加密的字符串长度 [英] Length of Encrypted String

问题描述

我需要建立一个数据库列将存储使用Triple DES加密的字符串。如何确定加密字符串列的长度？

I need to create a database column which will store a string encrypted using Triple DES. How do I determine the length of the encrypted string column?

（有答案的算法比三重DES等也欢迎。）

(Answers for algorithms other than Triple DES are also welcome.)

推荐答案

块密码，如DES，3DES和AES可以在字节块，才能操作。 DES和3DES上的8个字节和AES上的16字节的块的块进行操作。

Block-ciphers such as DES, 3DES and AES can only operate on blocks of bytes. DES and 3DES operate on block of 8 bytes and AES on blocks of 16 bytes.

要解决这个问题，你通常应用可逆的填充你的明文加密之前。这将主要永远是PKCS-padding（也称为PKCS5-或PKCS7，填充）。

To handle this, you usually apply a reversible padding to your plaintext before encrypting it. It will mostly always be "PKCS"-padding (also called PKCS5- or PKCS7-padding).

PKCS-填充增加了至少一个字节，使得所述填塞文本的长度整除与块长度（8字节为3DES）。填充字节的值是增加的字节数。 FX。 ABCDEF被填充到ABCDEF0505050505和0011223344556677被填充到0011223344566770808080808080808.请注意，这是很容易去除：你刚才看的填充字节的最后一个字节，验证它是1块长之间，并删除字节数从端（验证每个除去字节具有正确的值）。

PKCS-padding adds at least one byte such that the padded text has a length divisible with the block-length (8 bytes for 3DES). The value of the padding-bytes is the number of bytes added. Fx. ABCDEF is padded to ABCDEF0505050505 and 0011223344556677 is padded to 0011223344566770808080808080808. Note that this is easy to remove: you just look at the final byte of the padded bytes, verify that it is between 1 and the block-length, and remove that number of bytes from the end (verifying that each removed byte has the correct value).

最后，要回答你的问题：假设你正在使用3DES与CBC加密和PKCS - 填充 - 你可能是 - 长度为n的字符串加密的长度是：

Finally, to answer your question: Assuming you are using 3DES with CBC encryption and PKCS-padding - which you probably are - the encryption of a string of length n will have length:

n + 8 - (n % 8)

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