Shift ECDSA inputs to match OpenSSL behavior

See also GH #986

2 files changed, 21 insertions, 0 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