Encrypting and decrypting a file with symmetric key

This post explains how to encrypt and decrypt a file using symmetric key. In symmetric algorithms the security depends on the strength of the key as the encryption and decryption key is the same. The strength of the key depends on the key size. Key size is the number of bits in the key. This key has to be agreed by both the sender and receiver before the communication starts. The security of the algorithm depends on the secrecy of the key.

Consider a file ‘secret.txt’ with the following content

Secret key is 98234

Now, we encrypt it with a symmetric key using gpg.

gpg --symmetric secret.txt

This asks for a pass-phrase, which is the key.
The cipher algorithm used here is CAST5. CAST5(or CAST-128) is a block cipher. Block ciphers divides the message into blocks and each block is encrypted at a time. The key used for encrypting each block is the same. CAST5 has a fixed block size of 8 bytes. You can read more about CAST5 here.

Now, the encrypted file ‘secret.txt.gpg’ is created.
The content of secret.txt.gpg is:

<8c>^M^D^C^C^BÊÏñT<96>ÅGc`É1    7NA!áW<95>i^Y<98>s1 -lÐ^\j°ã[;$qs^GsÛ^?^<9d>    /n<99>ÞÜ<87>äâýD,^OM£¸ù

To decrypt, type the following command

gpg --decrypt secret.txt.gpg

This outputs the following

gpg: CAST5 encrypted data
gpg: encrypted with 1 passphrase
Secret key is 98234
gpg: WARNING: message was not integrity protected

Symmetric keys are useful when to protect any files in your own PC.

References: http://www.noah.org/wiki/GPG_notes

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