[erlang-questions] 3des decryption in erlang

[Ingela Andin]

Hi!


2015-08-11 6:22 GMT+02:00 Alex Xu <xuxb1979@REDACTED>:

> Thank you for your rapid answer, Sean.
>
> I’ve tried block_decrypt yesterday, but I couldn’t find the counterpart of
> ECB mode decryption.
>
> I tried des_ede3, des3_cbc, de3_cbf. For des_ede3/des3_cbc, the output
> seams the same, and only the first 8 bytes output result are correct.
>
> For des3_cbf, the output are completely incorrect.
>
> So I’m still confused how to do DES3 ECB decryption with block_decrypt?
>


It actually looks like this algorithm was missed when making the new
crypto-API. I think it could probably be added quite easily to the new API
but this will probably not be highly prioritized
by Ericsson.

Regards Ingela Erlang/OTP Team - Ericsson AB



> On Aug 10, 2015, at 21:54, Sean Cribbs <seancribbs@REDACTED> wrote:
>
> Xiaobin,
>
> Here's what I came up with:
>
>     crypto:start(),
>     %% To decrypt the text, note Key and IV must be defined in this scope
>     Unencoded = base64:decode(Value),
>     Cleartext = crypto:block_decrypt(des3_cbc, Key, IV, Unencoded),
>     %% To unpad the text, see https://github.com/camshaft/pkcs7.erl
>     pkcs7:unpad(Cleartext)
>
> The main thing to note is the difference in how you use the crypto module.
> In Erlang, you don't need to initialize, decrypt, and cleanup in separate
> steps. You do however, need to make sure the crypto application is started
> before you try this. Generally, you would make crypto a dependency (see
> http://erlang.org/doc/man/app.html) of the application that contains this
> code and it would be started automatically when your release is booted.
>
> On Mon, Aug 10, 2015 at 7:59 AM, Xiaobin Xu <xuxb1979@REDACTED> wrote:
>
>> Hi, all,
>>
>>    For some reason i have to decrypt a message that is encrypted using
>> 3DES algorithm, and I've PHP function example how to decrypt the message:
>>
>> public  function decrypt($value) {
>> $td = mcrypt_module_open ( MCRYPT_3DES, '', MCRYPT_MODE_ECB, '' );
>> mcrypt_generic_init ( $td, $this->key,$this->iv );
>> $ret = trim ( mdecrypt_generic ( $td, base64_decode ( $value ) ) );
>> $ret = $this->UnPaddingPKCS7 ( $ret );
>> mcrypt_generic_deinit ( $td );
>> mcrypt_module_close ( $td );
>> return $ret;
>> }
>>
>>
>> private  function UnPaddingPKCS7($data) {
>> $padlen = ord (substr($data, (strlen( $data )-1), 1 ) );
>> if ($padlen > 8 )
>> return $data;
>>
>> for($i = -1*($padlen-strlen($data)); $i < strlen ( $data ); $i ++) {
>> if (ord ( substr ( $data, $i, 1 ) ) != $padlen)return false;
>> }
>>
>> return substr ( $data, 0, -1*($padlen-strlen ( $data ) ) );
>> }
>>
>>    I googled and read crypto module document for a couple hours, and got
>> no idea how to translate these two functions into erlang.
>>
>>    Any ideas?
>>
>>
>>    Thanks,
>>     Xiaobin
>>
>>
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>>
>>
>
>
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