Ubuntu – side-by-side same file GUI text editor

Another method is to use split. Split the file into 8 pieces and manipulate the files with a editor. After that, you reassemble the files again.

split -b 53750k <your-file>

cat xa* > <your-file>


SYNOPSIS
       split [OPTION]... [INPUT [PREFIX]]

-a, --suffix-length=N
              use suffixes of length N (default 2)

       -b, --bytes=SIZE
              put SIZE bytes per output file

       -C, --line-bytes=SIZE
              put at most SIZE bytes of lines per output file

       -d, --numeric-suffixes
              use numeric suffixes instead of alphabetic

       -l, --lines=NUMBER
              put NUMBER lines per output file

Vi/Vim

Just use vim or vi which offers file encryption with blowfish when using -x option.

create a file for encryption as follows:

vim -x filename.txt

Then it will prompt to enter encryption key

Enter encryption key:

Once a file has been encrypted by Vim once, you never need to use the -x option when opening that file again. Vim will automatically recognize it as an encrypted file and do the right thing.

Because Blowfish is a symmetric key encryption system, the same key is used for both encryption and decryption. When Vim opens a file for the first time with the -x option, the first thing it will do is ask you to give it a key you can use to encrypt and decrypt the file, with this prompt:

Need encryption key for "abc.txt"
Enter encryption key:

After entering the key, you will then be asked to confirm the key, to ensure you did not mistype it.

Enter same key again:

Then it will open as normally as usual.

Read more here

CryptoTE

According to the website.

CryptoTE is a text editor with integrated strong cryptography. 
It is based on the popular Scintilla widget and automatically stores 
text data in secure encrypted container files. 
Compared to other "password keeper" programs, CryptoTE does not force 
any structure upon your data: it works with plain ASCII text 
and does not require you to fill in grids, key-value attributes,descriptions etc. 
Encryption is transparently performed using the 
highly-secure Serpent cipher. The editing interface is thoroughly 
optimized for speed and ease of use. 
Multiple subfiles, Quick-Find and a two-click random password generator 
make daily use very convenient.

for ubuntu see.