在Java中从私有获取公钥 [英] Get public key from private in Java

问题描述

我记得很久以前用OpenSSL这么做了，但我想知道它是否可能以及如何，我从来没有在java上使用过Cryptography。

I remember do this long time ago with OpenSSL, but I want to know if it's possible and how, I've never used Cryptography on java.

推荐答案

您无法直接从另一个生成任一密钥。这在数学上是不可能的。如果您的密钥blob包含两者公钥和私钥，则可以相对轻松地提取其中任何一个。

You cannot generate either key directly from the other. It is mathematically impossible. If you had a key blob that contained both the public and private keys, you could extract either one of them with relative ease.

编辑，2017年：多年以后对加密的理解要好得多，而且我现在很清楚这个答案并不是真的正确。

EDIT, 2017: Many years and a much better understanding of crypto later, and it's now clear to me that this answer isn't really correct.

引用维基百科：

公钥由模数n和公共（或加密）指数e组成。私钥由模数n和私有（或解密）指数d组成，它必须保密。 p，q和λ（n）也必须保密，因为它们可用于计算d。

The public key consists of the modulus n and the public (or encryption) exponent e. The private key consists of the modulus n and the private (or decryption) exponent d, which must be kept secret. p, q, and λ(n) must also be kept secret because they can be used to calculate d.

公共模数n可以计算为p× Q值。原始私钥中唯一缺少的是e，但是这个值通常被选为65537，如果不是，你仍然可以从d和λ（n）计算e。

The public modulus n can be computed as p × q. The only thing missing from a raw private key is e, but this value is usually selected as 65537, and if not you can still compute e from d and λ(n).

但是，许多私钥存储格式实际上包含公共模数n和其他组件，因此您可以直接提取值。

However, many private key storage formats actually contain the public modulus n alongside the other components, so you can just do a direct extraction of the values.

编辑，2018年：仍然为此付出了挫折，这是正确的！我将这个答案留下来，这样人们就能明白为什么我本来就错了，并提醒自己将来不要错。

EDIT, 2018: Still getting downvotes for this, and rightly so! I'm leaving this answer up so people can see why I was originally wrong, and to remind myself not to be wrong in future.

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