This is the first crypto challenge in set 1 from cryptopals, which is the qualifying set.

The difficulty level is relatively easy, to solve this problem I have used the programming language Go.

Problem description

Convert a string with hex values to a base64 string. The rules are:

Always operate on raw bytes, never on encoded strings. Only use hex and base64 for pretty-printing.

The input is:

49276d206b696c6c696e6720796f757220627261696e206c696b65206120706f69736f6e6f7573206d757368726f6f6d

which should give the following output

SSdtIGtpbGxpbmcgeW91ciBicmFpbiBsaWtlIGEgcG9pc29ub3VzIG11c2hyb29t

Solution

The go language has great support for this kind of problems through their standard libraries in encoding package.

There are two of interest in this case.

import (
    "encoding/base64"
    "encoding/hex"
)

The names of these packages are pretty much self-explanatory, base64 implements base64 encoding and hex implements hexadecimal encoding and decoding.

First step is to convert the hexadecimal string to a slice of bytes. The hex package has a perfect function for this, DecodeString, which takes a hexadecimal string.

    bytes, err := hex.DecodeString(theString)
    if err != nil {
        fmt.Println("Error decoding hexadecimal string:", err)
        return
    }

The last step of the problem is to encode this slice of bytes to base64. Naturally we will look after a good function for this job in the base64 package. There are two candidates for this, EncodeToString and Encode. We will go with EncodeToString, since we want to print a string to see if we got the correct result.

str := base64.StdEncoding.EncodeToString(bytes)

Now we have all the pieces to solve the problem, and the program could look like this.

package main

import (
    "encoding/base64"
    "encoding/hex"
    "fmt"
)

func convertToBase64(hexString string) (string, error) {
    bytes, err := hex.DecodeString(hexString)
    if err != nil {
        return "", err
    }

    return base64.StdEncoding.EncodeToString(bytes), nil
}

func main() {
    theString := "49276d206b696c6c696e6720796f757220627261696e206c696b65206120706f69736f6e6f7573206d757368726f6f6d" // Replace with your hex string

    str, err := convertToBase64(theString)
    if err != nil {
        fmt.Println("Error decoding hexadecimal string:", err)
        return
    }

    fmt.Println("Base64:", str)
}

Conclusion

The problem is easy to solve when having access to Go's standard packages and its great documentation.

References

• Set 1 - Challenge 1
• The Base16, Base32, and Base64 Data Encodings
• encoding/base64
• encoding/hex