What is an Encrypted File?
Encryption is the process of encoding data in such a way that only authorized parties can access it. It is used to protect the confidentiality and integrity of digital data.
There are two main types of encryption:
- Symmetric-key encryption uses the same key to encrypt and decrypt data. Common algorithms include AES, DES, 3DES, etc. (Source)
- Asymmetric or public-key encryption uses a public key to encrypt and a private key to decrypt. RSA is a commonly used public-key algorithm. (Source)
In addition to encryption, cryptographic hash functions like MD5, SHA-1 are used to generate a fixed-length digest of the input data. This is used to verify data integrity.
The main purposes of encryption are:
- Confidentiality – prevent unauthorized access
- Integrity – detect changes or tampering
- Authentication – prove identity
Reasons to Decrypt a File
There are several common reasons why someone may need to decrypt a file:
- To access the content in an encrypted file – Encryption prevents anyone without the decryption key from viewing the contents of a file. Decrypting the file allows you to view and access the original plain text or data again.
- To edit or modify an encrypted file – While a file is encrypted, it is read-only. Decrypting the file allows you to freely edit the contents again.
- To retrieve a lost encryption key – If the key used to encrypt a file is lost, decrypting the file by brute force may be the only way to recover the contents.
- To transfer an encrypted file to another system – Encrypted files may need to be decrypted before being opened on a new device or operating system.
In summary, the main reasons are to regain access to and control over the contents of an encrypted file when the encryption is preventing normal use and editing. Decryption returns the file to its original accessible form.
Locating the Encrypted File
The first step to decrypting a file is identifying that it is encrypted. There are a few ways to determine if a file is encrypted:
Check the file extension – Encrypted files often have extensions like .enc, .encrypted, .crypt, etc. This is not a definitive indicator though, as the original file extension may remain.
View the file properties – Right click the file, go to Properties > Details and look at the information there. Encrypted files may show the Type of file as “Encrypted”.
Attempt to open the file – Trying to open the encrypted file in its associated application will likely result in an error about the file being unreadable.
Once you’ve identified an encrypted file, you need to understand where it’s stored to locate it. It could be stored locally on a hard drive, on a network drive, or in cloud storage like Dropbox. You’ll want to note the file location and name when finding an encrypted file.
It’s also helpful to try and determine the encryption method used if possible. Sometimes encryption software leaves behind metadata about the algorithm used. This can aid in decryption later.
By fully locating and inspecting an encrypted file, you’ll gather key information needed for the decryption process. As this blog explains, this entry point analysis sets the stage for actually reversing the encryption.
Obtaining the Decryption Key
To decrypt an encrypted file, you need access to the decryption key that was used to encrypt it. There are a few different ways to obtain the decryption key:
Decryption key provided by creator: If you encrypted the file yourself, you should have the decryption key stored somewhere secure like a password manager. The person who encrypted the file may also be able to provide the key to decrypt it.
Brute force attack on password: You can attempt to crack the encryption password through brute force, trying millions of different password combinations. However this is very computationally intensive and could take an extremely long time to succeed, if at all.
Access recovery key or certificate: Some encryption methods like public key encryption use a public/private key pair. The private key serves as the decryption key. You may be able to access a recovery key or certificate that allows you to recover the private key and decrypt the file.
Using Encryption Software
There are a few options for decrypting encrypted files using software on your computer:
Built-in OS Decryption Tools
Operating systems like Windows and macOS have built-in tools to decrypt files encrypted by the OS’s own encryption methods. For example, on Windows 10 the BitLocker tool can decrypt files encrypted with BitLocker (Source 1). On macOS, the FileVault decryption tool can decrypt files encrypted with FileVault (Source 2). These built-in OS tools make it easy to decrypt files encrypted by the default disk encryption methods.
Third Party Decryption Software
There are many third party encryption/decryption programs available as well, both free and paid. Some popular options include VeraCrypt, AxCrypt, and Boxcryptor. These programs support decrypting files encrypted with a variety of methods like AES, Twofish, AES-Twofish, and more (Source 1). Third party software provides more flexibility to handle diverse encryption methods beyond the built-in OS tools.
When choosing third party decryption software, it’s important to pick a program that is compatible with the specific encryption algorithm used to encrypt the file originally. Checking the encryption method details can help narrow down software options.
Decrypting the File
Once you have obtained the decryption key or password, you can then decrypt the encrypted file using encryption software. Here are the general steps to decrypt a file:
- Import the encrypted file into your decryption software. Many encryption tools like VeraCrypt allow you to open and decrypt encrypted containers or drives.
- If prompted, enter the decryption key or password. This allows the software to decrypt the contents of the file.
- Follow any additional prompts from the decryption software to complete the decryption process. This may involve selecting options or destination folders.
- After decryption completes, export or save the now decrypted file to your desired location. The file should no longer be encrypted.
As an example, you can use VeraCrypt to decrypt an encrypted container file. Open VeraCrypt, select the “Select File” option, then browse to and select your encrypted container. Enter the password when prompted to decrypt the container. Finally, mount the container as a drive letter to access the decrypted contents (Microsoft).
Storing the Decrypted File
Once a file has been decrypted, you will need to decide where to store the unencrypted version. There are several options:
Save decrypted file locally or to external media
You can save the decrypted file to your local hard drive or an external storage device like a USB drive or external hard drive. This keeps the unencrypted file on hardware you control and limits access to anyone with physical access. However, it is still vulnerable to hardware failure, theft, or accidental deletion, so maintain regular backups.
Upload decrypted file to cloud storage
Cloud storage services like Dropbox or Google Drive allow access from many devices and provide secure remote backups. Enable encryption options offered by the cloud provider for an added layer of protection. Restrict permissions to limit which cloud users can view or edit the file. Keep in mind there are risks in allowing a third party to store unencrypted data.
Control access with permissions
Regardless of where you store the decrypted file, restrict access through user and group permissions. Allow only authorized users to view, edit, copy or delete the unencrypted file. Frequently review and update permissions to reflect staff changes.
Disposing of the Encrypted File
Once you have obtained the decrypted version of the file, you may no longer need the original encrypted file. It is recommended to securely delete the encrypted file to prevent unauthorized access.
You can permanently delete the encrypted file by using a secure deletion tool like Heidi Eraser (source). This overwrites the file’s data making it unrecoverable. Follow these steps:
- Download and install Heidi Eraser from the developer’s website.
- Locate the encrypted file you want to delete.
- Right click the file and choose “Wipe with Heidi Eraser”.
- Select a secure deletion method like “Gutmann 35 Pass”.
- Click “Erase” to permanently wipe the file.
Securely deleting the original encrypted file helps mitigate security risks by preventing the encrypted data from being recovered if the storage media fell into the wrong hands.
Mitigating Security Risks
When handling decrypted files, it’s important to take steps to mitigate security risks and prevent unauthorized access to sensitive data. Here are some tips:
– Avoid storing unencrypted sensitive data longer than absolutely necessary. Only keep decrypted versions of files for as long as you need to access or modify them. Delete or securely overwrite decrypted files when you no longer need them.
– Consider using new encryption on the decrypted file if you need to store it long term. This will ensure continued protection of the data if the original encryption is compromised. Use strong encryption algorithms like AES-256.
– Frequently update passwords, passphrases, and encryption keys to maintain security. This will limit the damage if any keys become compromised. Use a password manager to generate and store strong, unique keys.
– Tightly limit access to decrypted files, such as allowing read-only access where possible. The fewer people who can access plaintext data, the lower the risk.
– Enable disk encryption on devices storing decrypted files to add an extra layer of protection.
Following security best practices around encryption and access control will help mitigate risks when handling decrypted data. The key is limiting exposure of plaintext data as much as possible.
When Decryption is Not Possible
In some cases, decrypting an encrypted file may not be possible. Common reasons decryption can fail include:
Lost passphrase and keys – Encryption keys, passphrases, or passwords used to encrypt the file may be lost or forgotten over time, making decryption impossible without the original credentials. Proper key management and recovery procedures should be in place to avoid this issue.
Unsupported or outdated encryption – As encryption technologies evolve, older formats can become unsupported, obsolete, or insecure. Trying to decrypt with aging software or incompatible newer software often fails. Upgrading encryption schemes before they fade out helps avoid these problems.
Legal or compliance barriers – Encrypted data may fall under legal holds, compliance rules, or regulations that bar access even with proper keys. Attempting decryption could lead to sanctions. Understanding the legal context is important before decryption.
When decryption fails for these reasons, recovery options are limited. Passphrase guessing, cryptanalysis attacks, and legal appeals may occasionally work as last resorts. But in most cases, the encrypted data will remain unreadable without the proper decryption keys and legal clearance.
