Use BCL CipherMode enum for AesCipher class

[scott-xu]

Create a dedicated AesCtrCipher class just like AesGcmCipher class.

Relate to #1560

[Rob-Hague]

Is there a benefit to this?

[scott-xu]

FYI: some previous discussion: #865 (comment)

[scott-xu]

The main purpose is to use BCL as possible as it can.

[Rob-Hague]

Thanks, I remember the previous discussion, but I don't think it is worth changing the API right now. Maybe at a later time

[Copilot]

Pull Request Overview

This PR refactors the cipher implementation to use the BCL CipherMode enum instead of a custom AesCipherMode enum, and creates a dedicated AesCtrCipher class similar to the existing AesGcmCipher class. The changes remove support for DES-EDE3-CFB encryption and eliminate custom cipher mode implementations in favor of BCL-provided functionality.

• Replaced custom AesCipherMode enum with BCL's System.Security.Cryptography.CipherMode
• Created dedicated AesCtrCipher class for CTR mode encryption
• Removed DES-EDE3-CFB support and associated test data
• Simplified cipher implementations by removing custom mode classes (CFB, CTR, OFB, CBC)

Reviewed Changes

Copilot reviewed 33 out of 33 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Renci.SshNet/Security/Cryptography/Ciphers/AesCipher.cs	Refactored to use BCL CipherMode and simplified implementation
src/Renci.SshNet/Security/Cryptography/Ciphers/AesCtrCipher.cs	New dedicated class for AES-CTR mode
src/Renci.SshNet/Security/Cryptography/Ciphers/TripleDesCipher.cs	Refactored to use BCL CipherMode
src/Renci.SshNet/PrivateKeyFile.*.cs	Updated cipher instantiation to use BCL CipherMode
src/Renci.SshNet/ConnectionInfo.cs	Updated cipher configurations to use new class constructors
test/**/*CipherTest*.cs	Updated tests to use BCL CipherMode and new AesCtrCipher class
Multiple cipher mode files	Removed custom implementations (CFB, CTR, OFB, CBC, CipherMode base)
test/Data/Key.RSA.Encrypted.Des.Ede3.CFB.*	Removed test data for unsupported DES-EDE3-CFB

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[scott-xu]

Is there a benefit to this?

One other benefit is the reduction of code. Less code means less maintenance.

[scott-xu]

IMHO, SSH.NET should be the client of SSH but not a cryptography library. That's the reason why I hide AesCipher and TripleDesCipher.

[scott-xu]

Could you please have another review of this PR please? @Rob-Hague