Say I have previously created a private/public key combination, and decided at the time to not protect the private key with a password. If I later decide to "beef up" security and use a password-protected private key instead, would I need to generate a new private/public key pair, or can I simply add a password to my existing private key?
Is the opposite possible as well, can I "remove" a password from an existing private key?
A word of caution: as stated in laverya's answer openssl encrypts the key in a way that (depending on your threat model) is probably not good enough any more.
Of course you can add/remove a passphrase at a later time.
add one (assuming it was an rsa key, else use dsa)
openssl rsa -aes256 -in your.key -out your.encrypted.key
mv your.encrypted.key your.key
chmod 600 your.key
the -aes256 tells openssl to encrypt the key with AES256.
As ArianFaurtosh has correctly pointed out: For the encryption algorithm you can use aes128, aes192, aes256, camellia128, camellia192, camellia256, des (which you definitely should avoid), des3 or idea
remove it
openssl rsa -in your.key -out your.open.key
you will be asked for your passphrase one last time
by omitting the -aes256 you tell openssl to not encrypt the output.
mv your.open.key your.key
chmod 600 your.key
While Guntbert's answer was good at the time, it's getting a little outdated. openssl rsa -aes256 creates an encrypted file using the md5 hash of your password as the encryption key, which is weaker than you would expect - and depending on your perspective that may in fact be worse than plaintext. (If you use the same password for your ssh key and your login, cracking the md5 hash will be significantly faster than attacking however your system stores the password - barring things like Windows XP)
A modern solution would be to use ssh-keygen -p -o -f PRIVATEKEY, which will allow you to enter a passphrase and then will overwrite the existing private key with the encrypted version. This uses the bcrypt pbkdf, which is FAR slower than md5 even when running at the default 16 rounds. 'Far slower' in this case means between a tenth and a half of a second, instead of a millionth of a second - not something you'll notice when logging in, but a massive difference when cracking passwords.
2^cost rounds to derive the key then 64 rounds of encrypting a known block. For the equivalent of a normal bcrypt cost like 14, it should be roughly 2^14/128 = 128, though it's not quite the same as openssh spends more time encrypting.
– AndrolGenhald
Feb 11 '19 at 20:06
-o. Some applications doesent recognize it.
– Kuronashi
Jul 29 '20 at 07:45
-o; if you want old format, which as explained is very weak, specify -m pem. Almost everything that can read OpenSSH-old (aka OpenSSL-legacy) can also read PKCS8 which as I noted on the other answer is more secure, especially since the improvements in OpenSSL 1.1.0 (2016); OpenSSH (since almost forever) can read PKCS8 but not write it.
– dave_thompson_085
Aug 09 '20 at 01:49
unable to load Private Key 139703991919744:error:0909006C:PEM routines:get_name:no start line:../crypto/pem/pem_lib.c:745:Expecting: ANY PRIVATE KEY)
– Binarus
Sep 25 '20 at 14:01
-o below 7.8 and by default since is not usable with OpenSSL at all. You can use ssh-keygen to convert it to 'old' format which is OpenSSL compatible. puttygen in recent versions of PuTTY can also handle it, and has several output options.
– dave_thompson_085
Dec 25 '20 at 05:18
-o for ages, so I lost patience several months ago and dumped it. Instead, I now have installed cygwin and use its SSH.
– Binarus
Dec 25 '20 at 14:20
When a private is "protected by a password", it merely means that the key bytes, as stored somewhere, are encrypted with a password-derived symmetric key. A private key is readily encodable as a sequence of bytes, and can be copied, encrypted and decrypted just like any file. The important point here is that the password is all about storage: when the private key is to be used (e.g. to sign something), then it is first decrypted in the RAM of some computer, which then proceeds to use the non-encrypted private key. Correspondingly, there is nothing special in a RSA key pair which would make it suitable or unsuitable for password protection. Password protection is really an orthogonal issue.
Of course, if a private key has ever been stored on some physical medium (say, a hard disk) without any extra protection, then it may have left exploitable traces there. Details depend a lot on what system is actually used for private key storage. For instance, Windows systems use DPAPI for storing user's private keys, and DPAPI makes some extra efforts at not letting stored keys leak (whether these efforts are successful remains to be proven).
openssl rsa supports only RSA keys and its encryption is susceptible to brute-forcing.
It's better to use openssl pkcs8 - it uses a key derivation function and supports RSA, ECC and Edwards keys:
openssl pkcs8 -topk8 -in source.key -out encrypted.key
For even better security use the scrypt KDF:
openssl pkcs8 -topk8 -scrypt -in source.key -out encrypted.key
To decrypt a pkcs8 encrypted key, drop the -topk8 flag:
openssl pkcs8 -in enc.key -out dec.key
rsa writes PKCS8 by default, though it remains as you say limited to RSA (and only the original rsaEncryption OID, not rsassa-pss).
– dave_thompson_085
Sep 21 '22 at 06:53
pkcs8 -topk8 [-v2 $cipher]and in 1.1.0+ add-niter $nwith the highest count you can afford (the default, and before 1.1.0 the only value, is 2048). – dave_thompson_085 Feb 12 '19 at 02:28openssl pkcs8 -topk8command in modern versions of openssl can do scrypt or bcrypt with some large number of iterations. Even in older versions, it can do pbkdf2 with 2048 iterations. Depending on your openssl version, you may be stuck with Triple-DES as the cipher. – Brian Minton May 17 '19 at 03:30