Securing Stream Ciphers (HMAC) - Computerphile

Bit flipping a stream cipher could help you hit the Jackpot! But not with HMAC. Dr Mike Pound explains.
Correction : "pseudo" is spelled incorrectly on the graphic.
/ computerphile
/ computer_phile
This video was filmed and edited by Sean Riley.
Computer Science at the University of Nottingham: bit.ly/nottscom...
Computerphile is a sister project to Brady Haran's Numberphile. More at www.bradyharan.com

[@ZehEduardo]

I want to meet this man. I feel like I could hear him talking from hours and not get bored.

[@JoeMarGonzales]

@urbex2007 I guess it's supposed to be that way. He never looks directly into the camera, he looks towards the hidden interviewer instead.

[@pierluiginegro772]

It's his enthusiasm that does the trick! I'm absolutely caught up in his speech, unable to stop listening, getting angry at the video ending so abruptly!

[@misspaula9186]

You can say that again

[@arrowtlg2646]

I took the Computer Security Module and audited the Cryptography modules this year at the university of nottingham, both of which were taught by Dr Mike Pound, and honestly it was amazing. He has a great way of explaining hard concepts in a way that's easy to understand.

[@dalegeddes687]

I'd love to meet this man too huge respect for Mike pound :)

[@StreuB1]

....I honestly get the feeling that Dr. Pound is wildly more intelligent that we realize or he portrays and he always seems like he has to hold back just a bit so he doesn't inadvertently say too much. Like, there is a line that shouldn't be crossed with regard to explaining things and he is constantly dancing with that line and at times has to stop for a moment and think "Ok, right, where am I?....oh, right, ok we can go further" lol Anyone else get that?? Hes awesome man.

[@minihjalte]

Brian Streufert what? He is obviously very intelligent, else he wouldnt have doctorate.

[@StreuB1]

Nooo....beyond that level even!

[@minihjalte]

Brian Streufert what level do you mean? Omnipotent/supernatural level?

[@FriedEgg101]

Would you say that you find his intelligence artificial?

[@kale.online]

I get what you mean. Hopefully like Robert Miles he starts his own channel

[@markusdemedeiros8513]

Dr. Pound! This guy could read me his shopping list and I would still watch it

[@Hexanitrobenzene]

I'm sure he would manage to make it hilarious :)

[@gatoradeee]

This bloke is a well-rounded pundit of computer science--vision, cryptography, algorithms, all the good stuff.

[@SmallSparrowsChannel]

Absolutely love these computerphile videos. To the team: please do not stop making more videos!

[@brooped]

Please sent Mike £100

[@Anvilshock]

Sneeze bent like 7FFF

[@kebman]

Thank you Please!

[@philtrem]

He wants it yesterday !

[@8b8b8b]

Please sent Mike £100 yesterday Please send Mike £100 today

[@downstream0114]

Plase send Mike £10M

[@jamesvanden6251]

HE'S BACK! ALL ABOARD THE TRAIN TO POUND TOWN...

[@amaarquadri]

The encryption related Computerphile videos are some of my favorite.

[@daft_punker]

This guy is my favorite!

[@OnlyUseMeEquip]

make a 2 hour video with this guy, i could listen to him all day

[@jony7779]

Great video. I love Mike Pound - he is my favorite person on computerphile

[@MikhailFederov]

Just wait until you meet Mike Hawk

[@Xe4ro]

Switching off is probably not what anyone has ever done that clicked on a Computerphile video, especially with Mike.😁

[@rogervanbommel1086]

Well, do not forget AES CBC is weird, it’s a block cipher, though because of the XOR chaining you can also do this there

[@brandonbirchall5389]

Love this series, but could you please cover Elliptic curve Cryptography? I feel like this channel explains everything the best, so it would be much appreciated.

[@foobargorch]

djb and Tanja Lange have a great intro to the subject from 31c3

[@cobaltno51]

@@foobargorch Thanks for that!

[@ArthurKhazbs]

Mike is implicitly asking us to send him £100

[@cronaut5429]

£900 *

[@NoNameAtAll2]

Sent*

[@yoavklein123]

Something I didn't understand. Around 5:00, after introducing the first "naive" solution to the problem of tampering the stream cipher - i.e. computing a hash - Dr. Mike rejects this immediately because the middle-man can easily tamper the message and then compute a hash. what I don't understand is how can he compute the hash of the unencrypted message? All he has is the encrypted message, and if the hash is of the un-encrypted, how can he compute this?

[@0xbenedikt]

I don’t understand either. Also if you send the hash encrypted too, there would in my mind absolutely be no way to reconstruct it

[@00a5-h8j]

This video seemed to end without much of a conclusion...

[@user-tr3qt3qs9t]

The seccond method can actually be just as secure if it is guaranteed that none of the valid messages can be a simple concatenation of 2 or more other valid messages. The easiest way to guarantee it is just to always put a total message length block somewhere inside the protected message. This method is a little bit faster than the HMAC.

[@kingofshorekishore]

its said if you can explain something in simple words, you dont understand the concept. But you sir are just a marvel. Thanks alot.

[@friewire]

I have an exam in computer security coming up soon. These videos are golden

[@mahneh7121]

Something implicit in the video: the key in a SC is the same length than the message. Fantastic expl. The guy is a genius

[@bhexric]

I didn't even know HMACs existed until I came across them at work - it's good to know a bit more about them.

[@DreamingWithEyesWide]

Dr. Mike is the best! And this totally reminds me of the Office, especially with the personal interviewing and the random zooms and close-ups 😂 I also love the washing machine animation!!

[@y__h]

Someday you should do a video on AEAD, which is more modern than plain old Stream cipher.

[@esmatsaidy]

I like the way how he comes to the topic, it's brilliant Mike!!!!

[@jordanwaeles]

Is it wrong to feel kinda in love with him? I might have completed my studies if he were my teacher.

[@Xathian]

He's the archetype for the perfect teacher to engage students in learning. He combines easy grasped examples with technical concepts and makes even very complex subjects easily understandable even to a slightly learned layman.

[@Destrolll]

5:13 But why don't we encrypt the checksum too? In other words be append hash of the message right *before* we do the encryption

[@0xbenedikt]

I’d love to know the reason too

[@dlovegrove11]

I feel like these videos always just scratch the surface and don't go in depth too often. I really would like to see some in depth content for the loyal subscribers!

[@marcthachxuanky5744]

This is the most amazingly clear and concise explanation, thanks

[@lynxtouch]

You're helping me so much studying for my Security+ exam!!!!

[@abnorcscreenname8489]

This guy is awesome. Make sure he keeps on making us videos!

[@boynedmaster]

just had an hmac nightmare, this came a bit too late

[@gcheese25]

Should have watched this video before my midterm...finally I feel like I know something. Thanks!

[@alloounou6900]

Actual HMAC stuff starts at 4:00 Until then, he's pointing out flaws with substitution in a stream cipher.

[@aparnasingh6829]

How are you able to explain topics so easily.Seriously, it seems like child's play when I understand things like this

[@dd9516]

I have a big ask. It may take several videos but I should love to see a simple but complete differential cryptanalysis. And a set on linear cryptanalysis. We hear about it all the time but it means very little to most of us. There are many books on it but these are hard to follow, a video would be much easier and better. Thanks! If you do it I shall sent a much bigger 'thank you', capital letters and all!

[@pinnekjtt8002]

I freaking love this dude. Why aint this guy my teacher.

[@msolomonbush]

I like the videos that Dr. Pound does on cryptography and cryptoanalytics, but it would be nice if he could get into more of how the math works behind the concepts. Not just describe the variables.

[@mechris13524]

I think in the video, we were shown this as a non-working solution: encrypt(message)|hash(encrypt(message)) Why couldn't you do this: encrypt(message|hash(message))

[@gryzman]

that's essentially what signing is, you take checksum and encrypt it with the private key - and what you get is a signature.

[@suncrasher4168]

but both keys are private...

[@TMaorgmrtrstogamFFDsKMO]

Greg Jaskiewicz j

[@TMaorgmrtrstogamFFDsKMO]

Greg Jaskiewicz o

[@unvergebeneid]

Christopher Smith, yes, I was definitely missing an explanation for why that is a bad idea!

[@Valery0p5]

You say hases aren't for disks, but I'm pretty sure full disk encryption needs to have something like that... I remember hmacs being mentioned in the WiiU external drive encryption scheme...

[@domenicgilardoni1802]

what exactly stops us from doing h(m|k) as opposed to h(k|m)? we don't need to worry about resuming the internal state of the hash function because that would only allow us to append to the key, not the message

[@jeanmarabou9774]

Why not just do SHA2( msg | key ) and not SHA2( key | msg ) like this the malicious appends can only get SHA2( msg | key | malicious ) but the check will be made SHA2( msg | malicious | key) ?

[@mouadjadil2958]

these guys are legends

[@blakezonca9865]

*Sees doc pound* *EXTERMINES THAT LIKE BUTTON*

[@tommyboy3164]

Thanks. Studying for CISSP. I just subscribed!

[@loudej]

Great video! Can you also do one talking about HMAC-before-encrypt vs encrypt-before-HMAC? encrypt(message)|hmac(key,encrypt(message)) vs encrypt(message|hmac(key,message))

[@hardikmaru3311]

Confidentiality isn't the objective of HMAC, so encrypting the final message doesn't make sense. HMAC provides authentication and integrity

[@thomasb1337]

@@hardikmaru3311 You would want to encrypt then HMAC the encrypted message. Also known EtA

[@gabrielazevedo6028]

The teaching is awesome and he explains greatly, but i can't help but to notice how every video seems like an episode from The Office

[@recklessroges]

I was just writing in the middle of writing an RFC6238 TOTP in C++ (because I can't find any that compile in Code::Blocks to run in cmd.exe. Obviously over on Linux its all just works.)

[@TotalImmort7l]

There is a guy named Please who pleases Mike. Anyways, what is special about CMAC and Poly1305?

[@Anvilshock]

This guy couldn't look any more mischievous if he wore a black henchman's mask befitting a 70s' Batman episode.

[@harshalbhoir1262]

thanks a lot this helped me in studying hmac for my exams

[@user-sc1gd6kd4u]

Is this part of a series? He mentions the SHA video as if I'm supposed to have seen it before, but I can't find an official playlist with computer security related videos från computerphile.

[@xBZZZZyt]

06:33 would adding length of message (excluding added length) in message prevent append attack?

[@yoavklein123]

The questions of the guy with the camera don't sound very clearly..

[@Bloodman42]

I like the void cube and ghost cube on the shelf. :D

[@johanneszwilling]

😏 Ghost Cube has finally been solved 🙌

[@coomservative]

why not just append a salt and signature of the message+salt after that BEFORE applying the cipherstream? The attacker could change the message, but they're going to have a pretty hard time guessing what the salt is to compute the hash and even if they did they wouldn't be able to XOR it to the state the recipient expects.

[@paulfragemann3333]

could you hash the message and append the hash before encrypting it and then verifying i after decryption, or are there any security implications that make that a bad idea?

[@sarainiaangelsong440]

Wait a sec? So a Block cipher changes multiple bits? so if I changed a single letter/bit, in the message that in turn makes the entire message of letters before and after get altered also??? I know of alot of coding systems but as far as I'm aware I think everything usually is 1 to 1, example if we used Enigma 1 letter in 1 letter out sorta like a substitution! If we Transposition it then the same amount of letters move around, If we playfair it then we still end up with same count of letters, in fact I can't think of any 1 situation where 1 plaintext letter is fully responsible for keeping the entire rest of the message mathematically in tact The only thing I can think of is... if you Take 8 Letters that are 8 Bits long in Ascii and Put the rows of numbers 1 by 1 under until you get like 8x8 thus 64 bits but instead grouping all the 1st bits and making an Ascii character out of then then all the 2nd bits until all 8 is done then substitute that maybe along with a 1 time pad, then I can see how maybe by chance changing 1 Ascii can sometimes Affect many characters, unless I'm not seeing something here? So example lets say A=001 B=010 C=100 R=111and our word was Cab then 100, 001, 010 if stacked could be 100, 001, 010 which doesn't change anything but lets try BAR, 010, 001, 111 we get now, 001, 101, 011 now this is kinda neat because we have new numbers which might be mapped to a question mark or something if this was 8 bit ascii but then math to it could change it yet again. I can't 100% see how a single change normally can affect the outcome of how a message works, but I can see in my example how sometimes hit or miss it can affect the entire message! I will have to look at block ciphers to maybe understand, but for most part from my experience the bulk of codes/encryption almost always is the Plain Text being heavily altered with some steps where if you isolated any letter usually, you get the plaintext of that specific letter! Equally speaking if deleting a letter anywhere or changing a letter anywhere also usually doesn't disturb a message most times in all of it's entirety. However what I can say is... if it is possible where 1 Letter is the only way to crack the code if unchanged and all characters have that exact same property and must be One! Then yes that would be 100% valuable and secure as you can just take that and put that through 1 to 1 ciphers knowing there was a step involved that binds all of them together as a whole as one unit.

[@baatar]

So, the outerpad and innerpad are just arbitrary constants that are used to derive the two separate keys which are then used to hash the message authentication code?

[@oriongalactic]

Why we just hash before encrypt all together message and hash? we could not compute a new hash for the changed message since we would need to know the encryption key

[@charan_75]

How do the sender and receiver get same key? Do they use key exchange protocols?

[@J2963-u3x]

Hash length extension attack video ?

[@PvblivsAelivs]

Someone just flipping bits like that doesn't have the old message or the new message to compute the hashes from. Plus the hash itself is likely to be encrypted. Here, it comes down to how much power you think the attacker has. If he can read all your messages and correctly guess all your keys, you're sunk anyway because he can do whatever he wants.

[@foobargorch]

it's still useful to ensure by construction that the code that reads these messages can't accidentally reveal any information (for example side channel or timing attacks could play into this), or be made to crash given tampered data, naive tamper resistance schemes do not necessarily meet these criteria even if they exclude tampering per se

[@JNET_Reloaded]

What about video encryption is this stream cypher?

[@fandingoORG]

Doc Pound is my jam

[@macdjord]

Why not just append the key to the end of the message before taking the hash instead of prepending it to the beginning? Wouldn't that solve length-extension?

[@DanDart]

The captions on this aren''t great, any way for me to edit?

[@loadedfries5764]

I've really enjoyed these videos that are related to information and security! I was wondering though, could you guys could make some videos in the "Essentials" series that cover the basics of those topics?

[@foobargorch]

Unfortunately it's a broad subject, and modern cryptography (elliptic curves, for example) has really changed a lot of what is considered "essential" compared to a few years ago. There are some great courses on youtube, but that's literally tens of hours of material...

[@johnbll106]

So I guess the length extension video never came out sed.

[@DynoosHD]

Why doesnt this work?: encrypt(message hash(message))

[@wapted]

well, it does work.

[@gajbooks]

It does, but I think the intent was to secure an arbitrary message without requiring knowledge of what cypher it was using. Other people could bit flip all they want but unless they can decode the stream in the first place they can't correctly re-encode a new hash.

[@ikkohmation]

It's a stream cipher, so you shouldn't be able to hash the whole message ? (I think)

[@PeregrineBF]

It makes you vulnerable to padding oracle and other chosen ciphertext attacks. That construction is called MAC then Encrypt, and is insecure if the encryption scheme is vulnerable to such attacks (many are, and are designed to use a MAC of the ciphertext to remove the vulnerability).

[@foobargorch]

this is worse than MAC-then-encrypt, since the hash isn't signed, a chosen plaintext attack will also allow forgery, for stream ciphers and non chaining block cipher modes: You choose m_1, and get c_1: c_1 = k_1 ^ (m_1 || H(m_1)) You can recover the key stream (but not with a chaining block cipher mode): k_1 = c_1 ^ (m_1 || H(m_1)) Forge ciphertext for arbitrary message m_2: c_2 = k_1 ^ (m_2 || H(m_2)) In other words, the hash only provides integrity checking, not authentication

[@Seegalgalguntijak]

I've seen SHA512 being used as a hash function. Does it have the same flaws as SHA256?

[@Tomyb15]

From 6:10 to 6:42 I got completely lost. I didn't understand what you were talking about and also couldn't make out some words. Could somebody explain it to me or just point me to the video I missed to understand that?

[@foobargorch]

The Merkle-Damgard construction is a way of taking what's called a compression function, which has nothing to do with compression like zip files - that takes two blocks of constant length and produces one block of constant length that is a "mix" of the two inputs, and extends it to any message length by splitting the message into blocks, and chaining the blocks together, so you "compress" an initialization vector with the first block, and then take the output of that and the next block and "compress" that... Since the final hash is output of the last compression operation, if you know the hash of a message, you can produce the hash of any of its suffixes, even if you don't know the original message. Using his example, you could add additional 0s to the amount, instead of changing the most significant digit, even if it was prefixed with a key. By adding another hashing step that also depends on the key, you can prevent this kind of tampering, which is known as a length extension attakc.

[@SeeingGreenDevils]

so why not just use digital signatures? when would we use HMAC as oppsed to using signtures?

[@TheStevenhull]

Gotta love the paper he uses with the sprocket holes. Who does that ?!?! LoL

[@maxijonson]

DONT QUIT THE VIDEO JUST YET 😂

[@f3lixhu123]

Well done ! really enjoyed your video.

[@Krissini]

thank you for that brilliant video!

[@bohitargep]

YAY he finally solved the ghost cube

[@Mathematicmaster]

0:53 Left upper corner, that ghost cube :D

[@all-rounderb5012]

Very few understand this.

[@darshanv1748]

If we insert the hash (k1|m) at the start of message itself won't it also solve the problem? May be it s not efficient to prepend than append but i think it'l also solve the problem, but i would like to know your opinion

[@mihaistoian_]

Thank you!

[@fayasamd5204]

Did i miss something or he never mention how we gonna send the shared secret key to the other person ?

[@kolrabi]

So... do you HMAC then encrypt or encrypt then HMAC? And for the love of Turing, please clean your screen. :)

[@stheil]

IIRC, both are valid approaches and both are used in practice.

[@bengrap0]

Both are used. However, the new consensus is to encrypt then hmac. It allows you to skip decryption when the hmac is invalid. Thus allowing you to break earlier in your algorithm, resulting in less implementation mistakes. For example with block ciphers it might still be possible to exploit a padding oracle if the attacker can distinguish between the decryption failing and the hmac check failing, which in the case of never touching the encrypted text can never happen.

[@foobargorch]

if you use a random ID, you need to keep track of what it's identifying, which means you need to store that data, and storage scales as the number of things you need to keep track of. Using an authenticated encryption lets you delegate the storage to an untrusted party, without revealing the contents and without compromising your trust in the data, and only need to store a small amount of unchanging data.

[@banderi002]

You can see at 5:44 the spider bite in Tobey Maguire's hand

[@songofyesterday]

Helps me study for CISSP, thanks!

[@peschebichsu]

There's no video about the length extension attack yet if i see right :(

[@d1ngo153]

idk why but this reminds me of the office

[@DrKaii]

Why do I get the feel that this guy was a car mechanic in a previous life?

[@Rsutton99]

Great vid! But, those fingerprints on Mike's monitor are really upsetting :p

[@DorthLous]

Psuedo?

[@KulasangarGowrisangar]

Does anyone know, what's inner pad and outer pad which he mentions at 8:10?

[@fahd2372]

Why not instead of hashing the encrypted message, instead hash the original message? That way a middle-man wont be able to compute the hash without decrypting the entire message.

[@islandfireballkill]

Why wouldn't you just take the approach of appending the secret key to the end of the message? You wouldn't know the internal state then.

[@islandfireballkill]

If the problem is that you could brute force much faster since you can precompute most of the hash, then why not pad the key to both sides of the message?

[@foobargorch]

HMAC is resistent to length extension attacks and would be even with concatenation, since the hash of the message is hashed again performing a length extension attack on HMAC requires a preimage attack on the hash function

[@foobargorch]

Having thought about the alternate scheme you proposed (key at both ends), I really can't come up with a reason as to why that's functionally different. For a Merkle-Damgard function you can think of an HMAC construction on top of it as if it's just like your scheme, but with an extra copy of the initialization vector added towards the end, and the argument order to the compression function getting flipped twice (I think) [edit top clarify: two applications of the compression functions are flipped, not all of them are flipped twice, which does nothing ;-)]. I think since the construction doesn't assume anything different about the two inputs to the compression function, swapping the arguments like that doesn't change the security assumptions, nor does adding the extra IV as a delimiter, so I think it's basically equivalent, in terms of the security guarantees. However I also don't see any benefit to your proposed schema, apart from very minor efficiency gains, the algorithmic complexity is on the same order. Bear in mind that I'm not an expert or anything, I just think you asked a very interesting question =)

[@iJuenger]

CBC mode for block ciphers

[@nickpelov]

What if instead of appending the key you prepend the key. It's secret so no one would know what the state would be after the key.

[@lonerboy013]

is it just me or are they spelling "pseudo" as "psuedo"?

[@Computerphile]

+lonerboy013 I thought that didn't quite look right! Should've checked! >Sean

[@gedeonandrasmusic]

"We can actually change the amount get sent, which unfortunately is a purely hypothetical example, I don't actually get any money" 😂

[@naboogom2733]

need more !!!

[@george-cristianbirzan8643]

Whose videos are referenced in the bit about hamming distance at 8:50, Dave Brelson is the best I can make out...

[@Computerphile]

+George-Cristian Bîrzan Professor Dave Brailsford has done a number of computerphile videos regarding hamming codes

[@george-cristianbirzan8643]

Right. Thanks! English pronunciation/spelling is weird. However, I always felt that guests on the show should be showcased more, nobody reads the description. I'll blame it on that! :-(

[@andrewcai8130]

I dunno about you guys, but I have trouble having a clear understanding of what this guy talks about when he explains things. It's hard to grasp. No disrespect though.