MD5 is 128 bits but why is it 32 characters?
Solution 1
32 chars as hexdecimal representation, thats 2 chars per byte.
Solution 2
I wanted summerize some of the answers into one post.
First, don't think of the MD5 hash as a character string but as a hex number. Therefore, each digit is a hex digit (0-15 or 0-F) and represents four bits, not eight.
Taking that further, one byte or eight bits are represented by two hex digits, e.g. b'1111 1111
' = 0xFF
= 255
.
MD5 hashes are 128 bits in length and generally represented by 32 hex digits.
SHA-1 hashes are 160 bits in length and generally represented by 40 hex digits.
For the SHA-2 family, I think the hash length can be one of a pre-determined set. So SHA-512 can be represented by 128 hex digits.
Again, this post is just based on previous answers.
Solution 3
A hex "character" (nibble) is different from a "character"
To be clear on the bits vs byte, vs characters.
- 1 byte is 8 bits (for our purposes)
- 8 bits provides
2**8
possible combinations: 256 combinations
When you look at a hex character,
- 16 combinations of
[0-9] + [a-f]
: the full range of0,1,2,3,4,5,6,7,8,9,a,b,c,d,e,f
- 16 is less than 256, so one one hex character does not store a byte.
- 16 is
2**4
: that means one hex character can store 4 bits in a byte (half a byte). - Therefore, two hex characters, can store 8 bits,
2**8
combinations. - A byte represented as a hex character is
[0-9a-f][0-9a-f]
and that represents both halfs of a byte (we call a half-byte a nibble).
When you look at a regular single-byte character, (we're totally going to skip multi-byte and wide-characters here)
- It can store far more than 16 combinations.
- The capabilities of the character are determined by the encoding. For instance, the ISO 8859-1 that stores an entire byte, stores all this stuff
- All that stuff takes the entire
2**8
range. - If a hex-character in an
md5()
could store all that, you'd see all the lowercase letters, all the uppercase letters, all the punctuation and things like¡°ÀÐàð
, whitespace like (newlines, and tabs), and control characters (which you can't even see and many of which aren't in use).
So they're clearly different and I hope that provides the best break down of the differences.
Solution 4
MD5 yields hexadecimal digits (0-15 / 0-F), so they are four bits each. 128 / 4 = 32 characters.
SHA-1 yields hexadecimal digits too (0-15 / 0-F), so 160 / 4 = 40 characters.
(Since they're mathematical operations, most hashing functions' output is commonly represented as hex digits.)
You were probably thinking of ASCII text characters, which are 8 bits.
Solution 5
One hex digit = 1 nibble (four-bits)
Two hex digits = 1 byte (eight-bits)
MD5 = 32 hex digits
32 hex digits = 16 bytes ( 32 / 2)
16 bytes = 128 bits (16 * 8)
The same applies to SHA-1 except it's 40 hex digits long.
I hope this helps.
hash_jr90
Updated on July 14, 2021Comments
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hash_jr90 almost 3 years
I read some docs about md5, it said that its 128 bits, but why is it 32 characters? I can't compute the characters.
- 1 byte is 8 bits
- if 1 character is 1 byte
- then 128 bits is 128/8 = 16 bytes right?
EDIT:
SHA-1 produces 160 bits, so how many characters are there?
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ba__friend about 13 years1 byte is represented by a two digit hexdecimal number, like 255 = ff.
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hash_jr90 about 13 yearsthank you so much.. i have much more clearer picture in my head now!!
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Koray Tugay almost 10 yearsso 1 byte is 2 chars which means 16 bits is 2 chars then 128/16 = 8. So 8 of 2 chars = 16 chars is required then? Why 32?
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Gaston Sanchez over 9 yearsBecause each hex character can be represented by 4 bits. So if it is 128 bits it is 128/4 = 32 hex characters. Even though each "char" will be encoded as a utf8 or ascii which will make the hex representation size 32*8= 256 bits.
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Roy Lee almost 8 yearsnote: ascii-charset contains non-printable symbols.
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Evan Carroll over 7 yearsThis is a really poor answer. My answer goes into details on this: stackoverflow.com/a/41618070/124486
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Evan Carroll over 7 yearsI tried to break this down a little more explicitly stackoverflow.com/a/41618070/124486
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David Klempfner about 6 years@KorayTugay "so 1 byte is 2 chars which means 16 bits is 2 chars" - 1 byte != 16 bits.
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KumarM over 5 yearsWhat does this "In which order is actually spec or platform dependent" actually mean? Would love to see this explained a bit more.
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Evan Carroll over 5 years@KumarM I'm going to delete that because I don't think it's actually relevant to the conversation at all, and it's badly worded.