[PATCH] sshsig: check hashalg before selecting the RSA signature algorithm

[Damien Miller]

Sorry, this now been committed and will be in openssh-10.0

On Sat, 23 Nov 2024, Morten Linderud wrote:

> Hi,
> 
> I sent this patch back inn april and I still have a need for this. Would it be
> possible to get any pointers how we can have `hashalg` selectable by `ssh-keygen -Y`?
> 
> -- 
> Morten Linderud
> PGP: 9C02FF419FECBE16
> 
> On Thu, Apr 11, 2024 at 09:16:39PM +0200, Morten Linderud wrote:
> > `ssh-keygen -Y sign` only selects the signing algorithm `rsa-sha2-512`
> > and this prevents ssh-agent implementations that can't support sha512
> > from signing messages.
> > 
> > An example of this is TPMs which mostly only really supports sha256
> > widely.
> > 
> > This change enables `ssh-keygen -Y sign` to honor the `hashalg` option
> > for the signing algorithm.
> > 
> > Signed-off-by: Morten Linderud <morten at linderud.pw>
> > ---
> >  sshsig.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/sshsig.c b/sshsig.c
> > index 470b286a3..033b43353 100644
> > --- a/sshsig.c
> > +++ b/sshsig.c
> > @@ -190,8 +190,14 @@ sshsig_wrap_sign(struct sshkey *key, const char *hashalg,
> >  	}
> >  
> >  	/* If using RSA keys then default to a good signature algorithm */
> > -	if (sshkey_type_plain(key->type) == KEY_RSA)
> > -		sign_alg = RSA_SIGN_ALG;
> > +	if (sshkey_type_plain(key->type) == KEY_RSA){
> > +		if (hashalg == NULL)
> > +			sign_alg = RSA_SIGN_ALG;
> > +		else if (strcmp(hashalg, "sha256") == 0)
> > +			sign_alg = "rsa-sha2-256";
> > +		else if (strcmp(hashalg, "sha512") == 0)
> > +			sign_alg = "rsa-sha2-512";
> > +	}
> >  
> >  	if (signer != NULL) {
> >  		if ((r = signer(key, &sig, &slen,
> > -- 
> > 2.44.0
> > _______________________________________________
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> > openssh-unix-dev at mindrot.org
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>