I know that AES 256 bit is far more than enough to secure a file. I just want to know if there are any vulnerabilities or known attacks which can make such this combination less secure "double encryption using AES using different key lengths (first 128 bit and then 256 bit)".   Say, for example, I have a program that can encrypt a file using AES 128 bit and another program which uses AES 256 bit. Also, I have assumed that either one of these encryption programs have a flaw.   Case 1: So, is it possible that if AES 256 bit program fails (because of the flaw in its implementation by the programmer), the attacker will only find the file encrypted by the strong AES 128 bit and won't be able to proceed forward?  Can Case 1 be true?  Please note that the keys are not related in any way. I'm not an expert in Cryptography and I'm trying to understand its basics. So, if you may please reply in simple terms, that would be much appreciated. Thank you. :)
When transfering data using TLS the browser and server agree the cipher suite to be used - so for example this could be chosen as AES-128 and is (probably) outside of my control. If I separately encrypt a file using AES-256 and then Alice downloads this via her browser (using AES 128) have I still effectively got AES-256 security?  My reasoning goes like this: even if the AES-128 encryption on the TLS link was broken by Eve then all she would get is the AES-256 encrypted file and she would then have to break that encryption to see the plaintext.  Of course the AES-256 key is sent to Alice over a separate channel.
What size images do I need for common print sizes, such as wallet size, 8x10, and 16x20?  Is there a general formula?  How could that size change if, for example, a 16x20 was printed on canvas rather than photopaper? How to uninstall Windows from a Dual Boot system? How to use the parskip package? (space in between paragraphs) How to find out who called a method? (I am not asking how to block my posts on my timeline.)  I am asking how I can block my activity, such as posting in open groups, making comments on people's posts, adding a friend, liking a photo, liking a post, etc., from people seeing it in their news feeds?  I don't want people seeing my interactions with other people. What type of control joint is best for my concrete project? I'm looking at getting a lot of concrete work done and am looking to educate myself past the minimal anecdotal information I posses so that I can have a better conversation with prospective contractors.    Expansion joints, saw cut or hand troweled - is this just preference or is there a good reason to go for one over another? From an aesthetic perspective I like hand troweled but I've been told that the saw cut can leave the concrete stronger as you are interacting with the wet concrete less.   Google doesn't seem to provide much info by way of POV on this, just that there are options, so I look to the community here to see if there is a collective wisdom.  For perspective, this is for a standard 4" thick driveway, 6" thick apron, and garage floor. I was viewing some old posts on Stack Overflow (two-year-old posts) and found that due to "fast technology trend changing," some of them are no longer valid. Stack Overflow (and the Stack Exchange sites generally) should have some sort of criteria to tag questions as Obsolete, or a vote-based system to mark them as such.  I am not in favour of closing obsolete questions. Proof that the relation $5 \mid (a + 4b)$ is symmetric and transitive Take the relation $R$ to be defined on the set of integers:  $$aRb \iff 5 \mid (a + 4b)$$  As part of a larger proof, I have to show that $R$ is both symmetric and transitive. I'm lost.  I see the first steps, but I can't find how to progress further. Here's what I have at this point:    Proof of Symmetry  We have to prove that if $5 \mid (a + 4b)$, then $5 \mid (b + 4a)$. Clearly, this is true if $a = b$, but apart from that, it's unclear in my mind.  Proof of Transitivity  We have to prove that if $5 \mid (a + 4x)$ and $5 \mid (x + 4b)$, then $5 \mid (a + 4b)$.    I've fiddled around with sample values, but I still don't see it. I'm pretty lost here. Thoughts? Why do we use 'did' with questions using the simple past tense? The problem of counting the solutions $(a_1,a_2,\ldots,a_n)$ with integer $a_i\geq0$ for $i\in\{1,2,\ldots,n\}$ such that $$a_1+a_2+a_3+....a_n=N$$ can be solved with a stars-and-bars argument. What is the solution if one adds the constraint that $a_i\leq r_i$ for certain integers $r_1,\ldots,r_n$?  e.g. for $n=3$, $N=6$ and $(r_1,r_2,r_3)=(3,3,2)$, the tuple $(a_1,a_2,a_3)=(2,3,1)$ is a solution, but $(2,1,3)$ is not a solution because $a_3=3&gt;2=r_3$. How to make dot match newline characters using regular expressions Do I need a swap partition? I have 4GB of RAM and I want to install Ubuntu. Do I need a to have a swap partition? Because my computer has 4GB RAM, I do not think it is necessary to have a swap partition. If I do need to have a swap partition, should it be a 4GB or an 8GB partition? A term for adoption of a made up term Is there a term for the acceptance of a made up term that then became the name of the object/idea/action once it was invented?   Example: Cyberspace. It was first used by William Gibson in his cyberpunk novels, but when actually invented, it now is widely used across the world (or at least across the United States).  Is there a term for this phenomenon? Proof of property of abelian group I want to show that $\forall x$,$y\in G$, $(xy)^2=x^2y^2 \iff G $ is an abelian group.  Proof of the backward direction seems easy enough:  Assume $G$ is an abelian group. Then consider $(xy)^2=(xy)(xy)$. By associativity, $(xy)(xy)=xyxy$. Since $G$ is abelian, $xyxy=xxyy=x^2y^2$.   Proof of the forward direction I attempt by contrapositive:  That is, assume $G$ is not an abelian group. Then, consider $(xy)^2=xyxy$ as above. Yet, $xyxy\neq xxyy=x^2y^2$ since we no longer have commutativity.  Is this proof by contrapositive completely correct? Simple Proof by induction: $9$ divides $n^3 + (n+1)^3 + (n+2)^3$ I'm trying to prove using induction that $9$ divides $n^3 + (n+1)^3 + (n+2)^3$ whenever $n$ is a non-negative integer. So far, I have:   Base case: $P(1) = (1) + (8) + (27) = 36, 36$ can be divided by $9$ so the base case is valid Inductive step: let $P(n)$ be the statement $9$ divides $n^3 + (n+1)^3 + (n+2)^3$. Assume $P(k)$ is true, so $9$ divides $k^3 + (k+1)^3 + (k+2)^3$.   And this is where I'm stuck. I'm not sure how to demonstrate that $9$ divides $P(n)$ when $n = k+1$. If someone could step me in the right direction that would be awesome. Thanks! Non-static variable cannot be referenced from a static context let $n\gt1$  be a fixed natural number, $S:=${$A $: $M_n (\mathbb{R})$ be all real matrix,define this meter for all $A=[a_{ij}]$ $B=[b_{ij}]$   $d(A,B)$:=max{|$a_{ij}-b_{ij}$|:i,j=1,2,2...,n} and $GL(n,\mathbb R)$ is set of all $n\times n$ non singular matrix how to prove that $GL(n,\mathbb R)$ is  not connected and open subset of $S$ ?  thanks in advance Intersections of all subgroups is a nontrivial subgroup, so every element has finite order. I need help to prove this result:  "Let $G$ be a group such that the intersection of all its subgroups other than $\{1\}$ is a subgroup different from $\{1\}$. Then all its elements have a finite order".  I know I must think of an element $g$ of infinite order. That will imply that every subgroup has infinite order (because it will contain an element like $g^k$). After this, I don't know which step I can take. Can someone help? When I try to remove Java with sudo apt-get remove jdk1.8.0-40 I get this:    Reading package lists... Done Building dependency tree        Reading state information... Done The following packages will be REMOVED:   jdk1.8.0-40 0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded. 1 not fully installed or removed. After this operation, 245 MB disk space will be freed. Do you want to continue? [Y/n] y (Reading database ... 231488 files and directories currently installed.) Removing jdk1.8.0-40 (1.8.040-1) ... find: `/usr/java/*': No such file or directory /var/lib/dpkg/info/jdk1.8.0-40.postrm: line 586: /usr/sbin/alternatives: No such file or directory /var/lib/dpkg/info/jdk1.8.0-40.postrm: line 598: /usr/sbin/alternatives: No such file or directory dpkg: error processing package jdk1.8.0-40 (--remove):  subprocess installed post-removal script returned error exit status 127 Errors were encountered while processing:  jdk1.8.0-40 E: Sub-process /usr/bin/dpkg returned an error code (1)   Output of  ls -1 /var/cache/apt/archives/*jdk*:  openjdk-7-jre_7u75-2.5.4-1~trusty1_amd64.deb openjdk-7-jre-headless_7u75-2.5.4-1~trusty1_amd64.deb   Output of sudo dpkg --purge --force-all jdk1.8.0-40:  (Reading database ... 231488 files and directories currently installed.) removing jdk1.8.0-40 (1.8.040-1) ... /var/lib/dpkg/info/jdk1.8.0-40.postrm: line 586: /usr/sbin/alternatives: No such file or directory /var/lib/dpkg/info/jdk1.8.0-40.postrm: line 598: /usr/sbin/alternatives: No such file or directory dpkg: error processing package jdk1.8.0-40 (--purge): subprocess installed post-removal script returned error exit status 127 Errors were encountered while processing:  jdk1.8.0-40   When trying the edit I get this:  (Reading database ... 231488 files and directories currently installed.) Removing jdk1.8.0-40 (1.8.040-1) ... /var/lib/dpkg/info/jdk1.8.0-40.postrm: line 586: /usr/sbin/alternatives: Permission denied /var/lib/dpkg/info/jdk1.8.0-40.postrm: line 598: /usr/sbin/alternatives: Permission denied dpkg: error processing package jdk1.8.0-40 (--remove):  subprocess installed post-removal script returned error exit status 126 Errors were encountered while processing:  jdk1.8.0-40 E: Sub-process /usr/bin/dpkg returned an error code (1) How to ensure that string interpolated into `sed` substitution escapes all metachars I have a script that reads a text stream and generates a file of sed commands that is later run with sed -f. The generated sed commands are like:  s/cid:image002\.gif@01CC3D46\.926E77E0/https:\/\/mysite.com\/files\/1922/g s/cid:image003\.gif@01CC3D46\.926E77E0/https:\/\/mysite.com\/files\/1923/g s/cid:image004\.jpg@01CC3D46\.926E77E0/https:\/\/mysite.com\/files\/1924/g   Assume the script which generates the sed commands is something like:  while read cid fileid do     cidpat="$(echo $cid | sed -e s/\\./\\\\./g)"     echo 's/'"$cidpat"'/https:\/\/mysite.com\/files\/'"$fileid"'/g' &gt;&gt; sedscr done   How can I improve the script to ensure all regex metacharacters in the cid string are escaped and interpolated properly?