From: Stefan Berger <[email protected]>
Adjust the calculation of the maximum signature size for support of
NIST P521. While existing curves may prepend a 0 byte to their coordinates
(to make the number positive), NIST P521 will not do this since only the
first bit in the most significant byte is used.
If the encoding of the x & y coordinates requires at least 128 bytes then
an additional byte is needed for the encoding of the length. Take this into
account when calculating the maximum signature size.
Signed-off-by: Stefan Berger <[email protected]>
Reviewed-by: Lukas Wunner <[email protected]>
Tested-by: Lukas Wunner <[email protected]>
---
crypto/asymmetric_keys/public_key.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
diff --git a/crypto/asymmetric_keys/public_key.c b/crypto/asymmetric_keys/public_key.c
index e5f22691febd..16cc0be28929 100644
--- a/crypto/asymmetric_keys/public_key.c
+++ b/crypto/asymmetric_keys/public_key.c
@@ -233,6 +233,7 @@ static int software_key_query(const struct kernel_pkey_params *params,
info->key_size = len * 8;
if (strncmp(pkey->pkey_algo, "ecdsa", 5) == 0) {
+ int slen = len;
/*
* ECDSA key sizes are much smaller than RSA, and thus could
* operate on (hashed) inputs that are larger than key size.
@@ -246,8 +247,19 @@ static int software_key_query(const struct kernel_pkey_params *params,
* Verify takes ECDSA-Sig (described in RFC 5480) as input,
* which is actually 2 'key_size'-bit integers encoded in
* ASN.1. Account for the ASN.1 encoding overhead here.
+ *
+ * NIST P192/256/384 may prepend a '0' to a coordinate to
+ * indicate a positive integer. NIST P521 never needs it.
*/
- info->max_sig_size = 2 * (len + 3) + 2;
+ if (strcmp(pkey->pkey_algo, "ecdsa-nist-p521") != 0)
+ slen += 1;
+ /* Length of encoding the x & y coordinates */
+ slen = 2 * (slen + 2);
+ /*
+ * If coordinate encoding takes at least 128 bytes then an
+ * additional byte for length encoding is needed.
+ */
+ info->max_sig_size = 1 + (slen >= 128) + 1 + slen;
} else {
info->max_data_size = len;
info->max_sig_size = len;
--
2.43.0
On Tue Mar 12, 2024 at 8:36 PM EET, Stefan Berger wrote:
> From: Stefan Berger <[email protected]>
>
> Adjust the calculation of the maximum signature size for support of
> NIST P521. While existing curves may prepend a 0 byte to their coordinates
> (to make the number positive), NIST P521 will not do this since only the
> first bit in the most significant byte is used.
>
> If the encoding of the x & y coordinates requires at least 128 bytes then
> an additional byte is needed for the encoding of the length. Take this into
> account when calculating the maximum signature size.
>
> Signed-off-by: Stefan Berger <[email protected]>
> Reviewed-by: Lukas Wunner <[email protected]>
> Tested-by: Lukas Wunner <[email protected]>
> ---
> crypto/asymmetric_keys/public_key.c | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/crypto/asymmetric_keys/public_key.c b/crypto/asymmetric_keys/public_key.c
> index e5f22691febd..16cc0be28929 100644
> --- a/crypto/asymmetric_keys/public_key.c
> +++ b/crypto/asymmetric_keys/public_key.c
> @@ -233,6 +233,7 @@ static int software_key_query(const struct kernel_pkey_params *params,
> info->key_size = len * 8;
>
> if (strncmp(pkey->pkey_algo, "ecdsa", 5) == 0) {
> + int slen = len;
> /*
> * ECDSA key sizes are much smaller than RSA, and thus could
> * operate on (hashed) inputs that are larger than key size.
> @@ -246,8 +247,19 @@ static int software_key_query(const struct kernel_pkey_params *params,
> * Verify takes ECDSA-Sig (described in RFC 5480) as input,
> * which is actually 2 'key_size'-bit integers encoded in
> * ASN.1. Account for the ASN.1 encoding overhead here.
> + *
> + * NIST P192/256/384 may prepend a '0' to a coordinate to
> + * indicate a positive integer. NIST P521 never needs it.
> */
> - info->max_sig_size = 2 * (len + 3) + 2;
> + if (strcmp(pkey->pkey_algo, "ecdsa-nist-p521") != 0)
> + slen += 1;
Just wondering the logic of picking between these:
1. "strncmp"
2. "strcmp"
Now the "ecdsa" is matched with strncmp and "ecdsa-nist-p521" is
compared with strcmp.
So is there a good reason to use different function in these
cases?
I'd guess both could be using strcmp since comparing against
constant...
> + /* Length of encoding the x & y coordinates */
> + slen = 2 * (slen + 2);
> + /*
> + * If coordinate encoding takes at least 128 bytes then an
> + * additional byte for length encoding is needed.
> + */
> + info->max_sig_size = 1 + (slen >= 128) + 1 + slen;
> } else {
> info->max_data_size = len;
> info->max_sig_size = len;
BR, Jarkko
On Tue Mar 19, 2024 at 12:42 AM EET, Stefan Berger wrote:
>
>
> On 3/18/24 17:12, Jarkko Sakkinen wrote:
> > On Tue Mar 12, 2024 at 8:36 PM EET, Stefan Berger wrote:
> >> From: Stefan Berger <[email protected]>
> >>
> >> Adjust the calculation of the maximum signature size for support of
> >> NIST P521. While existing curves may prepend a 0 byte to their coordinates
> >> (to make the number positive), NIST P521 will not do this since only the
> >> first bit in the most significant byte is used.
> >>
> >> If the encoding of the x & y coordinates requires at least 128 bytes then
> >> an additional byte is needed for the encoding of the length. Take this into
> >> account when calculating the maximum signature size.
> >>
> >> Signed-off-by: Stefan Berger <[email protected]>
> >> Reviewed-by: Lukas Wunner <[email protected]>
> >> Tested-by: Lukas Wunner <[email protected]>
> >> ---
> >> crypto/asymmetric_keys/public_key.c | 14 +++++++++++++-
> >> 1 file changed, 13 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/crypto/asymmetric_keys/public_key.c b/crypto/asymmetric_keys/public_key.c
> >> index e5f22691febd..16cc0be28929 100644
> >> --- a/crypto/asymmetric_keys/public_key.c
> >> +++ b/crypto/asymmetric_keys/public_key.c
> >> @@ -233,6 +233,7 @@ static int software_key_query(const struct kernel_pkey_params *params,
> >> info->key_size = len * 8;
> >>
> >> if (strncmp(pkey->pkey_algo, "ecdsa", 5) == 0) {
> >> + int slen = len;
> >> /*
> >> * ECDSA key sizes are much smaller than RSA, and thus could
> >> * operate on (hashed) inputs that are larger than key size.
> >> @@ -246,8 +247,19 @@ static int software_key_query(const struct kernel_pkey_params *params,
> >> * Verify takes ECDSA-Sig (described in RFC 5480) as input,
> >> * which is actually 2 'key_size'-bit integers encoded in
> >> * ASN.1. Account for the ASN.1 encoding overhead here.
> >> + *
> >> + * NIST P192/256/384 may prepend a '0' to a coordinate to
> >> + * indicate a positive integer. NIST P521 never needs it.
> >> */
> >> - info->max_sig_size = 2 * (len + 3) + 2;
> >> + if (strcmp(pkey->pkey_algo, "ecdsa-nist-p521") != 0)
> >> + slen += 1;
> >
> > Just wondering the logic of picking between these:
> >
> > 1. "strncmp"
> > 2. "strcmp"
> >
>
> strncmp: prefix-matching
> strcmp: full string matching
Right, in first case is necessary because strcmp() would return "-1" for
the substring.
BR, Jarkko
> -----Original Message-----
> From: Stefan Berger <[email protected]>
> Sent: Wednesday, March 13, 2024 12:06 AM
> To: [email protected]; [email protected];
> [email protected]; [email protected]
> Cc: [email protected]; [email protected];
> [email protected]; Bharat Bhushan <[email protected]>;
> [email protected]; Stefan Berger <[email protected]>
> Subject: [EXTERNAL] [PATCH v6 12/13] crypto: asymmetric_keys - Adjust
> signature size calculation for NIST P521
> ----------------------------------------------------------------------
> From: Stefan Berger <[email protected]>
>
> Adjust the calculation of the maximum signature size for support of NIST
> P521. While existing curves may prepend a 0 byte to their coordinates (to
> make the number positive), NIST P521 will not do this since only the first bit in
> the most significant byte is used.
>
> If the encoding of the x & y coordinates requires at least 128 bytes then an
> additional byte is needed for the encoding of the length. Take this into account
> when calculating the maximum signature size.
>
> Signed-off-by: Stefan Berger <[email protected]>
> Reviewed-by: Lukas Wunner <[email protected]>
> Tested-by: Lukas Wunner <[email protected]>
> ---
> crypto/asymmetric_keys/public_key.c | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/crypto/asymmetric_keys/public_key.c
> b/crypto/asymmetric_keys/public_key.c
> index e5f22691febd..16cc0be28929 100644
> --- a/crypto/asymmetric_keys/public_key.c
> +++ b/crypto/asymmetric_keys/public_key.c
> @@ -233,6 +233,7 @@ static int software_key_query(const struct
> kernel_pkey_params *params,
> info->key_size = len * 8;
>
> if (strncmp(pkey->pkey_algo, "ecdsa", 5) == 0) {
> + int slen = len;
> /*
> * ECDSA key sizes are much smaller than RSA, and thus could
> * operate on (hashed) inputs that are larger than key size.
> @@ -246,8 +247,19 @@ static int software_key_query(const struct
> kernel_pkey_params *params,
> * Verify takes ECDSA-Sig (described in RFC 5480) as input,
> * which is actually 2 'key_size'-bit integers encoded in
> * ASN.1. Account for the ASN.1 encoding overhead here.
> + *
> + * NIST P192/256/384 may prepend a '0' to a coordinate to
> + * indicate a positive integer. NIST P521 never needs it.
> */
> - info->max_sig_size = 2 * (len + 3) + 2;
> + if (strcmp(pkey->pkey_algo, "ecdsa-nist-p521") != 0)
> + slen += 1;
> + /* Length of encoding the x & y coordinates */
> + slen = 2 * (slen + 2);
> + /*
> + * If coordinate encoding takes at least 128 bytes then an
> + * additional byte for length encoding is needed.
> + */
> + info->max_sig_size = 1 + (slen >= 128) + 1 + slen;
Is "(slen >= 128)" valid for P192/256/384 also?
Thanks
-Bharat
> } else {
> info->max_data_size = len;
> info->max_sig_size = len;
> --
> 2.43.0