[erlang-questions] 3des decryption in erlang

[Alex Xu]

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? 


> 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 <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 <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 <mailto: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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