GPG Cheat Sheet

Quick'n easy gpg cheatsheet
If you found this page, hopefully it's what you were looking for. It's just a brief explanation of some of the command line functionality from gnu privacy guard (gpg).


Filenames are italicized (loosely, some aren't, sorry), so if you see something italicized, think "put my filename there."
I've used User Name as being the name associated with the key. Sorry that isn't very imaginative. I *think* gpg is pretty wide in it's user assignments, ie. the name for my private key is Charles Lockhart, but I can reference that by just putting in Lockhart. That doesn't make any sense, sorry.
to create a key:
gpg --gen-keygenerally you can select the defaults.
to export a public key into file public.key:
gpg --export -a "User Name" > public.key
This will create a file called public.key with the ascii representation of the public key for User Name. This is a variation on:
gpg --export
which by itself is basically going to print out a bunch of crap to your screen. I recommend against doing this.
gpg --export -a "User Name"
prints out the public key for User Name to the command line, which is only semi-useful
to export a private key:
gpg --export-secret-key -a "User Name" > private.key
This will create a file called private.key with the ascii representation of the private key for User Name.
It's pretty much like exporting a public key, but you have to override some default protections. There's a note (*) at the bottom explaining why you may want to do this.
to import a public key:
gpg --import public.key
This adds the public key in the file "public.key" to your public key ring.
to import a private key:
NOTE: I've been informed that the manpage indicates that "this is an obsolete option and is not used anywhere." So this may no longer work.
gpg --allow-secret-key-import --import private.key
This adds the private key in the file "private.key" to your private key ring. There's a note (*) at the bottom explaining why you may want to do this.
to delete a public key (from your public key ring):
gpg --delete-key "User Name"This removes the public key from your public key ring.
NOTE! If there is a private key on your private key ring associated with this public key, you will get an error! You must delete your private key for this key pair from your private key ring first.
to delete an private key (a key on your private key ring):
gpg --delete-secret-key "User Name"This deletes the secret key from your secret key ring.
To list the keys in your public key ring:
gpg --list-keys
To list the keys in your secret key ring:
gpg --list-secret-keys
To generate a short list of numbers that you can use via an alternative method to verify a public key, use:
gpg --fingerprint > fingerprintThis creates the file fingerprint with your fingerprint info.
To encrypt data, use:
gpg -e -u "Sender User Name" -r "Receiver User Name" somefile
There are some useful options here, such as -u to specify the secret key to be used, and -r to specify the public key of the recipient.
As an example: gpg -e -u "Charles Lockhart" -r "A Friend" mydata.tar
This should create a file called "mydata.tar.gpg" that contains the encrypted data. I think you specify the senders username so that the recipient can verify that the contents are from that person (using the fingerprint?).
NOTE!: mydata.tar is not removed, you end up with two files, so if you want to have only the encrypted file in existance, you probably have to delete mydata.tar yourself.
An interesting side note, I encrypted the preemptive kernel patch, a file of 55,247 bytes, and ended up with an encrypted file of 15,276 bytes.
To decrypt data, use:
gpg -d mydata.tar.gpgIf you have multiple secret keys, it'll choose the correct one, or output an error if the correct one doesn't exist. You'll be prompted to enter your passphrase. Afterwards there will exist the file "mydata.tar", and the encrypted "original," mydata.tar.gpg.
NOTE: when I originally wrote this cheat sheet, that's how it worked on my system, however it looks now like "gpg -d mydata.tar.gpg" dumps the file contents to standard output. The working alternative (worked on my system, anyway) would be to use "gpg -o outputfile -d encryptedfile.gpg", or using mydata.tar.gpg as an example, I'd run "gpg -o mydata.tar -d mydata.tar.gpg". Alternatively you could run something like "gpg -d mydata.tar.gpg > mydata.tar" and just push the output into a file. Seemed to work either way.
Ok, so what if you're a paranoid bastard and want to encrypt some of your own files, so nobody can break into your computer and get them? Simply encrypt them using yourself as the recipient.
I haven't used the commands:
gpg --edit-key
gpg --gen-revoke

• --gen-revoke creates a revocation certificate, which when distributed to people and keyservers tells them that your key is no longer valid, see http://www.gnupg.org/gph/en/manual/r721.html
• --edit-key allows you do do an assortment of key tasks, see http://www.gnupg.org/gph/en/manual/r899.html

Sharing Secret Keys

NOTE!: the following use cases indicate why the secret-key import/export commands exist, or at least a couple ideas of what you could do with them. HOWEVER, there's some logistics required for sharing that secret-key. How do you get it from one computer to another? I guess encrypting it and sending it by email would probably be ok, but I wouldn't send it unencrypted with email, that'd be DANGEROUS.
Use Case *.1 : Mentioned above were the commands for exporting and importing secret keys, and I want to explain one reason of why maybe you'd want to do this. Basically if you want one key-pair for all of your computers (assuming you have multiple computers), then this allows you export that key-pair from the original computer and import it to your other computers.
Use Case *.2 : Mentioned above were the commands for exporting and importing secret keys, and I want to explain one reason of why maybe you'd want to do this. Basically, if you belonged to a group, and wanted to create a single key-pair for that group, one person would create the key-pair, then export the public and private keys, give them to the other members of the group, and they would all import that key-pair. Then a member of the group or someone outside could use the group public key, encrypt the message and/or data, and send it to members of the group, and all of them would be able to access the message and/or data. Basically you could create a simplified system where only one public key was needed to send encrypted stuffs to muliple recipients.

Curl to get SSL Cert

http://www.shellhacks.com/en/HowTo-Check-SSL-Certificate-Expiration-Date-from-the-Linux-Shell


echo | openssl s_client -connect <hostname>:443 2>/dev/null | openssl x509 -noout -issuer -subject -dates

Mysql Date

mysql> SELECT DATE_SUB(LAST_DAY(NOW() - INTERVAL 1 MONTH),INTERVAL DAY(LAST_DAY(NOW() - INTERVAL 1 MONTH))-1 DAY);
+-----------------------------------------------------------------------------------------------------+
| DATE_SUB(LAST_DAY(NOW() - INTERVAL 1 MONTH),INTERVAL DAY(LAST_DAY(NOW() - INTERVAL 1 MONTH))-1 DAY) |
+-----------------------------------------------------------------------------------------------------+
| 2016-08-01                                                                                          |
+-----------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT DATE_SUB(LAST_DAY(NOW()),INTERVAL DAY(LAST_DAY(NOW()))-1 DAY);
+---------------------------------------------------------------+
| DATE_SUB(LAST_DAY(NOW()),INTERVAL DAY(LAST_DAY(NOW()))-1 DAY) |
+---------------------------------------------------------------+
| 2016-09-01                                                    |
+---------------------------------------------------------------+
1 row in set (0.00 sec)

Sending Mail in Unix

echo "test" | mutt -e "my_hdr From:Test Support <support@test.com>" -s "test" "recipient@test.com" -a /tmp/mailmessage.txt

Spring AOP

1. AOP stands for Aspect Oriented Programming
2. The key unit of modularity in OOP is the class, whereas in AOP the unit of modularity is the aspect.
3. Aspects enable the modularization of concerns such as transaction management that cut across multiple types and objects.
4. Spring AOP is implemented in pure Java.
5. Spring AOP currently supports only method execution join points (advising the execution of methods on Spring beans). Field interception is not implemented, although support for field interception could be added without breaking the core Spring AOP APIs. If you need to advise field access and update join points, consider a language such as AspectJ.

AOP concepts

1. Aspect: a modularization of a concern that cuts across multiple classes. Transaction management is a good example of a crosscutting concern in enterprise Java applications. In Spring AOP, aspects are implemented using regular classes (the schema-based approach) or regular classes annotated with the @Aspect annotation (the @AspectJ style).
2. Join point: a point during the execution of a program, such as the execution of a method or the handling of an exception. In Spring AOP, a join point always represents a method execution.
3. Advice: action taken by an aspect at a particular join point. Different types of advice include "around," "before" and "after" advice. Many AOP frameworks, including Spring, model an advice as an interceptor, maintaining a chain of interceptors around the join point.
4. Pointcut: a predicate that matches join points. Advice is associated with a pointcut expression and runs at any join point matched by the pointcut (for example, the execution of a method with a certain name). The concept of join points as matched by pointcut expressions is central to AOP, and Spring uses the AspectJ pointcut expression language by default.
5. Introduction: declaring additional methods or fields on behalf of a type. Spring AOP allows you to introduce new interfaces (and a corresponding implementation) to any advised object. For example, you could use an introduction to make a bean implement an IsModified interface, to simplify caching. (An introduction is known as an inter-type declaration in the AspectJ community.)
6. Target object: object being advised by one or more aspects. Also referred to as the advised object. Since Spring AOP is implemented using runtime proxies, this object will always be a proxied object.
7. AOP proxy: an object created by the AOP framework in order to implement the aspect contracts (advise method executions and so on). In the Spring Framework, an AOP proxy will be a JDK dynamic proxy or a CGLIB proxy.
8. Weaving: linking aspects with other application types or objects to create an advised object. This can be done at compile time (using the AspectJ compiler, for example), load time, or at runtime. Spring AOP, like other pure Java AOP frameworks, performs weaving at runtime.

Types of advice:

• Before advice: Advice that executes before a join point, but which does not have the ability to prevent execution flow proceeding to the join point (unless it throws an exception).
• After returning advice: Advice to be executed after a join point completes normally: for example, if a method returns without throwing an exception.
• After throwing advice: Advice to be executed if a method exits by throwing an exception.
• After (finally) advice: Advice to be executed regardless of the means by which a join point exits (normal or exceptional return).
• Around advice: Advice that surrounds a join point such as a method invocation. This is the most powerful kind of advice. Around advice can perform custom behavior before and after the method invocation. It is also responsible for choosing whether to proceed to the join point or to shortcut the advised method execution by returning its own return value or throwing an exception.

Around advice is the most general kind of advice. if you need only to update a cache with the return value of a method, you are better off implementing an after returning advice than an around advice, although an around advice can accomplish the same thing. Using the most specific advice type provides a simpler programming model with less potential for errors. For example, you do not need to invoke the proceed() method on the JoinPoint used for around advice, and hence cannot fail to invoke it.

In Spring 2.0, all advice parameters are statically typed, so that you work with advice parameters of the appropriate type (the type of the return value from a method execution for example) rather than Object arrays.

The concept of join points, matched by pointcuts, is the key to AOP which distinguishes it from older technologies offering only interception. Pointcuts enable advice to be targeted independently of the Object-Oriented hierarchy. For example, an around advice providing declarative transaction management can be applied to a set of methods spanning multiple objects (such as all business operations in the service layer).

Declaring an aspect

With the @AspectJ support enabled, any bean defined in your application context with a class that is an @AspectJ aspect (has the @Aspect annotation) will be automatically detected by Spring and used to configure Spring AOP. 

package org.xyz;
import org.aspectj.lang.annotation.Aspect;

@Aspect
public class TestAspect {

}

NOTE : You may register aspect classes as regular beans in your Spring XML configuration, or autodetect them through classpath scanning - just like any other Spring-managed bean. However, note that the @Aspect annotation is not sufficient for autodetection in the classpath: For that purpose, you need to add a separate @Component annotation (or alternatively a custom stereotype annotation that qualifies, as per the rules of Spring’s component scanner).

In Spring AOP, it is not possible to have aspects themselves be the target of advice from other aspects. The @Aspect annotation on a class marks it as an aspect, and hence excludes it from auto-proxying.

 Declaring a pointcut

Recall that pointcuts determine join points of interest, and thus enable us to control when advice executes. Spring AOP only supports method execution join points for Spring beans, so you can think of a pointcut as matching the execution of methods on Spring beans. A pointcut declaration has two parts: a signature comprising a name and any parameters, and a pointcut expression that determines exactly which method executions we are interested in. In the @AspectJ annotation-style of AOP, a pointcut signature is provided by a regular method definition, and the pointcut expression is indicated using the @Pointcut annotation (the method serving as the pointcut signature must have a void return type).

@Pointcut("execution(* transfer(..))")// the pointcut expression
private void anyOldTransfer() {}// the pointcut signature

Supported Pointcut Designators (PCD)

Spring AOP supports the following AspectJ pointcut designators (PCD) for use in pointcut expressions:

• execution - for matching method execution join points, this is the primary pointcut designator you will use when working with Spring AOP
• within - limits matching to join points within certain types (simply the execution of a method declared within a matching type when using Spring AOP)
• this - limits matching to join points (the execution of methods when using Spring AOP) where the bean reference (Spring AOP proxy) is an instance of the given type
• target - limits matching to join points (the execution of methods when using Spring AOP) where the target object (application object being proxied) is an instance of the given type
• args - limits matching to join points (the execution of methods when using Spring AOP) where the arguments are instances of the given types
• @target - limits matching to join points (the execution of methods when using Spring AOP) where the class of the executing object has an annotation of the given type
• @args - limits matching to join points (the execution of methods when using Spring AOP) where the runtime type of the actual arguments passed have annotations of the given type(s)
• @within - limits matching to join points within types that have the given annotation (the execution of methods declared in types with the given annotation when using Spring AOP)
• @annotation - limits matching to join points where the subject of the join point (method being executed in Spring AOP) has the given annotation

Declaring advice

Advice is associated with a pointcut expression, and runs before, after, or around method executions matched by the pointcut. The pointcut expression may be either a simple reference to a named pointcut, or a pointcut expression declared in place.

Before advice

Before advice is declared in an aspect using the @Before annotation:

After returning advice

After returning advice runs when a matched method execution returns normally. It is declared using the @AfterReturning annotation:

After throwing advice

After throwing advice runs when a matched method execution exits by throwing an exception. It is declared using the @AfterThrowing annotation:

After (finally) advice

After (finally) advice runs however a matched method execution exits. It is declared using the @After annotation. After advice must be prepared to handle both normal and exception return conditions. It is typically used for releasing resources, etc.

Around advice

The final kind of advice is around advice. Around advice runs "around" a matched method execution.

Resources:

http://docs.spring.io/spring/docs/current/spring-framework-reference/html/aop.html

For loop in Java

In java 8 there is a new way to use for loop. There is a new foreach() method in java.util.stream.Stream class.

Pre JDK 5
      for(int i=0; i < countries.size(); i++)
      {
          System.out.println(countries.get(i));
      }

In JDK5
    for(String country : countries){
             System.out.println(country);
    }

This was much shorter, cleaner and less error prone than previous one and everybody loved it. You don't need to keep track of index, you don't need to call size() method in every step and it was less error prone than previous one, but it was still imperative. You are telling compiler what to do and how to do like traditional for loop.

JDK8
       countries.stream().forEach(str -> System.out.println(str));

This code has reduced looping over collection or list into just one line. To add into, If you want to perform some pre processing task e.g. filtering, conversion or mapping, you can also do this in same line, as shown below :

    countries.stream()
         .filter(country -> country.contains("n"))
         .forEach(str -> System.out.println(str));

Adding Global Variables in Eclipse

If you need to add a global variable in eclipse follow the below steps :

eclipse -- Window -- Preferences
Type Classpath in the search box
go to classpath variable -- click new