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?
MD5 processes an arbitrary-length message into a fixed-length output of 128 bits, typically represented as a sequence of 32 hexadecimal digits.
- 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.
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.
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)!
A 128-bit MD5 hash is represented as a sequence of 32 hexadecimal digits.
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).
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