MD Operation

The message digest (MD) algorithm allows a fixed-length digest to be generated from data of arbitrary size by using the hash algorithm. The MD algorithm is also referred to as a hash algorithm or a one-way hash algorithm.

When the same digest algorithm is used, the generated digest (hash value) has the following features:

• The same message always results in the same hash value.

• The digest generated is of the fixed length no matter the length of messages. (The digest length is determined by the algorithm used).

• It is almost impossible to find two different messages with the same hash value. (The probability still exists, depending on the length of the digest.)

Supported Algorithms and Specifications

The Supported Type column in the following table lists the algorithm to be used when a Md instance is created.

How to Develop

During the MD operation, you can use update() to pass in all the data at a time or pass in data by segment. For the same piece of data, the result will be the same no matter how the data is passed. Use the appropriate method based on the data size.

The following provides examples of MD operations with different data passing methods.

MD (Passing In Full Data)

1. Use cryptoFramework.createMd with the MD algorithm SHA256 to create a message digest (Md) instance.

2. Use Md.update to pass in the full data. The data to be passed in by a single update() operation is not size bound.

3. Use Md.digest to generate an MD.

4. Use Md.getMdLength to obtain the MD length, in bytes.

Example: Pass in the full data to calculate an MD using await.

import cryptoFramework from '@ohos.security.cryptoFramework';
import buffer from '@ohos.buffer';

async function doMd() {
  let mdAlgName = "SHA256"; // Algorithm to use.
  let message = "mdTestMessgae"; // Message to be digested.
  let md = cryptoFramework.createMd(mdAlgName);
  // If the data to be processed is short, use update() to pass in the full data at a time. The data to be passed in by a single **update()** operation is not size bound.
  await md.update({ data: new Uint8Array(buffer.from(message, 'utf-8').buffer) });
  let mdResult = await md.digest();
  console.info('Md result:' + mdResult.data);
  let mdLen = md.getMdLength();
  console.info("md len: " + mdLen);
}

MD (Passing In Data by Segment)

1. Use cryptoFramework.createMd with the MD algorithm SHA256 to create a message digest (Md) instance.

2. Call Md.update multiple times to pass in 20 bytes each time.

3. Use Md.digest to generate an MD.

4. Use Md.getMdLength to obtain the MD length, in bytes.

Example: Pass in data by segment to calculate an MD using await.

import cryptoFramework from '@ohos.security.cryptoFramework';
import buffer from '@ohos.buffer';

async function doLoopMd() {
  let mdAlgName = "SHA256"; // Algorithm to use.
  let md = cryptoFramework.createMd(mdAlgName);
  // In this example, the message is of 43 bytes. After decoded in UTF-8 format, the message is also of 43 bytes.
  let messageText = "aaaaa.....bbbbb.....ccccc.....ddddd.....eee";
  let messageData = new Uint8Array(buffer.from(messageText, 'utf-8').buffer);
  let updateLength = 20; // Pass in 20 bytes each time. You can set this parameter as required.
  for (let i = 0; i < messageData.length; i += updateLength) {
    let updateMessage = messageData.subarray(i, i + updateLength);
    let updateMessageBlob: cryptoFramework.DataBlob = { data: updateMessage };
    await md.update(updateMessageBlob);
  }
  let mdOutput = await md.digest();
  console.info("md result: " + mdOutput.data);
  let mdLen = md.getMdLength();
  console.info("md len: " + mdLen);
}