improving passphrase protected private keys

[Sven Kieske]

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Hi,

I got a proposal
for a slight different default
private key encryption protocol.

Here is my understanding what
ssh-keygen currently does.

According to this article:

http://martin.kleppmann.com/2013/05/24/improving-security-of-ssh-private-keys.html

when you create a new key with a
passphrase to protect it, ssh-keygen
uses a hard-coded openssl call to do this:

It encrypts the private key with AES-128 in
CBC mode, and generates the encryption key
the following way:

1. "Append the first 8 bytes of the IV
to the passphrase, without a separator
(serves as a salt)."

2. "Take the MD5 hash of the resulting string (once)."

So my proposal is, to alter this
by using PKCS 8 as defined in RFC 5208
as is described in the above article.

This currently works already by converting
your key manually:

openssl pkcs8 -topk8 -v2 des3 \
    -in test_rsa_key.old -passin 'pass:super secret passphrase' \
    -out test_rsa_key -passout 'pass:super secret passphrase'

I didn't find any contradicting documentation
or stuff inside the SSH RFCs why this is not the default
yet.

I know this is just a little hardening
and just covers cases where your encrypted
private key gets stolen and is harder to bruteforce
due to the use of PBKDF2 instead of MD5.

What do you think about this?
Is there some error in my information
which prevents this from being the default
way ssh keys are generated?

Does this not work on all supported
plattforms?

Please keep me CC'ed in your answers
as I'm not subscribed to the list.

kind regards

Sven Kieske

PS: Thanks for this awesome free software
and your work!
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