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Fix ECDSA corner case: missing reduction mod N

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No security issue, can cause valid signatures to be rejected.

Reported by DualTachyon on github.
This commit is contained in:
Manuel Pégourié-Gonnard 2013-10-29 10:45:28 +01:00 committed by Paul Bakker
parent 60b1d10131
commit 178d9bac3c
2 changed files with 10 additions and 2 deletions
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1
ChangeLog
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@ -16,6 +16,7 @@ Bugfix
* Misc fixes and additions to dependency checks * Misc fixes and additions to dependency checks
* Const correctness * Const correctness
* cert_write with selfsign should use issuer_name as subject_name * cert_write with selfsign should use issuer_name as subject_name
* Fix ECDSA corner case: missing reduction mod N (found by DualTachyon)
= PolarSSL 1.3.1 released on 2013-10-15 = PolarSSL 1.3.1 released on 2013-10-15
Features Features

11
library/ecdsa.c
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@ -68,12 +68,13 @@ int ecdsa_sign( ecp_group *grp, mpi *r, mpi *s,
{ {
/* /*
* Steps 1-3: generate a suitable ephemeral keypair * Steps 1-3: generate a suitable ephemeral keypair
* and set r = xR mod n
*/ */
key_tries = 0; key_tries = 0;
do do
{ {
MPI_CHK( ecp_gen_keypair( grp, &k, &R, f_rng, p_rng ) ); MPI_CHK( ecp_gen_keypair( grp, &k, &R, f_rng, p_rng ) );
MPI_CHK( mpi_copy( r, &R.X ) ); MPI_CHK( mpi_mod_mpi( r, &R.X, &grp->N ) );
if( key_tries++ > 10 ) if( key_tries++ > 10 )
{ {
@ -176,7 +177,13 @@ int ecdsa_verify( ecp_group *grp,
} }
/* /*
* Step 6: check that xR == r * Step 6: convert xR to an integer (no-op)
* Step 7: reduce xR mod n (gives v)
*/
MPI_CHK( mpi_mod_mpi( &R.X, &R.X, &grp->N ) );
/*
* Step 8: check if v (that is, R.X) is equal to r
*/ */
if( mpi_cmp_mpi( &R.X, r ) != 0 ) if( mpi_cmp_mpi( &R.X, r ) != 0 )
{ {