How to create a secure random AES key in Java

Question

What is the recommended way of generating a secure  random AES key in Java  using the standard JDK   In other posts  I have found this  but using a SecretKeyFactory might be a better idea   KeyGenerator keyGen   KeyGenerator getInstance  AES    SecureRandom random   new SecureRandom       cryptograph  secure random  keyGen init random    SecretKey secretKey   keyGen generateKey      It would be great if the answer included an explanation of why it is a good way of generating the random key  Thanks

User · Answer

Lots of good advince in the other posts  This is what I use   Key key  SecureRandom rand   new SecureRandom    KeyGenerator generator   KeyGenerator getInstance  AES    generator init 256  rand   key   generator generateKey      If you need another randomness provider  which I sometime do for testing purposes  just replace rand with  MySecureRandom rand   new MySecureRandom

User · Answer

I would use your suggested code  but with a slight simplification   KeyGenerator keyGen   KeyGenerator getInstance  AES    keyGen init 256      for example SecretKey secretKey   keyGen generateKey      Let the provider select how it plans to obtain randomness - don t define something that may not be as good as what the provider has already selected   This code example assumes  as Maarten points out below  that you ve configured your java security file to include your preferred provider at the top of the list  If you want to manually specify the provider  just call KeyGenerator getInstance  AES    providerName      For a truly secure key  you need to be using a hardware security module  HSM  to generate and protect the key  HSM manufacturers will typically supply a JCE provider that will do all the key generation for you  using the code above

User · Answer

Using KeyGenerator would be the preferred method  As Duncan indicated  I would certainly give the key size during initialization  KeyFactory is a method that should be used for pre-existing keys   OK  so lets get to the nitty-gritty of this  In principle AES keys can have any value  There are no  weak keys  as in  3 DES  Nor are there any bits that have a specific meaning as in  3 DES parity bits  So generating a key can be as simple as generating a byte array with random values  and creating a SecretKeySpec around it   But there are still advantages to the method you are using  the KeyGenerator is specifically created to generate keys  This means that the code may be optimized for this generation  This could have efficiency and security benefits  It might be programmed to avoid a timing side channel attacks that would expose the key  for instance  Note that it may already be a good idea to clear any byte   that hold key information as they may be leaked into a swap file  this may be the case anyway though    Furthermore  as said  not all algorithms are using fully random keys  So using KeyGenerator would make it easier to switch to other algorithms  More modern ciphers will only accept fully random keys though  this is seen as a major benefit over e g  DES   Finally  and in my case the most important reason  it that the KeyGenerator method is the only valid way of handling AES keys within a secure token  smart card  TPM  USB token or HSM   If you create the byte   with the SecretKeySpec then the key must come from memory  That means that the key may be put in the secure token  but that the key is exposed in memory regardless  Normally  secure tokens only work with keys that are either generated in the secure token or are injected by e g  a smart card or a key ceremony  A KeyGenerator can be supplied with a provider so that the key is directly generated within the secure token   As indicated in Duncan s answer  always specify the key size  and any other parameters  explicitly  Do not rely on provider defaults as this will make it unclear what your application is doing  and each provider may have its own defaults

[java] How to create a secure random AES key in Java?

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