问题
What is the maximum length of the string that can have md5 hashed? Or: If it has no limit, and if so what will be the max length of the md5 output value?
回答1:
MD5 processes an arbitrary-length message into a fixed-length output of 128 bits, typically represented as a sequence of 32 hexadecimal digits.
回答2:
- The length of the message is unlimited.
Append Length
A 64-bit representation of b (the length of the message before the padding bits were added) is appended to the result of the previous step. In the unlikely event that b is greater than 2^64, then only the low-order 64 bits of b are used.
- The hash is always 128 bits. If you encode it as a hexdecimal string you can encode 4 bits per character, giving 32 characters.
- MD5 is not encryption. You cannot in general "decrypt" an MD5 hash to get the original string.
See more here.
回答3:
You can have any length, but of course, there can be a memory issue on the computer if the String input is too long. The output is always 32 characters.
回答4:
The algorithm has been designed to support arbitrary input length. I.e you can compute hashes of big files like ISO of a DVD...
If there is a limitation for the input it could come from the environment where the hash function is used. Let's say you want to compute a file and the environment has a MAX_FILE limit.
But the output string will be always the same: 32 hex chars (128 bits)!
回答5:
A 128-bit MD5 hash is represented as a sequence of 32 hexadecimal digits.
回答6:
There is no limit to the input of md5 that I know of. Some implementations require the entire input to be loaded into memory before passing it into the md5 function (i.e., the implementation acts on a block of memory, not on a stream), but this is not a limitation of the algorithm itself. The output is always 128 bits. Note that md5 is not an encryption algorithm, but a cryptographic hash. This means that you can use it to verify the integrity of a chunk of data, but you cannot reverse the hashing. Also note that md5 is considered broken, so you shouldn't use it for anything security-related (it's still fine to verify the integrity of downloaded files and such).
回答7:
You may want to use SHA-1 instead of MD5, as MD5 is considered broken.
You can read more about MD5 vulnerabilities in this Wikipedia article.
来源：https://stackoverflow.com/questions/3394503/maximum-length-for-md5-input-output