{{comic
| number = 2828
| date = September 13, 2023
| title = Exoplanet Observation
| image = exoplanet_observation_2x.png
| imagesize = 295x272px
| noexpand = true
| titletext = NASA prefers to say that their rovers are 'looking for signs of past life on Mars' and not 'ghost hunting.'
}}

==Explanation==
{{incomplete|Created by an ALIEN GHOST EXORCIST - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

Spectral analysis as used in the study of exoplanets and other distant objects in space refers to using the intensities of different frequencies of light (and other electromagnetic frequencies) from stars which they occlude to infer data about their chemical composition. This is used to detect certain gases in the atmosphere, such as oxygen, which might suggest life. On the day before this comic was published, NASA announced that spectral analysis of the exoplanet K2-18 b showed abundance of methane and carbon dioxide, and shortage of ammonia, supporting the hypothesis that it may have a water ocean; and there was possible detection of dimethyl sulfide, which on Earth is only produced by life.[https://www.nasa.gov/goddard/2023/webb-discovers-methane-carbon-dioxide-in-atmosphere-of-k2-18b]

"Spectral" can also be used to refer to ghosts or "spectres". Here it is suggested that the planet may have ghosts in its atmosphere due to the light that they absorb, similar to how gases may absorb certain wavelengths. Like many atmospheric gases, ghosts are typically invisible{{Citation needed}}, however they may have their own absorption spectrum distinguishing them from normal gases. The presence of ghosts in an exoplanet's atmosphere can also be considered evidence of (past) alien life.

The title text refers to the search for past life on Mars. It draws similarities between looking for real signs such as specific molecules or fossils (which NASA is actually doing), and looking for fictional ghosts of dead creatures (which would also be an indication of past life). In a very loose sense, real signs such as gases or fossils could be considered "ghosts" of past life.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:: [Cueball is holding a stick and pointing at a slide with two ghosts and a planet/star]
:: Cueball: When the planet passed in front of its host star, some of the light was absorbed by ghosts, indicating that the planet is likely haunted.

:: [Caption below the panel]
:: Exoplanet Spectral Analysis

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Astronomy]]
[[Category:Aliens]]

Talk:2828: Exoplanet Observation

2023-09-13T15:43:49Z

Hddqsb:

2022-11-21T15:36:07Z

Hddqsb: /* Explanation */ remove footnote saying UTF-8 encoding of null character is zero byte (probably too technical for most readers, easier to just gloss over it), convert other footnote to parenthetical, reword

{{comic
| number = 2700
| date = November 18, 2022
| title = Account Problems
| image = account_problems_2x.png
| imagesize = 538x272px
| noexpand = true
| titletext = My password is just every Unicode codepoint concatenated into a single UTF-8 string.
}}

==Explanation==
{{incomplete|Created by a VISIBLE ZERO WIDTH SPACE. Please change this comment when editing this page. Do NOT delete this tag too soon.}}

[[Cueball]] asks [[Ponytail]] to help him because he can't log in to his account. Having attempted to fix [[:Category:Cueball Computer Problems|Cueball's tech issues]] in the past, Ponytail replies with dread. Cueball promises that "It's a normal problem this time", and Ponytail agrees to look at it. But then Cueball reveals that he has included a {{w|Null character|null string terminator character}} in his password when creating an account and now he can't log in. Ponytail responds in disbelief, and Cueball defends his actions by saying that the instructions said to use special characters.

In computer systems, every {{w|Character (computing)|"character"}} (letter, digit, punctuation, etc.) is represented as an integer. For example the lowercase letter 'a' is represented as the number 97, and the digit '1' is represented as the number 49 (when using the {{w|ASCII}} character encoding or {{w|Unicode}} character encoding). A {{w|String (computer science)|"string"}} refers to a sequence of characters, and can be used to store arbitrary text (for example names, messages, passwords). Strings can be arbitrarily long, so some mechanism must be used to record their length. One approach is to store the length explicitly; this representation is often called a {{w|String_(computer_science)#Length-prefixed|Pascal string}} (after the programming language {{w|Pascal}}, which uses this representation). Another approach is to mark the end of the string using a specific character, usually the {{w|null character}} (which is represented as the number 0); such strings are called {{w|null-terminated string}}s, and are used by the {{w|C (programming language)|C programming language}}. Both approaches have advantages and disadvantages. A limitation of null-terminated strings is that they cannot be used to represent text containing embedded null characters. This is usually not a problem, because normal text never contains null characters. However, if somehow a null character were to end up in the middle of the string, it would cause problems: any code that uses that string would assume this null character marks the end of the string, so the string would effectively be truncated.

Account registration systems often place requirements on passwords in an attempt to encourage users to pick stronger passwords. For example, they might ask that the password include at least one "special character" (such as <code>!@#$%^&*</code>). Cueball misunderstood this requirement as referring to characters such as the null character (which is more accurately referred to as a {{w|ASCII#Control_characters|control character}}). Cueball managed to type the null character as part of his password somehow (on some systems it is possible to type the null character using {{w|Null_character#Representation|certain keyboard shortcuts}} such as <code>Ctrl</code>+<code>Space</code>, <code>Ctrl</code>+<code>@</code>, <code>Ctrl</code>+<code>2</code>, or <code>Alt+0</code> {{w|Alt_code|using the number pad}}), but the software running the registration system was poorly written and could not cope with this – it allowed him to create an account with that password, but then when he tried to log in with the same password the system didn't accept it.

It's unclear how that particular situation might arise in real software, but here is a similar situation that can easily happen in practice: Suppose a website's registration form allows the user's new password to have up to 20 characters, but due to a programmer error the login page only accepts passwords with up to 18 characters. If the user picks a medium-length password (say with 12 characters), all is well. But if the user picks a password with 20 characters, they will be able to register but they won't be able to log in (which is what happened to Cueball).

The title text describes a different situation, where a person's password is "just" every Unicode character concatenated into a single string. {{w|Unicode}} is a standard for representing characters from many writing systems, and it has 149,186 characters[https://en.wikipedia.org/wiki/Unicode#Versions] as at the time of this comic (with new characters being added over time). A password consisting of all of those characters would be extremely long; it would be impractical to type by hand, and would be too long for pretty much all account registration systems. (A "codepoint" is the number assigned to a character, and {{w|UTF-8}} is a common encoding system for representing a Unicode codepoint as a sequence of {{w|byte}}s.) Also, since Unicode includes the null character, the password would have the same issue as Cueball's password. Further, if the account registration system treats the null character as a string terminator (as in C), then the password would be equivalent to an empty password (assuming it contains the Unicode codepoints in order, starting with the null character).

==Trivia==

* User input containing unsafe characters has previously appeared in the famous comic [[327: Exploits of a Mom]].

* There are several techniques that can be used to safely handle passwords and other user inputs that might contain unsafe characters such as the null character:
** Validate: Check whether the user input contains unsafe characters, and if it does display an error message to the user.
** Sanitize: Remove unsafe characters from the user input to prevent them from causing problems.
** Encode/quote/escape: Replace each unsafe character with an appropriate sequence of characters (depending on the context). For example, a null character can be included in a {{w|URL}} by encoding it as <code>%00</code>. This technique is not very relevant to password handling, but is relevant for example when {{w|Cross-site_scripting#Non-persistent_(reflected)|including user input in generated web pages}} or [https://www.cloudflare.com/learning/security/threats/sql-injection/ passing user inputs to database queries].
** For the specific case of null characters: Use a string representation that supports null characters (e.g. Pascal strings), and be very careful not to pass such strings to functions that can't handle embedded null characters.

* Failure to handle strings containing null characters correctly can result in security vulnerabilities. For example, including a null character in crafted input may allow a user to read or write files that they are not supposed to be able to access.[https://insecure.org/news/P55-07.txt][https://elixirforum.com/t/static-and-session-security-fixes-for-plug/3913]

* Here are some additional situations where passwords with special characters might stop working:
** The registration form allows passwords to contain null characters, but the login form strips null characters (for example because it was written by a different developer/team, or because it has been updated over time). When Cueball tries to log in, the login form strips the null characters, so the resulting password doesn't match the stored password (which contains a null character).
** The password system allows the input of Unicode characters at first, but is later changed to only accept ASCII passwords. Users who included non-ASCII characters like é or ö in their password are locked out of their account because they are no longer allowed to type those characters.

* The number of the xkcd comic is 2700. When interpreting this as two concatenated octal numbers \27 + \00 it represents both the {{w|End-of-Transmission-Block_character|ETB}} as well as the null character, both of these characters possibly leading to problems when processed in legacy systems (e.g. mainframe computers). When interpreting 2700 as hexadecimal 0x27 + 0x00 numbers it represents the ' character and the null character - a sequence that could lead to [[327: Exploits of a Mom|SQL injection]] when it is placed in unescaped form inside of a SQL command.

==Transcript==
:[Cueball carries an open laptop over to Ponytail, holding in in both hands. The screen shows a box filling the screen with some text on lines. Ponytail is sitting in an office chair with her laptop at her desk. She has turned her head away from the computer looking at Cueball's screen.]
:Cueball: Can you help me with my account?
:Ponytail: Oh no.

:[Cueball holds his laptop up in front of Ponytail who has turned the chair so she faces him, with her hands in her lap. Her table is not drawn.]
:Cueball: No no, I promise it's a normal problem this time.
:Ponytail: Okay. Fine. What is it?

:[Cueball holds both hands out palm up towards Ponytail who is sitting with his laptop in her lap typing on it.]
:Cueball: I included a null string terminator as part of my password, and now I can't-
:Ponytail: '''''How?!'''''
:Cueball: They said to use special characters!

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Cueball Computer Problems]]