[Openvpn-devel,v2,for,2.4] Handle PSS padding in cryptoapicert

Message ID 1564346061-5683-1-git-send-email-selva.nair@gmail.com
State New
Headers show
Series
  • [Openvpn-devel,v2,for,2.4] Handle PSS padding in cryptoapicert
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Commit Message

Selva Nair July 28, 2019, 8:34 p.m.
From: Selva Nair <selva.nair@gmail.com>

For PSS padding, CNG requires the digest to be signed
and the digest algorithm in use, which are not accessible
via the rsa_sign and rsa_priv_enc callbacks of OpenSSL.
This patch uses the EVP_KEY interface to hook to
evp_pkey_sign callback if OpenSSL version is > 1.1.0.

Mapping of OpenSSL hash algorithm types to CNG is moved
to a function for code-reuse.

To test, both the server and client should be built with
OpenSSL 1.1.1 and use TLS version >= 1.2

Tested on Windows 7 client against a Linux server.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
---
v2: rebased to release/2.4 after siglen -> *siglen change

Essentially, commits 0cab3475a and ce1c1bee in master
combined and adapted for 2.4.
Required on Windows when built with OpenSSL 1.1.1 and
cryptoapicert is in use against a 1.1.1 peer.

 src/openvpn/cryptoapi.c | 377 ++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 329 insertions(+), 48 deletions(-)

Patch

diff --git a/src/openvpn/cryptoapi.c b/src/openvpn/cryptoapi.c
index 35a9ebc..7f2c3c0 100644
--- a/src/openvpn/cryptoapi.c
+++ b/src/openvpn/cryptoapi.c
@@ -39,6 +39,7 @@ 
 #ifdef ENABLE_CRYPTOAPI
 
 #include <openssl/ssl.h>
+#include <openssl/evp.h>
 #include <openssl/err.h>
 #include <windows.h>
 #include <wincrypt.h>
@@ -101,6 +102,12 @@  static ERR_STRING_DATA CRYPTOAPI_str_functs[] = {
     { 0, NULL }
 };
 
+/* Global EVP_PKEY_METHOD used to override the sign operation */
+static EVP_PKEY_METHOD *pmethod;
+static int (*default_pkey_sign_init) (EVP_PKEY_CTX *ctx);
+static int (*default_pkey_sign) (EVP_PKEY_CTX *ctx, unsigned char *sig,
+                                 size_t *siglen, const unsigned char *tbs, size_t tbslen);
+
 typedef struct _CAPI_DATA {
     const CERT_CONTEXT *cert_context;
     HCRYPTPROV_OR_NCRYPT_KEY_HANDLE crypt_prov;
@@ -108,6 +115,80 @@  typedef struct _CAPI_DATA {
     BOOL free_crypt_prov;
 } CAPI_DATA;
 
+/**
+ * Translate OpenSSL padding type to CNG padding type
+ * Returns 0 for unknown/unsupported padding.
+ */
+static DWORD
+cng_padding_type(int padding)
+{
+    DWORD pad = 0;
+
+    switch (padding)
+    {
+        case RSA_NO_PADDING:
+            pad = BCRYPT_PAD_NONE;
+            break;
+
+        case RSA_PKCS1_PADDING:
+            pad = BCRYPT_PAD_PKCS1;
+            break;
+
+        case RSA_PKCS1_PSS_PADDING:
+            pad = BCRYPT_PAD_PSS;
+            break;
+
+        default:
+            msg(M_WARN|M_INFO, "cryptoapicert: unknown OpenSSL padding type %d.",
+                padding);
+    }
+
+    return pad;
+}
+
+/**
+ * Translate OpenSSL hash OID to CNG algorithm name. Returns
+ * "UNKNOWN" for unsupported algorithms and NULL for MD5+SHA1
+ * mixed hash used in TLS 1.1 and earlier.
+ */
+static const wchar_t *
+cng_hash_algo(int md_type)
+{
+    const wchar_t *alg = L"UNKNOWN";
+    switch (md_type)
+    {
+        case NID_md5:
+            alg = BCRYPT_MD5_ALGORITHM;
+            break;
+
+        case NID_sha1:
+            alg = BCRYPT_SHA1_ALGORITHM;
+            break;
+
+        case NID_sha256:
+            alg = BCRYPT_SHA256_ALGORITHM;
+            break;
+
+        case NID_sha384:
+            alg = BCRYPT_SHA384_ALGORITHM;
+            break;
+
+        case NID_sha512:
+            alg = BCRYPT_SHA512_ALGORITHM;
+            break;
+
+        case NID_md5_sha1:
+        case 0:
+            alg = NULL;
+            break;
+
+        default:
+            msg(M_WARN|M_INFO, "cryptoapicert: Unknown hash type NID=0x%x", md_type);
+            break;
+    }
+    return alg;
+}
+
 static char *
 ms_error_text(DWORD ms_err)
 {
@@ -217,25 +298,44 @@  rsa_pub_dec(int flen, const unsigned char *from, unsigned char *to, RSA *rsa, in
  * Sign the hash in 'from' using NCryptSignHash(). This requires an NCRYPT
  * key handle in cd->crypt_prov. On return the signature is in 'to'. Returns
  * the length of the signature or 0 on error.
+ * Only RSA is supported and padding should be BCRYPT_PAD_PKCS1 or
+ * BCRYPT_PAD_PSS.
  * If the hash_algo is not NULL, PKCS #1 DigestInfo header gets added
- * to 'from', else it is signed as is.
- * For now we support only RSA and the padding is assumed to be PKCS1 v1.5
+ * to |from|, else it is signed as is. Use NULL for MD5 + SHA1 hash used
+ * in TLS 1.1 and earlier.
+ * In case of PSS padding, |saltlen| should specify the size of salt to use.
+ * If |to| is NULL returns the required buffer size.
  */
 static int
 priv_enc_CNG(const CAPI_DATA *cd, const wchar_t *hash_algo, const unsigned char *from,
-             int flen, unsigned char *to, int tlen, int padding)
+             int flen, unsigned char *to, int tlen, DWORD padding, DWORD saltlen)
 {
     NCRYPT_KEY_HANDLE hkey = cd->crypt_prov;
     DWORD len = 0;
     ASSERT(cd->key_spec == CERT_NCRYPT_KEY_SPEC);
 
-    msg(D_LOW, "Signing hash using CNG: data size = %d", flen);
-
-    BCRYPT_PKCS1_PADDING_INFO padinfo = {hash_algo};
     DWORD status;
 
-    status = NCryptSignHash(hkey, padding? &padinfo : NULL, (BYTE*) from, flen,
-                            to, tlen, &len, padding? BCRYPT_PAD_PKCS1 : 0);
+    msg(D_LOW, "Signing hash using CNG: data size = %d padding = %lu", flen, padding);
+
+    if (padding == BCRYPT_PAD_PKCS1)
+    {
+        BCRYPT_PKCS1_PADDING_INFO padinfo = {hash_algo};
+        status = NCryptSignHash(hkey, &padinfo, (BYTE *)from, flen,
+                                to, tlen, &len, padding);
+    }
+    else if (padding == BCRYPT_PAD_PSS)
+    {
+        BCRYPT_PSS_PADDING_INFO padinfo = {hash_algo, saltlen};
+        status = NCryptSignHash(hkey, &padinfo, (BYTE *)from, flen,
+                                to, tlen, &len, padding);
+    }
+    else
+    {
+        RSAerr(RSA_F_RSA_OSSL_PRIVATE_ENCRYPT, RSA_R_UNKNOWN_PADDING_TYPE);
+        return 0;
+    }
+
     if (status != ERROR_SUCCESS)
     {
         SetLastError(status);
@@ -261,16 +361,19 @@  rsa_priv_enc(int flen, const unsigned char *from, unsigned char *to, RSA *rsa, i
         RSAerr(RSA_F_RSA_OSSL_PRIVATE_ENCRYPT, ERR_R_PASSED_NULL_PARAMETER);
         return 0;
     }
+
+    if (cd->key_spec == CERT_NCRYPT_KEY_SPEC)
+    {
+        return priv_enc_CNG(cd, NULL, from, flen, to, RSA_size(rsa),
+                            cng_padding_type(padding), 0);
+    }
+
     if (padding != RSA_PKCS1_PADDING)
     {
         /* AFAICS, CryptSignHash() *always* uses PKCS1 padding. */
         RSAerr(RSA_F_RSA_OSSL_PRIVATE_ENCRYPT, RSA_R_UNKNOWN_PADDING_TYPE);
         return 0;
     }
-    if (cd->key_spec == CERT_NCRYPT_KEY_SPEC)
-    {
-        return priv_enc_CNG(cd, NULL, from, flen, to, RSA_size(rsa), padding);
-    }
 
     /* Unfortunately, there is no "CryptSign()" function in CryptoAPI, that would
      * be way to straightforward for M$, I guess... So we have to do it this
@@ -333,12 +436,13 @@  rsa_priv_enc(int flen, const unsigned char *from, unsigned char *to, RSA *rsa, i
     return len;
 }
 
-/*
+/**
  * Sign the hash in |m| and return the signature in |sig|.
  * Returns 1 on success, 0 on error.
  * NCryptSignHash() is used to sign and it is instructed to add the
  * the PKCS #1 DigestInfo header to |m| unless the hash algorithm is
  * the MD5/SHA1 combination used in TLS 1.1 and earlier versions.
+ * OpenSSL exercises this callback only when padding is PKCS1 v1.5.
  */
 static int
 rsa_sign_CNG(int type, const unsigned char *m, unsigned int m_len,
@@ -355,44 +459,16 @@  rsa_sign_CNG(int type, const unsigned char *m, unsigned int m_len,
         return 0;
     }
 
-    switch (type)
+    alg = cng_hash_algo(type);
+    if (alg && wcscmp(alg, L"UNKNOWN") == 0)
     {
-        case NID_md5:
-            alg = BCRYPT_MD5_ALGORITHM;
-            break;
-
-        case NID_sha1:
-            alg = BCRYPT_SHA1_ALGORITHM;
-            break;
-
-        case NID_sha256:
-            alg = BCRYPT_SHA256_ALGORITHM;
-            break;
-
-        case NID_sha384:
-            alg = BCRYPT_SHA384_ALGORITHM;
-            break;
-
-        case NID_sha512:
-            alg = BCRYPT_SHA512_ALGORITHM;
-            break;
-
-        case NID_md5_sha1:
-            if (m_len != SSL_SIG_LENGTH)
-            {
-                RSAerr(RSA_F_RSA_SIGN, RSA_R_INVALID_MESSAGE_LENGTH);
-                return 0;
-            }
-            /* No DigestInfo header is required -- set alg-name to NULL */
-            alg = NULL;
-            break;
-        default:
-            msg(M_WARN, "cryptoapicert: Unknown hash type NID=0x%x", type);
-            RSAerr(RSA_F_RSA_SIGN, RSA_R_UNKNOWN_ALGORITHM_TYPE);
-            return 0;
+        RSAerr(RSA_F_RSA_SIGN, RSA_R_UNKNOWN_ALGORITHM_TYPE);
+        return 0;
     }
 
-    *siglen = priv_enc_CNG(cd, alg, m, (int)m_len, sig, RSA_size(rsa), padding);
+    *siglen = priv_enc_CNG(cd, alg, m, (int)m_len, sig, RSA_size(rsa),
+                           cng_padding_type(padding), 0);
+
     return (*siglen == 0) ? 0 : 1;
 }
 
@@ -518,6 +594,176 @@  find_certificate_in_store(const char *cert_prop, HCERTSTORE cert_store)
     return rv;
 }
 
+#if (OPENSSL_VERSION_NUMBER >= 0x10100000L)
+
+static const CAPI_DATA *
+retrieve_capi_data(EVP_PKEY *pkey)
+{
+    const CAPI_DATA *cd = NULL;
+
+    if (pkey && EVP_PKEY_id(pkey) == EVP_PKEY_RSA)
+    {
+        RSA *rsa = EVP_PKEY_get0_RSA(pkey);
+        if (rsa)
+        {
+            cd = (CAPI_DATA *)RSA_meth_get0_app_data(RSA_get_method(rsa));
+        }
+    }
+    return cd;
+}
+
+static int
+pkey_rsa_sign_init(EVP_PKEY_CTX *ctx)
+{
+    EVP_PKEY *pkey = EVP_PKEY_CTX_get0_pkey(ctx);
+
+    if (pkey && retrieve_capi_data(pkey))
+    {
+        return 1; /* Return success */
+    }
+    else if (default_pkey_sign_init)  /* Not our key. Call the default method */
+    {
+        return default_pkey_sign_init(ctx);
+    }
+    return 1;
+}
+
+/**
+ * Implementation of EVP_PKEY_sign() using CNG: sign the digest in |tbs|
+ * and save the the signature in |sig| and its size in |*siglen|.
+ * If |sig| is NULL the required buffer size is returned in |*siglen|.
+ * Returns 1 on success, 0 or a negative integer on error.
+ */
+static int
+pkey_rsa_sign(EVP_PKEY_CTX *ctx, unsigned char *sig, size_t *siglen,
+              const unsigned char *tbs, size_t tbslen)
+{
+    EVP_PKEY *pkey = NULL;
+    const CAPI_DATA *cd = NULL;
+    EVP_MD *md = NULL;
+    const wchar_t *alg = NULL;
+
+    int padding;
+    int hashlen;
+    int saltlen;
+
+    pkey = EVP_PKEY_CTX_get0_pkey(ctx);
+    if (pkey)
+    {
+        cd = retrieve_capi_data(pkey);
+    }
+
+    /*
+     * We intercept all sign requests, not just the one's for our key.
+     * Check the key and call the saved OpenSSL method for unknown keys.
+     */
+    if (!pkey || !cd)
+    {
+        if (default_pkey_sign)
+        {
+            return default_pkey_sign(ctx, sig, siglen, tbs, tbslen);
+        }
+        else  /* This should not happen */
+        {
+            msg(M_FATAL, "cryptopaicert: Unknown key and no default sign operation to fallback on");
+            return -1;
+        }
+    }
+
+    if (!EVP_PKEY_CTX_get_rsa_padding(ctx, &padding))
+    {
+        padding = RSA_PKCS1_PADDING; /* Default padding for RSA */
+    }
+
+    if (EVP_PKEY_CTX_get_signature_md(ctx, &md))
+    {
+        hashlen = EVP_MD_size(md);
+        alg = cng_hash_algo(EVP_MD_type(md));
+
+        /*
+         * alg == NULL indicates legacy MD5+SHA1 hash, else alg should be a valid
+         * digest algorithm.
+         */
+        if (alg && wcscmp(alg, L"UNKNOWN") == 0)
+        {
+            RSAerr(RSA_F_PKEY_RSA_SIGN, RSA_R_UNKNOWN_ALGORITHM_TYPE);
+            return -1;
+        }
+    }
+    else
+    {
+        msg(M_NONFATAL, "cryptoapicert: could not determine the signature digest algorithm");
+        RSAerr(RSA_F_PKEY_RSA_SIGN, RSA_R_UNKNOWN_ALGORITHM_TYPE);
+        return -1;
+    }
+
+    if (tbslen != (size_t)hashlen)
+    {
+        RSAerr(RSA_F_PKEY_RSA_SIGN, RSA_R_INVALID_DIGEST_LENGTH);
+        return -1;
+    }
+
+    /* If padding is PSS, determine parameters to pass to CNG */
+    if (padding == RSA_PKCS1_PSS_PADDING)
+    {
+        /*
+         * Ensure the digest type for signature and mask generation match.
+         * In CNG there is no option to specify separate hash functions for
+         * the two, but OpenSSL supports it. However, I have not seen the
+         * two being different in practice. Also the recommended practice is
+         * to use the same for both (rfc 8017 sec 8.1).
+         */
+        EVP_MD *mgf1md;
+        if (!EVP_PKEY_CTX_get_rsa_mgf1_md(ctx, &mgf1md)
+            || EVP_MD_type(mgf1md) != EVP_MD_type(md))
+        {
+            msg(M_NONFATAL, "cryptoapicert: Unknown MGF1 digest type or does"
+                " not match the signature digest type.");
+            RSAerr(RSA_F_PKEY_RSA_SIGN, RSA_R_UNSUPPORTED_MASK_PARAMETER);
+        }
+
+        if (!EVP_PKEY_CTX_get_rsa_pss_saltlen(ctx, &saltlen))
+        {
+            msg(M_WARN|M_INFO, "cryptoapicert: unable to get the salt length from context."
+                " Using the default value.");
+            saltlen = -1;
+        }
+
+        /*
+         * In OpenSSL saltlen = -1 indicates to use the size of the digest as
+         * size of the salt. A value of -2 or -3 indicates maximum salt length
+         * that will fit. See RSA_padding_add_PKCS1_PSS_mgf1() of OpenSSL.
+         */
+        if (saltlen == -1)
+        {
+            saltlen = hashlen;
+        }
+        else if (saltlen < 0)
+        {
+            const RSA *rsa = EVP_PKEY_get0_RSA(pkey);
+            saltlen = RSA_size(rsa) - hashlen - 2; /* max salt length for RSASSA-PSS */
+            if (RSA_bits(rsa) &0x7) /* number of bits in the key not a multiple of 8 */
+            {
+                saltlen--;
+            }
+        }
+
+        if (saltlen < 0)
+        {
+            RSAerr(RSA_F_PKEY_RSA_SIGN, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
+            return -1;
+        }
+        msg(D_LOW, "cryptoapicert: PSS padding using saltlen = %d", saltlen);
+    }
+
+    *siglen = priv_enc_CNG(cd, alg, tbs, (int)tbslen, sig, *siglen,
+                           cng_padding_type(padding), (DWORD)saltlen);
+
+    return (*siglen == 0) ? 0 : 1;
+}
+
+#endif /* OPENSSL_VERSION >= 1.1.0 */
+
 int
 SSL_CTX_use_CryptoAPI_certificate(SSL_CTX *ssl_ctx, const char *cert_prop)
 {
@@ -620,10 +866,45 @@  SSL_CTX_use_CryptoAPI_certificate(SSL_CTX *ssl_ctx, const char *cert_prop)
     /* For CNG, set the RSA_sign method which gets priority over priv_enc().
      * This method is called with the raw hash without the digestinfo
      * header and works better when using NCryptSignHash() with some tokens.
+     * However, if PSS padding is in use, openssl does not call this
+     * function but adds the padding and then calls rsa_priv_enc()
+     * with padding set to NONE which is not supported by CNG.
+     * So, when posisble (OpenSSL 1.1.0 and up), we hook on to the sign
+     * operation in EVP_PKEY_METHOD struct.
      */
     if (cd->key_spec == CERT_NCRYPT_KEY_SPEC)
     {
+#if (OPENSSL_VERSION_NUMBER < 0x10100000L)
         RSA_meth_set_sign(my_rsa_method, rsa_sign_CNG);
+#else
+        /* pmethod is global -- initialize only if NULL */
+        if (!pmethod)
+        {
+            pmethod = EVP_PKEY_meth_new(EVP_PKEY_RSA, 0);
+            if (!pmethod)
+            {
+                SSLerr(SSL_F_SSL_CTX_USE_CERTIFICATE_FILE, ERR_R_MALLOC_FAILURE);
+                goto err;
+            }
+            const EVP_PKEY_METHOD *default_pmethod = EVP_PKEY_meth_find(EVP_PKEY_RSA);
+            EVP_PKEY_meth_copy(pmethod, default_pmethod);
+
+            /* We want to override only sign_init() and sign() */
+            EVP_PKEY_meth_set_sign(pmethod, pkey_rsa_sign_init, pkey_rsa_sign);
+            EVP_PKEY_meth_add0(pmethod);
+
+            /* Keep a copy of the default sign and sign_init methods */
+
+#if (OPENSSL_VERSION_NUMBER < 0x1010009fL)   /* < version 1.1.0i */
+            /* The function signature is not const-correct in these versions */
+            EVP_PKEY_meth_get_sign((EVP_PKEY_METHOD *)default_pmethod, &default_pkey_sign_init,
+                                   &default_pkey_sign);
+#else
+            EVP_PKEY_meth_get_sign(default_pmethod, &default_pkey_sign_init,
+                                   &default_pkey_sign);
+#endif
+        }
+#endif /* (OPENSSL_VERSION_NUMBER < 0x10100000L) */
     }
 
     rsa = RSA_new();