RSA on Android is different from PC

Question

I searched a lot, but I did not find a useful answer. I am developing a Java and Android security application. I found some problem when I create an RSA key on a PC and pass the RSA public key on Android. When I encrypt my data with the public key on android, and I decrypt the data using the private key on the PC, it shows the Badpadding exception. I am searching on google about this exception. I found some user, I have the same problem, but the answer is useless. They told me to use Base64 for encoding, but the key is still wrong. here is my code.

public void generateKeys(){ try { KeyPairGenerator generator; generator = KeyPairGenerator.getInstance("RSA"); generator.initialize(1024, new SecureRandom()); KeyPair pair = generator.generateKeyPair(); pubKey = pair.getPublic(); privKey = pair.getPrivate(); //================================================ System.out.println("My Public KEY "+pubKey.toString()); System.out.println("My Private KEY "+privKey.toString()); byte[]pk_byte = pubKey.getEncoded(); byte[]sk_byte = privKey.getEncoded(); FileOutputStream keyfos_pub = new FileOutputStream(new File(Environment.getExternalStorageDirectory(),"RSAKey/publicKey.txt")); keyfos_pub.write(pk_byte); keyfos_pub.close(); FileOutputStream keyfos_pri = new FileOutputStream(new File(Environment.getExternalStorageDirectory(),"RSAKey/privateKey.txt")); keyfos_pri.write(sk_byte); keyfos_pri.close(); } catch (FileNotFoundException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (NoSuchAlgorithmException e) { // TODO Auto-generated catch block e.printStackTrace(); } } 

When I print the key, it looks like this.

 RSA Private CRT Key modulus: dba017a7653612c53f3a46bb99f5098c7bf9012d06f3d20f8f01a67d17aeefd83070a2ff181468679a2d5abae6f7c074b7b3888a4a57533acc3d2d1dfdd86cdc032d3eafd6cf8423edfa3451333bf1d7377352dd3feb9268032831abd72c5c932fed9b4c667f3da062f4beb7a321f2c434d232ee746885dce13a1656eb42fe6b public exponent: 10001 private exponent: 72f10f5019653a5fa6de9e0432e80e4e4ad79fe8ebd99157793598e7a10c7a14000e0310548ee1b14efc6b9ea3a08845cd9a955c22da1f0207c000abd09d83228e05a42f9bd88733afc2a4c1e30770e7ac3c41c55adae734e4f8126da494a88f362d716d6b37a2d7bfbe53824cfb9dc7a0289aa432226bbece49129f47de2a61 primeP: efa2665a1e08dce4450ad608e17df1e3650ab2cfc44e9b3b6a346611b270b245e21c28f71971d5062368e66b99297085f23104577e529ef08b263e63387cf17b primeQ: ea9fde740a15d7f68f5aadb72a588f65da3761119b0c73930f111a43d513d19f42185afcd0291f03a9861a7757108f0e83d4d904ddf541a2debd79344387bbd1 primeExponentP: 45202ab84a3bb244a2e9fa4dfb861235cf5ae3b3ed63e381a32454613c8127dbe2daceb26103a638ac14418bbe55e6e0acb99910081f9b3bb65824dd08597a6b primeExponentQ: 53ca665a90a37e6e1a822ad9e8309a7da871f0a3a5f8cb69b08ece0f7d90476395ea36cc64d6dab1a72032617e6176859b852f3ff2b8bd091a7d164518fce791 crtCoefficient: 695a3707d033aa2474a7a64959589842099a220540167b48f42bbbd1d786d1a84c378f9a1bc0311948287cbf7669aa44156dd9f24bfe88baf931e5da70a06c4 RSA Public Key modulus: dba017a7653612c53f3a46bb99f5098c7bf9012d06f3d20f8f01a67d17aeefd83070a2ff181468679a2d5abae6f7c074b7b3888a4a57533acc3d2d1dfdd86cdc032d3eafd6cf8423edfa3451333bf1d7377352dd3feb9268032831abd72c5c932fed9b4c667f3da062f4beb7a321f2c434d232ee746885dce13a1656eb42fe6b public exponent: 10001 

I send my key to the PC and print it as if it

 Sun RSA private CRT key, 1024 bits modulus: 133510687177139403090984227659818165988216147752229771757259279456909046924729578431711819486905574365970242930474912203422914618872480707080105400973005394104312017850522474151319747965391515392091886771183127658574637486403332621957057719375091145573274843838361168501101321026667287976397870949812555235301 public exponent: 65537 private exponent: 9980146428442039393666657480590341260299844482325612277626580558453963728645653672535456363067433797990268399780224833064816191126482490562360748269935354262219600344146818595176602910809143102802601846580823241077395526479491554799584457750285244532837191028964347562514222968145677820868899082320352227173 prime p: 12299841631131712301068734715117232218530765196368758184959862403767638338760002562429845237522849739220962134079733702092483313582445665328865919334315231 prime q: 10854667172235374767842313747899047475769598966944705527971116954964144369647441254504149073040778536441723902849901407264741891511747045046848533162966971 prime exponent p: 9681923647204217071082501281466465900019303827612974308340469989251337818636053102684450861548877311242977166683023364260601327205871598989662862911311993 prime exponent q: 1654443602597908945419791446477006656323336803344838236704510234877044083623118096514059921732827819407607611968318128037101282547428799502158627916518563 crt coefficient: 8339065711012644104593134475846199184972354402766421557421676186133596071951423663692155855342215056476322973047333984032026031966395156454879551656339085 

It is clear that the key has been changed, because the word "1024 bits" appears in the key, and the number of modules does not match

How can I deal with this problem. I would like a professional to help me.

Answer 1

It looks like a different key. Try again, as you are likely to send the old key or something else. Make sure that you do not restore the key pair, this will be different every time. Use the private key to print the module as BigInteger ; BigInteger instances will always be printed as decimals for easy comparison. Or even better, use BigInteger.toString(16) to get the hexadecimal representation.

If you use a good random number generator (for example, by default in Java), the module must be unique for a key pair. Therefore, it or the SHA1 hash modulo is usually used to identify the key.

Answer 2

This is the same thing, but Android prints it in base 16 (hexadecimal), while your Java desktop prints it in base 10 (decimal). You will see output options toString() between different implementations.

For example, look at the common exponent and note that hex 0x10001 is decimal 65537 . This metric is commonly known as RSA F4. However, since this public metric is used for a large number of keys, you cannot use it to distinguish between keys.