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SHA-1 deprecation: allow it in key exchange

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By default, keep allowing SHA-1 in key exchange signatures. Disabling
it causes compatibility issues, especially with clients that use
TLS1.2 but don't send the signature_algorithms extension.

SHA-1 is forbidden in certificates by default, since it's vulnerable
to offline collision-based attacks.
This commit is contained in:
Gilles Peskine 2017-05-12 13:16:40 +02:00 committed by Manuel Pégourié-Gonnard
parent 682df09159
commit 5d2511c4d4
7 changed files with 32 additions and 17 deletions
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9
ChangeLog
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@ -4,12 +4,9 @@ mbed TLS 2.x.x branch released xxxx-xx-xx
Security
* SHA-1 deprecation: remove it from the default allowed hash
algorithms for certificate verification and TLS 1.2 handshake
signatures. It can be turned back on at compile time with
MBEDTLS_TLS_DEFAULT_ALLOW_SHA1 or explicitly with ssl_conf functions.
* Removed RIPEMD-160 from the default hash algorithms for
certificate verification.
* Removed SHA-1 and RIPEMD-160 from the default hash algorithms for
certificate verification. SHA-1 can be turned back on with a compile-time
option if needed.
Bugfix
* Remove invalid use of size zero arrays in ECJPAKE test suite.

23
include/mbedtls/config.h
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@ -2638,13 +2638,24 @@
//#define MBEDTLS_X509_MAX_FILE_PATH_LEN 512 /**< Maximum length of a path/filename string in bytes including the null terminator character ('\0'). */
/**
* Allow SHA-1 in the default TLS configuration for certificate signing and
* TLS 1.2 handshake signature. Without this build-time option, SHA-1
* support must be activated explicitly through mbedtls_ssl_conf_cert_profile
* and mbedtls_ssl_conf_sig_hashes. The use of SHA-1 in TLS <= 1.1 and in
* HMAC-SHA-1 for XXX_SHA ciphersuites is always allowed by default.
* Allow SHA-1 in the default TLS configuration for certificate signing.
* Without this build-time option, SHA-1 support must be activated explicitly
* through mbedtls_ssl_conf_cert_profile. Turning on this option is not
* recommended because of it is possible to generte SHA-1 collisions, however
* this may be safe for legacy infrastructure where additional controls apply.
*/
// #define MBEDTLS_TLS_DEFAULT_ALLOW_SHA1
// #define MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_CERTIFICATES
/**
* Allow SHA-1 in the default TLS configuration for TLS 1.2 handshake
* signature and ciphersuite selection. Without this build-time option, SHA-1
* support must be activated explicitly through mbedtls_ssl_conf_sig_hashes.
* The use of SHA-1 in TLS <= 1.1 and in HMAC-SHA-1 is always allowed by
* default. At the time of writing, there is no practical attack on the use
* of SHA-1 in handshake signatures, hence this option is turned on by default
* for compatibility with existing peers.
*/
#define MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_KEY_EXCHANGE
/* \} name SECTION: Customisation configuration options */

2
include/mbedtls/x509_crt.h
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@ -651,7 +651,7 @@ int mbedtls_x509write_crt_pem( mbedtls_x509write_cert *ctx, unsigned char *buf,
}
#endif
#ifndef MBEDTLS_TLS_DEFAULT_ALLOW_SHA1
#ifndef MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_CERTIFICATES
/* The test infrastructure requires a positive define */
#define MBEDTLS_X509__DEFAULT_FORBID_SHA1
#endif

2
library/ssl_tls.c
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@ -7162,7 +7162,7 @@ static int ssl_preset_default_hashes[] = {
MBEDTLS_MD_SHA256,
MBEDTLS_MD_SHA224,
#endif
#if defined(MBEDTLS_SHA1_C) && defined(MBEDTLS_TLS_DEFAULT_ALLOW_SHA1)
#if defined(MBEDTLS_SHA1_C) && defined(MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_KEY_EXCHANGE)
MBEDTLS_MD_SHA1,
#endif
MBEDTLS_MD_NONE

2
library/x509_crt.c
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@ -85,7 +85,7 @@ static void mbedtls_zeroize( void *v, size_t n ) {
*/
const mbedtls_x509_crt_profile mbedtls_x509_crt_profile_default =
{
#if defined(MBEDTLS_TLS_DEFAULT_ALLOW_SHA1)
#if defined(MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_CERTIFICATES)
/* Allow SHA-1 (weak, but still safe in controlled environments) */
MBEDTLS_X509_ID_FLAG( MBEDTLS_MD_SHA1 ) |
#endif

9
tests/ssl-opt.sh
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@ -2908,12 +2908,19 @@ run_test "Per-version suites: TLS 1.2" \
# Test for ClientHello without extensions
requires_gnutls
run_test "ClientHello without extensions" \
run_test "ClientHello without extensions, SHA-1 allowed" \
"$P_SRV debug_level=3" \
"$G_CLI --priority=NORMAL:%NO_EXTENSIONS:%DISABLE_SAFE_RENEGOTIATION" \
0 \
-s "dumping 'client hello extensions' (0 bytes)"
requires_gnutls
run_test "ClientHello without extensions, SHA-1 forbidden in certificates on server" \
"$P_SRV debug_level=3 key_file=data_files/server2.key crt_file=data_files/server2.crt allow_sha1=0" \
"$G_CLI --priority=NORMAL:%NO_EXTENSIONS:%DISABLE_SAFE_RENEGOTIATION" \
0 \
-s "dumping 'client hello extensions' (0 bytes)"
# Tests for mbedtls_ssl_get_bytes_avail()
run_test "mbedtls_ssl_get_bytes_avail: no extra data" \

2
tests/suites/test_suite_x509parse.data
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@ -432,7 +432,7 @@ depends_on:MBEDTLS_SHA1_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_SHA1_C:MBEDTLS_RSA_C:MBEDT
x509_verify:"data_files/cert_sha1.crt":"data_files/test-ca.crt":"data_files/crl.pem":"NULL":0:0:"compat":"NULL"
X509 Certificate verification #14 (Valid Cert SHA1 Digest allowed in compile-time default profile)
depends_on:MBEDTLS_SHA1_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_SHA1_C:MBEDTLS_RSA_C:MBEDTLS_PKCS1_V15:MBEDTLS_TLS_DEFAULT_ALLOW_SHA1
depends_on:MBEDTLS_SHA1_C:MBEDTLS_PEM_PARSE_C:MBEDTLS_SHA1_C:MBEDTLS_RSA_C:MBEDTLS_PKCS1_V15:MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_CERTIFICATES
x509_verify:"data_files/cert_sha1.crt":"data_files/test-ca.crt":"data_files/crl.pem":"NULL":0:0:"default":"NULL"
X509 Certificate verification #14 (Valid Cert SHA1 Digest forbidden in default profile)