Add a warning about the usage of the montgomery functions (if the inputs

[openssl.git] / doc / crypto / BN_mod_mul_montgomery.pod

diff --git a/doc/crypto/BN_mod_mul_montgomery.pod b/doc/crypto/BN_mod_mul_montgomery.pod
index 7d25dfbac49155f3478536b737be4801c422ccfd..ed4af7a214ed1bd94cc183fe5dc52498d6e8fc9e 100644 (file)
--- a/doc/crypto/BN_mod_mul_montgomery.pod
+++ b/doc/crypto/BN_mod_mul_montgomery.pod
@@ -29,41 +29,42 @@ BN_from_montgomery, BN_to_montgomery - Montgomery multiplication
 =head1 DESCRIPTION
 
 These functions implement Montgomery multiplication. They are used
-automatically when BN_mod_exp(3) is called with suitable input,
-but they may be useful when several operations are to be perfomed
+automatically when L<BN_mod_exp(3)|BN_mod_exp(3)> is called with suitable input,
+but they may be useful when several operations are to be performed
 using the same modulus.
 
 BN_MONT_CTX_new() allocates and initializes a B<BN_MONT_CTX> structure.
 BN_MONT_CTX_init() initializes an existing uninitialized B<BN_MONT_CTX>.
 
-BN_MONT_CTX_set() sets up the B<mont> structure from the modulus B<m>
+BN_MONT_CTX_set() sets up the I<mont> structure from the modulus I<m>
 by precomputing its inverse and a value R.
 
-BN_MONT_CTX_copy() copies the B<N_MONT_CTX> B<from> to B<to>.
+BN_MONT_CTX_copy() copies the B<BN_MONT_CTX> I<from> to I<to>.
 
 BN_MONT_CTX_free() frees the components of the B<BN_MONT_CTX>, and, if
 it was created by BN_MONT_CTX_new(), also the structure itself.
 
-BN_mod_mul_mont() computes Mont(B<a>,B<b>):=B<a>*B<b>*R^-1 and places
-the result in B<r>.
+BN_mod_mul_montgomery() computes Mont(I<a>,I<b>):=I<a>*I<b>*R^-1 and places
+the result in I<r>.
 
-BN_from_montgomery() performs the Montgomery reduction B<r> = B<a>*R^-1.
+BN_from_montgomery() performs the Montgomery reduction I<r> = I<a>*R^-1.
 
-BN_to_montgomery() computes Mont(B<a>,R^2).
+BN_to_montgomery() computes Mont(I<a>,R^2), i.e. I<a>*R.
+Note that I<a> must be non-negative and smaller than the modulus.
 
-For all functions, B<ctx> is a previously allocated B<BN_CTX> used for
+For all functions, I<ctx> is a previously allocated B<BN_CTX> used for
 temporary variables.
 
 The B<BN_MONT_CTX> structure is defined as follows:
 
  typedef struct bn_mont_ctx_st
         {
-        int use_word;   /* 0 for word form, 1 for bignum form */
         int ri;         /* number of bits in R */
         BIGNUM RR;      /* R^2 (used to convert to Montgomery form) */
         BIGNUM N;       /* The modulus */
-        BIGNUM Ni;      /* The inverse of N (bignum form) */
-        BN_ULONG n0;    /* The inverse of N in word form */
+        BIGNUM Ni;      /* R*(1/R mod N) - N*Ni = 1
+                         * (Ni is only stored for bignum algorithm) */
+        BN_ULONG n0;    /* least significant word of Ni */
         int flags;
         } BN_MONT_CTX;
 
@@ -77,11 +78,17 @@ on error.
 BN_MONT_CTX_init() and BN_MONT_CTX_free() have no return values.
 
 For the other functions, 1 is returned for success, 0 on error.
-The error codes can be obtained by ERR_get_error(3).
+The error codes can be obtained by L<ERR_get_error(3)|ERR_get_error(3)>.
+
+=head1 WARNING
+
+The inputs must be reduced modulo B<m>, otherwise the result will be
+outside the expected range.
 
 =head1 SEE ALSO
 
-bn(3), err(3), BN_add(3), BN_CTX_new(3)
+L<bn(3)|bn(3)>, L<err(3)|err(3)>, L<BN_add(3)|BN_add(3)>,
+L<BN_CTX_new(3)|BN_CTX_new(3)>
 
 =head1 HISTORY