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AES-256 requires a 256-bit key, period. However there are different ways of building that 256-bit key. One way is to generate 256 random bits and take them as the key. You need to store these 256 bits somewhere, or you won't be able to decrypt what you've encrypted. Allows you to encrypt and decrypt files or strings using the OpenSSL AES-256-CBC cipher and SHA1 digest algorithms. It is interoperable with the openssl command line tool which makes it a good introduction to using OpenSSL for ciphers.

1. 256 Bit Aes Key Example
2. Aes Key Absent
3. Openssl Generate 256 Bit Aes Key
4. Generate 256 Bit Aes Key Openssl Version

If you have someone’s public SSH key, you can use OpenSSL to safely encrypt a file and send it to them over an insecure connection (i.e. the internet). They can then use their private key to decrypt the file you sent.

If you encrypt/decrypt files or messages on more than a one-off occasion, you should really use GnuPGP as that is a much better suited tool for this kind of operations. But if you already have someone’s public SSH key, it can be convenient to use it, and it is safe.

There is a limit to the maximum length of a message – i.e. size of a file – that can be encrypted using asymmetric RSA public key encryption keys (which is what SSH keys are). For this reason, we’ll actually generate a 256 bit key to use for symmetric AES encryption and then encrypt/decrypt that symmetric AES key with the asymmetric RSA keys. This is how encrypted connections usually work, by the way.

Encrypt a file using a public SSH key

Generate the symmetric key (32 bytes gives us the 256 bit key):

You should only use this key this one time, by the way. If you send something to the recipient at another time, don’t reuse it.

Encrypt the file you’re sending, using the generated symmetric key:

In this example secretfile.txt is the unencrypted secret file, and secretfile.txt.enc is the encrypted file. The encrypted file can be named whatever you like.

Encrypt the symmetric key, using the recipient’s public SSH key:

Replace recipients-key.pub with the recipient’s public SSH key.

Delete the unencrypted symmetric key, so you don’t leave it around:

Now you can send the encrypted secret file (secretfile.txt.enc) and the encrypted symmetric key (secret.key.enc) to the recipient. It is even safe to upload the files to a public file sharing service and tell the recipient to download them from there.

Decrypt a file encrypted with a public SSH key

First decrypt the symmetric.key:

The recipient should replace ~/.ssh/id_rsa with the path to their secret key if needed. But this is the path to where it usually is located.

Now the secret file can be decrypted, using the symmetric key:

Again, here the encrypted file is secretfile.txt.enc and the unencrypted file will be named secretfile.txt

This post briefly describes how to utilise AES to encrypt and decrypt files with OpenSSL.

AES - Advanced Encryption Standard (also known as Rijndael).

Generating a public key from a private key. The Public/Private key can be used in place of a password so that no username/password is required to connect to the server via SSH. Search Bluehost Web Hosting Help SSH Access - Generating a Public/Private Key Using a Public/Private key to authenticate when logging into SSH can provide added convenience or added security. Instead the unique public and private key provide the secure authentication.

OpenSSL - Cryptography and SSL/TLS Toolkit

256 Bit Aes Key Example

We’ll walk through the following steps:

• Generate an AES key plus Initialization vector (iv) with openssl and
• how to encode/decode a file with the generated key/iv pair

Note: AES is a symmetric-key algorithm which means it uses the same key during encryption/decryption.

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Generating key/iv pair

We want to generate a 256-bit key and use Cipher Block Chaining (CBC).

The basic command to use is openssl enc plus some options:

• -P — Print out the salt, key and IV used, then exit
• -k <secret> or -pass pass:<secret> — to specify the password to use
• -aes-256-cbc — the cipher name

Note: We decided to use no salt to keep the example simple.

Issue openssl enc --help for more details and options (e.g. other ciphernames, how to specify a salt, …).

Encoding

Let's start with encoding Hello, AES! contained in the text file message.txt:

Decoding

Decoding is almost the same command line - just an additional -d for decrypting:

Aes Key Absent

Note: Beware of the line breaks

Openssl Generate 256 Bit Aes Key

While working with AES encryption I encountered the situation where the encoder sometimes produces base 64 encoded data with or without line breaks..

Generate 256 Bit Aes Key Openssl Version

Short answer: Yes, use the OpenSSL -A option.