RSAEncrypt long strings

General
#1

I am trying to encrypt strings with RSAEncrypt and Decrypt by using the example at http://documentation.xojo.com/index.php/Crypto.RSADecrypt

It works well with the 14 characters sample message, so I modified it to accept longer strings to encrypt/decrypt as follow :

[code] dim kMessage as string = "this is a test this is a test this is a test this is a test this is a test this is a test this is a test this is a test "

If Crypto.RSAGenerateKeyPair( 1024, privateKey, publicKey ) Then
// 1024-bit private and public keys were generated
End If

If Crypto.RSAGenerateKeyPair( 1024, privateKey, publicKey ) Then
// 1024-bit private and public keys were generated

Dim msg As New MemoryBlock(len(kMessage))
msg.StringValue(0, len(kMessage)) = kMessage

// Encrypt msg using the publicKey
Dim encryptedData As MemoryBlock = Crypto.RSAEncrypt( msg, publicKey )

If encryptedData <> Nil Then
  MsgBox("Successfully encrypted.")
  
  // Now decrypt
  Dim decryptedData As MemoryBlock = Crypto.RSADecrypt( encryptedData, privateKey )
  Msgbox("DecryptedData=" + decryptedData.StringValue(0, len(kMessage)))
End If

End If[/code]

I replaced the fixed length of the memoryblock by len(kMessage) .
When I run it with the message "this is a test … as it shows, I get a Crypto exception “RSA/OAET - MGF1(SHA-1): message length of 120 exceeds the maximum of 86 for this public key”.

What would be the best approach ?

  • Is there a way to set the maximum for a public key ?
  • Should I divide my message in 86 character clusters to encrypt and decrypt ?
  • Is there another, better method ?
#2

You will need to encrypt in multiple clusters, however the maximum length is defined by the bit size of the key. Unfortunately, I haven’t been able to figure out the math which dictates the maximum length. A solution I’ve used in the past is to keep adding characters to the string until I get the exception. Then I known the maximum and know how many chunks I’ll need to create.

I’d be very happy to know a formula to determine the maximum length based on the key size.

#3

OAEP use a hash function with output length as h bits; this implies a size limit of floor(n/8)-2*ceil(h/8)-2: still for a 1024-bit RSA key, with SHA-256 as hash function (h = 256), this means binary messages up to 60 bytes. I’m not sure if it applies to OAET as well. Will play around for a bit and test. It should work the same substituting 256 for 1 in the case of SHA-1.

[quote=77997:@Thom McGrath]You will need to encrypt in multiple clusters, however the maximum length is defined by the bit size of the key. Unfortunately, I haven’t been able to figure out the math which dictates the maximum length. A solution I’ve used in the past is to keep adding characters to the string until I get the exception. Then I known the maximum and know how many chunks I’ll need to create.

I’d be very happy to know a formula to determine the maximum length based on the key size.[/quote]

#4

I am going to divide the string into 85 bytes chunks, then, with a one byte header for the cluster length, as the last one may range in size from 1 to 85.

#5

It looks like the header size is 42 (go figure):

1024 ÷ 8 = 128 - 42 = 86 2048 ÷ 8 = 256 - 42 = 214 4096 ÷ 8 = 512 - 42 = 470

#6

Isn’t that always, ultimately, the answer?

#7

it just made me chuckle. I wonder if the crypto++ people went out of their way to make that be.

#8

Makes you wonder if you can find “Oh no not again” or “So long and thanks for all the fish” in the source code :stuck_out_tongue:

#9

I faced the same problem. Hmm… 86 bytes max

#10

Keep in mind that RSA encryption is generally very slow. If you have a lot of data, consider using something like blowfish to encrypt the data and then use RSA to encrypt the blowfish key.

#11

Given the hassle to put an app that uses encryption in the MAS, I have simply abandoned the project since :stuck_out_tongue:

#12