OpenSSL is a public-key crypto library (plus some other random stuff). To generate the private (and public key): The private key is encoded with Base64. Open up a terminal and navigate to where the file is. Encrypt large file using OpenSSL Now we are ready to decrypt large file using OpenSSL encryption tool: $ openssl smime -encrypt -binary -aes-256-cbc -in large_file.img -out large_file.img.dat -outform DER public-key.pem The above command have encrypted your large_file.img and store it as large_file.img.dat: This post is not associated with my employer. Here is how I create my key pair. Ran the following command to get the .pem version of the key: openssl rsa -in public -pubout > file.pem But doing so says the following: unable to load Private Key Encrypt a file using a public SSH key Generate the symmetric key (32 bytes gives us the 256 bit key): $ openssl rand -out secret.key 32 You should only use this key this one time, by the way. Here is how you encrypt files with OpenSSL. a RSA public key). Since 175 characters is 1400 bits, even a small RSA key will be able to encrypt it. openssl rsautl: Encrypt and decrypt files with RSA keys. Two approaches to do this with OpenSSL: (1) generate a random key to be used with a symmetric cipher to encrypt the message and then encrypt the key with RSA; (2) use the smime operation, which combines RSA and a symmetric cipher to automate approach 1. password (not shared with recipient) using recipient’s RSA public key, encrypt the large file using a key derived from this secret password and then send the encrypted secret password and encrypted file to the recipient. Note that although the steps used in both outputs are the same, the actual values differ (i.e. NOTE: For this example, let’s assume that the recipient has generated a The openssl_public_encrypt() function will encrypt the data with public key.. If you want to encrypt a file with an RSA public in order to send private message to the owner of the public key, you can use the OpenSSL "rsault -encrypt" command as shown below: C:\Users\fyicenter>type clear.txt The quick brown fox jumped over the lazy dog. In the example we’ll walkthrough how to encrypt a file using a symmetric key. public_encrypt function encrypts message using public_key.pem file To view the values: To sign the message you need to calculate its hash and then encrypt that hash using your private key. It makes no sense to encrypt a file with a private key.. How do I do public-key encryption with openssl? You now have some data in file.txt, lets encrypt it using OpenSSL and the public key: $ openssl rsautl -encrypt -inkey public.pem -pubin -in file.txt -out file.ssl This creates an encrypted version of file.txt calling it file.ssl, if you look at this file it’s just binary junk, nothing very useful to anyone. All content copyright James Fisher 2017. Step 1: Encrypting your file. Here’s how to do the basics: key generation, encryption and decryption. Furthermore, DES and AES are block ciphers. Tagged . To create a hash of a message (without encrypting): OpenSSL has an option to calculate the hash and then sign it: To encrypt the message using RSA, use the recipients public key: Note that direct RSA encryption should only be used on small files, with length less than the length of the key. Package the encrypted key file with the encrypted data. Using a private key to attach a tag to a file that guarantees that the file was provided by the holder of the private key is called signing, and the tag is called a signature.. The solution is to generate a strong random password, use that password to encrypt the file with AES-256 in CBC mode (as above), then encrypt that password with a public RSA key. That's why we can't directly encrypt a large file using rsautl. Using a private key to attach a tag to a file that guarantees that the file was provided by the holder of the private key is called signing, and the tag is called a signature.. openssl_public_encrypt () encrypts data with public key and stores the result into crypted. Now we are ready to encrypt this file with public key: $ openssl rsautl -encrypt -inkey public_key.pem -pubin -in encrypt.txt -out encrypt.dat $ ls encrypt.dat encrypt.txt private_key.pem public_key.pem $ file encrypt.dat encrypt.dat: data. -decrypt . OpenSSL is a public-key crypto library (plus some other random stuff). openssl rsautl -decrypt -inkey id_rsa.pem -in key.bin.enc -out key.bin openssl enc -d -aes-256-cbc -in SECRET_FILE.enc -out SECRET_FILE -pass file:./key.bin Notes You should always verify the hash of the file with the recipient or sign it with your private key, so the other person knows it actually came from you. 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