RSA File De- and Encryption

Docu for encrypt and decrypt a large file with AES and RSA

Keypairs

Generate RSA Keypairs

//generates a private Key with 8196 Bit. 
openssl genrsa -out private.pem 8196

//strips out the public key from the private key
openssl rsa -in private.pem -out public.pem -outform PEM -pubout

Note: Currently 16384 is the max modulo lenght

Generate AES Key

//generate a Radnom 32 Byte (256 Bit) AES Key an save the key to the aesKey.txt file
openssl rand -base64 32 | cut -c1-31 > aesKey.txt

Encryption

Encrypt File with AES Key

//encrypt the file.txt with the generated AES Key to the file.enc
openssl enc -aes-256-cbc -salt -in file.txt -out file.enc -pass file:./aesKey.txt

Encrypt AES Key with RSA Public Key

//encrpyt the AES Key with the RSA Public Key and save the outcome int the aesKey.txt.crypted file. 
openssl rsautl -encrypt -inkey public.pem -pubin -in aesKey.txt -out aesKey.txt.crypted

Generate a Signature for the file.txt

//Generate the signature.txt for the file.txt
openssl dgst -sha256 -sign private.pem -out signature.txt file.txt

You can now send the file.enc, aesKey.txt.crypted, signature.txt and the public.pem via email or something similar. Dont send the private.pem!

Decryption

Decrypt AES Key with RSA Private Key

//decrypt the AES Key with the Private RSA Key and save the result in aesKey.txt.decrypted
openssl rsautl -decrypt -inkey private.pem -in aesKey.txt.crypted -out aesKey.txt.decrypted

Decrypt File with AES Key

//decrypt the encrypted file with the decrypted AES Key
openssl enc -d -aes-256-cbc -in file.enc -out file.txt.decrypted -pass file:./aesKey.txt.decrypted
//The file.txt.decrypted and file.txt should be te same

Verify the signature for the recieved file.txt and the signature.txt

openssl dgst -sha256 -verify public.pem -signature signature.txt file.txt
# in case of success: prints "Verified OK"
# in case of failure: prints "Verification Failure"

Stackoverflow: Digital signature for a file using openssl

Source

Public – Private key encryption using OpenSSL

An Introduction to the OpenSSL command line tool - Digital signatures

carlj/rsa-encryption.md