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Securing Applications and Data

AP Cybersecurity · Topic 5

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5.1

Application and Data Vulnerabilities and Attacks
Syllabus
Learning ObjectiveEssential Knowledge

5.1.A
Explain how adversaries can exploit application and file vulnerabilities to cause loss, damage, disruption, or destruction.

  • 5.1.A.1 An adversary can read any unencrypted files if they have access to the device or drive storing the files.
  • 5.1.A.2 Computers have standard users and administrative users. Administrative users have access to control system settings and can typically access any files or applications on a system. If regular users are given administrative privileges on a computer, and an adversary can compromise a user’s account, then the adversary will have elevated privileges on the system.
  • 5.1.A.3 When access control settings are weakly configured, many users often have permission to view and sometimes even edit files on a system. Adversaries can take advantage of weak access control settings to steal or destroy files or disrupt an application.

5.1.B
Explain how application attacks exploit vulnerabilities.

  • 5.1.B.1 Applications are programs that run instructions on computers; they are executable data. Some applications run locally on a user’s computer, while other applications, like web applications, run on a server and are accessed by users through a network.
  • 5.1.B.2 Many applications take user input through open-ended input fields where users can type characters (e.g., letters, numbers, punctuation). Developers should include user input checks in their application, such as numeric input when asked for a number of items, to ensure that the user input matches what is expected; the application should reject input outside of the expected parameters. This process of verifying that user input meets expected criteria before processing it is called data validation. Applications that fail to validate user input are vulnerable to injection-type attacks, where adversaries insert unexpected character strings in input fields to alter the behavior of a program.
  • 5.1.B.3 Structured query language (SQL) is a computer language used to request information from databases and make changes to databases or entries in databases. Applications that query a database using unvalidated or unsanitized input from users are vulnerable.
  • 5.1.B.4 An SQL-injection attack places SQL commands and control characters into a user-input field in an application, which can lead to a breach of confidentiality by causing the application to return more information than it should, or a breach of integrity by modifying or deleting data in the database.
  • 5.1.B.5 Websites are written using hypertext markup language (HTML), and many websites use Javascript to create dynamic content on websites or web applications. Because Javascript commands run in the browser of the user visiting the website, those commands can access sensitive data stored in the browser like usernames, passwords, and cryptographic keys.
  • 5.1.B.6 A cross site scripting (XSS) attack injects malicious code into a website that a user’s browser then executes. The malicious code can be embedded in a link the user clicks (a Type I or Reflected XSS attack) or it can be inserted onto a website through a comment field, forum post, or visitor log, which would affect any user visiting that website (a Type II or Stored XSS attack).
  • 5.1.B.7 When applications take user input, that input is written to a buffer. A buffer is a designated section of computer memory with a fixed size. If the amount of data the user enters exceeds the size of the buffer, it can overflow into adjacent memory locations and overwrite other parts of the computer’s memory.
  • 5.1.B.8 A buffer overflow attack feeds more data into memory than was allotted, which can cause a system to crash or to execute code outside the scope of a program’s security policy, effectively allowing the adversary to perform unauthorized actions on a computer, such as accessing, modifying, or deleting files.
  • 5.1.B.9 The files that run web applications are stored in directories on servers. When users access web applications, their browsers send GET requests using hypertext transfer protocol (HTTP). A GET request accesses a file somewhere in the filesystem of the server.
  • 5.1.B.10 In a directory traversal attack, adversaries modify URLs and GET requests to attempt to access sensitive data (e.g., usernames and passwords) on a server’s file system.
    • Illustrative examples for 5.1.B.10:
      • A web server stores images for a website it hosts in the /var/www/images/ directory. An adversary modifies a URL requesting an image to ../../../etc/passwd. The .. moves one directory up in the file system; so the three consecutive .. returns the path to the root, and from there the adversary is attempting to access the passwd file that would return a list of all the authorized usernames on the device.

5.1.C
Assess and document risks from application and data vulnerabilities.

  • 5.1.C.1 Data security risks can involve a compromise of confidentiality when unauthorized persons can access sensitive data, integrity when data can be manipulated or altered from its intended state, and availability when data can be destroyed or encrypted to prevent others from accessing it.
  • 5.1.C.2 High risks from data vulnerabilities often involve highly sensitive data (e.g., data that is governed by laws or regulations) that could be compromised through a highly likely exploit.
    • Illustrative examples for 5.1.C.2:
      • The company developing the next jet engine that will be used by the Air Force in its planes is storing the technical specifications for the engine on an unencrypted drive.
  • 5.1.C.3 Moderate risks from data vulnerabilities often involve sensitive data not having strong enough encryption or strict enough access controls.
    • Illustrative examples for 5.1.C.3:
      • A company stores its customers’ PII in a spreadsheet, and the spreadsheet is encrypted using a small key.
  • 5.1.C.4 Low risks from data vulnerabilities often involve less sensitive information being encrypted with shorter keys or having access controls that are not strict enough.
    • Illustrative examples for 5.1.C.4:
      • An organization’s CEO stores his private memos to his executive staff on a company share drive that is unencrypted and has no access controls.

Source: College Board AP Course and Exam Description

SQL injection

Applications 应用程序 are the programs that run on computers, and data is what they process - both are prime targets. If files are stored unencrypted, anyone with access to the drive can read them. If a normal user is given administrative 管理性 privileges, an adversary who steals that account gains sweeping power.

The biggest application danger is bad user input. When a program does not check what a user types, an adversary can slip in commands - an injection attack 注入攻击. Data validation 数据验证 (checking input meets expected rules) is the defense. Key attacks:

  • SQL injection SQL注入 - inserting SQL commands into an input field to read or change a database.
  • Cross-site scripting (XSS) 跨站脚本 - injecting malicious script into a website that runs in another user's browser.

What a SQL injection actually looks like

SQL is a language for querying a database, and its control words are conventionally written in capital letters (SQL itself ignores their case, so select works too) — SELECT, FROM, WHERE, IN, OR, AND. A login form usually builds a query by pasting what you typed into one:

SELECT * FROM users WHERE name = 'alice' AND password = 'secret'

An attacker types SQL into the field instead of a name. Two tricks do most of the damage:

  • A condition that is always true. In the password field of the unsafe query shown, entering ' OR '1'='1 changes its logic to (name = 'alice' AND password = '') OR '1'='1', which is true for every row. The same text in the name field does not automatically bypass the remaining password check.
  • A double dash, which begins a comment in SQL. Entering admin' -- ends the name string and comments out the whole rest of the line, including the password check, so the query becomes … WHERE name = 'admin' and the attacker is logged in as the administrator without a password.

The defence is not to filter for the word SELECT. It is to stop the input being treated as code at all: use parameterised queries 参数化查询 (also called prepared statements), where the database is given the query and the values separately and never mixes them, and add input validation to reject characters the field has no reason to contain.

  • Buffer overflow 缓冲区溢出 - sending more data than a memory buffer 缓冲区 can hold, so it overflows into nearby memory and may run the adversary's code.
  • Directory traversal 目录遍历 - using ../ sequences in a URL to reach files outside the intended folder, such as /etc/passwd.

Rate data risk using both likelihood and impact. Sensitivity, exposure, access controls, encryption and credible threats matter; low-value data or a short key alone does not determine a fixed risk rating.

SQL injection: input pasted into a query changes the query itself
SQL injection: input pasted into a query changes the query itself
Vocabulary Train
English
Applications/ˌæplɪˈkeɪʃnz/
administrative/ədˈmɪnɪstrətɪv/
injection attack/ɪnˈdʒekʃn əˈtæk/
Data validation/ˈdeɪtə ˌvælɪˈdeɪʃn/
Cross-site scripting (XSS)/krɒs saɪt ˈskrɪptɪŋ/
parameterised queries/ˌpærəˈmetəraɪzd ˈkwɪərɪz/
Buffer overflow/ˈbʌfə ˌəʊvəˈfləʊ/
buffer/ˈbʌfə/
Directory traversal/daɪˈrektəri træˈvɜːsl/
SQL injection/ˌes kjuː ˈel ɪnˈdʒekʃn/
5.2

Protecting Applications and Data: Managerial Controls and Access Controls
Syllabus
Learning ObjectiveEssential Knowledge

5.2.A
Explain how the state or classification of data impacts the type and degree of security applied to that data.

  • 5.2.A.1 Organizations implement specific security controls to comply with legal requirements based on the types of data they collect, store, process, and transmit.
  • 5.2.A.2 Data can be classified by their state.
    • Data at rest are stored on a drive. It is important to protect the physical drive storing the data from destruction or theft. Data at rest can also be encrypted so that if an adversary steals it, they can’t immediately read the data.
    • Data in transit are being sent from one device to another. If the data are being transferred over physical media (e.g., cables) it is important to protect the media. Data in transit can also be encrypted so that if an adversary intercepts it, they can’t immediately read the data.
    • Data in use are being processed by software or a person. Access controls can be used to limit who or what has the ability to use data in different ways (e.g., view or edit). Data must be unencrypted to be used.
  • 5.2.A.3 Organizations often categorize data according to their sensitivity and prioritize a higher degree of security for more sensitive information.
  • 5.2.A.4 Laws and regulations can require certain types of data to be stored, transmitted, and handled according to specific rules.
    • Personally identifiable information (PII) is any data that allows someone to be identified and includes (but is not limited to): name, signature, phone number, address, biometric data (e.g., fingerprints), social security number, date of birth, and email address. The protection of this data is covered by many laws but most notably The Privacy Act of 1974 and for children under the age of 13 the Children’s Online Privacy Protection Act of 1998.
    • Protected health information (PHI) is any data related to an individual’s health, treatment, payment for healthcare at any time and includes (but is not limited to): test results, treatment records, hospital records, doctor visit notes, and health provider payment records. The protection of PHI is included in the Health Insurance Portability and Accountability Act of 1996.
    • Payment card information (PCI) is the data collected by organizations to process payments via cards (e.g., credit cards) and includes the following: name, account number, expiration date, address, and CVV code. The protection of this data is regulated by the Payment Card Industry Data Security Standard (PCI-DSS).
  • 5.2.A.5 Organizations that collect regulated data will label them and have policies that comply with the legal or regulatory requirements for the safe storage, transmission, and handling of these data.

5.2.B
Identify managerial controls related to application and data security.

  • 5.2.B.1 A cryptography policy will describe the acceptable encryption protocols and key parameters for an organization and may include:
    • A list of encryption algorithms approved for specific uses
    • Minimum or maximum key lengths
    • Cryptographic key-generation requirements and parameters
    • Cryptographic key-storage requirements
  • 5.2.B.2 A web application security policy will outline the requirements and parameters for testing and mitigating web application vulnerabilities in an organization, and it may include:
    • Parameters for when an application is subject to a security assessment
    • Timelines for remediating vulnerabilities based on level of risk
    • Parameters for how an application security assessment is to be carried out (e.g., using specific tools or according to specific frameworks)

5.2.C
Determine an appropriate access control model to protect applications and data.

  • 5.2.C.1 Access control enforces which users or applications (called subjects) can access, modify, add, or remove (called operations) which files or applications (called objects). Access control models describe how to determine which subjects have what type of access to which objects.
  • 5.2.C.2 Role-based access control (RBAC) assigns every subject to a role and defines which roles have which types of access to which objects.
    • Illustrative examples for 5.2.C.2:
      • An example of a role at a company might be “accountant,” and one type of object could be the payroll software. Role-based access could be used to ensure that only subjects who are assigned to the role of “accountant” have access to the payroll software object.
  • 5.2.C.3 Rule-based access control (RuBAC) checks a set of rules to determine what type of access a subject should have for a specific object and then allows or denies types of access based on the rules. This access control model is typically layered on top of another access control model.
    • Illustrative examples for 5.2.C.3:
      • There is a rule that prohibits subjects (even those who would normally have access) from accessing a certain database (the object) outside of local working hours. When a subject attempts to access the database, even if they are authorized to access it, they will be denied access if it is outside the time designated by the rule.
  • 5.2.C.4 Discretionary access control (DAC) gives individual subjects the ability to set the type of access that other subjects have on objects they own. In DAC models some subjects are designated as administrators or super users, and they have the ability to override the access controls established by other subjects.
    • Illustrative examples for 5.2.C.4:
      • Bob creates a file (an object) and decides to give Alice permission to edit the file, to give Frank permission to view the file only, and to deny everyone else access to the file altogether.
  • 5.2.C.5 Mandatory access control (MAC) follows strict rules for which types of access each subject level has for objects that are above their level, at their level, or below their level. Subject and object levels are assigned by an external administrator.
  • 5.2.C.6 The Bell-LaPadula model is a MAC model that is often used by governments and military organizations to control the security of information. This model has the following two important properties:
    • i. The Simple Security Property states that subjects may not read objects that are above their level.
    • ii. The * (Star) Security Property states that subjects may not write to objects below their level.
    • These rules taken together are often summarized as “write up, read down” (WURD).
  • 5.2.C.7 The principle of least privilege is the idea that entities should be given exactly as much access as they need to perform their function and no more.

5.2.D
Configure access control settings on a Linux-based system.

  • 5.2.D.1 Authorization is when an entity is granted permission to have a certain type of access to a resource. Access controls are put in place to control which users have what types of access to which data.
  • 5.2.D.2 There are three types of access to a file in Linux that can be set, and they always come in the following order:
    • i. Read access allows a user to view the contents of a file.
    • ii. Write access allows a user to make changes to a file.
    • iii. Execute access allows a user to run a binary file such as a program.
    • These are abbreviated rwx, respectively. If a user only has read and execute permissions (not write), then it would display as r-x. The - symbol indicates the absence of that permission.
  • 5.2.D.3 There are three default entities for which permissions are set and always in this order: (1) the file owner, (2) the file group, and (3) all other users. The three sets are displayed with no spaces (e.g., rwxrwxrwx).
  • 5.2.D.4 To view the current permission settings for a file, use the command ls -l, which will show the current settings for the default entities. If there is a + symbol at the end of the permissions, this means that other permissions have been set for that file and it can be viewed with the getfacl command.
  • 5.2.D.5 To modify the permission settings for a file, use the chmod command. This command can be used with the numeric method or the symbolic method.
  • 5.2.D.6 To use chmod in the numeric method the syntax is chmod ### filename. Each of the three ### represents one of the three entities mentioned above (the owner, the group, other nongroup users).
    • The first # = the owner
    • The second # = the group
    • The third # = other nongroup users
    • The permission for each entity is determined by adding up the values for the types of access to be granted:
    • 0 = no permissions
    • 1 = execute
    • 2 = write
    • 4 = read
    • Therefore 3 sets permission to write and execute, 5 sets permission to read and execute, 6 sets permission to read and write, and 7 sets permission to read, write, and execute.
    • Illustrative examples for 5.2.D.6:
      • The command chmod 750 test would set the permissions for the owner to read, write, and execute, for the group to read and execute, and for everyone else to no access at all.
      • The command chmod 543 test would set the permissions for the owner to read and execute, for the group to read only, and for everyone else to write and execute.
      • The command chmod 777 test would set the permissions for all three entities to read, write, and execute for the file test.
  • 5.2.D.7 To use chmod in the symbolic method the syntax is chmod entity +(or –) permission filename. The entities are the user owner, the group, and other nongroup users. Each entity is represented with a single letter.
    • u = user owner
    • g = group
    • o = others
    • a = all
    • Permission can be either added or removed to any combination of entities.
      • = add the permission
    • – = remove the permission
    • The permissions that can be set are read, write, and execute.
    • r = read
    • w = write
    • x = execute
    • Entities and permissions can be combined in a single command. To add the read and execute permissions for the group and user owner for a file called testfile, the command would be chmod ug+rx testfile.

Source: College Board AP Course and Exam Description

Data is classified by its state - at rest 静态数据 (stored on a drive), in transit 传输中数据 (moving between devices), and in use 使用中数据 (being processed). Data at rest and in transit can be encrypted so a thief cannot read it; conventional processing normally decrypts data in use, so access controls and protection of memory and execution are also needed.

Some data types are regulated 受监管 - the law dictates how they must be stored, transmitted and handled - so an organisation must achieve compliance 合规 by matching its controls to the rules. The course pairs these data types with example U.S. legal frameworks or industry standards. Their scope matters; these are not universal one-to-one rules:

Data type What it is Example framework and scope
personally identifiable information (PII) 个人身份信息 information identifying or linked to a person Privacy Act: U.S. federal-agency records; COPPA: covered online collection from children under 13
protected health information (PHI) 受保护健康信息 identifiable health information in covered contexts HIPAA: covered entities and business associates, with defined scope and exclusions
payment card information (PCI) 支付卡信息 payment-card account and authentication data PCI DSS: an industry security standard for relevant card-data environments

An organisation that collects regulated data must label it and hold policies that keep its storage, transmission and handling compliant - the higher the sensitivity, the higher the required degree of security.

Access control decides which subjects (users) may perform which operations on which objects (files). Four models:

  • Role-based (RBAC) 基于角色的访问控制 - access follows your role (all "accountants" reach the payroll software).
  • Rule-based (RuBAC) 基于规则的访问控制 - access follows conditions (only during business hours), layered on another model.
  • Discretionary (DAC) 自主访问控制 - the owner of a file decides who else may use it.
  • Mandatory (MAC) 强制访问控制 - a central administrator sets strict levels; the Bell-LaPadula model summarises it as "write up, read down".
Four access-control models decide who reaches which object, and how
Four access-control models decide who reaches which object, and how

A guiding idea across all models is the principle of least privilege 最小权限原则 - give each entity exactly the access it needs and no more.

On a Linux system, each file has three permissions - read (r), write (w), execute (x) - for three groups: the owner, the group, and others. The chmod command sets them with numbers, adding 4 (read) + 2 (write) + 1 (execute). So chmod 640 means owner read+write (6), group read (4), others nothing (0).

Linux file permissions: read/write/execute for owner, group, and others
Linux file permissions: read/write/execute for owner, group, and others

Worked example. A principal wants only herself to read and edit a file, her staff group to read it, and no one else to touch it. Read+write = 4+2 = 6 for the owner, read = 4 for the group, nothing = 0 for others, giving chmod 640 file. The listing then shows -rw-r-----. To also let the owner run the file as a program you would add execute (7 = 4+2+1), giving chmod 740.

Explore

Which access-control model fits the rule?

Each access-control model has a different decider: RBAC by your role, RuBAC by a condition, DAC by the file's owner, and MAC by a central administrator's levels.

Vocabulary Train
English
at rest/æt rest/
in transit/ɪn ˈtrænsɪt/
in use/ɪn juːs/
regulated/ˈreɡjʊleɪtɪd/
compliance/kəmˈplaɪəns/
personally identifiable information (PII)/ˈpɜːsənəli aɪˈdentɪfaɪəbl ˌɪnfəˈmeɪʃn/
protected health information (PHI)/prəˈtektɪd helθ ˌɪnfəˈmeɪʃn/
payment card information (PCI)/ˈpeɪmənt kɑːd ˌɪnfəˈmeɪʃn/
Role-based (RBAC)/rəʊl beɪst/
Rule-based (RuBAC)/ruːl beɪst/
Discretionary (DAC)/dɪˈskreʃənəri/
Mandatory (MAC)/ˈmændətəri/
principle of least privilege/ˈprɪnsɪpl ɒv liːst ˈprɪvɪlɪdʒ/
5.3

Protecting Stored Data with Cryptography
Syllabus
Learning ObjectiveEssential Knowledge

5.3.A
Explain how encryption can be used to protect files.

  • 5.3.A.1 The purpose of cryptography is to hide information. A cryptographic algorithm defines a process for encrypting and decrypting information. Encryption is the process of hiding the information, and decryption is the process of reversing the encryption to retrieve the original information.
  • 5.3.A.2 An encryption algorithm defines a process for combining the information to be encrypted with a predefined key. The information to be encrypted is called the plaintext. The output of the encryption algorithm is called the ciphertext.
  • 5.3.A.3 The number of possible keys that can be used in an encryption algorithm is called the keyspace. The larger the keyspace, the longer it will take an adversary to discover the correct key by random chance.
  • 5.3.A.4 Cryptographic algorithms are classified by whether they use one key or two keys.
    • Symmetric encryption algorithms use the same key to encrypt and decrypt information.
    • Asymmetric encryption algorithms use two different keys—one to encrypt information and the other to decrypt information.
  • 5.3.A.5 Cryptographic algorithms are also classified by whether they process information one bit at a time or in fixed-size chunks of bits.
    • Block encryption handles information in fixed-size chunks called blocks, producing an output block for each input block.
    • Stream encryption handles input information continuously, producing output one element at a time.

5.3.B
Apply symmetric encryption algorithms to encrypt and decrypt data.

  • 5.3.B.1 Computer-based encryption algorithms operate on binary data. The most common symmetric encryption algorithm is the Advanced Encryption Standard (AES). AES encryption is used to secure Wi-Fi transmissions, internet browsing, file encryption on disks, and hardware-level encryption on processors.
  • 5.3.B.2 AES is a symmetric key block cipher that encrypts data in 128-bit blocks (16 bytes). AES can operate with keys of varying lengths. Longer keys produce more secure encryption but require more time to encrypt and decrypt.
  • 5.3.B.3 Symmetric encryption and decryption can be performed using the command line, specialized software, or web-based tools.
    • On a command line interface, users can encrypt or decrypt with OpenSSL.
    • Specialized software like AES Crypt is an open source tool that can encrypt and decrypt files.
    • There are many web-based tools for encrypting and decrypting files.
  • 5.3.B.4 Using OpenSSL in a CLI, a user can encrypt and decrypt a file using the following commands (note that the encryption key is derived from the password provided):
    • To encrypt a file named test with AES using a 128-bit key, use the command: openssl enc -aes-128-cbc -e -in test -k password -out test.enc
    • To decrypt the encrypted file using the same key, use the command: openssl enc -aes-128-cbc -d -in test.enc -k password -out text

Source: College Board AP Course and Exam Description

An Enigma machine: cryptography protects stored and transmitted data from eavesdroppers
An Enigma machine: cryptography protects stored and transmitted data from eavesdroppers
Symmetric vs asymmetric encryption
Hashing and the avalanche effect

Cryptography 密码学 uses mathematical techniques to protect information and support checks such as authenticity and integrity. An encryption algorithm combines the plaintext 明文 with a key 密钥 to produce ciphertext 密文; decryption reverses it. The keyspace 密钥空间 is the set of possible keys. A fully variable n-bit key has $2^n$ possible values. A larger space can increase exhaustive-search work, but practical security also depends on the algorithm and implementation.

Symmetric encryption 对称加密 uses the same key to encrypt and decrypt. The standard is AES 高级加密标准, a block cipher 分组密码 that works on 128-bit blocks and secures Wi-Fi, browsing, and stored files. Because both sides need the same secret key, sharing that key safely is the challenge.

A World War II Enigma cipher machine with keys and rotors
The Enigma machine scrambled messages with rotors — an early, breakable example of encryption
Explore

Encrypt a message by shifting letters

Encryption combines plaintext with a key to make ciphertext. In this simple cipher the key is the shift amount; only someone who knows the shift can decrypt the message back.

Vocabulary Train
English
Cryptography/krɪpˈtɒɡrəfi/
plaintext/ˈpleɪntekst/
key/kiː/
ciphertext/ˈsaɪfətekst/
keyspace/ˈkiːspeɪs/
Symmetric encryption/sɪˈmetrɪk enˈkrɪpʃn/
AES/ˌeɪ iː ˈes/
block cipher/blɒk ˈsaɪfə/
5.4

Asymmetric Cryptography
Syllabus
Learning ObjectiveEssential Knowledge

5.4.A
Determine the appropriate asymmetric key to use when sending or receiving encrypted data.

  • 5.4.A.1 Asymmetric encryption allows users to communicate securely without prearranging a shared secret key.
  • 5.4.A.2 When using asymmetric encryption, each entity that will be receiving data must first generate a key pair. Key pairs are binary strings of equal length that are generated at the same time through a mathematical process. One key is designated as the public key and the other as the private key. The keys are mathematical inverses of each other— each key reverses its partner. Either key can be used to encrypt information, but only the other key in the key pair will then be able to decrypt it.
  • 5.4.A.3 Once the receiver generates the key pair, the private key must be stored securely. If the private key is exposed, shared, stolen, corrupted, or compromised the key pair must be deleted and a new key pair must be generated, because the security of the encryption algorithm rests on the security of the private key. The public key is published for anyone to view and use.
  • 5.4.A.4 To send information securely to someone, the sender will use the receiver’s public key to encrypt the data and send it. Only the receiver who has the private key will be able to decrypt and read the information.

5.4.B
Explain why the length of a key impacts the security of encrypted data.

  • 5.4.B.1 Longer keys result in larger keyspaces. For binary keys, an n-bit length key has a keyspace of $2^n$.
  • 5.4.B.2 Using an application to randomly guess an n-bit length encryption key means that on average an adversary will be able to guess the correct key in $2^n \div 2$ (or $2^{n-1}$) guesses.
  • 5.4.B.3 Although longer keys are more secure, they also require more time to encrypt and decrypt messages.
  • 5.4.B.4 Computational processing power and efficiency continue to improve, allowing software to guess keys faster. Key-length recommendations for both symmetric and asymmetric encryption algorithms are periodically increased to account for increased processing power.
  • 5.4.B.5 Key-length comparison is only valid when comparing keys for the same cryptographic algorithm.
    • Illustrative examples for 5.4.B.5:
      • An AES 256-bit key is more secure than an AES 128-bit key.
      • An RSA 4096-bit key is more secure than an RSA 2048-bit key.
      • RSA and AES keys cannot be directly compared to one another in determining the level of security.

5.4.C
Apply asymmetric encryption algorithms to encrypt and decrypt data.

  • 5.4.C.1 Common asymmetric encryption algorithms include RSA and elliptic curve cryptography (ECC). Asymmetric algorithms are used in many applications, including digital signatures and digital certificates.
  • 5.4.C.2 As with symmetric encryption, asymmetric encryption and decryption can be performed using the command line, specialized software, or web-based tools.
    • On a command line interface, users can encrypt or decrypt with OpenSSL.
    • Specialized software like RSA Encryption Tool is an open source tool that can encrypt and decrypt files.
    • There are many web-based tools for encrypting and decrypting files.
  • 5.4.C.3 In a CLI, a user can generate an asymmetric key pair and encrypt or decrypt files as necessary.
    • To generate a 2048-bit RSA key pair and save the key to a file named rsa.pem use the command: openssl genrsa -out rsa.pem 2048
    • To extract the public key from rsa.pem into a file named public.pem, use the command: openssl rsa -pubout -in rsa.pem -outform PEM -out public.pem
    • To encrypt the file test using RSA encryption and the key file public.pem, use the command: openssl pkeyutl -encrypt -pubin -inkey public.pem -in test -out test.enc
    • To decrypt the test.enc file using the rsa.pem file, run the command: openssl pkeyutl -decrypt -inkey rsa.pem -in test.enc -out test

Source: College Board AP Course and Exam Description

Asymmetric encryption 非对称加密 solves the key-sharing problem with a key pair 密钥对 - a public key 公钥 anyone may see and a private key 私钥 kept secret. Each scheme specifies how its keys are used. For public-key encryption, I encrypt with your public key and you decrypt with your private key. Digital signatures use a private signing key and public verification key; signing is not generally “encrypting with the private key”. The sender must also establish that the public key really belongs to the intended recipient.

Asymmetric encryption: encrypt with the public key, decrypt with the private key
Asymmetric encryption: encrypt with the public key, decrypt with the private key

Longer keys mean larger keyspaces and more security, but slower encryption. Common asymmetric algorithms are RSA and elliptic curve cryptography (ECC) 椭圆曲线密码学, used in digital signatures and certificates. Remember: you can only compare key lengths within the same algorithm - an RSA 4096-bit key is not directly comparable to an AES 256-bit key.

A padlock: cryptography locks data so only someone with the matching key can open it
A padlock: cryptography locks data so only someone with the matching key can open it
Vocabulary Train
English
Asymmetric encryption/ˌeɪsɪˈmetrɪk enˈkrɪpʃn/
key pair/kiː peə/
public key/ˈpʌblɪk kiː/
private key/ˈpraɪvət kiː/
elliptic curve cryptography (ECC)/ɪˈlɪptɪk kɜːv krɪpˈtɒɡrəfi/
5.5

Protecting Applications
Syllabus
Learning ObjectiveEssential Knowledge

5.5.A
Identify the application security principles of secure by design and security by default.

  • 5.5.A.1 Secure by design is an initiative that encourages companies to include security in all phases of product development including design. When organizations implement secure by design, security is a design principle not just a technical feature.
  • 5.5.A.2 Secure by design includes three design principles:
    • i. Companies should take ownership of customer security outcomes. Companies should build products that meet the security needs of their customers.
    • ii. Companies should embrace radical transparency and accountability. Sharing relevant security-related product news and updates quickly increases security for everyone.
    • iii. Companies should build organizational structure and leadership to implement secure by design. Companies need leaders who are focused on security and have a security-first posture.
  • 5.5.A.3 Secure by design includes the concept of secure by default, which is the idea that security features for software and devices should be enabled by default. Devices and software should be secure to use out of the box, with security features already enabled.

5.5.B
Explain how user input sanitization protects applications.

  • 5.5.B.1 When users enter input into an application, the application typically encases that input in special characters to process it. The characters that encase the user input are called control characters and include the single quote, the double quote, and the semicolon.
  • 5.5.B.2 When creating a program that takes user input, programmers should use a function to verify that user input meets their expected criteria and does not include any control characters that could be used to manipulate the system. This verification function can sanitize user input by removing potentially malicious characters, or it can give the user an error and force the user to provide different input. This can protect against many application attacks, including:
    • SQL injection attacks
    • XSS attacks
    • Directory traversal attacks

Source: College Board AP Course and Exam Description

Two design principles keep applications safe from the start. Secure by design 安全设计 builds security into every phase of development, not as an afterthought. Secure by default 默认安全 means the product ships with its security features already enabled - safe straight out of the box.

Secure by design rests on three principles a company must adopt: (1) take ownership of its customers' security outcomes rather than shifting blame onto users, (2) embrace radical transparency and accountability – sharing security-relevant news and updates quickly so everyone becomes safer, and (3) build the organisational structure and leadership that makes security a first-class goal.

Input sanitization 输入清理 transforms input for a specific destination; validation checks an allowed type, length, range or format. Quotes and semicolons are printable special characters 特殊字符, not necessarily control characters. A legitimate name such as O'Neill should remain intact. A single blacklist is not a universal defense: SQL injection needs parameterised queries with separately bound values; XSS needs safe text rendering or output encoding for the destination context (and a suitable HTML sanitizer when HTML is allowed); directory traversal needs resolved paths confined to an allowed directory. For example, 0 OR 1=1 can change an unsafe numeric SQL expression without using quotes or semicolons. Use fixed SQL such as SELECT name FROM pupils WHERE name = ? and bind the entered name separately.

Building security into every stage of an application's life cycle
Building security into every stage of an application's life cycle
Vocabulary Train
English
Secure by design/sɪˈkjʊə baɪ dɪˈzaɪn/
Secure by default/sɪˈkjʊə baɪ dɪˈfɒlt/
input sanitization/ˈɪnpʊt ˌsænɪtaɪˈzeɪʃn/
special characters/ˈspeʃl ˈkærɪktəz/
5.6

Detecting Attacks on Data and Applications
Syllabus
Learning ObjectiveEssential Knowledge

5.6.A
Explain how to detect attacks on data.

  • 5.6.A.1 Devices track and log when data are accessed and by whom. The process of recording and monitoring user activities is called accounting. Analysis of these logs can reveal malicious activity when an adversary attempts to access, copy, move, or delete data. Suspicious activity can include:
    • Accessing files that aren’t typically accessed
    • Accessing files or applications outside of a user’s normal patterns (including time of day, location, and device type)
    • Attempts to delete or copy sensitive files
  • 5.6.A.2 A honeypot is a file that appears as if it contains valuable data (e.g., credit card information, PII, passwords), but the data in the file are fake. A system can alert defenders if someone attempts to access the honeypot. Since the honeypot is a fake file, there is no legitimate reason to be accessing it, and any attempted access would be an indicator of malicious activity.
  • 5.6.A.3 Cryptographic hash functions can generate a digest for data and can reveal if data have been altered. If a file has changed unexpectedly, this can be a sign of malicious activity.

5.6.B
Determine controls for detecting attacks against applications or data.

  • 5.6.B.1 Cost is a criterion in determining detective controls. Detective controls like honeypots and using hash values to check data integrity are inexpensive. Some organizations invest in third-party data loss prevention (DLP) services, which monitor data access, usage, and transmission by users throughout the organization to detect suspicious activity; DLP services provide strong detection capabilities at a higher cost.
  • 5.6.B.2 Sensitivity or criticality of data or applications is a criterion in determining detective controls. More sensitive or critical data or applications are more likely targets of an adversary and should be monitored more closely.
  • 5.6.B.3 Classification of data is a criterion in determining detective controls. Data that have been classified as private, educational, healthcare, or financial often have legal or regulatory detection and monitoring requirements.

5.6.C
Evaluate the impact of a method for detecting attacks against an application or data.

  • 5.6.C.1 To operate at an effective speed, log analysis needs to be augmented with some automation. Honeypots offer near instantaneous detection capabilities.
  • 5.6.C.2 Some DLP tools, honeypots, and realtime automated log analysis provide alerts as an attack is happening. These tools allow for a prompt response that can stop an attack before it does more harm. Retrospective log analysis and the use of cryptographic hashes to verify data integrity identify attacks after they have occurred.
  • 5.6.C.3 False negatives can occur in applications and data attack detection. Cryptographic hash functions only detect if data have been altered. An adversary could view and steal data without altering it, and a cryptographic hash function would not detect this. Honeypots cannot detect adversaries that do not attempt to access them.

5.6.D
Identify whether a file has been altered by verifying its hash.

  • 5.6.D.1 Cryptographic hash functions can help identify changes in a file because they are repeatable: the same input always produces the same output for a given hash function.
  • 5.6.D.2 Hashes can be calculated using the command line on a computer, a website, or specialized software.
    • In Windows Powershell, if a user wanted to generate the SHA256 hash for a file named testfile, they would use the command: Get-FileHash testfile -Algorithm SHA256
    • In BASH the same could be accomplished with the command: sha256sum testfile
    • In zsh, the common command line terminal on Apple computers, this could be accomplished with the command: shasum -a 256 testfile
  • 5.6.D.3 A file can be hashed and its hash output recorded. Then it can be hashed again later, and the second hash output can be compared to the previous hash output for the same file. If a file’s hash changes, then the file was altered between when the first and second hashes were generated.

5.6.E
Apply detection techniques to identify and report indicators of application attacks by analyzing log files.

  • 5.6.E.1 SQL injection attacks can be detected by reviewing application and server logs of user input for SQL control words and symbols such as:
    • A single (') or double (") quote character
    • Boolean conditions like OR 1=1
    • A double dash (which indicates a comment in SQL): --
    • SQL control words (always in capital letters) like WHERE, IN, FROM
  • 5.6.E.2 XSS attacks can be detected by reviewing user input for suspicious tags, particularly the tag.
  • 5.6.E.3 For web applications, buffer overflows can be detected by checking the amount of data the user is sending to the web application in their request. The fields commonly checked are the URL length, cookie length, query string length, and total request length. Long strings in any of these fields can be an indicator of an attempted buffer overflow attack.
  • 5.6.E.4 Directory traversal attacks can be detected by reviewing application and server logs. HTTP GET requests that include paths with sequences of ../ are indicators of an adversary attempting a directory traversal.

Source: College Board AP Course and Exam Description

To detect data attacks, systems perform accounting 审计记录 - logging who accessed what and when. But logs are huge, so log analysis must be automated to run at a useful speed; a human reading raw logs is far too slow. A clever complement is a honeypot 蜜罐 - a fake file that looks valuable; unexpected access gives an alert to investigate. Administrators or scanners may also access a decoy, so an alert alone is not proof of an attack. Watch especially for attempts to delete or copy sensitive files. Cryptographic hashes also help: re-hash a file and compare - if the digest changed, the file was altered.

Choosing detective controls means weighing cost (honeypots are cheap; a data loss prevention (DLP) 数据泄露防护 service is powerful but pricey) against the sensitivity of the data. To read a specific attack from logs, look for its signature: SQL injection shows OR 1=1 and --; XSS shows <script> tags; directory traversal shows ../ sequences; a buffer overflow shows unusually long input strings.

Checking that a file has not been altered

A cryptographic hash turns a file of any size into a short fixed-length value. A change normally gives a different digest, but collisions exist. A match to the publisher's trusted full digest is strong evidence that the bytes match, rather than mathematical proof. You do this at the command line:

Shell Command
BASH (Linux, and most servers) sha256sum testfile
zsh, the usual terminal on Apple computers shasum -a 256 testfile

Both print the SHA-256 hash of testfile. If it differs from the published value by even one character, the file has been altered — by corruption in transit, or by an attacker who replaced it.

⚠️ A hash proves integrity, not authenticity. An attacker who can replace the file on a web page can usually replace the published hash beside it too; that is why a signed hash, or one fetched over a separate trusted channel, is stronger evidence.

Vocabulary Train
English
accounting/əˈkaʊntɪŋ/
honeypot/ˈhʌnɪpɒt/
data loss prevention (DLP)/ˈdeɪtə lɒs prɪˈvenʃn/
5.6

Exam tips
  • Match each application attack to its evidence in a log: OR 1=1 / -- = SQL injection; <script> = XSS; ../ = directory traversal; very long input = buffer overflow.
  • Learn the four access-control models by their decider: RBAC = your role, RuBAC = a condition, DAC = the file's owner, MAC = a central admin. Least privilege underlies them all.
  • Read Linux permissions by adding 4+2+1 per group - chmod 750 = owner rwx (7), group r-x (5), others none (0). Practice converting both ways.
  • Symmetric = one shared key (fast, AES); asymmetric = a public/private key pair (solves key sharing, RSA/ECC). Encrypt with the recipient's public key.
  • Match input defenses to the destination: parameter binding for SQL, context-specific output protection for XSS, and allowed-directory checks for paths. Input sanitization is not one universal character filter; a honeypot is a detective control.

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