Function decodeBase64

src/platforms/esp/32/ota/ota_impl.cpp.hpp:287–351

@brief Simple Base64 decoder for Basic Auth @param input Base64 encoded string @param output Output buffer (must be at least (inputLen * 3 / 4) + 1 bytes) @param max_output_len Maximum size of output buffer @return Length of decoded data, or -1 on error

/// @param max_output_len Maximum size of output buffer
/// @return Length of decoded data, or -1 on error
int decodeBase64(const char* input, char* output, size_t max_output_len) {
    static const char base64_chars[] =
        "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

    if (!input || !output || max_output_len == 0) {
        return -1;
    }

    int in_len = strlen(input);
    if (in_len == 0) {
        output[0] = '\0';
        return 0;
    }

    int out_len = 0;

    for (int i = 0; i < in_len && i + 3 < in_len; i += 4) {
        // Get 4 base64 characters
        const char* pos_a = strchr(base64_chars, input[i]);
        const char* pos_b = strchr(base64_chars, input[i + 1]);
        const char* pos_c = (input[i + 2] == '=') ? nullptr : strchr(base64_chars, input[i + 2]);
        const char* pos_d = (input[i + 3] == '=') ? nullptr : strchr(base64_chars, input[i + 3]);

        // Check for invalid characters
        if (!pos_a || !pos_b) {
            return -1;
        }

        u32 sextet_a = pos_a - base64_chars;
        u32 sextet_b = pos_b - base64_chars;
        u32 sextet_c = pos_c ? (pos_c - base64_chars) : 0;
        u32 sextet_d = pos_d ? (pos_d - base64_chars) : 0;

        // Combine into 3 bytes
        u32 triple = (sextet_a << 18) | (sextet_b << 12) | (sextet_c << 6) | sextet_d;

        // Write output bytes with bounds checking
        if (out_len + 1 <= (int)max_output_len) {
            output[out_len++] = (triple >> 16) & 0xFF;
        } else {
            return -1;  // Buffer overflow
        }

        if (input[i + 2] != '=' && out_len + 1 <= (int)max_output_len) {
            output[out_len++] = (triple >> 8) & 0xFF;
        } else if (input[i + 2] != '=') {
            return -1;  // Buffer overflow
        }

        if (input[i + 3] != '=' && out_len + 1 <= (int)max_output_len) {
            output[out_len++] = triple & 0xFF;
        } else if (input[i + 3] != '=') {
            return -1;  // Buffer overflow
        }
    }

    // Null terminate
    if (out_len < (int)max_output_len) {