homepage
Open menu
Go one level top
  • Train and Certify
    • Overview
    • Get Started in Cyber
    • Courses
    • GIAC Certifications
    • Training Roadmap
    • OnDemand
    • Live Training
    • Summits
    • Cyber Ranges
    • College Degrees & Certificates
    • Scholarship Academies
    • NICE Framework
    • Specials
  • Manage Your Team
    • Overview
    • Group Purchasing
    • Why Work with SANS
    • Build Your Team
    • Hire Cyber Talent
    • Team Development
    • Private Training
    • Security Awareness Training
    • Leadership Training
    • Industries
  • Resources
    • Overview
    • Internet Storm Center
    • White Papers
    • Webcasts
    • Tools
    • Newsletters
    • Blog
    • Podcasts
    • Posters & Cheat Sheets
    • Summit Presentations
    • Security Policy Project
  • Focus Areas
    • Cyber Defense
    • Cloud Security
    • Digital Forensics & Incident Response
    • Industrial Control Systems
    • Cyber Security Leadership
    • Offensive Operations
  • Get Involved
    • Overview
    • Join the Community
    • Work Study
    • Teach for SANS
    • CISO Network
    • Partnerships
    • Sponsorship Opportunities
  • About
    • About SANS
    • Our Founder
    • Instructors
    • Mission
    • Diversity
    • Awards
    • Contact
    • Frequently Asked Questions
    • Customer Reviews
    • Press
  • SANS Sites
    • GIAC Security Certifications
    • Internet Storm Center
    • SANS Technology Institute
    • Security Awareness Training
  • Search
  • Log In
  • Join
    • Account Dashboard
    • Log Out
  1. Home >
  2. Blog >
  3. Known plaintext analysis of encoded strings
John Jarocki

Known plaintext analysis of encoded strings

August 27, 2008

notice-doc

As a child, my first introduction to ciphers came in the form of Edgar Allen Poe's The Gold Bug. The tale of pirates, treasure, and ciphers gripped my ten-year-old imagination and held me spellbound — sparking an interest in puzzles and codes that would later make computer science the obvious choice for my career. 

Recently the Gold Bug returned to my thoughts while digging through a new bit of malware. Our team obtained an email from "IRS.com" with an attached Microsoft Word document. Inside that attachment seemed to be an embedded PDF document, but opening that embedded document actually extracted a trojan executable (C__Adobe_Acrobat_Reader.exe).

Looking at printable strings found in this executable, I came across some text that looked like gibberish (shown here). One line in particular jumped off the screen:

encoded-strings

I remembered seeing similar strings in the past and with help from Google, was able to find a posting at the Internet Storm Center that mentioned the string "iuug;" 

As I was staring at the portion of the packet trace in the ISC posting containing the string, it occurred to me that iuuq; could be read as http: and that this data was encoded using ROT-1, a Caesar substitution cipher.

0a69 7575 713b 3030 3836 2f32 3337 2f33   .iuuq;0086/237/3
322f 3237 3330 6267 6730 656a 7330 6d70    2/2730bgg0ejs0mp
686a 2f66 7966 0a0d 0a30 0d0a 0d0a         hj/fyf...0....

Shifting "i" one letter to the left in the ASCII "alphabet" produced "h", "u" became "t",  and so on.

After writing a very small perl script (to which I added a line to also translate hex-encoded characters), I was able to decode the strings from the captured executable.

#!/usr/bin/perl
#
# Ridiculously simple script to reverse ROT-1 encoding
# and HEX-escaped characters.
#
# Example input:  MisdpMpoh!>!#iuuq;00xxx/mjkjo/hpw/do0uphbp0xjo43/fyf#
# Example output: LhrcoLong = "http://www.lijin.gov.cn/togao/win32.exe"

while (  ) {
      s/(.)/chr(ord($1)-1)/ge;
      s/%(..)/chr(hex($1))/eg;
      print;
}

After writing this script, I found it useful in other situations. For example, using it on strings extracted from a dump of Windows physical memory turned up interesting information. Since not every piece of malware uses ROT-1 to encode data, it occurred to me that I should generalize the approach.

In the Gold Bug, frequency analysis was used to crack the cipher. In the case of the data contained in malware samples, more than 26 letters are used, and some non-alphabetic characters are used frequently.

Instead of frequency analysis of letters, my new script focused on frequency of common strings. I picked three that appear frequently in malware:

  • http: 
  • IP addresses in the form "W.X.Y.Z."
  • .exe

To keep things simple, I focused only on ROT encoding and wrote a new perl script to try a range of ROT encodings and count matches against those three strings. The script returned this output for me on a bit of malware that I analyzed:

Statistics:

ROT(1) \.[Ee][Xx][Ee]\b = 14
ROT(1) \b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b = 1
ROT(98) \b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b = 1
ROT(1) \bhttp:// = 6

The result: with fourteen .exe matches, an IP address, and six apparent URLs, it looks like the strings in this malware sample are encoded with ROT-1. It's also possible that some data is encoded ROT-98, but with only one match... pretty unlikely. This script is just a start. The next step would be to add other encoding schemes, such as XOR. This is my favorite kind of script because it simply mines data for interesting patterns, leaving me with more time for other tasks.

kpa

John Jarocki, GCFA Silver #2161, is an Information Security Analyst specializing in intrusion detection, forensics, and malware analysis. He also holds GCIA, GCIH, GCFW and GSEC certifications and is a board member of NM InfraGard.

Share:
TwitterLinkedInFacebook
Copy url Url was copied to clipboard
Subscribe to SANS Newsletters
Receive curated news, vulnerabilities, & security awareness tips
United States
Canada
United Kingdom
Spain
Belgium
Denmark
Norway
Netherlands
Australia
India
Japan
Singapore
Afghanistan
Aland Islands
Albania
Algeria
American Samoa
Andorra
Angola
Anguilla
Antarctica
Antigua and Barbuda
Argentina
Armenia
Aruba
Austria
Azerbaijan
Bahamas
Bahrain
Bangladesh
Barbados
Belarus
Belize
Benin
Bermuda
Bhutan
Bolivia
Bonaire, Sint Eustatius, and Saba
Bosnia And Herzegovina
Botswana
Bouvet Island
Brazil
British Indian Ocean Territory
Brunei Darussalam
Bulgaria
Burkina Faso
Burundi
Cambodia
Cameroon
Cape Verde
Cayman Islands
Central African Republic
Chad
Chile
China
Christmas Island
Cocos (Keeling) Islands
Colombia
Comoros
Cook Islands
Costa Rica
Croatia (Local Name: Hrvatska)
Curacao
Cyprus
Czech Republic
Democratic Republic of the Congo
Djibouti
Dominica
Dominican Republic
East Timor
East Timor
Ecuador
Egypt
El Salvador
Equatorial Guinea
Eritrea
Estonia
Ethiopia
Falkland Islands (Malvinas)
Faroe Islands
Fiji
Finland
France
French Guiana
French Polynesia
French Southern Territories
Gabon
Gambia
Georgia
Germany
Ghana
Gibraltar
Greece
Greenland
Grenada
Guadeloupe
Guam
Guatemala
Guernsey
Guinea
Guinea-Bissau
Guyana
Haiti
Heard And McDonald Islands
Honduras
Hong Kong
Hungary
Iceland
Indonesia
Iraq
Ireland
Isle of Man
Israel
Italy
Jamaica
Jersey
Jordan
Kazakhstan
Kenya
Kingdom of Saudi Arabia
Kiribati
Korea, Republic Of
Kosovo
Kuwait
Kyrgyzstan
Lao People's Democratic Republic
Latvia
Lebanon
Lesotho
Liberia
Liechtenstein
Lithuania
Luxembourg
Macau
Macedonia
Madagascar
Malawi
Malaysia
Maldives
Mali
Malta
Marshall Islands
Martinique
Mauritania
Mauritius
Mayotte
Mexico
Micronesia, Federated States Of
Moldova, Republic Of
Monaco
Mongolia
Montenegro
Montserrat
Morocco
Mozambique
Myanmar
Namibia
Nauru
Nepal
Netherlands Antilles
New Caledonia
New Zealand
Nicaragua
Niger
Nigeria
Niue
Norfolk Island
Northern Mariana Islands
Oman
Pakistan
Palau
Palestine
Panama
Papua New Guinea
Paraguay
Peru
Philippines
Pitcairn
Poland
Portugal
Puerto Rico
Qatar
Reunion
Romania
Russian Federation
Rwanda
Saint Bartholemy
Saint Kitts And Nevis
Saint Lucia
Saint Martin
Saint Vincent And The Grenadines
Samoa
San Marino
Sao Tome And Principe
Senegal
Serbia
Seychelles
Sierra Leone
Sint Maarten
Slovakia (Slovak Republic)
Slovenia
Solomon Islands
South Africa
South Georgia and the South Sandwich Islands
South Sudan
Sri Lanka
St. Helena
St. Pierre And Miquelon
Suriname
Svalbard And Jan Mayen Islands
Swaziland
Sweden
Switzerland
Taiwan
Tajikistan
Tanzania
Thailand
Togo
Tokelau
Tonga
Trinidad And Tobago
Tunisia
Turkey
Turkmenistan
Turks And Caicos Islands
Tuvalu
Uganda
Ukraine
United Arab Emirates
United States Minor Outlying Islands
Uruguay
Uzbekistan
Vanuatu
Vatican City
Venezuela
Vietnam
Virgin Islands (British)
Virgin Islands (U.S.)
Wallis And Futuna Islands
Western Sahara
Yemen
Yugoslavia
Zambia
Zimbabwe

Tags:
  • Digital Forensics and Incident Response

Related Content

Blog
Vote_now.png
Digital Forensics and Incident Response
April 24, 2022
Which DFIR Summit Mascots do you want to see as Lego giveaways this year? Vote now!
To celebrate the 15th year of the DFIR Summit, we are letting you choose your favorite Summit mascot over the years. Which will make our Lego set?
Viv_Ross_370x370.png
Viviana Ross
read more
Blog
Untitled_design-43.png
Digital Forensics and Incident Response, Cybersecurity and IT Essentials, Industrial Control Systems Security, Purple Team, Open-Source Intelligence (OSINT), Penetration Testing and Ethical Hacking, Cyber Defense, Cloud Security, Security Management, Legal, and Audit
December 8, 2021
Good News: SANS Virtual Summits Will Remain FREE for the Community in 2022
They’re virtual. They’re global. They’re free.
Emily Blades
read more
Blog
Digital Forensics and Incident Response
February 1, 2010
It's the little things (Part One)
For forensic analysts working in Windows environments, .lnk shortcut files and the thumbprint caches are valuable sources for details about missing data. Individuals wanting to hide their activities may flush their browser cache, Temp files, use, and even wipe the drive free space. However, they...
SANS_DFIR-370x370.png
SANS DFIR
read more
  • Register to Learn
  • Courses
  • Certifications
  • Degree Programs
  • Cyber Ranges
  • Job Tools
  • Security Policy Project
  • Posters & Cheat Sheets
  • White Papers
  • Focus Areas
  • Cyber Defense
  • Cloud Security
  • Cyber Security Leadership
  • Digital Forensics
  • Industrial Control Systems
  • Offensive Operations
Subscribe to SANS Newsletters
Receive curated news, vulnerabilities, & security awareness tips
United States
Canada
United Kingdom
Spain
Belgium
Denmark
Norway
Netherlands
Australia
India
Japan
Singapore
Afghanistan
Aland Islands
Albania
Algeria
American Samoa
Andorra
Angola
Anguilla
Antarctica
Antigua and Barbuda
Argentina
Armenia
Aruba
Austria
Azerbaijan
Bahamas
Bahrain
Bangladesh
Barbados
Belarus
Belize
Benin
Bermuda
Bhutan
Bolivia
Bonaire, Sint Eustatius, and Saba
Bosnia And Herzegovina
Botswana
Bouvet Island
Brazil
British Indian Ocean Territory
Brunei Darussalam
Bulgaria
Burkina Faso
Burundi
Cambodia
Cameroon
Cape Verde
Cayman Islands
Central African Republic
Chad
Chile
China
Christmas Island
Cocos (Keeling) Islands
Colombia
Comoros
Cook Islands
Costa Rica
Croatia (Local Name: Hrvatska)
Curacao
Cyprus
Czech Republic
Democratic Republic of the Congo
Djibouti
Dominica
Dominican Republic
East Timor
East Timor
Ecuador
Egypt
El Salvador
Equatorial Guinea
Eritrea
Estonia
Ethiopia
Falkland Islands (Malvinas)
Faroe Islands
Fiji
Finland
France
French Guiana
French Polynesia
French Southern Territories
Gabon
Gambia
Georgia
Germany
Ghana
Gibraltar
Greece
Greenland
Grenada
Guadeloupe
Guam
Guatemala
Guernsey
Guinea
Guinea-Bissau
Guyana
Haiti
Heard And McDonald Islands
Honduras
Hong Kong
Hungary
Iceland
Indonesia
Iraq
Ireland
Isle of Man
Israel
Italy
Jamaica
Jersey
Jordan
Kazakhstan
Kenya
Kingdom of Saudi Arabia
Kiribati
Korea, Republic Of
Kosovo
Kuwait
Kyrgyzstan
Lao People's Democratic Republic
Latvia
Lebanon
Lesotho
Liberia
Liechtenstein
Lithuania
Luxembourg
Macau
Macedonia
Madagascar
Malawi
Malaysia
Maldives
Mali
Malta
Marshall Islands
Martinique
Mauritania
Mauritius
Mayotte
Mexico
Micronesia, Federated States Of
Moldova, Republic Of
Monaco
Mongolia
Montenegro
Montserrat
Morocco
Mozambique
Myanmar
Namibia
Nauru
Nepal
Netherlands Antilles
New Caledonia
New Zealand
Nicaragua
Niger
Nigeria
Niue
Norfolk Island
Northern Mariana Islands
Oman
Pakistan
Palau
Palestine
Panama
Papua New Guinea
Paraguay
Peru
Philippines
Pitcairn
Poland
Portugal
Puerto Rico
Qatar
Reunion
Romania
Russian Federation
Rwanda
Saint Bartholemy
Saint Kitts And Nevis
Saint Lucia
Saint Martin
Saint Vincent And The Grenadines
Samoa
San Marino
Sao Tome And Principe
Senegal
Serbia
Seychelles
Sierra Leone
Sint Maarten
Slovakia (Slovak Republic)
Slovenia
Solomon Islands
South Africa
South Georgia and the South Sandwich Islands
South Sudan
Sri Lanka
St. Helena
St. Pierre And Miquelon
Suriname
Svalbard And Jan Mayen Islands
Swaziland
Sweden
Switzerland
Taiwan
Tajikistan
Tanzania
Thailand
Togo
Tokelau
Tonga
Trinidad And Tobago
Tunisia
Turkey
Turkmenistan
Turks And Caicos Islands
Tuvalu
Uganda
Ukraine
United Arab Emirates
United States Minor Outlying Islands
Uruguay
Uzbekistan
Vanuatu
Vatican City
Venezuela
Vietnam
Virgin Islands (British)
Virgin Islands (U.S.)
Wallis And Futuna Islands
Western Sahara
Yemen
Yugoslavia
Zambia
Zimbabwe
  • © 2022 SANS™ Institute
  • Privacy Policy
  • Contact
  • Careers
  • Twitter
  • Facebook
  • Youtube
  • LinkedIn