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Super Size The Strength Of Your OpenSSH Private Keys

In a previous post, about 18 months ago, I blogged about how you can increase the strength of your OpenSSH private keys by using openssl(1) to convert them to PKCS#8 format. However, as of OpenSSH verison 6.5, there is a new private key format for private keys, as well as a new key type. The new key type is ed25519. Without going into the details of the strengths of ed25519 over RSA, I do want to identify a new encryption method for your private keys.

In previous versions of OpenSSH, if you provided a passphrase to encrypt your private key, it was converted into a cipher key by first hashing it with MD5, then encrypting your private key. Unfortunately in this case, as usually the problem with all hashed password storage, MD5 is fast, fast, fast. While MD5 is a cryptographic one-way hashing function, it can fall victim to rainbow table attacks, as well as just plain old brute forcing. With a few GPUs, and the right software algorithm, it's not unheard of to try billions of passwords per second to try and achieve the correct hash, or in this case a cipher key.

Key derivation functions (KDFs) however, can be resource intensive, and slow. One in particular is bcrypt, which is very similar to the bcrypt one-way hashing function. With OpenSSH 6.5, when generating ed25519 keys, the bcrypt pbkdf is the default function for creating that cipher key based on your passphrase. To further protect you from brute force searching on your passphrase, ssh-keygen(1) will apply 16 rounds to the bcrypt pbkdf before creating the cipher key which is used to encrypt the private key on disk. On my ThinkPad T61, this takes approximately 1/3 of a second to complete all 16 rounds, or about 50 per second. This is a far cry from the millions I know my T61 can do with MD5.

However, this isn't even the bread and butter of the post: You can convert your existing keys to the new format with OpenSSH 6.5. This means your old DSA and RSA keys, and even the newer ECDSA keys, can all be converted to take advantage of the new format.

Further, you don't have to take the default 16 rounds of encrypting your key. Instead, you can increase that if you want to be a bit more paranoid. Suppose I wish to apply 100 rounds instead of the default 16- a factor of over 6x. To do this, for each of your private keys, run the following:

$ ssh-keygen -o -p -a 64 -f id_rsa
Enter old passphrase: 
Key has comment 'rsa w/o comment'
Enter new passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved with the new passphrase.

At this point, it will take approximately 2 seconds on my T61 to complete the rounds, and encrypt the key. This can be verified whet creating an SSH agent, and adding my key to the agent:

$ eval $(ssh-agent)
Agent pid 17202
$ ssh-add
Enter passphrase for /home/aaron/.ssh/id_rsa: 
Identity added: /home/aaron/.ssh/id_rsa (/home/aaron/.ssh/id_rsa)

When adding my passphrase, it takes a full 2 seconds before returning to a shell on the remote server. Of course, feel free to increase the rounds count. 1000 rounds would take me a full 20 seconds. Probably not sufficient for day-to-day use while at work, but could be applicable in other cases.

When you look at your private keys, with the old version, the header would look something like this:

Proc-Type: 4,ENCRYPTED
DEK-Info: AES-128-CBC,DF7C541751D59241F15DA424506137CE

If you converted your key to PKCS#8 with openssl(1), then your headers would look something like this:

(base64 output)

However, with the new OpenSSH key format, encrypted keys now look like:

(base64 output)

With bcrypt as the new encrypted storage format, and the ability to adjust the number of rounds, as well as convert older keys, this is a big win for security. Well done OpenSSH team!

UPDATE: It should be noted that when using this new on-disk encrypted format, your OpenSSH private key will no longer be compatible with openssl(1), as previously, the private key was stored in PEM format. Further, using the "ed25519" key type means using the new format automatically, as openssl(1) does not support the ed25519 algorithm.

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