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libsec: move zero check to curve25519_dh_finish()

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As checking for all zero has to be done in a timing-safe
way to avoid a side channel, it is best todo this here
instead of letting the caller deal with it.

This adds a return type of int to curve25519_dh_finish()
where returning 0 means we got a all zero shared key.

RFC7748 states:

The check for the all-zero value results from the fact
that the X25519 function produces that value if it
operates on an input corresponding to a point with small
order, where the order divides the cofactor of the curve.
This commit is contained in:
cinap_lenrek 2021-06-20 14:41:26 +00:00
parent 6dd2c638b6
commit 57d95c7325
5 changed files with 10 additions and 14 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

2
sys/include/ape/libsec.h
View file

@ -583,7 +583,7 @@ void curve25519(uchar mypublic[32], uchar secret[32], uchar basepoint[32]);
/* Curve25519 diffie hellman */
void curve25519_dh_new(uchar x[32], uchar y[32]);
void curve25519_dh_finish(uchar x[32], uchar y[32], uchar z[32]);
int curve25519_dh_finish(uchar x[32], uchar y[32], uchar z[32]);
/* password-based key derivation function 2 (rfc2898) */
void pbkdf2_x(uchar *p, ulong plen, uchar *s, ulong slen, ulong rounds, uchar *d, ulong dlen,

2
sys/include/libsec.h
View file

@ -575,7 +575,7 @@ void curve25519(uchar mypublic[32], uchar secret[32], uchar basepoint[32]);
/* Curve25519 diffie hellman */
void curve25519_dh_new(uchar x[32], uchar y[32]);
void curve25519_dh_finish(uchar x[32], uchar y[32], uchar z[32]);
int curve25519_dh_finish(uchar x[32], uchar y[32], uchar z[32]);
/* password-based key derivation function 2 (rfc2898) */
void pbkdf2_x(uchar *p, ulong plen, uchar *s, ulong slen, ulong rounds, uchar *d, ulong dlen,

3
sys/src/cmd/ssh.c
View file

@ -600,7 +600,8 @@ Next1: switch(recvpkt()){
if((S = ssh2rsasig(sig, nsig)) == nil)
sysfatal("bad server signature");
curve25519_dh_finish(x, ys, z);
if(!curve25519_dh_finish(x, ys, z))
sysfatal("unlucky shared key");
K = betomp(z, 32, nil);
nk = (mpsignif(K)+8)/8;

5
sys/src/libsec/port/curve25519_dh.c
View file

@ -3,6 +3,7 @@
#include <libsec.h>
static uchar nine[32] = {9};
static uchar zero[32] = {0};
void
curve25519_dh_new(uchar x[32], uchar y[32])
@ -20,7 +21,7 @@ curve25519_dh_new(uchar x[32], uchar y[32])
y[31] |= b & 0x80;
}
void
int
curve25519_dh_finish(uchar x[32], uchar y[32], uchar z[32])
{
/* remove the random bit */
@ -31,4 +32,6 @@ curve25519_dh_finish(uchar x[32], uchar y[32], uchar z[32])
memset(x, 0, 32);
memset(y, 0, 32);
return tsmemcmp(z, zero, 32) != 0;
}

12
sys/src/libsec/port/tlshand.c
View file

@ -948,7 +948,6 @@ Out:
static Bytes*
tlsSecECDHEc(TlsSec *sec, int curve, Bytes *Ys)
{
static char zero[32] = {0};
ECdomain *dom = &sec->ec.dom;
ECpriv *Q = &sec->ec.Q;
ECpub *pub;
@ -967,10 +966,7 @@ tlsSecECDHEc(TlsSec *sec, int curve, Bytes *Ys)
Yc = newbytes(32);
curve25519_dh_new(sec->X, Yc->data);
Z = newbytes(32);
curve25519_dh_finish(sec->X, Ys->data, Z->data);
// rfc wants us to terminate the connection if
// shared secret == all zeroes.
if(tsmemcmp(Z->data, zero, Z->len) == 0){
if(!curve25519_dh_finish(sec->X, Ys->data, Z->data)){
freebytes(Yc);
freebytes(Z);
return nil;
@ -2573,7 +2569,6 @@ tlsSecECDHEs1(TlsSec *sec)
static int
tlsSecECDHEs2(TlsSec *sec, Bytes *Yc)
{
static char zero[32] = {0};
ECdomain *dom = &sec->ec.dom;
ECpriv *Q = &sec->ec.Q;
ECpoint K;
@ -2591,10 +2586,7 @@ tlsSecECDHEs2(TlsSec *sec, Bytes *Yc)
return -1;
}
Z = newbytes(32);
curve25519_dh_finish(sec->X, Yc->data, Z->data);
// rfc wants us to terminate the connection if
// shared secret == all zeroes.
if(tsmemcmp(Z->data, zero, Z->len) == 0){
if(!curve25519_dh_finish(sec->X, Yc->data, Z->data)){
werrstr("unlucky shared key");
freebytes(Z);
return -1;