Voyage: Live Long Enough to Live Forever by Ray Kurzweil and Terry Grossman, M.D. They describe ways how to “live forever” referring to exponential growth of technology and scientific discoveries. It is based on three pillars: old way to prevent aging (consume healthy food and drinks, exercise, etc.), biotechnology and nanobots.

If you don’t want to read the whole book, the authors offer a short guide at

Modulo operation

For positive integers it is intuitive:  a mod n gives the remainder when a is divided by n.

Example: 7 mod 4 = 3


For negative integers we use the following formula (it actually holds for all integers) a = q . n + r, where we require modulus n > 0.

Example: -11 mod 7 = 3

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