Convert Encryption key to string (Decode the string)

Convert Encryption key to string or Decode the string or Covert MD5 hash to string :

Blob targetBlob = Blob.valueOf('ad89a9b94b5efd734e0c75a1480c20a4AD64439');

Blob hash = Crypto.generateDigest('MD5', targetBlob);

string hexDigest = EncodingUtil.convertToHex(hash);

system.debug('hexDigest==>'+hexDigest);