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commit 3462280f2e23e16adf3ed5176e0f2413d8861320
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Author: NIIBE Yutaka <gniibe@fsij.org>
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Date: Fri May 21 11:15:07 2021 +0900
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cipher: Fix ElGamal encryption for other implementations.
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* cipher/elgamal.c (gen_k): Remove support of smaller K.
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(do_encrypt): Never use smaller K.
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(sign): Folllow the change of gen_k.
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--
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Cherry-pick master commit of:
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632d80ef30e13de6926d503aa697f92b5dbfbc5e
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This change basically reverts encryption changes in two commits:
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74386120dad6b3da62db37f7044267c8ef34689b
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78531373a342aeb847950f404343a05e36022065
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Use of smaller K for ephemeral key in ElGamal encryption is only good,
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when we can guarantee that recipient's key is generated by our
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implementation (or compatible).
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For detail, please see:
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Luca De Feo, Bertram Poettering, Alessandro Sorniotti,
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"On the (in)security of ElGamal in OpenPGP";
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in the proceedings of CCS'2021.
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CVE-id: CVE-2021-33560
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GnuPG-bug-id: 5328
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Suggested-by: Luca De Feo, Bertram Poettering, Alessandro Sorniotti
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Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>
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diff --git a/cipher/elgamal.c b/cipher/elgamal.c
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index 9835122f..eead4502 100644
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--- a/cipher/elgamal.c
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+++ b/cipher/elgamal.c
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@@ -66,7 +66,7 @@ static const char *elg_names[] =
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static int test_keys (ELG_secret_key *sk, unsigned int nbits, int nodie);
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-static gcry_mpi_t gen_k (gcry_mpi_t p, int small_k);
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+static gcry_mpi_t gen_k (gcry_mpi_t p);
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static gcry_err_code_t generate (ELG_secret_key *sk, unsigned nbits,
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gcry_mpi_t **factors);
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static int check_secret_key (ELG_secret_key *sk);
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@@ -189,11 +189,10 @@ test_keys ( ELG_secret_key *sk, unsigned int nbits, int nodie )
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/****************
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* Generate a random secret exponent k from prime p, so that k is
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- * relatively prime to p-1. With SMALL_K set, k will be selected for
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- * better encryption performance - this must never be used signing!
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+ * relatively prime to p-1.
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*/
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static gcry_mpi_t
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-gen_k( gcry_mpi_t p, int small_k )
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+gen_k( gcry_mpi_t p )
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{
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gcry_mpi_t k = mpi_alloc_secure( 0 );
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gcry_mpi_t temp = mpi_alloc( mpi_get_nlimbs(p) );
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@@ -202,18 +201,7 @@ gen_k( gcry_mpi_t p, int small_k )
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unsigned int nbits, nbytes;
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char *rndbuf = NULL;
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- if (small_k)
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- {
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- /* Using a k much lesser than p is sufficient for encryption and
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- * it greatly improves the encryption performance. We use
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- * Wiener's table and add a large safety margin. */
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- nbits = wiener_map( orig_nbits ) * 3 / 2;
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- if( nbits >= orig_nbits )
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- BUG();
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- }
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- else
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- nbits = orig_nbits;
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-
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+ nbits = orig_nbits;
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nbytes = (nbits+7)/8;
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if( DBG_CIPHER )
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@@ -492,7 +480,7 @@ do_encrypt(gcry_mpi_t a, gcry_mpi_t b, gcry_mpi_t input, ELG_public_key *pkey )
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* error code.
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*/
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- k = gen_k( pkey->p, 1 );
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+ k = gen_k( pkey->p );
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mpi_powm (a, pkey->g, k, pkey->p);
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/* b = (y^k * input) mod p
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@@ -608,7 +596,7 @@ sign(gcry_mpi_t a, gcry_mpi_t b, gcry_mpi_t input, ELG_secret_key *skey )
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*
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*/
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mpi_sub_ui(p_1, p_1, 1);
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- k = gen_k( skey->p, 0 /* no small K ! */ );
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+ k = gen_k( skey->p );
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mpi_powm( a, skey->g, k, skey->p );
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mpi_mul(t, skey->x, a );
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mpi_subm(t, input, t, p_1 );
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