SHA-1 deprecation: allow it in key exchange

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.

ChangeLog

@@ -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.

include/mbedtls/config.h

@@ -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 */
 

include/mbedtls/x509_crt.h

@@ -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

library/ssl_tls.c

@@ -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

library/x509_crt.c

@@ -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

tests/ssl-opt.sh

@@ -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" \

tests/suites/test_suite_x509parse.data

@@ -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)