加密哈希和碰撞
Last updated
Last updated
Different input messages are expected to produce different output hash values (message digest).
预期中不同的输入信息会产生不同的输出哈希值（信息摘要）。
A collision means the same hash value for two different inputs. For simple hash functions it is easy to reach a collision. For example, assume a hash function h(text) sums of all character codes in a text. It will produce the same hash value (collision) for texts holding the same letters in different order, i.e. h('abc') == h('cab') == h('bca'). To avoid collisions, cryptographers have designed collision-resistant hash functions.
碰撞表示不同的两个输入具有相同的哈希值。对于简单的哈希函数，很容易产生碰撞。例如，假设一个哈希函数 h(text) 是文本中所有字符编码的和。在文本为相同字母的不同顺序组合时，它将产生相同的哈希值（碰撞），即 h('abc') == h('cab') == h('bca')。为了避免碰撞，密码学家设计了抗碰撞哈希函数。
Collisions in the cryptographic hash functions are extremely unlikely to be found, so crypto hashes are considered to almost uniquely identify their corresponding input. Moreover, it is extremely hard to find an input message that hashes to given value.
加密哈希函数的碰撞极难出现，因此加密哈希被认为几乎惟一地标识了它们对应的输入，而且很难找出某个给定的哈希值对应的输入信息。
Cryptographic hash functions are one-way hash functions, which are infeasible to invert. The chance to find a collision (by brute force) for a strong cryptographic hash function (like SHA-256) is extremely little. Let's define this in more details:
加密哈希函数是单向哈希函数，不可逆推。为强加密哈希函数（如 SHA-256）找出碰撞（通过暴力）的机会非常小。下面来让我们更详细地定义它：
Let's have hash value h=hash(p) for certain strong cryptographic hash function hash.
我们用 h=hash(p) 来表示某个强加密哈希函数得到的哈希值。
It is expected to be extremely hard to find an input p', such that hash(p')=h.
预期中很难找到输入 p' 使得 hash(p')=h。
For most modern strong cryptographic hash functions there are no known collisions.
对于大多数现代强加密哈希函数，不存在已知的碰撞。
The ideal cryptographic hash function should have the following properties:
理想的加密哈希函数应具有以下属性：
Deterministic: the same input message should always result in the same hash value.
确定性：相同的输入信息应始终产生相同的哈希值。
Quick: it should be fast to compute the hash value for any given message.
快速：计算任何给定信息的哈希值应该很快。
Hard to analyze: a small change to the input message should totally change the output hash value.
难于分析：对输入信息的微小更改将完全改变输出的哈希值。
Irreversible: generating a valid input message from its hash value should be infeasible. This means that there should be no significantly better way than brute force (try all possible input messages).
不可逆：从其哈希值生成有效的输入信息应该是不可行的。这意味着没有比暴力破解更好的方法（即尝试所有可能的输入信息）。
No collisions: it should be extremely hard (or practically impossible) to find two different messages with the same hash.
没有碰撞：找到具有相同哈希值的两个不同信息应该非常困难（或几乎不可能）。
Modern cryptographic hash functions (like SHA2 and SHA3) match the above properties and are used widely in cryptography.
现代加密哈希函数（如 SHA2 和 SHA3）符合上述性质，在密码学中得到了广泛的应用。
