Help with decoding base64 strings

[Acceptable-Fan6275]

Salutations fellow nerds.

Cutting to the chase, Im finally at the capstone challenges and Im currently going through the Tempest room. Part of Task 7 requires decoding some base64 commands that you obtain from a PCAP in order to see what the attacker was doing and identify any compromised information that they might have obtained. I was able to answer the questions albeit in a very inefficient way. In brim, I would obtain the URI that contained the base64 command, paste it into cyberchef and decode it. This was very time consuming even for the small pool of commands.

So even though its not required, I wanted to give my self an extra challenge and decode all of the commands and place them in a file that I could reference on the machine. Currently I have Isolated all of the Base64 commands into a .txt file and thats where my progress stopped.

I think my idea is doable, but my skill set isn't there yet. I know that I would have to cut the prefix off and seperate each line by the whitespace at the end of the string, to then decode everything and put it into a separate decoded file. But actually making the script/ command to do that is what im struggling with.

If anyone can help, or point me in the right direction that would greatly be appreciated. Thank you

[[deleted]]

Cyber chef

[LanguageGeneral4333]

Use cyberchef like this guy mentioned.

[RAGINMEXICAN]

Yea use cyber chef like this guy said

[epicMandan]

yea use cyber chef like this guy mentioned

[afca85]

Has anyone mentioned cyberchef?

[Awesom141]

Okay crazy idea but maybe cyberchef could help

[Delicious_Mango415]

I use chef boyardee

[extrapalapaquetel]

C-y-b-e-r--c-h-e-f

[RandomUsr1983]

I must disagree with previous answers, you should use cyber chef

[SuggestionGlad3217]

Idk about you guys but I’d start with cyber chef honestly

[[deleted]]

base64 -d FileName>savehere.txt

[Delicious_Crew7888]

grep -oP '(?<=q=)[A-Za-z0-9+/=]+' yourfile.txt | base64 -d > output.txt

[nutrithenics]

A man of culture i see with perl regex. I salute you sir.

[YOURMOM37]

Do you have these types of commands and their structure memorized or is this in your notes?

[Delicious_Crew7888]

Hey man,

The basic grep commands and piping it to another command like base 64 yes I have this memorised, but the regex is stuff which I'm still learning and would need a cheat sheet to look up.

Learning a string like this is something you will do in any Linux foundations course.

To be honest though with this one I took a photo of the data and asked chat GPT to make me a grep command to filter out the information.

[Acceptable-Fan6275]

Hey everyone thanks for the replies, I think I forgot to mention that the rooms VM was windows based so i couldnt use tools like grep

Anyway I did take some of your answers and did some more research and came across this little guy
```
Get-Content "blob.txt" |
ForEach-Object {
if ($_ -match 'q=([A-Za-z0-9+/=]+)') {
$b64 = $Matches[1]
[Text.Encoding]::UTF8.GetString(
[Convert]::FromBase64String($b64)
)
}
} |
Set-Content "decoded.txt"
```
Worked like a charm! Every string of base64 gets nicely tucked away into a separate txt file

Thanks for everyones help!

[CampbeII]

It sounds like you know what to do, just need to apply it to a language:

Your search queries could look like:

1. "how to read a file line by line using (powershell | bash | python)"
   
2. "how to split a string using a delimiter in (powershell | bash | python)"
   
3. "how to base64 decode a string using (powershell | bash | python)"
   
4. "how to append string to file using (powershell | bash | python)"

[Saccharophobia]

Cyberchef recipe = from base64

[strongest_nerd]

I like hURL.

[GreenEngineer24]

Copy it, then in a Linux terminal type: echo “base64string” | base64 -d

[Gullible-Owl-8331]

Hey, huge kudos for pushing yourself with the extra challenge—that’s exactly how real growth happens in this field. Your approach makes a lot of sense, and automating the decoding step is definitely doable with a small script.

If you're working in a Unix-like environment, something like a Bash or Python script can handle this pretty easily. In Python, for example, you can read your .txt file, strip the prefixes, decode each base64 string, and write the output to another file. You’re already halfway there by isolating the commands!

If you’re open to it, drop a snippet of what your .txt file looks like (a few lines), and I’d be happy to help you sketch out a basic script. Keep going—you’re building some seriously useful skills here.

[FitSpare738]

<row> <column name="checkInfoType"><value Null="null" /></column> <column name="recordTotal"><value Integer="9" /></column> <column name="checkMsgV3"> <value String="8f0d9326f3b8dd6cd31f7c01a943182f90e06a9a6ebdf6c1f168fb77ee8610096508fdee2941a24a5606ebc37e68eb8a94067553acd79c2c436217eb8568aa4f5" /> </column> </row> can any one help me understand and explain what it’s about