uk – If a US citizen didn’t realize that they overstayed and then found out, what are they expected to do?

Are you expected to leave immediately if you have already been existing in the country for a few years? Do you have some time allowance for when you’re expected to make your way out? If you realize that you’ve overstayed and would like to do the right thing, what is the time frame that you’re expected to act in?

20.04 – VScode installed from snap : command “code” not working as expected

I just installed Visual Studio code from snap : sudo snap install code --classic

When I type code . in the terminal, nothing occurs it just returns to new command line without any error but won’t open vscode as I expected.

When I type which code i got this output : /snap/bin/code

I tried these solution :

How To Open Visual Studio Code Using Terminal

But nothing help. Because the command is found, it just doesn’t open the program.

I also tried to sudo snap remove code and installed it from the .deb package following those steps :

But i got the same behavior : nothing append when typing code . command.

I can open visual studio from the main menu by clicking on the vscode icon….but i used the ‘code’ command everywhere and it is very frustrating to not be able to use it.

please help !

mysql – After changing engine from myisam to innodb tables are larger in size, is this an expected behaviour?

I updated a WordPress database from myisam to innodb on a server using apache and mysql, and I noticed that some tables, such as wp-posts grew in size after running the ALTER TABLE wp_posts ENGINE=InnoDB; command, about 20MB. So, this is the first time I’ve done this, and I’m wondering if this is an expected behaviour?


caching – How do I decide an initial in-memory cache size given my DB size and expected load throughput?

(Purely for learning purposes)

Say the DB contains 1 billion rows with 200 bytes per row = 200 GB of data.

The traffic at peak is 1000 requests/s, with each request asking for one DB row.

What cache size would I begin with to ease off the load on the DB? I realize that this is determined best empirically and can be tuned as time goes on.

Caches are usually not too large given memory constraint (unless you go for a distributed cache like redis), so we can’t have the in-memory cache be more than say 200 MB of space, which accounts for way less than 1% of the DB size and seems too small. The cache might just spend all its time being 100% occupied with 95% misses and evicting entries and caching new entries using a simple LRU scheme.

Perhaps there’s no point bothering to cache anything in-memory here. In that case, how would you go about coming up with an initial cache size in a redis cache?

Proving Expected Value is Maximum of this Probability Function

Suppose we had m many dice each of which had n many sides. If we roll the m dice, then the probability that k of those dice have the same face value of, say r, where 1<= r <= n, should be

((n-1)^(m-k)(m C k))/n^m.

So I was looking for what value k maximizes this probability and through some trials it looks like it is when k=m/n, or rather the closest integer to m/n.

After looking this up (I have never formally taken stats or prob theory), I read that m/n is the expected value. This makes sense, but I was hoping someone could provide an intuitive derivation or proof that the expected value is indeed m/n and why it does maximize the probability.

Any insight on this would be greatly appreciated. Thank you!

rsa – Cryptoki PKCS11 C_Decrypt returns shorter key (decrypted messaged) than expected

First off let me say I’m fully aware this question can only be answered by the token vendor but I have already contacted them and with the whole COVID situation my hopes of hearing from them soon are not high (nor that I need to, this is just to satisfy my own curiosity). What I’m hoping to get as an answer is maybe somebody with a similar experience.

I have several tokens (smart cards with crypto capabilities based on an MCU from ST Micro., ST19WLxx to be more precise) where I have stored certificates, mostly for authentication and digital signature purposes. But they can also be used for decryption so I decided to give that a try. The original idea was to generate a symmetric key for disk encryption purposes. Then I would use the public key on the card to encrypt it and the private key to decrypt it to unlock access to a partition on my disk.

This is all standard practice and should be pretty straight forward but for completeness, I will guide you through the steps (on Linux; although I also tried with Windows with the same end result):

1) I generated a random symmetric key, 245 bytes long to account for the fact that I will be using RSA-PKCS padding, the only one supported by the card and considering the RSA keys are 2048 bit long:

$ dd if=/dev/urandom of=./symmetric_key bs=1 count=245

2) I extract the public key from the card, once I got its ID:

$ pkcs11-tool -r --login --id $KEY_ID --type pubkey --module -o public_key.der

3) I convert the public key format to pem:

$ openssl rsa -pubin -in pub_key.der -inform DER -outform PEM -out pub_key.pem

4) I encrypt the symmetric key from step one using my public key:

$ openssl rsautl -inkey ./pub_key.pem -pubin -encrypt -pkcs -in ./symmetryc_key -out ./encrypted_key.pkcs1

5) And finally, I decrypt with the private key on my card:

$ pkcs11-tool --id $KEY_ID --decrypt --login -m RSA-PKCS --module --input-file encrypted_key.pkcs1 --output-file decrypted_key

With that, I should recover the original symmetric_key again on decrypted_key. Unfortunately, this is not what happens. Instead, my decrypted_key is only 102 bytes long.

If I examine the key I can see I’m getting only the last 102 bytes of the original key, the remaining 143 are lost.

This is an example of symmetric_key (sample output from step 1):

00000000  77 1a e4 f3 71 c1 23 c8  0a 47 17 87 d3 c6 ad 31  |w...q.#..G.....1|
00000010  2b 43 94 f9 1f 41 a0 c7  4f 80 5c 00 51 bb 6b b6  |+C...A..O..Q.k.|
00000020  a4 4c 87 5b 5c 5c 28 ef  d3 b7 d1 85 a2 3a c0 87  |.L.(\(......:..|
00000030  f1 25 38 b7 b9 28 d7 5f  e4 a1 da 4d 0a 71 f2 85  |.%8..(._...M.q..|
00000040  89 0e bb a4 2b 58 3e 18  90 c6 be 75 22 78 27 d7  |....+X>....u"x'.|
00000050  36 4a 95 74 aa fe e3 c1  d1 f6 02 a0 26 18 28 e2  |6J.t........&.(.|
00000060  14 9c 46 58 ea d1 b6 b6  1f d6 86 f6 9f f7 29 c7  |..FX..........).|
00000070  0e bd 50 8e dd ce 34 65  3f 7a 32 e3 3a 28 4c 3a  |..P...4e?z2.:(L:|
00000080  8d 47 36 9c ab af d0 db  bf d0 db f1 ca 32 be 97  |.G6..........2..|
00000090  62 4e c4 6a 79 b3 1a 3a  2b 2c 11 69 84 9b d5 65  |bN.jy..:+,.i...e|
000000a0  d6 75 b5 00 05 42 c5 8f  cd 82 6a 09 9a 50 07 2b  |.u...B....j..P.+|
000000b0  04 86 0d 15 92 e3 8b cf  fb 97 1c 9e f7 6f 22 51  |.............o"Q|
000000c0  e1 45 00 64 45 3d 4b 38  a6 7f f0 aa 7e 12 bb 26  |.E.dE=K8....~..&|
000000d0  85 91 a4 5c 9e dd 59 6a  f6 85 c2 2b 38 4d 2b c2  |.....Yj...+8M+.|
000000e0  f1 2f 71 d0 21 46 1b d2  fd 57 03 66 2f b1 c1 0f  |./q.!F...W.f/...|
000000f0  51 53 9d 22 4e                                    |QS."N|

And the corresponding output from decrypting on step 5:

00000000  97 62 4e c4 6a 79 b3 1a  3a 2b 2c 11 69 84 9b d5  |.bN.jy..:+,.i...|
00000010  65 d6 75 b5 00 05 42 c5  8f cd 82 6a 09 9a 50 07  |e.u...B....j..P.|
00000020  2b 04 86 0d 15 92 e3 8b  cf fb 97 1c 9e f7 6f 22  |+.............o"|
00000030  51 e1 45 00 64 45 3d 4b  38 a6 7f f0 aa 7e 12 bb  |Q.E.dE=K8....~..|
00000040  26 85 91 a4 5c 9e dd 59  6a f6 85 c2 2b 38 4d 2b  |&.....Yj...+8M+|
00000050  c2 f1 2f 71 d0 21 46 1b  d2 fd 57 03 66 2f b1 c1  |../q.!F...W.f/..|
00000060  0f 51 53 9d 22 4e                                 |.QS."N|

First thing I thought was: “huh? software/driver issue. But I have access to the driver code and after staring at it and messing with it for quite a long while I am almost completely sure there is nothing wrong with it.

The major clue that makes me think this is a firmware issue (I don’t have access to the code inside the card’s MCU) comes from a very careful examination of the APDU frames that the card exchanges with the host: there are no errors anywhere, I always get the magic 0x9000 everything is fine message from the card and the frame where I receive the decrypted data is short (it’s actually 20 or so bytes longer than 102, but there are headers and a secure channel involved so part of the message is encrypted) and comes announced with the correct number of bytes (SW=0x6179).

I did many more things, like: testing on Windows, trying keys and text messages with different lengths (the decryption works fine up to messages of 102 bytes, longer than that and they get truncated), using different cards with the same hardware and firmware version, using different cards with different hardware and firmware versions (not that dissimilar after all because I got the same problem), getting all debug info from the driver to see if I was getting any hidden errors…

Considering RSA-OAEP is not supported by this card (or at least not documented) and the problems associated with RSA-PKCS I guess it’s best to let this old dog sleep and not try to teach it new tricks.

But as I said: I’m curious, have you ever encounter something like this? Is there something else I can do to be sure this is a firmware issue? I guess in part I refuse to believe something so fundamental has been lurking undetected for so long (this hardware has been in use for many years by a significant amount of people). Maybe there is something wrong with my setup or understanding of the problem after all.

debug – Why Do Pages Always Work Aa Expected When Logged Into The Dashboard?

I am finding that all the changes I make and preview when logged into the dashboard are just as I would like them. Once I log out of the dashboard all hell breaks loose. Pages don’t display my latest changes, new passwords aren’t accepted etc. But all works while in the dashboard!

What have I tried?

  • I don’t have any caching plugins
  • I DO have a cache menu item in the dashboard which I have clicked ‘Purge All’
  • I have entered CTRL-F5 to reload the page from the server as well as its resources
  • Clearing the cache from my hosting service admin page.

I am using Chrome version 83.

My site is NOT using CloudFlare…

numerical integration – Nintegrate providing results much smaller than what is expected

This is the first time I am posting something here. So, apologies if I make any mistake. I am dealing with a numerical integration in Mathematica-11.0 that has a Bessel function in it. In order to solve it I am using the Module function in Mathematica:

yldLst(((Alpha)_)?NumericQ, ((Beta)_)?NumericQ, (T1_)?NumericQ, (TRH_)?NumericQ, 
   (mx_)?NumericQ, (m(Phi)_)?NumericQ) := Module({yldX}, 
   yldX = NIntegrate((1/(512*Pi^6))*((amp1(s, (Alpha), (Beta), mx, m(Phi))*(1/(4*s))*
         (s - m(Phi)^2)*(s - mx^2)*(1/Sqrt(s))*BesselK(1, Sqrt(s)/T))/
      {s, If(m(Phi)^2 > mx^2, m(Phi)^2, mx^2, 0), Infinity}, {T, T1, TRH}, 
      PrecisionGoal -> 10); {T1, yldX}) 

Here amp1(s, (Alpha), (Beta), mx, m(Phi)) is the a polynomial of the following form:

amp(s_, (Alpha)_, (Beta)_, mx_, m(Phi)_) := (-(1/128))*

(((Alpha)^2*(3*mx^4*m(Phi)^4 - 14*mx^4*m(Phi)^2*s + 2*mx^2*m(Phi)^4*s + 19*mx^4*s^2 - 
       12*mx^2*m(Phi)^2*s^2 + 3*m(Phi)^4*s^2 - 6*mx^2*s^3 + 10*m(Phi)^2*s^3 - 5*s^4 + 
       4*(mx^2 - s)*(m(Phi)^2 - s)*((-mx^2)*(m(Phi)^2 - 3*s) - s*(m(Phi)^2 + 3*s))*
        Cos((Theta)) + (mx^2 - s)^2*(m(Phi)^2 - s)^2*Cos(2*(Theta))) + 
     (3*mx^4*m(Phi)^4 - 30*mx^4*m(Phi)^2*s + 2*mx^2*m(Phi)^4*s + 35*mx^4*s^2 + 
       4*mx^2*m(Phi)^2*s^2 + 3*m(Phi)^4*s^2 - 22*mx^2*s^3 + 10*m(Phi)^2*s^3 - 5*s^4 + 
       4*(mx^2 - s)*(m(Phi)^2 - s)*((-mx^2)*(m(Phi)^2 - 7*s) - s*(m(Phi)^2 + 3*s))*
        Cos((Theta)) + (mx^2 - s)^2*(m(Phi)^2 - s)^2*Cos(2*(Theta)))*(Alpha)^2*(Beta)^2)/
    (s*((mx^2 - s)*(m(Phi)^2 - s) - (mx^2 - s)*(m(Phi)^2 - s)*Cos((Theta)))))

amp1(s_, (Alpha)_, (Beta)_, mx_, m(Phi)_) := 

If(s > mx^2 && s > m(Phi)^2, 
   Integrate(amp(s, (Alpha), (Beta), mx, m(Phi))*Sin((Theta))*2*Pi, {(Theta), 10^(-2), Pi}), 0)

For mx=m(Phi)=0 one can perform this integral analytically using Integrate, as the lower limit on s becomes zero, also the expression for amp1(s, (Alpha), (Beta), mx, m(Phi)) becomes simpler. But if I use mX=m(Phi)=0 and use NIntegrate, the result is at least an order different from what I get by using Integrate. Also NIntegrate is giving the slow convergence warning for mX=m(Phi)=0. So I am not sure if I can rely on NIntegrate results for non-zero values of mx,m(Phi) as the result I am getting for non-zero mX,m(Phi) are way smaller than what I expect. How can I ensure the consistency of the code?

Thank you so much for the help.

thailand – Are taxi drivers in Bangkok expected to round up the taxi fare to the nearest multiple of 10 THB?

For example, if the taxi fare is 53 THB and I give 60 THB (for lack of coins), the taxi drivers typically will not give any change back. I wonder whether this is because I don’t look Thai or because it is the tradition here for taxi fares not to give change back if below 10 THB.