63 lines
2.5 KiB
C#
63 lines
2.5 KiB
C#
using System.Security.Cryptography;
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using System.Text;
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namespace RelayServer.Services.Chat;
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/// <summary>
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/// AES-GCM-256 only (no RSA). Used exclusively for "at-rest" encryption of channel messages
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/// in the SurrealDB channel_messages table.
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///
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/// Why a separate service from E2EeHelper:
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/// - E2EeHelper is for *transit* between a specific sender and a specific recipient — it
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/// wraps an ephemeral AES key with the recipient's RSA public key.
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/// - ChannelCryptoService is for *storage* — the server is both the encryptor and the
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/// decryptor, and it stores the symmetric channel key in server_encryption_keys.KeyBase64.
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/// There's no recipient to wrap for.
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///
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/// Server flow for a chat message:
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/// incoming SocketEncryptedMessage (encrypted with server's RSA public key, by client)
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/// → E2EeHelper.DecryptForRecipient(serverPrivateKey) → plaintext
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/// → ChannelCryptoService.Encrypt(channelDbKey) → stored ciphertext
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/// → … later, on history fetch …
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/// → ChannelCryptoService.Decrypt(channelDbKey) → plaintext
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/// → E2EeHelper.EncryptForRecipient(clientPublicKey) → delivered ciphertext
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/// </summary>
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public sealed class ChannelCryptoService
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{
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public string GenerateKey()
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{
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return Convert.ToBase64String(RandomNumberGenerator.GetBytes(32));
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}
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public (string cipherText, string nonce, string tag) Encrypt(string plainText, string keyBase64)
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{
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var key = Convert.FromBase64String(keyBase64);
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var nonce = RandomNumberGenerator.GetBytes(12);
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var plainBytes = Encoding.UTF8.GetBytes(plainText);
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var cipherBytes = new byte[plainBytes.Length];
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var tag = new byte[16];
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using var aes = new AesGcm(key, 16);
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aes.Encrypt(nonce, plainBytes, cipherBytes, tag);
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return (
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Convert.ToBase64String(cipherBytes),
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Convert.ToBase64String(nonce),
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Convert.ToBase64String(tag)
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);
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}
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public string Decrypt(string cipherTextBase64, string nonceBase64, string tagBase64, string keyBase64)
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{
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var key = Convert.FromBase64String(keyBase64);
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var nonce = Convert.FromBase64String(nonceBase64);
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var tag = Convert.FromBase64String(tagBase64);
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var cipherBytes = Convert.FromBase64String(cipherTextBase64);
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var plainBytes = new byte[cipherBytes.Length];
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using var aes = new AesGcm(key, 16);
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aes.Decrypt(nonce, cipherBytes, tag, plainBytes);
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return Encoding.UTF8.GetString(plainBytes);
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}
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} |