Fix ecp_gen_keypair()
Too few tries caused failures for some curves (esp. secp224k1)
diff --git a/ChangeLog b/ChangeLog
index 2f6a3c5..7f7726b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
PolarSSL ChangeLog (Sorted per branch, date)
+= PolarSSL 1.3 branch
+Bugfix
+ * ecp_gen_keypair() does more tries to prevent failure because of
+ statistics
+
= PolarSSL 1.3.4 released on 2014-01-27
Features
* Support for the Koblitz curves: secp192k1, secp224k1, secp256k1
diff --git a/library/ecp.c b/library/ecp.c
index b1c4548..a27d30e 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -1796,7 +1796,16 @@
MPI_CHK( mpi_read_binary( d, rnd, n_size ) );
MPI_CHK( mpi_shift_r( d, 8 * n_size - grp->nbits ) );
- if( count++ > 10 )
+ /*
+ * Each try has at worst a probability 1/2 of failing (the msb has
+ * a probability 1/2 of being 0, and then the result will be < N),
+ * so after 30 tries failure probability is a most 2**(-30).
+ *
+ * For most curves, 1 try is enough with overwhelming probability,
+ * since N starts with a lot of 1s in binary, but some curves
+ * such as secp224k1 are actually very close to the worst case.
+ */
+ if( ++count > 30 )
return( POLARSSL_ERR_ECP_RANDOM_FAILED );
}
while( mpi_cmp_int( d, 1 ) < 0 ||