About one line in an implementation of MD5 - c

Question

I'm confused by one line of code in an implementation of MD5,
void MD5_Update(MD5_CTX *ctx, const void *data, unsigned long size)
{
MD5_u32plus saved_lo;
unsigned long used, available;
saved_lo = ctx->lo;
if ((ctx->lo = (saved_lo + size) & 0x1fffffff) < saved_lo)
ctx->hi++;
ctx->hi += size >> 29;
used = saved_lo & 0x3f;
if (used)
{
available = 64 - used;
if (size < available)
{
memcpy(&ctx->buffer[used], data, size);
return;
}
memcpy(&ctx->buffer[used], data, available);
data = (const unsigned char *)data + available;
size -= available;
body(ctx, ctx->buffer, 64);
}
if (size >= 64)
{
data = body(ctx, data, size & ~(unsigned long)0x3f);
size &= 0x3f;
}
memcpy(ctx->buffer, data, size);
}
The question line is if ((ctx->lo = (saved_lo + size) & 0x1fffffff) < saved_lo), it seems the 'size' counts bytes, but the 'ctx->lo' and 'saved_lo' count bits. Why add them together? There are also some similar codes in Github, and also some projects use these code. So anyone can give some explanation?

Answer 1

The remarks about "bit counters" are likely misleading - ctx->hi and ctx->lo count bytes, just like size does.
You correctly notice that you're just adding size (bytes) to ctx->lo (and then checking for overflow/propagating overflow into ctx->hi). The overflow check is pretty simple - lo is used as a 29-bit integer, and if the result after adding/masking is less than the original value, then overflow occurred.
The checks around used are also evidence for ctx->lo and ctx->hi being byte counters -- body processes data 64 bytes at a time, and the lo counter is ANDed with 0x3F (i.e. 63).