An MD5 hash for any given message is 16 bytes (128 bits) in size and is represented by a unique 32 digit Hexadecimal number.

Erlang has a built-in-function to calculate the MD5, which returns the hash in the form of a binary data-structure.

```
$ erlang:md5("hello").
<93,65,64,42,188,75,42,118,185,113,157,145,16,23,197,146>
```

But what we usually need is a string representation of the Hexadecimal value. Hence, we need to convert the Erlang Binary to a Hex-string, for which I didn’t find any BIF.

So first of all, lets convert the Binary to a list (of integers)- so that we can process it easily. Erlang has a function for converting binary to list:

```
$ binary_to_list(<93,65,64,42,188,75,42,118,185,113,157,145,16,23,197,146>).
[93,65,64,42,188,75,42,118,185,113,157,145,16,23,197,146]
```

Now, we have to convert each of the integers in the list into its hex equivalent. How do you convert an integer in Decimal system to a Hexadecimal system?

Eg. Take an integer 230. Divide it by 16.

230 div 16 = 14 In Hex, 14 is E

230 rem 16 = 6 In Hex, 6 is 6

So 230 in Hex is E6.

Now we will have to do the same for every integer in the list. I did it using the *lists:map* function and the applying the *int_to_hex* conversion to every integer:

```
$ lists:map(fun(X) ->
int_to_hex(X) end, L).
```

This will actually return a list of integers representing the (hex) string, which is how a string is represented in Erlang – a list of Integers.

The complete code is below:

-module(md5). -export([md5_hex/1]). md5_hex(S) -> Md5_bin = erlang:md5(S), Md5_list = binary_to_list(Md5_bin), lists:flatten(list_to_hex(Md5_list)). list_to_hex(L) -> lists:map(fun(X) -> int_to_hex(X) end, L). int_to_hex(N) when N < 256 -> [hex(N div 16), hex(N rem 16)]. hex(N) when N < 10 -> $0+N; hex(N) when N >= 10, N < 16 -> $a + (N-10).

Output:

```
$ md5:md5_hex("hello").
"5d41402abc4b2a76b9719d911017c592"
```

Line 10, why write `fun(X) -> int_to_hex(X) end` instead of the simpler (and maybe faster?) `fun int_to_hex/1`?

The whole “int_to_hex” code could also be replaced with `fun(X) -> io_lib:format(“~.16b”, [X]) end`, but I’m not sure it would be much better (if better at all).

And using binary comprehension, we can do away with the rest of the code and use:

md5_hex(S) ->

lists:flatten(

[io_lib:format("~.16b",[N]) || <> <= erlang:md5(S)]).

wathout: oneliner

>> io:format(lists:flatten( array:to_list(array:new([{size,16},{default,"~.16b"}])) “~n” ), binary_to_list(erlang:md5(“hello”))).

5d41402abc4b2a76b9719d911017c592

I know this looks awful. But look at io:format documentation, it can

print hex numbers.

Or a much shorter and slightly more obscure version:

-module(md5).

-export([md5_hex/1]).

md5_hex(S) -> [hex(N) || <> <= erlang:md5(S)].

hex(N) when N $0 N;

hex(N) when N >= 10, N $a (N-10).

Whoops my comment does not look right because of angle brackets I’ll try once more:

module(md5).

-export([md5_hex/1]).

md5_hex(S) -> [hex(N) || <<N:4>> <= erlang:md5(S)].

hex(N) when N < 10 -> $0 N;

hex(N) when N >= 10, N < 16 -> $a (N-10).

You can also use list comprehensions to make it simpler:

14> MD5 = fun(S) -> [ HexChar || <> <= erlang:md5(S), [HexChar] <- io_lib:format(“~.16b”, [Nibble]) ] end.

#Fun

15> MD5(“hello”).

“5d41402abc4b2a76b9719d911017c592″

Looks like your website eats up characters. Please advise as to how to format my reply appropriately. Thank-you.

Thanks guys. All of your solutions are very much valid and appreciated. I am learning my way through, as I go.

[...] %% Crypto doesn’t have a hexdigest method. I found the code below %% here %% Convert Integer from the SHA to Hex list_to_hex(L)-> lists:map(fun(X) -> int_to_hex(X) end, L). [...]

I am also learning Erlang and was having a problem doing this. Thank you for the post and also thanks to other commenters.

[...] Sudarshan Acharya » Blog Archive » MD5 in Erlang [...]

1> lists:flatten([io_lib:format("~2.16.0b",[N])||N<-binary_to_list(erlang:md5(“abc”))]).

“900150983cd24fb0d6963f7d28e17f72″

Or use a little bit of magic bit syntax:

1> <> = erlang:md5(“abc”), lists:flatten(io_lib:format(“~32.16.0b”, [M])).

“900150983cd24fb0d6963f7d28e17f72″

Read more about bit syntax here: http://erlang.org/documentation/doc-5.4.12/doc/programming_examples/bit_syntax.html

Nice form that strips out random characters within *lesser than* and *greater *than*

the left hand side of the = should be: lesser_than M colon 128 greater_than

Thanks

Erlang has great bignum support. All the mapping/comprehension solutions miss the point.

Here’s the canonical conversion function:

hexstring(<>) ->

lists:flatten(io_lib:format(“~32.16.0b”, [X])).

As you can see, it takes a 128-bit binary value, and formats it as a big-endian unsigned 0-padded base-16 lowercase number. Which is exactly what you want.

hexstring(erlang:md5(“foobar”)).

“3858f62230ac3c915f300c664312c63f”

Blech. Crappy tag stripping…

The argument doing the match is:

< < X:128/big-unsigned-integer > >

@JonWatte, Interesting. Would have saved some trouble! Thanks for your post on the topic. I am linking it here: http://www.enchantedage.com/node/199