Re: [PATCH 11/18] crypto: rename des-rfb cipher to just des

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Jul 07, 2021 at 02:47:15PM +0200, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
> 
> > Currently the crypto layer exposes support for a 'des-rfb'
> > algorithm which is just normal single-DES, with the bits
> > in each key byte reversed. This special key munging is
> > required by the RFB protocol password authentication
> > mechanism.
> >
> > Since the crypto layer is generic shared code, it makes
> > more sense to do the key byte munging in the VNC server
> > code, and expose normal single-DES support.
> >
> > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> > ---
> 
> [...]
> 
> > diff --git a/qapi/crypto.json b/qapi/crypto.json
> > index 7116ae9a46..6b3fadabac 100644
> > --- a/qapi/crypto.json
> > +++ b/qapi/crypto.json
> > @@ -66,7 +66,7 @@
> >  # @aes-128: AES with 128 bit / 16 byte keys
> >  # @aes-192: AES with 192 bit / 24 byte keys
> >  # @aes-256: AES with 256 bit / 32 byte keys
> > -# @des-rfb: RFB specific variant of single DES. Do not use except in VNC.
> > +# @des: DES with 56 bit / 8 byte keys. Do not use except in VNC.
> >  # @3des: 3DES(EDE) with 192 bit / 24 byte keys (since 2.9)
> >  # @cast5-128: Cast5 with 128 bit / 16 byte keys
> >  # @serpent-128: Serpent with 128 bit / 16 byte keys
> > @@ -80,7 +80,7 @@
> >  { 'enum': 'QCryptoCipherAlgorithm',
> >    'prefix': 'QCRYPTO_CIPHER_ALG',
> >    'data': ['aes-128', 'aes-192', 'aes-256',
> > -           'des-rfb', '3des',
> > +           'des', '3des',
> >             'cast5-128',
> >             'serpent-128', 'serpent-192', 'serpent-256',
> >             'twofish-128', 'twofish-192', 'twofish-256']}
> 
> Is enum value "des-rfb" part of any external interface?

Strictly speaking, yes, but in reality it doesn't matter.


The only place in QEMU that actually uses DES-RFB is the
VNC server code. That is an indirect usage when the user
sets the "password" option flag in QemuOpts. The fact that
it uses DES-RFB is an internal impl detail.

The one place that does publically expose ability to set a
field using the QCryptoCipherAlgorithm enum type is the
LUKS support in the block layer:

{ 'struct': 'QCryptoBlockCreateOptionsLUKS',
  'base': 'QCryptoBlockOptionsLUKS',
  'data': { '*cipher-alg': 'QCryptoCipherAlgorithm',
            '*cipher-mode': 'QCryptoCipherMode',
            '*ivgen-alg': 'QCryptoIVGenAlgorithm',
            '*ivgen-hash-alg': 'QCryptoHashAlgorithm',
            '*hash-alg': 'QCryptoHashAlgorithm',
            '*iter-time': 'int'}}

eg exposed on CLI as:

  $ qemu-img create -f luks -o cipher-alg=NNN foo.luks 1G

or equivalant with QMP blockdev-create

While the QMP schema allows any valid QCryptoCipherAlgorithm
string to be set, the actual implementation does not.

The crypto/block-luks.c code has a map between cipher algs
and LUKS format algoritm names:


static const QCryptoBlockLUKSCipherNameMap
qcrypto_block_luks_cipher_name_map[] = {
    { "aes", qcrypto_block_luks_cipher_size_map_aes },
    { "cast5", qcrypto_block_luks_cipher_size_map_cast5 },
    { "serpent", qcrypto_block_luks_cipher_size_map_serpent },
    { "twofish", qcrypto_block_luks_cipher_size_map_twofish },
};

If it isn't in that table, it can't be used. IOW, the only
scenario we're affecting in this rename is one which would
already result in an error condition

Original behaviour:

 $ qemu-img create -f luks --object secret,id=sec0,data=123 -o cipher-alg=des-rfb,key-secret=sec0 demo.luks 1G
Formatting 'demo.luks', fmt=luks size=1073741824 key-secret=sec0 cipher-alg=des-rfb
qemu-img: demo.luks: Algorithm 'des-rfb' not supported

New behaviour:

$ qemu-img create -f luks --object secret,id=sec0,data=123 -o cipher-alg=des-rfb,key-secret=sec0 demo.luks 1G
Formatting 'demo.luks', fmt=luks size=1073741824 key-secret=sec0 cipher-alg=des-fish
qemu-img: demo.luks: Invalid parameter 'des-rfb'

I considered this incompatibility to be acceptable, and thus
not worth going through a deprecation dance.

Regards,
Daniel
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