How to encrypt and decrypt plain text with a RSA k

Question

I am trying to write a utility program which encrypts and decrypts plain text files using a RSA key pair. The RSA keys were generated with ssh-keygen and are stored in .ssh, as usual.

I am having trouble understanding how to do that with the Go language crypto and crypto/rsa packages? The documentation on those is a little sparse (even more so because I am new to encryption) and there are very few examples. I checked the rsa_test.go file for any clues, but it only confused me more.

In short I am trying to load the public/private key pair from the id_rsa and id_rsa.pub files in .ssh and use them to encrypt/decrypt a plain text file.

Thank you in advance!

Answer 1

Keep in mind that RSA is not designed to be a block cipher. Usually RSA is used to encrypt a symmetric key that is then used to encrypt the data. With that in mind, however, here is a program which can use an RSA private key to encrypt data that can be decrypted by itself:

package main

import (
    "crypto/rand"
    "crypto/rsa"
    "crypto/sha1"
    "crypto/x509"
    "encoding/pem"
    "flag"
    "io/ioutil"
    "log"
)

// Command-line flags
var (
    keyFile = flag.String("key", "id_rsa", "Path to RSA private key")
    inFile  = flag.String("in", "in.txt", "Path to input file")
    outFile = flag.String("out", "out.txt", "Path to output file")
    label   = flag.String("label", "", "Label to use (filename by default)")
    decrypt = flag.Bool("decrypt", false, "Decrypt instead of encrypting")
)

func main() {
    flag.Parse()

    // Read the input file
    in, err := ioutil.ReadFile(*inFile)
    if err != nil {
        log.Fatalf("input file: %s", err)
    }

    // Read the private key
    pemData, err := ioutil.ReadFile(*keyFile)
    if err != nil {
        log.Fatalf("read key file: %s", err)
    }

    // Extract the PEM-encoded data block
    block, _ := pem.Decode(pemData)
    if block == nil {
        log.Fatalf("bad key data: %s", "not PEM-encoded")
    }
    if got, want := block.Type, "RSA PRIVATE KEY"; got != want {
        log.Fatalf("unknown key type %q, want %q", got, want)
    }

    // Decode the RSA private key
    priv, err := x509.ParsePKCS1PrivateKey(block.Bytes)
    if err != nil {
        log.Fatalf("bad private key: %s", err)
    }

    var out []byte
    if *decrypt {
        if *label == "" {
            *label = *outFile
        }
        // Decrypt the data
        out, err = rsa.DecryptOAEP(sha1.New(), rand.Reader, priv, in, []byte(*label))
        if err != nil {
            log.Fatalf("decrypt: %s", err)
        }
    } else {
        if *label == "" {
            *label = *inFile
        }
        out, err = rsa.EncryptOAEP(sha1.New(), rand.Reader, &priv.PublicKey, in, []byte(*label))
        if err != nil {
            log.Fatalf("encrypt: %s", err)
        }
    }

    // Write data to output file
    if err := ioutil.WriteFile(*outFile, out, 0600); err != nil {
        log.Fatalf("write output: %s", err)
    }
}