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home:gerritbeine:forwards
compat-openssl096g
openssl-0.9.6g.CBC-timing-attack.dif
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File openssl-0.9.6g.CBC-timing-attack.dif of Package compat-openssl096g
--- ../openssl-0.9.6h/CHANGES Thu Dec 5 22:40:48 2002 +++ ./CHANGES Tue Feb 19 00:00:00 2003 @@ -1,3 +1,19 @@ + Change from security patch [19 Feb 2003] + + (Please consider installing OpenSSL 0.9.7a or OpenSSL 0.9.6i. + These versions already include this change; do not try to apply + the patch to them!) + + *) In ssl3_get_record (ssl/s3_pkt.c), minimize information leaked + via timing by performing a MAC computation even if incorrrect + block cipher padding has been found. This is a countermeasure + against active attacks where the attacker has to distinguish + between bad padding and a MAC verification error. (CAN-2003-0078) + + [Bodo Moeller; problem pointed out by Brice Canvel (EPFL), + Alain Hiltgen (UBS), Serge Vaudenay (EPFL), and + Martin Vuagnoux (EPFL, Ilion)] + OpenSSL CHANGES _______________ --- ../openssl-0.9.6h/ssl/s3_pkt.c Mon May 6 12:42:56 2002 +++ ./ssl/s3_pkt.c Wed Feb 18 00:00:00 2003 @@ -238,6 +238,8 @@ unsigned int mac_size; int clear=0; size_t extra; + int decryption_failed_or_bad_record_mac = 0; + unsigned char *mac = NULL; rr= &(s->s3->rrec); sess=s->session; @@ -353,8 +355,11 @@ /* SSLerr() and ssl3_send_alert() have been called */ goto err; - /* otherwise enc_err == -1 */ - goto decryption_failed_or_bad_record_mac; + /* Otherwise enc_err == -1, which indicates bad padding + * (rec->length has not been changed in this case). + * To minimize information leaked via timing, we will perform + * the MAC computation anyway. */ + decryption_failed_or_bad_record_mac = 1; } #ifdef TLS_DEBUG @@ -380,28 +385,46 @@ SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_PRE_MAC_LENGTH_TOO_LONG); goto f_err; #else - goto decryption_failed_or_bad_record_mac; + decryption_failed_or_bad_record_mac = 1; #endif } /* check the MAC for rr->input (it's in mac_size bytes at the tail) */ - if (rr->length < mac_size) + if (rr->length >= mac_size) { + rr->length -= mac_size; + mac = &rr->data[rr->length]; + } + else + { + /* record (minus padding) is too short to contain a MAC */ #if 0 /* OK only for stream ciphers */ al=SSL_AD_DECODE_ERROR; SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_LENGTH_TOO_SHORT); goto f_err; #else - goto decryption_failed_or_bad_record_mac; + decryption_failed_or_bad_record_mac = 1; + rr->length = 0; #endif } - rr->length-=mac_size; i=s->method->ssl3_enc->mac(s,md,0); - if (memcmp(md,&(rr->data[rr->length]),mac_size) != 0) + if (mac == NULL || memcmp(md, mac, mac_size) != 0) { - goto decryption_failed_or_bad_record_mac; + decryption_failed_or_bad_record_mac = 1; } } + if (decryption_failed_or_bad_record_mac) + { + /* A separate 'decryption_failed' alert was introduced with TLS 1.0, + * SSL 3.0 only has 'bad_record_mac'. But unless a decryption + * failure is directly visible from the ciphertext anyway, + * we should not reveal which kind of error occured -- this + * might become visible to an attacker (e.g. via a logfile) */ + al=SSL_AD_BAD_RECORD_MAC; + SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_DECRYPTION_FAILED_OR_BAD_RECORD_MAC); + goto f_err; + } + /* r->length is now just compressed */ if (s->expand != NULL) { @@ -443,19 +466,12 @@ return(1); -decryption_failed_or_bad_record_mac: - /* Separate 'decryption_failed' alert was introduced with TLS 1.0, - * SSL 3.0 only has 'bad_record_mac'. But unless a decryption - * failure is directly visible from the ciphertext anyway, - * we should not reveal which kind of error occured -- this - * might become visible to an attacker (e.g. via logfile) */ - al=SSL_AD_BAD_RECORD_MAC; - SSLerr(SSL_F_SSL3_GET_RECORD,SSL_R_DECRYPTION_FAILED_OR_BAD_RECORD_MAC); f_err: ssl3_send_alert(s,SSL3_AL_FATAL,al); err: return(ret); } +const char *CAN_2003_0078_patch_ID="CAN-2003-0078 patch 2003-02-19"; static int do_uncompress(SSL *ssl) {
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