For one of the Matasano crypto challenges, I had to decrypt the text which was encrypted using AES in ECB mode. Everything about AES is actually documented by the National Institute of Standards and Technology. You can get all the algorithms behind AES encryption. It is probably not a good idea to implement it from scratch.Openssl has a well tested and widely used library which works.

OpenSSL 3.0 is the next major version of OpenSSL that is currently in development and includes the new FIPS Object Module. A pre-release version of this is available below. This is for testing only. It should not be used in production. Information and notes about OpenSSL 3.0 are available on the OpenSSL Wiki. The OpenSSL library uses the included full-strength general-purpose cryptography library, which can be used by itself. OpenSSL 1.1.1k on 32-bit and 64-bit PCs This download is licensed as freeware for the Windows (32-bit and 64-bit) operating system on a laptop or desktop PC from programming software without restrictions.

This Openssl library page gives a complete example of how to use them. There are a few preparatory steps before you can use the instructions though. These instructions are for Ubuntu like Linux distributions. These worked well on my Raspberry Pi too.

Installing Openssl library

  • Check openssl tool which is a collection of Openssl from the LibreSSL project and Cygwin libraries (2.5 MB).
  • Some people have offered to provide OpenSSL binary distributions for selected operating systems. The condition to get a link here is that the link is stable and can provide continued support for OpenSSL for a while. Note: many Linux distributions come with pre-compiled OpenSSL packages.

Following command installs all the C libraries needed to use Openssl with your C code.

For example, you will want to include the following header files:

Compiling your C program with the Openssl library

Next, you can follow the instructions from the Openssl crypto library page to create your C program. I have an example program in my CrytopalsGithub repository. While linking the program you need to provide the ssl and crypto library names. Following command should do it:

A few pointers on the do_crypt function

  • If you are going to use the do_crypt function for decrypting a text encrypted using electronic code book (ECB) mode, you should remove the following assert line since there is no Initialization Vector for ECB.
  • The example code operates on the raw data. So, if you are trying to decrypt the data which is base64 encoded, your first step should be to convert it into raw data.

Deciding on Key Generation Options

When generating a key, you have to decide three things: the key algorithm, the key size, and whether to use a passphrase.

Key Algorithm

For the key algorithm, you need to take into account its compatibility. For this reason, we recommend you use RSA. However, if you have a specific need to use another algorithm (such as ECDSA), you can use that too, but be aware of the compatibility issues you might run into.

Note: This guide only covers generating keys using the RSA algorithm.

Openssl library c++

Key Size

For the key size, you need to select a bit length of at least 2048 when using RSA and 256 when using ECDSA; these are the smallest key sizes allowed for SSL certificates. Unless you need to use a larger key size, we recommend sticking with 2048 with RSA and 256 with ECDSA.

Note: In older versions of OpenSSL, if no key size is specified, the default key size of 512 is used. Any key size lower than 2048 is considered unsecure and should never be used.

Passphrase

Openssl Library Required

For the passphrase, you need to decide whether you want to use one. If used, the private key will be encrypted using the specified encryption method, and it will be impossible to use without the passphrase. Because there are pros and cons with both options, it's important you understand the implications of using or not using a passphrase. In this guide, we will not be using a passphrase in our examples.

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