Support messages that exceed the stack size

Previous code allocated an array on the stack of mlen bytes, but
it should be possible to also sign heap-space messages. By relying
on the fact that sm and m fit the message + signature, we move
the message so that 4*n bytes of prefix can be added.

hash.c

@@ -1,12 +1,13 @@
+#include <stdint.h>
+#include <string.h>
+#include <openssl/sha.h>
+
 #include "hash_address.h"
 #include "xmss_commons.h"
 #include "params.h"
 #include "hash.h"
 #include "fips202.h"
 
-#include <stdint.h>
-#include <openssl/sha.h>
-
 void addr_to_bytes(unsigned char *bytes, const uint32_t addr[8])
 {
     int i;
@@ -20,22 +21,13 @@ static int core_hash(const xmss_params *params,
                      const unsigned char *key, unsigned int keylen,
                      const unsigned char *in, unsigned long long inlen, int n)
 {
-    unsigned long long i = 0;
     unsigned char buf[inlen + n + keylen];
 
     /* We arrange the input into the hash function to be of the form:
-     *  toByte(X, 32) || KEY || M
+     *  toByte(X, 32) || KEY || M */
-     */
-
     ull_to_bytes(buf, n, type);
-
+    memcpy(buf + n, key, keylen);
-    for (i=0; i < keylen; i++) {
+    memcpy(buf + keylen + n, in, inlen);
-        buf[i+n] = key[i];
-    }
-
-    for (i=0; i < inlen; i++) {
-        buf[keylen + n + i] = in[i];
-    }
 
     if (n == 32 && params->func == XMSS_SHA2) {
         SHA256(buf, inlen + keylen + n, out);
@@ -70,6 +62,44 @@ int h_msg(const xmss_params *params,
     return core_hash(params, out, 2, key, keylen, in, inlen, params->n);
 }
 
+/*
+ * Computes the message hash using R, the public root, the index of the leaf
+ * node, and the message. Notably, it requires m_with_prefix to have 4*n bytes
+ * of space before the message, to use for the prefix. This is necessary to
+ * prevent having to move the message around (and thus allocate memory for it).
+ */
+int hash_message(const xmss_params *params, unsigned char *out,
+                 const unsigned char *R, const unsigned char *root,
+                 unsigned long long idx,
+                 unsigned char *m_with_prefix, unsigned long long mlen)
+{
+    /* We're creating a hash using input of the form:
+       toByte(X, 32) || R || root || index || M */
+    ull_to_bytes(m_with_prefix, params->n, 2);
+    memcpy(m_with_prefix + params->n, R, params->n);
+    memcpy(m_with_prefix + 2 * params->n, root, params->n);
+    ull_to_bytes(m_with_prefix + 3 * params->n, params->n, idx);
+
+    /* Since the message can be bigger than the stack, this cannot use the
+     * core_hash function. */
+    if (params->n == 32 && params->func == XMSS_SHA2) {
+        SHA256(m_with_prefix, mlen + 4*params->n, out);
+    }
+    else if (params->n == 32 && params->func == XMSS_SHAKE) {
+        shake128(out, 32, m_with_prefix, mlen + 4*params->n);
+    }
+    else if (params->n == 64 && params->func == XMSS_SHA2) {
+        SHA512(m_with_prefix, mlen + 4*params->n, out);
+    }
+    else if (params->n == 64 && params->func == XMSS_SHAKE) {
+        shake256(out, 64, m_with_prefix, mlen + 4*params->n);
+    }
+    else {
+        return 1;
+    }
+    return 0;
+}
+
 /**
  * We assume the left half is in in[0]...in[n-1]
  */

hash.h

@@ -23,4 +23,9 @@ int hash_f(const xmss_params *params,
            unsigned char *out, const unsigned char *in,
            const unsigned char *pub_seed, uint32_t addr[8]);
 
+int hash_message(const xmss_params *params, unsigned char *out,
+                 const unsigned char *R, const unsigned char *root,
+                 unsigned long long idx,
+                 unsigned char *m_with_prefix, unsigned long long mlen);
+
 #endif

test/xmss.c

@@ -1,9 +1,11 @@
 #include <stdio.h>
 #include <stdint.h>
 #include <string.h>
+#include <stdlib.h>
 
 #include "../xmss.h"
 #include "../params.h"
+#include "../randombytes.h"
 
 #define MLEN 32
 #define SIGNATURES 16
@@ -33,12 +35,14 @@ int main()
 
     unsigned char pk[XMSS_OID_LEN + params.pk_bytes];
     unsigned char sk[XMSS_OID_LEN + params.sk_bytes];
-    unsigned char m[MLEN];
+    unsigned char *m = malloc(MLEN);
-    unsigned char sm[params.sig_bytes + MLEN];
+    unsigned char *sm = malloc(params.sig_bytes + MLEN);
-    unsigned char mout[params.sig_bytes + MLEN];
+    unsigned char *mout = malloc(params.sig_bytes + MLEN);
     unsigned long long smlen;
     unsigned long long mlen;
 
+    randombytes(m, MLEN);
+
     XMSS_KEYPAIR(pk, sk, oid);
 
     printf("Testing %d %s signatures.. \n", SIGNATURES, XMSS_VARIANT);
@@ -106,5 +110,9 @@ int main()
         }
     }
 
+    free(m);
+    free(sm);
+    free(mout);
+
     return 0;
 }

ui/open.c

@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <stdlib.h>
 
 #include "../params.h"
 #include "../xmss.h"
@@ -49,8 +50,8 @@ int main(int argc, char **argv) {
     XMSS_PARSE_OID(&params, oid);
 
     unsigned char pk[XMSS_OID_LEN + params.pk_bytes];
-    unsigned char sm[smlen];
+    unsigned char *sm = malloc(smlen);
-    unsigned char m[smlen];
+    unsigned char *m = malloc(smlen);
     unsigned long long mlen;
 
     fseek(keypair_file, 0, SEEK_SET);
@@ -70,5 +71,8 @@ int main(int argc, char **argv) {
     fclose(keypair_file);
     fclose(sm_file);
 
+    free(m);
+    free(sm);
+
     return ret;
 }

ui/sign.c

@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <stdlib.h>
 
 #include "../params.h"
 #include "../xmss.h"
@@ -56,8 +57,8 @@ int main(int argc, char **argv) {
     XMSS_PARSE_OID(&params, oid_sk);
 
     unsigned char sk[XMSS_OID_LEN + params.sk_bytes];
-    unsigned char m[mlen];
+    unsigned char *m = malloc(mlen);
-    unsigned char sm[params.sig_bytes + mlen];
+    unsigned char *sm = malloc(params.sig_bytes + mlen);
     unsigned long long smlen;
 
     /* fseek back to start of sk. */
@@ -74,4 +75,9 @@ int main(int argc, char **argv) {
 
     fclose(keypair_file);
     fclose(m_file);
+
+    free(m);
+    free(sm);
+
+    return 0;
 }

xmss_commons.c

@@ -115,25 +115,6 @@ void l_tree(const xmss_params *params,
     memcpy(leaf, wots_pk, params->n);
 }
 
-/**
- * Computes the randomized message hash.
- */
-void hash_message(const xmss_params *params, unsigned char *mhash,
-                  const unsigned char *R, const unsigned char *root,
-                  unsigned long long idx,
-                  const unsigned char *m, unsigned long long mlen)
-{
-    unsigned char hash_key[3*params->n];
-
-    /* Compute hash key. */
-    memcpy(hash_key, R, params->n);
-    memcpy(hash_key + params->n, root, params->n);
-    ull_to_bytes(hash_key + 2*params->n, params->n, idx);
-
-    /* Hash the message using the randomized hash key. */
-    h_msg(params, mhash, m, mlen, hash_key, 3*params->n);
-}
-
 /**
  * Computes a root node given a leaf and an auth path
  */
@@ -228,9 +209,13 @@ int xmssmt_core_sign_open(const xmss_params *params,
     /* Convert the index bytes from the signature to an integer. */
     idx = bytes_to_ull(sm, params->index_bytes);
 
+    /* Put the message all the way at the end of the m buffer, so that we can
+     * prepend the required other inputs for the hash function. */
+    memcpy(m + params->sig_bytes, sm + params->sig_bytes, *mlen);
+
     /* Compute the message hash. */
     hash_message(params, mhash, sm + params->index_bytes, pk, idx,
-                 sm + params->sig_bytes, *mlen);
+                 m + params->sig_bytes - 4*params->n, *mlen);
     sm += params->index_bytes + params->n;
 
     /* For each subtree.. */

xmss_commons.h

@@ -41,14 +41,6 @@ void l_tree(const xmss_params *params,
             unsigned char *leaf, unsigned char *wots_pk,
             const unsigned char *pub_seed, uint32_t addr[8]);
 
-/**
- * Computes the randomized message hash.
- */
-void hash_message(const xmss_params *params, unsigned char *mhash,
-                  const unsigned char *R, const unsigned char *root,
-                  unsigned long long idx,
-                  const unsigned char *m, unsigned long long mlen);
-
 /**
  * Verifies a given message signature pair under a given public key.
  * Note that this assumes a pk without an OID, i.e. [root || PUB_SEED]

xmss_core.c

@@ -189,10 +189,14 @@ int xmssmt_core_sign(const xmss_params *params,
     uint32_t ots_addr[8] = {0};
     set_type(ots_addr, XMSS_ADDR_TYPE_OTS);
 
+    /* Already put the message in the right place, to make it easier to prepend
+     * things when computing the hash over the message. */
+    memcpy(sm + params->sig_bytes, m, mlen);
+    *smlen = params->sig_bytes + mlen;
+
     /* Read and use the current index from the secret key. */
     idx = (unsigned long)bytes_to_ull(sk, params->index_bytes);
     memcpy(sm, sk, params->index_bytes);
-    sm += params->index_bytes;
 
     /*************************************************************************
      * THIS IS WHERE PRODUCTION IMPLEMENTATIONS WOULD UPDATE THE SECRET KEY. *
@@ -202,11 +206,12 @@ int xmssmt_core_sign(const xmss_params *params,
 
     /* Compute the digest randomization value. */
     ull_to_bytes(idx_bytes_32, 32, idx);
-    prf(params, sm, idx_bytes_32, sk_prf, params->n);
+    prf(params, sm + params->index_bytes, idx_bytes_32, sk_prf, params->n);
 
     /* Compute the message hash. */
-    hash_message(params, mhash, sm, pub_root, idx, m, mlen);
+    hash_message(params, mhash, sm + params->index_bytes, pub_root, idx,
-    sm += params->n;
+                 sm + params->sig_bytes - 4*params->n, mlen);
+    sm += params->index_bytes + params->n;
 
     set_type(ots_addr, XMSS_ADDR_TYPE_OTS);
 
@@ -232,8 +237,5 @@ int xmssmt_core_sign(const xmss_params *params,
         sm += params->tree_height*params->n;
     }
 
-    memcpy(sm, m, mlen);
-    *smlen = params->sig_bytes + mlen;
-
     return 0;
 }

xmss_core_fast.c

@@ -393,6 +393,8 @@ int xmss_core_sign(const xmss_params *params,
                    unsigned char *sm, unsigned long long *smlen,
                    const unsigned char *m, unsigned long long mlen)
 {
+    const unsigned char *pub_root = sk + params->index_bytes + 3*params->n;
+
     uint16_t i = 0;
 
     // Extract SK
@@ -408,8 +410,6 @@ int xmss_core_sign(const xmss_params *params,
     unsigned char idx_bytes_32[32];
     ull_to_bytes(idx_bytes_32, 32, idx);
 
-    unsigned char hash_key[3*params->n];
-
     // Update SK
     sk[0] = ((idx + 1) >> 24) & 255;
     sk[1] = ((idx + 1) >> 16) & 255;
@@ -432,12 +432,14 @@ int xmss_core_sign(const xmss_params *params,
     // Message Hash:
     // First compute pseudorandom value
     prf(params, R, idx_bytes_32, sk_prf, params->n);
-    // Generate hash key (R || root || idx)
+
-    memcpy(hash_key, R, params->n);
+    /* Already put the message in the right place, to make it easier to prepend
-    memcpy(hash_key+params->n, sk+4+3*params->n, params->n);
+     * things when computing the hash over the message. */
-    ull_to_bytes(hash_key+2*params->n, params->n, idx);
+    memcpy(sm + params->sig_bytes, m, mlen);
-    // Then use it for message digest
+
-    h_msg(params, msg_h, m, mlen, hash_key, 3*params->n);
+    /* Compute the message hash. */
+    hash_message(params, msg_h, R, pub_root, idx,
+                 sm + params->sig_bytes - 4*params->n, mlen);
 
     // Start collecting signature
     *smlen = 0;
@@ -554,6 +556,8 @@ int xmssmt_core_sign(const xmss_params *params,
                      unsigned char *sm, unsigned long long *smlen,
                      const unsigned char *m, unsigned long long mlen)
 {
+    const unsigned char *pub_root = sk + params->index_bytes + 3*params->n;
+
     uint64_t idx_tree;
     uint32_t idx_leaf;
     uint64_t i, j;
@@ -566,7 +570,6 @@ int xmssmt_core_sign(const xmss_params *params,
     // Init working params
     unsigned char R[params->n];
     unsigned char msg_h[params->n];
-    unsigned char hash_key[3*params->n];
     unsigned char ots_seed[params->n];
     uint32_t addr[8] = {0, 0, 0, 0, 0, 0, 0, 0};
     uint32_t ots_addr[8] = {0, 0, 0, 0, 0, 0, 0, 0};
@@ -599,13 +602,14 @@ int xmssmt_core_sign(const xmss_params *params,
     // First compute pseudorandom value
     ull_to_bytes(idx_bytes_32, 32, idx);
     prf(params, R, idx_bytes_32, sk_prf, params->n);
-    // Generate hash key (R || root || idx)
-    memcpy(hash_key, R, params->n);
-    memcpy(hash_key+params->n, sk+params->index_bytes+3*params->n, params->n);
-    ull_to_bytes(hash_key+2*params->n, params->n, idx);
 
-    // Then use it for message digest
+    /* Already put the message in the right place, to make it easier to prepend
-    h_msg(params, msg_h, m, mlen, hash_key, 3*params->n);
+     * things when computing the hash over the message. */
+    memcpy(sm + params->sig_bytes, m, mlen);
+
+    /* Compute the message hash. */
+    hash_message(params, msg_h, R, pub_root, idx,
+                 sm + params->sig_bytes - 4*params->n, mlen);
 
     // Start collecting signature
     *smlen = 0;