This removes both uses of SHA-1 in the file format: it was used as the
MAC protecting the key file against tamperproofing, and also used in
the key derivation step that converted the user's passphrase to cipher
and MAC keys.
The MAC is simply upgraded from HMAC-SHA-1 to HMAC-SHA-256; it is
otherwise unchanged in how it's applied (in particular, to what data).
The key derivation is totally reworked, to be based on Argon2, which
I've just added to the code base. This should make stolen encrypted
key files more resistant to brute-force attack.
Argon2 has assorted configurable parameters for memory and CPU usage;
the new key format includes all those parameters. So there's no reason
we can't have them under user control, if a user wants to be
particularly vigorous or particularly lightweight with their own key
files. They could even switch to one of the other flavours of Argon2,
if they thought side channels were an especially large or small risk
in their particular environment. In this commit I haven't added any UI
for controlling that kind of thing, but the PPK loading function is
all set up to cope, so that can all be added in a future commit
without having to change the file format.
While I'm at it, I've also switched the CBC encryption to using a
random IV (or rather, one derived from the passphrase along with the
cipher and MAC keys). That's more like normal SSH-2 practice.
When I transcribed the code into this document, I misread 'put_data'
as 'put_string' in several places, and documented SSH-style string
length headers that do not actually exist in the format.
Somebody on comp.security.ssh asked about it recently, and I decided
that storing it in a comment in the key file was not really good
enough. Also, that comment was incomplete (it listed the private key
formats for RSA and DSA but not any of the newer ECC key types, simple
as their private-key formats may be).
