Encrypt entire CSV file, but decrypt only specific fields?

Question

I have some data in CSV file, say data.csv with columns age, sex, country and status. Is it possible I could encrypt the entire CSV file some way which allows me to fetch only selected columns, say country and status, from the encrypted file and then decrypt to get back only those fields in clear text?

I am talking something similar to column-based encryption available in some databases but I cannot use any databases.

One idea I have is while encryption i encrypt each field column one by one, put a delimeter and then encrypt other field and append to the previously encrypted field but i am certain its a bad approach.

Answer 1

Transpose the CSV before encrypting. So each column is its own contiguous block.

So the CSV:

a,b,c
1,2,3
4,5,6
7,8,9

is stored as three blocks. The format of these only requires that you have some way of finding the beginning of each column in the file and that you have some way of delimiting values. I am not specifying an encoding. But one could use CSV encoding for the values where the spaces in the following are actually commas and the values are a single line.

a 1 4 7
b 2 5 8
c 3 6 9

Each of the lines represents a block where the part in brackets is encrypted:

a [1 4 7]
b [2 5 8]
c [3 6 9]

Then you can iterate over the rows by doing decryption steps on each column and merging their output to produce rows as the values may be of variable length the amount of decryption steps on a column depends on the length of the value in that column.