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asp.net – Compute SHA256 Hash in Android/Java and C#

Posted by: admin April 23, 2020 Leave a comment

Questions:

I am trying to generate a SHA256 hash in android, that I then pass to an ASP.NET Web API web service and compare the hash there. As such, I need to construct a hash in Android, that given the same inputs in ASP.NET will generate an equivalent hash. I’m pulling my hair out trying to figure out what I’m doing wrong.

Here’s the Android code:

public String computeHash(String input) throws NoSuchAlgorithmException{
    MessageDigest digest = MessageDigest.getInstance("SHA-256");
    digest.reset();
    try{
      digest.update(input.getBytes("UTF-8"));
    } catch (UnsupportedEncodingException e){
      e.printStackTrace();
    }

    byte[] byteData = digest.digest(input.getBytes());
    StringBuffer sb = new StringBuffer();

    for (int i = 0; i < byteData.length; i++){
      sb.append(Integer.toString((byteData[i] & 0xff) + 0x100, 16).substring(1));
    }
    return sb.toString();
}

AND HERE’s THE CODE ON THE SERVER (c#):

    private static string ComputeHash(string input, HashAlgorithm algorithm)
    {

        Byte[] inputBytes = Encoding.UTF8.GetBytes(input);
        Byte[] hashedBytes = algorithm.ComputeHash(inputBytes);

        StringBuilder sb = new StringBuilder();

        for (int i = 0; i < hashedBytes.Length; i++)
        {
            sb.Append(String.Format("{0:x2}", hashedBytes[i]));
        }

        return sb.ToString();
    }

UPDATE:
Here is the corrected Android/Java implementation (thank you Nikolay Elenkov):

public String computeHash(String input) throws NoSuchAlgorithmException, UnsupportedEncodingException{
    MessageDigest digest = MessageDigest.getInstance("SHA-256");
    digest.reset();

    byte[] byteData = digest.digest(input.getBytes("UTF-8"));
    StringBuffer sb = new StringBuffer();

    for (int i = 0; i < byteData.length; i++){
      sb.append(Integer.toString((byteData[i] & 0xff) + 0x100, 16).substring(1));
    }
    return sb.toString();
}
How to&Answers:

Your Java code is wrong: you are adding the input bytes twice. If you are calculating this in one go, you need to either call only digest(bytes) or call digest() after update(bytes);

Answer:

I was looking for a Kotlin version for my Android app.

I could not find one; here is what I came up with:

fun String.getSha256(): String {
    val digest = MessageDigest.getInstance("SHA-256").apply { reset() }
    val byteData: ByteArray = digest.digest(this.toByteArray())
    return StringBuffer().apply {
        byteData.forEach {
            append(((it.toInt() and 0xff) + 0x100).toString(16).substring(1))
        }
    }.toString()
}