* How to get my krb5 crypto lib upstream?
@ 2023-05-31 17:02 ` David Howells
[not found] ` <726863.1685563684@warthog.procyon.org.uk>
0 siblings, 1 reply; 2+ messages in thread
From: David Howells @ 2023-05-31 17:02 UTC (permalink / raw)
To: Herbert Xu, Chuck Lever
Cc: dhowells, linux-afs, linux-nfs, linux-crypto, linux-kernel
Hi Herbert, Chuck,
I'm wondering how to make progress on getting my krb5 crypto lib upstream.
Can I push it as it stands and then we try and build it into the crypto API
for it later? That would allow me to push my rxgk implementation for AF_RXRPC
and at least allow userspace to use that.
As far as building a crypto API around it goes, we need four interfaces:
(1) Key generation. We may need to generate a {cipher,hash} key pair {Ke,Ki}
or just a hash key Kc. We might conceivably want both.
At the moment, I return a prepared cipher or a prepared hash. I don't
deal with the key pairing here as it makes testing a bit more awkward.
int crypto_krb5_get_Kc(const struct krb5_enctype *krb5,
const struct krb5_buffer *TK,
u32 usage,
struct krb5_buffer *key,
struct crypto_shash **_shash,
gfp_t gfp);
int crypto_krb5_get_Ke(const struct krb5_enctype *krb5,
const struct krb5_buffer *TK,
u32 usage,
struct krb5_buffer *key,
struct crypto_sync_skcipher **_ci,
gfp_t gfp);
int crypto_krb5_get_Ki(const struct krb5_enctype *krb5,
const struct krb5_buffer *TK,
u32 usage,
struct krb5_buffer *key,
struct crypto_shash **_shash,
gfp_t gfp);
(2) PRF+ generation. This takes some a key and a metadata blob and generates
a new blob that then gets used as a key.
int crypto_krb5_calc_PRFplus(const struct krb5_enctype *krb5,
const struct krb5_buffer *K,
unsigned int L,
const struct krb5_buffer *S,
struct krb5_buffer *result,
gfp_t gfp);
(3) Encrypt and Decrypt.
Encrypt has to be parameterised to take a specific confounder for testing
and generate a random one for normal operation. The IV is fixed all
zeroes in the cases I've implemented. When testing, decrypt should
perhaps be passed the confounder to check it.
When encrypting, the output buffer will be larger than the input buffer
(or, at least, room must be set aside) so that a confounder, padding and
a checksum can be inserted.
When decrypting, we either want to provide a separate output buffer so
that the confounder and checksum can be stripped off, or we need to be
able to find out where the decrypted payload plus the padding will be (we
can't work out how much padding there is - that's left to the caller).
At the moment, I pass a single buffer descriptor, providing encrypt with
extra space front and back and providing decrypt with somewhere to save
offset and length:
ssize_t crypto_krb5_encrypt(const struct krb5_enctype *krb5,
struct krb5_enc_keys *keys,
struct scatterlist *sg, unsigned int nr_sg,
size_t sg_len,
size_t data_offset, size_t data_len,
bool preconfounded);
int crypto_krb5_decrypt(const struct krb5_enctype *krb5,
struct krb5_enc_keys *keys,
struct scatterlist *sg, unsigned int nr_sg,
size_t *_offset, size_t *_len,
int *_error_code);
I also allow a krb5/gssapi error code to be returned so that it can be
used in the generation of abort messages. This needs sorting out
better. It may be that only one code is actually relevant to this and
that the caller generates all the rest as it checks the metadata.
The AEAD interface might suffice as it stands if we pass in the keys
already generated and passed in as a single key blob. We don't want an
IV generator as the protocol defines the IV to use.
(4) GetMIC and VerifyMIC.
Both of these need parameterising with extra metadata that will get
inserted into the hash before the data is hashed. GetMIC will insert the
checksum into the buffer and VerifyMIC will check it and strip it off.
I'm not sure that the hash API is suitable for this. AEAD might suit for
GetMIC at least, but using AEAD for VerifyMIC would lead to an extraneous
copy I'd prefer to avoid.
ssize_t crypto_krb5_get_mic(const struct krb5_enctype *krb5,
struct crypto_shash *shash,
const struct krb5_buffer *metadata,
struct scatterlist *sg, unsigned int nr_sg,
size_t sg_len,
size_t data_offset, size_t data_len);
int crypto_krb5_verify_mic(const struct krb5_enctype *krb5,
struct crypto_shash *shash,
const struct krb5_buffer *metadata,
struct scatterlist *sg, unsigned int nr_sg,
size_t *_offset, size_t *_len,
int *_error_code);
There's a lot to be said in using an asynchronous overrideable interface for
encrypt and decrypt. The problem is that we want to do simultaneous hash and
crypt if we can. I think the Intel AES/SHA instructions permit this to be
done and there is sufficient register space to do it - and I *think* that it
may be possible to offload this to the Intel QAT or the Intel IAA on the
latest 4th Gen Xeons - and maybe NICs that can handle NFS/SMB offload.
I think we'll perhaps need a "krb5 encoding type" API that can provide key
derivation, PRF+ and information - something along the following lines:
struct krb5_enctype {
int etype; // Encryption (key) type
int ctype; // Checksum type
const char *name; // "Friendly" name
const char *encrypt_name; // Crypto encrypt name
const char *cksum_name; // Crypto checksum name
const char *hash_name; // Crypto hash name
u16 block_len; // Length of encryption block
u16 conf_len; // Length of confounder
u16 cksum_len; // Length of checksum
u16 key_bytes; // Length of raw key
u16 key_len; // Length of final key
u16 hash_len; // Length of hash
u16 prf_len; // Length of PRF() result
u16 Kc_len; // Length of Kc
u16 Ke_len; // Length of Ke
u16 Ki_len; // Length of Ki
bool keyed_cksum; // T if a keyed cksum
bool pad; // T if should pad
};
We need to be able to look the encoding up by ID, not by name.
David
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