大叔又来发武林秘笈了。

占个位，仔细看

菜农都闭关好久了，又要闭馆

给自己扫盲一下。

Trapdoor function

From Wikipedia, the free encyclopedia

A trapdoor function is a function that is easy to compute in one direction, yet believed to be difficult to compute in the opposite direction (finding its inverse) without special information, called the "trapdoor". Trapdoor functions are widely used in cryptography.

In mathematical terms, if f is a trapdoor function there exists some secret information y, such that given f(x) and y it is easy to compute x. Consider a padlock and its key. It is trivial to change the padlock from open to closed without using the key, by pushing the shackle into the lock mechanism. Opening the padlock easily, however, requires the key to be used. Here the key is the trapdoor.

Trapdoor functions came to prominence in cryptography in the mid-1970s with the publication of asymmetric (or public key) encryption techniques by Diffie, Hellman, and Merkle. Indeed, Diffie and Hellman first coined the term (Diffie and Hellman, 1976). Several function classes have been proposed, and it soon became obvious that trapdoor functions are harder to find than was initially thought. For example, an early suggestion was to use schemes based on the subset sum problem. This turned out -- rather quickly -- to be unsuitable.

As of 2004[update], the best known trapdoor function (family) candidates are the RSA and Rabin families of functions. Both are written as exponentiation modulo a composite number, and both are related to the problem of prime factorization.

Functions related to the hardness of the discrete logarithm problem (either modulo a prime or in a group defined over an elliptic curve) are not known to be trapdoor functions, because there is no known "trapdoor" information about the group that enables the efficient computation of discrete logs. However, the discrete logarithm problem can be used as the basis for a trapdoor when the related problems called the computational Diffie-Hellman problem (CDH) and/or its decisional variant are used. The semantically secure version of the ElGamal Cryptosystem relies on the Decision Diffie-Hellman problem (DDH). The Digital Signature Algorithm is based on CDH in a prime order subgroup.

A trapdoor in cryptography has the very specific aforementioned meaning and is not to be confused with a backdoor (these are frequently used interchangeably and this is incorrect). A backdoor is a deliberate mechanism that is added to a cryptographic algorithm (e.g., a key pair generation algorithm, digital signing algorithm, etc.) or operating system, for example, that permits one or more unauthorized parties to bypass or subvert the security of the system in some fashion.

陷门和后门不是一回事（A trapdoor in cryptography ... is not to be confused with a backdoor）。百度百科上把陷门搞成后门了。看来还是西方人严谨。

陷门函数不好找（trapdoor functions are harder to find than was initially thought）。最著名的陷门函数是RSA和Rabin陷门函数。不知菜农搞的是不是这个级别的东西？

另外，量子计算离实用还早着呢。菜农很超前。

每次看兰州的文章，我都为自己的语文阅读能力的低下感到深深的自责：（

兰州是谁呀

lss应该是打算说楼主（lz）

解开陷门小数点将会“农转非”，“菜农要做城里人”～～～

红杏出墙记？

主题本想冠以“红杏出墙记”，可恨被密界高人“按在墙头上”～～～

http://bbs.pediy.com/showthread.php?t=101847

引用:
【注意】菜地公告：即日起开辟《菜农奔甲子系列之》栏目  

1. 這是密碼版，不是開心農場，版友不是農夫，不種菜也不擔心菜被偷。
建議 HotPower 大哥是否更換一下標題會比較合適一點!?

2. 建議儘量不要搞外部連結，及讓人有一種宣傳廣告嫌疑的感覺。

3. 再次謝謝熱情的參與及配合。



本主题原名被密界高人“规劝更名”，还在敢“红杏出墙记”？？？不想在密界混了？？？

菜地公告：即日起开辟《菜农奔甲子系列之》栏目
量子时代送臭蛋，菜农红杏想出墙。
还是谷歌早看出来了，百度没发现~~~
最新版的HotWC3网上在线CRC运算器V5.01

谢谢分享！！！！！！！！！！！

学习中……。