From 3462280f2e23e16adf3ed5176e0f2413d8861320 Mon Sep 17 00:00:00 2001 From: NIIBE Yutaka Date: Fri, 21 May 2021 11:15:07 +0900 Subject: [PATCH] cipher: Fix ElGamal encryption for other implementations. * cipher/elgamal.c (gen_k): Remove support of smaller K. (do_encrypt): Never use smaller K. (sign): Folllow the change of gen_k. -- Cherry-pick master commit of: 632d80ef30e13de6926d503aa697f92b5dbfbc5e This change basically reverts encryption changes in two commits: 74386120dad6b3da62db37f7044267c8ef34689b 78531373a342aeb847950f404343a05e36022065 Use of smaller K for ephemeral key in ElGamal encryption is only good, when we can guarantee that recipient's key is generated by our implementation (or compatible). For detail, please see: Luca De Feo, Bertram Poettering, Alessandro Sorniotti, "On the (in)security of ElGamal in OpenPGP"; in the proceedings of CCS'2021. CVE-id: CVE-2021-33560 GnuPG-bug-id: 5328 Suggested-by: Luca De Feo, Bertram Poettering, Alessandro Sorniotti Signed-off-by: NIIBE Yutaka --- cipher/elgamal.c | 24 ++++++------------------ 1 file changed, 6 insertions(+), 18 deletions(-) diff --git a/cipher/elgamal.c b/cipher/elgamal.c index 9835122f..eead4502 100644 --- a/cipher/elgamal.c +++ b/cipher/elgamal.c @@ -66,7 +66,7 @@ static const char *elg_names[] = static int test_keys (ELG_secret_key *sk, unsigned int nbits, int nodie); -static gcry_mpi_t gen_k (gcry_mpi_t p, int small_k); +static gcry_mpi_t gen_k (gcry_mpi_t p); static gcry_err_code_t generate (ELG_secret_key *sk, unsigned nbits, gcry_mpi_t **factors); static int check_secret_key (ELG_secret_key *sk); @@ -189,11 +189,10 @@ test_keys ( ELG_secret_key *sk, unsigned int nbits, int nodie ) /**************** * Generate a random secret exponent k from prime p, so that k is - * relatively prime to p-1. With SMALL_K set, k will be selected for - * better encryption performance - this must never be used signing! + * relatively prime to p-1. */ static gcry_mpi_t -gen_k( gcry_mpi_t p, int small_k ) +gen_k( gcry_mpi_t p ) { gcry_mpi_t k = mpi_alloc_secure( 0 ); gcry_mpi_t temp = mpi_alloc( mpi_get_nlimbs(p) ); @@ -202,18 +201,7 @@ gen_k( gcry_mpi_t p, int small_k ) unsigned int nbits, nbytes; char *rndbuf = NULL; - if (small_k) - { - /* Using a k much lesser than p is sufficient for encryption and - * it greatly improves the encryption performance. We use - * Wiener's table and add a large safety margin. */ - nbits = wiener_map( orig_nbits ) * 3 / 2; - if( nbits >= orig_nbits ) - BUG(); - } - else - nbits = orig_nbits; - + nbits = orig_nbits; nbytes = (nbits+7)/8; if( DBG_CIPHER ) @@ -492,7 +480,7 @@ do_encrypt(gcry_mpi_t a, gcry_mpi_t b, gcry_mpi_t input, ELG_public_key *pkey ) * error code. */ - k = gen_k( pkey->p, 1 ); + k = gen_k( pkey->p ); mpi_powm (a, pkey->g, k, pkey->p); /* b = (y^k * input) mod p @@ -608,7 +596,7 @@ sign(gcry_mpi_t a, gcry_mpi_t b, gcry_mpi_t input, ELG_secret_key *skey ) * */ mpi_sub_ui(p_1, p_1, 1); - k = gen_k( skey->p, 0 /* no small K ! */ ); + k = gen_k( skey->p ); mpi_powm( a, skey->g, k, skey->p ); mpi_mul(t, skey->x, a ); mpi_subm(t, input, t, p_1 ); -- 2.30.2