5 files changed, 27 insertions, 6 deletions
diff --git a/src/lib/math/bigint/bigint.cpp b/src/lib/math/bigint/bigint.cpp
index 694c7afab..fd967e66e 100644
--- a/src/lib/math/bigint/bigint.cpp
+++ b/src/lib/math/bigint/bigint.cpp
@@ -93,6 +93,18 @@ BigInt::BigInt(const uint8_t input[], size_t length, Base base)
    *this = decode(input, length, base);
    }
 
+BigInt::BigInt(const uint8_t buf[], size_t length, size_t max_bits)
+   {
+   const size_t max_bytes = std::min(length, (max_bits + 7) / 8);
+   *this = decode(buf, max_bytes);
+
+   const size_t b = this->bits();
+   if(b > max_bits)
+      {
+      *this >>= (b - max_bits);
+      }
+   }
+
 /*
 * Construct a BigInt from an encoded BigInt
 */
diff --git a/src/lib/math/bigint/bigint.h b/src/lib/math/bigint/bigint.h
index 56b907d80..3d6626e4d 100644
--- a/src/lib/math/bigint/bigint.h
+++ b/src/lib/math/bigint/bigint.h
@@ -86,6 +86,15 @@ class BOTAN_PUBLIC_API(2,0) BigInt final
      BigInt(const uint8_t buf[], size_t length, Base base);
 
      /**
+     * Create a BigInt from an integer in a byte array
+     * @param buf the byte array holding the value
+     * @param length size of buf
+     * @param max_bits if the resulting integer is more than max_bits,
+     *        it will be shifted so it is at most max_bits in length.
+     */
+     BigInt(const uint8_t buf[], size_t length, size_t max_bits);
+
+     /**
      * Create a BigInt from an array of words
      * @param words the words
      * @param length number of words
diff --git a/src/lib/pubkey/dsa/dsa.cpp b/src/lib/pubkey/dsa/dsa.cpp
index 9249cd0d5..982e1b931 100644
--- a/src/lib/pubkey/dsa/dsa.cpp
+++ b/src/lib/pubkey/dsa/dsa.cpp
@@ -105,7 +105,7 @@ DSA_Signature_Operation::raw_sign(const uint8_t msg[], size_t msg_len,
    {
    const BigInt& q = m_group.get_q();
 
-   BigInt i(msg, msg_len);
+   BigInt i(msg, msg_len, q.bits());
 
    while(i >= q)
       i -= q;
@@ -167,7 +167,7 @@ bool DSA_Verification_Operation::verify(const uint8_t msg[], size_t msg_len,
 
    BigInt r(sig, q_bytes);
    BigInt s(sig + q_bytes, q_bytes);
-   BigInt i(msg, msg_len);
+   BigInt i(msg, msg_len, q.bits());
 
    if(r <= 0 || r >= q || s <= 0 || s >= q)
       return false;
diff --git a/src/lib/pubkey/ecdsa/ecdsa.cpp b/src/lib/pubkey/ecdsa/ecdsa.cpp
index 6ff02e8c9..f35693189 100644
--- a/src/lib/pubkey/ecdsa/ecdsa.cpp
+++ b/src/lib/pubkey/ecdsa/ecdsa.cpp
@@ -81,7 +81,7 @@ secure_vector<uint8_t>
 ECDSA_Signature_Operation::raw_sign(const uint8_t msg[], size_t msg_len,
                                     RandomNumberGenerator& rng)
    {
-   const BigInt m(msg, msg_len);
+   BigInt m(msg, msg_len, m_group.get_order_bits());
 
 #if defined(BOTAN_HAS_RFC6979_GENERATOR)
    const BigInt k = generate_rfc6979_nonce(m_x, m_group.get_order(), m, m_rfc6979_hash);
@@ -134,7 +134,7 @@ bool ECDSA_Verification_Operation::verify(const uint8_t msg[], size_t msg_len,
    if(sig_len != m_group.get_order_bytes() * 2)
       return false;
 
-   const BigInt e(msg, msg_len);
+   const BigInt e(msg, msg_len, m_group.get_order_bits());
 
    const BigInt r(sig, sig_len / 2);
    const BigInt s(sig + sig_len / 2, sig_len / 2);
diff --git a/src/lib/pubkey/ecgdsa/ecgdsa.cpp b/src/lib/pubkey/ecgdsa/ecgdsa.cpp
index db790b0d1..12962d18c 100644
--- a/src/lib/pubkey/ecgdsa/ecgdsa.cpp
+++ b/src/lib/pubkey/ecgdsa/ecgdsa.cpp
@@ -57,7 +57,7 @@ secure_vector<uint8_t>
 ECGDSA_Signature_Operation::raw_sign(const uint8_t msg[], size_t msg_len,
                                      RandomNumberGenerator& rng)
    {
-   const BigInt m(msg, msg_len);
+   const BigInt m(msg, msg_len, m_group.get_order_bits());
 
    BigInt k = BigInt::random_integer(rng, 1, m_group.get_order());
 
@@ -107,7 +107,7 @@ bool ECGDSA_Verification_Operation::verify(const uint8_t msg[], size_t msg_len,
    if(sig_len != m_group.get_order_bytes() * 2)
       return false;
 
-   const BigInt e(msg, msg_len);
+   const BigInt e(msg, msg_len, m_group.get_order_bits());
 
    const BigInt r(sig, sig_len / 2);
    const BigInt s(sig + sig_len / 2, sig_len / 2);