Is there a standard way to represent the SHA1 hash as a C string and how to convert it?

Question

This question is about creating a SHA-1 hash from an array of data in C using OpenSSL .

It returns an array of 20 bytes containing a hash. Is there a standard way to represent data in string form, and not in binary format?

If so, is there a function in OpenSSL to convert to the specified string format?

If not, how to do it? Of course, I can come up with my own encoding, use base64 or not, but is there any canonical format?

Answer 1

Usually, hashes are represented as a sequence of hexadecimal digits (of course, two bytes each). You can write code to easily write such a thing using ostringstream with right modifiers:

 #include <string> #include <sstream> #include <iomanip> std::string GetHexRepresentation(const unsigned char * Bytes, size_t Length) { std::ostringstream os; os.fill('0'); os<<std::hex; for(const unsigned char * ptr=Bytes;ptr<Bytes+Length;ptr++) os<<std::setw(2)<<(unsigned int)*ptr; return os.str(); } 

Answer 2

The standard way to represent hashes is in hexadecimal strings.
In C, you can use printf("%02x", byte) to get the hexadecimal representation of each byte.

An example for MD5 should be easy to adapt it for SHA:

http://en.literateprograms.org/MD5_sum_ (C, _OpenSSL)

Answer 3

Here is an example for C:

 //function void convertSHA1BinaryToCharStr(const unsigned char * const hashbin, char * const hashstr) { for(int i = 0; i<20; ++i) { sprintf(&hashstr[i*2], "%02X", hashbin[i]); } hashstr[40]=0; } //Example call. hashbin is the 20byte hash array. char hashstr[41]; convertSHA1BinaryToCharStr(hashbin, hashstr); printf("%s\n", hashstr); 

Answer 4

Privacy Enhanced Mail (or PEM) seems to have set the standard for storing textual representations of cryptographic data. PEM stores the actual binary fragment in Base64, but also has a text header and footer.