When a BN_CTX is used for temporary workspace, the variables are sometimes

left in an inconsistent state when they are released for later reuse. This
change resets the BIGNUMs when they are released back to the context.

crypto/bn/bn_ctx.c

@@ -167,6 +167,19 @@ void BN_CTX_end(BN_CTX *ctx)
 
 	ctx->too_many = 0;
 	ctx->depth--;
+	/* It appears some "scrapbook" uses of BN_CTX result in BIGNUMs being
+	 * left in an inconsistent state when they are released (eg. BN_div).
+	 * These can trip us up when they get reused, so the safest fix is to
+	 * make sure the BIGNUMs are made sane when the context usage is
+	 * releasing them. */
 	if (ctx->depth < BN_CTX_NUM_POS)
+#if 0
 		ctx->tos = ctx->pos[ctx->depth];
+#else
+		{
+		while(ctx->tos > ctx->pos[ctx->depth])
+			/* This ensures the BIGNUM is sane(r) for reuse. */
+			ctx->bn[--(ctx->tos)].top = 0;
+		}
+#endif
 	}