I don’t have a lot of time to write; I’m having my fifth (I think) upper endoscopy done tomorrow, which means that the day’s going to be a wash; and Yom Kippur is thursday, and I need to cook, so between the personal crap and work, I’m not going to have much time for blogging. So I’m trying to make use of the time I have to write one short but (hopefully) interesting post.<\/p>\n
One thing that I’ve mentioned in passing is the distinction between message confidentiality<\/em>, and message integrity<\/em>.<\/p>\n Confidentiality is most of what we’ve been talking about Integrity is something very different. Integrity guarantees <\/p>\n Suppose you’re using CBC mode for a message in DES, and you’ve Can you tell that there was a corruption in the message? If the plaintext was something like human language, you can see that it makes no sense. But that’s relying on our meta-knowledge about In CBC mode – and in fact in all of the modes we’ve looked at, an attacker can change bits of the ciphertext, corrupting the message, and we can’t tell<\/em>. That’s why we say that How can we get around that? The easiest thing is to add something called a message authentication code<\/em> (MAC). It’s easiest to understand the MAC using a bit of notation: MAC functions need to have some basic properties to be If you’re familiar with digital signatures, some of this Next post, I’ll go through one or two MAC algorithms, and then talk about modes of operation that incorporate MAC-like integrity I don’t have a lot of time to write; I’m having my fifth (I think) upper endoscopy done tomorrow, which means that the day’s going to be a wash; and Yom Kippur is thursday, and I need to cook, so between the personal crap and work, I’m not going to have much time for blogging. […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[82],"tags":[],"class_list":["post-690","post","type-post","status-publish","format-standard","hentry","category-cryptography"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_shortlink":"https:\/\/wp.me\/p4lzZS-b8","jetpack_sharing_enabled":true,"jetpack_likes_enabled":true,"_links":{"self":[{"href":"http:\/\/www.goodmath.org\/blog\/wp-json\/wp\/v2\/posts\/690","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.goodmath.org\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.goodmath.org\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.goodmath.org\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.goodmath.org\/blog\/wp-json\/wp\/v2\/comments?post=690"}],"version-history":[{"count":0,"href":"http:\/\/www.goodmath.org\/blog\/wp-json\/wp\/v2\/posts\/690\/revisions"}],"wp:attachment":[{"href":"http:\/\/www.goodmath.org\/blog\/wp-json\/wp\/v2\/media?parent=690"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.goodmath.org\/blog\/wp-json\/wp\/v2\/categories?post=690"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.goodmath.org\/blog\/wp-json\/wp\/v2\/tags?post=690"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
\nso far. Confidentially provides a guarantee that when you send an encrypted message, no one but your intended recipient is able
\nto read the plaintext.<\/p>\n
\nthat if you send an encrypted message, there’s no way that the encrypted message could have been tampered with after you encrypted it, without the recipient knowing it.<\/p>\n
\ngot an attacker who wants to screw up your message. What happens if the attacker flips a couple of bits in one block? First, the plaintext for that block will be corrupted. Second, the
\nciphertext for that block is used for decrypting the next block – so the plaintext for the next block will also be corrupted.<\/p>\n
\nthe structure\/nature of the plaintext. In fact, just looking at
\nthe encrypted message, there is no possible way<\/em> for us to detect tampering!<\/p>\n
\nthese modes of operation can’t provide any guarantees of message
\nintegrity.<\/p>\n
\nA MAC is basically a hash-code: a short string appended to the
\nmessage which in some way summarizes the message, so that if any
\npart of the message was changed, the MAC will not match the message, and so we’ll know that the message was corrupted. <\/p>\n
\nMac = M(T, K), where “M” is the MAC function, T is the plaintext of the message, and K is an encryption key. So you take your plaintext and your key, feed it to M, and get back a short string which is the MAC for your message.<\/p>\n
\nsuccessful in protecting integrity:<\/p>\n\n
\na significant change in the MAC.<\/li>\n
\nability to compute O(T) for an arbitrary set of chosen
\nplaintexts, it must be difficult to guess the MAC for a
\nparticular message without submitting it to O. (That is, even
\nin the scenario of a ideal chosen plaintext attack,
\nwhere you can choose any set of messages you want, and
\nget the MAC for those messages, that won’t help you to guess
\nthe MAC of any message outside of that set.)<\/li>\n
\nU such that M(T,K)=M(U,K).<\/li>\n<\/ol>\n
\nshould be ringing bells. The idea of a digital signature is very
\nclose to this. The main difference is that a digital signature
\nis asymmetric: the sender and the receiver have different<\/em> keys. The sender has a private key, and signs the message;
\neveryone else has a public key, and can verify that the sender
\nsigned the message. With MACs, both the sender and the receiver have the same key<\/em>. MACs provide no guarantee of who originated the message; they just guarantee that the message
\nwasn’t changed between the time it was encrypted by the sender
\nand decrypted by the reciever.<\/p>\n
\nguarantees.<\/p>\n","protected":false},"excerpt":{"rendered":"