Add ECDSA signature primitive.

include/polarssl/ecdsa.h

@@ -52,6 +52,24 @@ int ecdsa_sign( const ecp_group *grp, mpi *r, mpi *s,
                 const mpi *d, const unsigned char *buf, size_t blen,
                 int (*f_rng)(void *, unsigned char *, size_t), void *p_rng );
 
+/**
+ * \brief           Verify ECDSA signature of a previously hashed message
+ *
+ * \param grp       ECP group
+ * \param buf       Message hash
+ * \param blen      Length of buf
+ * \param Q         Public key to use for verification
+ * \param r         First integer of the signature
+ * \param s         Second integer of the signature
+ *
+ * \return          0 if successful,
+ *                  POLARSSL_ERR_ECP_BAD_INPUT_DATA if signature is invalid
+ *                  or a POLARSSL_ERR_ECP_XXX or POLARSSL_MPI_XXX error code
+ */
+int ecdsa_verify( const ecp_group *grp,
+                  const unsigned char *buf, size_t blen,
+                  const ecp_point *Q, const mpi *r, const mpi *s);
+
 /**
  * \brief          Checkup routine
  *

library/ecdsa.c

@@ -35,6 +35,17 @@
 
 #include "polarssl/ecdsa.h"
 
+/*
+ * Derive a suitable integer for group grp from a buffer of length len
+ * SEC1 4.1.3 step 5 aka SEC1 4.1.4 step 3
+ */
+static int derive_mpi( const ecp_group *grp, mpi *x,
+                       const unsigned char *buf, size_t blen )
+{
+    size_t n_size = (grp->nbits + 7) / 8;
+    return( mpi_read_binary( x, buf, blen > n_size ? n_size : blen ) );
+}
+
 /*
  * Compute ECDSA signature of a hashed message (SEC1 4.1.3)
  * Obviously, compared to SEC1 4.1.3, we skip step 4 (hash message)
@@ -44,7 +55,6 @@ int ecdsa_sign( const ecp_group *grp, mpi *r, mpi *s,
                 int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
 {
     int ret, key_tries, sign_tries;
-    size_t n_size;
     ecp_point R;
     mpi k, e;
 
@@ -72,8 +82,7 @@ int ecdsa_sign( const ecp_group *grp, mpi *r, mpi *s,
         /*
          * Step 5: derive MPI from hashed message
          */
-        n_size = (grp->nbits + 7) / 8;
+        MPI_CHK( derive_mpi( grp, &e, buf, blen ) );
-        MPI_CHK( mpi_read_binary( &e, buf, blen > n_size ? n_size : blen ) );
 
         /*
          * Step 6: compute s = (e + r * d) / k mod n
@@ -97,6 +106,75 @@ cleanup:
     return( ret );
 }
 
+/*
+ * Verify ECDSA signature of hashed message (SEC1 4.1.4)
+ * Obviously, compared to SEC1 4.1.3, we skip step 2 (hash message)
+ */
+int ecdsa_verify( const ecp_group *grp,
+                  const unsigned char *buf, size_t blen,
+                  const ecp_point *Q, const mpi *r, const mpi *s)
+{
+    int ret;
+    mpi e, s_inv, u1, u2;
+    ecp_point R, P;
+
+    ecp_point_init( &R ); ecp_point_init( &P );
+    mpi_init( &e ); mpi_init( &s_inv ); mpi_init( &u1 ); mpi_init( &u2 );
+
+    /*
+     * Step 1: make sure r and s are in range 1..n-1
+     */
+    if( mpi_cmp_int( r, 1 ) < 0 || mpi_cmp_mpi( r, &grp->N ) >= 0 ||
+        mpi_cmp_int( s, 1 ) < 0 || mpi_cmp_mpi( s, &grp->N ) >= 0 )
+    {
+        return( POLARSSL_ERR_ECP_BAD_INPUT_DATA );
+    }
+
+    /*
+     * Additional precaution: make sure Q is valid
+     */
+    MPI_CHK( ecp_check_pubkey( grp, Q ) );
+
+    /*
+     * Step 3: derive MPI from hashed message
+     */
+    MPI_CHK( derive_mpi( grp, &e, buf, blen ) );
+
+    /*
+     * Step 4: u1 = e / s mod n, u2 = r / s mod n
+     */
+    MPI_CHK( mpi_inv_mod( &s_inv, s, &grp->N ) );
+
+    MPI_CHK( mpi_mul_mpi( &u1, &e, &s_inv ) );
+    MPI_CHK( mpi_mod_mpi( &u1, &u1, &grp->N ) );
+
+    MPI_CHK( mpi_mul_mpi( &u2, r, &s_inv ) );
+    MPI_CHK( mpi_mod_mpi( &u2, &u2, &grp->N ) );
+
+    /*
+     * Step 5: R = u1 G + u2 Q
+     */
+    MPI_CHK( ecp_mul( grp, &R, &u1, &grp->G ) );
+    MPI_CHK( ecp_mul( grp, &P, &u2, Q ) );
+    MPI_CHK( ecp_add( grp, &R, &R, &P ) );
+
+    if( ecp_is_zero( &R ) )
+        return( POLARSSL_ERR_ECP_BAD_INPUT_DATA );
+
+    /*
+     * Step 6: check that xR == r
+     */
+    if( mpi_cmp_mpi( &R.X, r ) != 0 )
+        return( POLARSSL_ERR_ECP_BAD_INPUT_DATA );
+
+cleanup:
+    ecp_point_free( &R ); ecp_point_free( &P );
+    mpi_free( &e ); mpi_free( &s_inv ); mpi_free( &u1 ); mpi_free( &u2 );
+
+    return( ret );
+}
+
+
 #if defined(POLARSSL_SELF_TEST)
 
 /*