Use Go to encrypt message with ssh-rsa public key which then can be decrypted using openssl rsautl -decrypt

Question

I have been trying to solve this for days. In Go code I am looking to take a ssh-rsa public key like:

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDGnnY4LuLq7Bs7VnFk2Vs6hNTmZLkUBRRhXNFyKZOCvmhKcM7BSHkGS7+phpIzj6mTOsJEBZKHQgac46COOT3ukO/farnnDz78KIq24U/+TZmyAyNNdzOVizK5aAApvpYTQpuSlIDDltLXQkPokedE/5vCIPiwVZW0TfqT/Rdy2XXwKewDQ05xvJhX3+nymZkyJX3GJ+pTfsDkKR+suSLDN3nupThPiWK5A1ZG9bbUkxHbsAXiTKS+qwADIWOtJvfNtPX54JjCo3Gh3/Fy0Ovxn3QSQlCF/IZNbSgm6R6adjaU4kXEF6zsLq+BjDKLtEA3A0tAIBj0T+DuuxpcV3aX

and message like: hello-world and encrypt that message with that key.

The private key is then going to be used to decrypt the key with openssl rsautl -decrypt -inkey privateKeyFile -in encryptedMsgFile

Answer 1

Encryption with SSH keys is described in detail in the post Encrypting Data With SSH Keys and Golang, in particular the import of the OpenSSH public key, for which the ssh package is applied. For encryption (with PKCS#1 v1.5 padding) the rsa package is used.

The following sample code illustrates this. Since the OpenSSL statement reads the ciphertext from a file, it makes most sense for the ciphertext to be stored in a binary file. For simplicity, however, in this example the cipher text is output hex encoded:

package main

import (
    "crypto/rand"
    "crypto/rsa"
    "encoding/hex"
    "fmt"

    "golang.org/x/crypto/ssh"
)

func main() {

    // Key import
    publicKey := "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC8NGcNWf6cUno2lpny36js9ynXCQ6kuJ14yIViuPwty0ZkfhpPdw3zVKWmKV0zS1/QyO6a5d85I9ywvEXHiZr4+fE4JRPxdEIQVGMz7OThs0XgluE4zZrw55ywMt/b6elPC2TkshUHQfDYQuIh1ZBqFOnvUIh8yyGc1L4sLACGY75tpvSE29RZQRmaZaV5YpzIM0SbpFirOqWsjKd3kLMjPqPr9+ISpqOP2tTfOfw9IW8gvRUYa2oRUEyE7Ju4QrwSWmKj9JK9KTeAzRiy5rsSW6issDaxQIYg2BNT8YLrNVm+FWpOVRud6KwohOpk8/YeM0EKEO3vAfGJH6tYT9Zv whatever"
    parsed, _, _, _, err := ssh.ParseAuthorizedKey([]byte(publicKey))
    if err != nil {
        panic(err)
    }
    parsedCryptoKey := parsed.(ssh.CryptoPublicKey)
    pubCrypto := parsedCryptoKey.CryptoPublicKey()
    pub := pubCrypto.(*rsa.PublicKey)

    // Encryption
    msg := "The quick brown fox jumps over the lazy dog"
    encryptedBytes, err := rsa.EncryptPKCS1v15(
        rand.Reader,
        pub,
        []byte(msg),
    )
    if err != nil {
        panic(err)
    }
    fmt.Println(hex.EncodeToString(encryptedBytes))
}

A possible hex encoded ciphertext is:

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

Note that this encryption is not deterministic, i.e. when you execute the code, you will get a different ciphertext.

---

Decryption is possible with the posted OpenSSL statement:

openssl rsautl -decrypt -inkey privateKeyFile -in encryptedMsgFile 

taking into account:

• encryptedMsgFile contains the binary data of the ciphertext, i.e. the hex decoding of 7610da... (if the ciphertext is stored directly as a binary file as mentioned above, encryptedMsgFile would correspond to this file).

• privateKeyFile contains the PEM encoded private PKCS#1 or PKCS#8 key. If your key is in OpenSSH's proprietary format (-----BEGIN OPENSSH PRIVATE KEY-----...), it must be converted to e.g. PKCS#8 format, e.g. with ssh_keygen:

  ssh-keygen -p -N "" -m pkcs8 -f data.key 
  

  Note that data.key contains the OpenSSH key and is overwritten with the PKCS#8 key. For the posted example, privateKeyFile is:

  -----BEGIN PRIVATE KEY-----
  MIIEvwIBADANBgkqhkiG9w0BAQEFAASCBKkwggSlAgEAAoIBAQC8NGcNWf6cUno2
  lpny36js9ynXCQ6kuJ14yIViuPwty0ZkfhpPdw3zVKWmKV0zS1/QyO6a5d85I9yw
  vEXHiZr4+fE4JRPxdEIQVGMz7OThs0XgluE4zZrw55ywMt/b6elPC2TkshUHQfDY
  QuIh1ZBqFOnvUIh8yyGc1L4sLACGY75tpvSE29RZQRmaZaV5YpzIM0SbpFirOqWs
  jKd3kLMjPqPr9+ISpqOP2tTfOfw9IW8gvRUYa2oRUEyE7Ju4QrwSWmKj9JK9KTeA
  zRiy5rsSW6issDaxQIYg2BNT8YLrNVm+FWpOVRud6KwohOpk8/YeM0EKEO3vAfGJ
  H6tYT9ZvAgMBAAECggEBAKP++alNuSpYSDxXAPD86cMLIL9LGiJ46Gb+PBSpYr04
  uy8IHz8NW++j2/AtbRQsYuKYpCn/koLE+CJc/GUCSDMaAJLO5FDq4EJAdm0hyNPP
  Fl28u6Z2qsOu3v8+ZYjIi8+f+xu4/c/kKs0Xgtq+sOdvL+WkBDrR+okhbFErSo3O
  z4SXkSTbRzKuFuZQA8HPi5kkDloEKhvZr3M0UyxUm6OtQTgW9mz3u+eabtBWVHEk
  3CY6AVi+FpKX1wj1j3gYCl5kjAxJALVuPXg/MhuiHBJODWWtsl4qnlrUqVn/qYwW
  G7GQb9dYjJK+WCjwG0VqoH2egEphaF4BrWMr6DLtseECgYEA5geNRjqKjmfxOlqm
  mA5BV08tey9C59Ef11aYmrx+ngkKDdHNvY2JOkpEA3huoa0sT4rRylmmYNBUVxnM
  UVqU8ppwRCfa58KTccopQO3BvSub74GvpwOX9MSn0CzW3brrFPfs72Lo3TAG4png
  ENufcF1PGqTDFXhQsQC8ZPJmWbECgYEA0XQBcF8tuEN1UHzPwnG35YPayRPusrxd
  3PD2BmOcp8B4DHS1KU2vJwu0nisi7S8eUbGxpRjwg3c5clQWFxWHuc//aRRb2QGl
  TR1XvtnU6qjGAnH7jcF+tsRlWx6Qc35InGodzgS8SzemNzXEOVmI7U4r7LmQe9Q1
  SX6dKzp/Gh8CgYB0bzwmYTmDLb/gDsSm0Qhn/k8CPID8QFGCuXWTVXgt3Ft3dUxT
  91GP7MmSjCJLuhFkzdq1Nz3NYYZfdFKEl3ovdtb+2MGocEgb3/2TvQVFEM7ko9ta
  iaogHm7nI9s67wNRYNFQttsyIr5JcyQExHZm9QQ2c1HAC1+kyL+TxVLjcQKBgQC7
  g0+Geq9Dt6DfXd3iBkzMfS7xtZaNDXY6xr57GdK1m+ndvN4zDAkyu5gHwjaSgQxz
  ttGDLMCl8abMY9si73ODNmNCf6d6r659Szey9PFY45/hsIm0bvYyScEzwjkwLG51
  Gct1FWg9LqTv6IKzlSSwzrskQzzGn0TVdzTd7pC7oQKBgQDGTj2gvzn4gwFcAQI6
  Zs4zbnXWwuQ0zt9y2Nzvs3Qwv8F2uryPnzv8DdbvdZ9dqnZCa8BvEALqfuOpKi4P
  5wpd3+Tw6eCwtNZwNZ+TJXLxXelmQBehM52RUb9wM+QzRBhIJayEuveNzJ9Irc14
  IDI76VfujESxC/qjZHFXkfdS3A==
  -----END PRIVATE KEY-----