Apply some suggestions from review

Co-authored-by: René Meusel <[email protected]>

src/lib/math/bigint/big_code.cpp

@@ -9,6 +9,7 @@
 
 #include <botan/hex.h>
 #include <botan/internal/divide.h>
+#include <botan/internal/stl_util.h>
 
 namespace Botan {
 
@@ -110,8 +111,9 @@ secure_vector<uint8_t> BigInt::encode_fixed_length_int_pair(const BigInt& n1, co
       throw Encoding_Error("encode_fixed_length_int_pair: values too large to encode properly");
    }
    secure_vector<uint8_t> output(2 * bytes);
-   n1.serialize_to(std::span{output}.subspan(0, bytes));
-   n2.serialize_to(std::span{output}.subspan(bytes, bytes));
+   BufferStuffer stuffer(output);
+   n1.serialize_to(stuffer.next(bytes));
+   n2.serialize_to(stuffer.next(bytes));
    return output;
 }
 
@@ -138,7 +140,9 @@ BigInt BigInt::decode(const uint8_t buf[], size_t length, Base base) {
 
          binary = hex_decode_locked(buf0_with_leading_0, 2);
 
-         binary += hex_decode_locked(cast_uint8_ptr_to_char(&buf[1]), length - 1, false);
+         if(length > 1) {
+            binary += hex_decode_locked(cast_uint8_ptr_to_char(&buf[1]), length - 1, false);
+         }
       } else {
          binary = hex_decode_locked(cast_uint8_ptr_to_char(buf), length, false);
       }

src/lib/pubkey/ec_group/ec_point.cpp

@@ -12,6 +12,7 @@
 #include <botan/numthry.h>
 #include <botan/rng.h>
 #include <botan/internal/ct_utils.h>
+#include <botan/internal/stl_util.h>
 
 namespace Botan {
 
@@ -595,22 +596,22 @@ std::vector<uint8_t> EC_Point::encode(EC_Point_Format format) const {
    const BigInt x = get_affine_x();
    const BigInt y = get_affine_y();
 
-   std::vector<uint8_t> result;
+   const size_t fe = (format == EC_Point_Format::Compressed) ? 1 : 2;
+
+   std::vector<uint8_t> result(1 + fe * p_bytes);
+   BufferStuffer stuffer(result);
 
    if(format == EC_Point_Format::Uncompressed) {
-      result.resize(1 + 2 * p_bytes);
-      result[0] = 0x04;
-      x.serialize_to(std::span{result}.subspan(1, p_bytes));
-      y.serialize_to(std::span{result}.subspan(1 + p_bytes, p_bytes));
+      stuffer.append(0x04);
+      x.serialize_to(stuffer.next(p_bytes));
+      y.serialize_to(stuffer.next(p_bytes));
    } else if(format == EC_Point_Format::Compressed) {
-      result.resize(1 + p_bytes);
-      result[0] = 0x02 | static_cast<uint8_t>(y.get_bit(0));
-      x.serialize_to(std::span{result}.subspan(1, p_bytes));
+      stuffer.append(0x02 | static_cast<uint8_t>(y.get_bit(0)));
+      x.serialize_to(stuffer.next(p_bytes));
    } else if(format == EC_Point_Format::Hybrid) {
-      result.resize(1 + 2 * p_bytes);
-      result[0] = 0x06 | static_cast<uint8_t>(y.get_bit(0));
-      x.serialize_to(std::span{result}.subspan(1, p_bytes));
-      y.serialize_to(std::span{result}.subspan(1 + p_bytes, p_bytes));
+      stuffer.append(0x06 | static_cast<uint8_t>(y.get_bit(0)));
+      x.serialize_to(stuffer.next(p_bytes));
+      y.serialize_to(stuffer.next(p_bytes));
    } else {
       throw Invalid_Argument("EC2OSP illegal point encoding");
    }

src/lib/pubkey/pubkey.cpp

@@ -16,6 +16,7 @@
 #include <botan/internal/fmt.h>
 #include <botan/internal/parsing.h>
 #include <botan/internal/pss_params.h>
+#include <botan/internal/stl_util.h>
 
 namespace Botan {
 
@@ -286,9 +287,11 @@ std::vector<uint8_t> der_encode_signature(std::span<const uint8_t> sig, size_t p
       throw Encoding_Error("Unexpected size for DER signature");
    }
 
+   BufferSlicer bs_sig(sig);
    std::vector<BigInt> sig_parts;
+   sig_parts.reserve(parts);
    for(size_t i = 0; i != parts; ++i) {
-      sig_parts.push_back(BigInt::from_bytes(sig.subspan(part_size * i, part_size)));
+      sig_parts.emplace_back(BigInt::from_bytes(bs_sig.take(part_size)));
    }
 
    std::vector<uint8_t> output;

src/lib/pubkey/rfc6979/rfc6979.cpp

@@ -21,7 +21,7 @@ RFC6979_Nonce_Generator::RFC6979_Nonce_Generator(std::string_view hash, const Bi
       m_rng_out(m_rlen) {
    m_hmac_drbg = std::make_unique<HMAC_DRBG>(MessageAuthenticationCode::create_or_throw(fmt("HMAC({})", hash)));
 
-   x.serialize_to(std::span{m_rng_in}.subspan(0, m_rlen));
+   x.serialize_to(std::span{m_rng_in}.first(m_rlen));
 }
 
 RFC6979_Nonce_Generator::~RFC6979_Nonce_Generator() = default;