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87 lines
3.9 KiB
Markdown
87 lines
3.9 KiB
Markdown
# Attested TLS (aTLS)
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In a confidential computing (CC) environment, attested TLS (aTLS) can be used to establish secure connections between two parties utilizing the remote attestation features of the CC components.
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aTLS modifies the TLS handshake by embedding an attestation statement into the TLS certificate.
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Instead of relying on a Certificate Authority, aTLS uses this attestation statement to establish trust in the certificate.
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The protocol can be used by clients to verify a server certificate, by a server to verify a client certificate, or for mutual verification (mutual aTLS).
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## Client side verification
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1. The client sends a ClientHello message, setting ServerName to a random nonce.
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2. The server generates an attestation statement using the clients nonce and its CC capabilities.
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* The attestation is embedded in the server certificate using x509 certificate extensions with an object identifier (OID) to identify the CC attestation type. Take a look at the [`variant`](../variant/variant.go) package for implementation details.
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3. The client verifies the attestation statement.
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4. If successful the client can trust the server to be running the expected configuration, and finish the TLS handshake.
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```mermaid
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sequenceDiagram
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participant Client
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participant Server
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Client->>Server: ClientHello(nonce)
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Server->>Client: ServerCertificate(AttestationStatement), ServerHelloDone
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Note over Client: Verify Attestation
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Client->>Server: ClientKeyExchange
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Client->>Server: ChangeCipherSpec, Finished
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Server->>Client: #
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```
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## Server side verification
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1. The client sends a ClientHello message
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2. The server sends back a certificate and a random nonce. The nonce is encoded as the Distinguished Name of an acceptable CA.
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3. The client does not verify the servers certificate, but uses the nonce to generate an attestation based on its CC capabilities.
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* The attestation is embedded in the client certificate using x509 certificate extensions with an OID to identify the CC attestation type.
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4. The server verifies the client's attestation statement.
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5. If successful the server can trust the client to be running the expected configuration, and finish the TLS handshake.
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```mermaid
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sequenceDiagram
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participant Client
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participant Server
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Client->>Server: ClientHello
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Server->>Client: ServerCertificate, AcceptableCAs(nonce), ServerHelloDone
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Client->>Server: ClientKeyExchange, ClientCertificate(AttestationStatement)
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Client->>Server: ChangeCipherSpec, Finished
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Note over Server: Verify Attestation
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Server->>Client: ChangeCipherSpec, Finished
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```
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## Mutual aTLS
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1. The client sends a ClientHello message, setting ServerName to a random nonce.
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2. The server generates an attestation statement using the clients nonce and its CC capabilities.
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* The attestation is embedded in the server certificate using x509 certificate extensions with an OID to identify the attestation type.
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* A nonce is encoded as the Distinguished Name of an acceptable CA.
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3. The client verifies the attestation statement.
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4. The client uses the nonce to generate an attestation based on its CC capabilities.
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* The attestation is embedded in the client certificate using x509 certificate extensions with an OID to identify the CC attestation type.
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5. The server verifies the client's attestation statement.
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6. If all verifications were successful, mutual trust in each others configuration is established, and the TLS handshake can be finished.
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```mermaid
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sequenceDiagram
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participant Client
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participant Server
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Client->>Server: ClientHello(nonce)
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Server->>Client: ServerCertificate(AttestationStatement), AcceptableCAs(nonce), ServerHelloDone
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Note over Client: Verify Attestation
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Client->>Server: ClientKeyExchange, ClientCertificate(AttestationStatement)
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Client->>Server: ChangeCipherSpec, Finished
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Note over Server: Verify Attestation
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Server->>Client: ChangeCipherSpec, Finished
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```
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