SSL Forward Proxy Overview

Secure Sockets Layer (SSL) is an application-level protocol that provides encryption technology for the Internet. SSL, also called Transport Layer Security (TLS), ensures the secure transmission of data between a client and a server through a combination of privacy, authentication, confidentiality, and data integrity. SSL relies on certificates and private-public key exchange pairs for this level of security.

Server authentication guards against fraudulent transmissions by enabling a Web browser to validate the identity of a Web server. Confidentiality mechanisms ensure that communications are private. SSL enforces confidentiality by encrypting data to prevent unauthorized users from eavesdropping on electronic communications. Finally, message integrity ensures that the contents of a communication have not been tampered with.

SSL forward proxy is a transparent proxy; that is, it performs SSL encryption and decryption between the client and the server, but neither the server nor the client can detect its presence. SSL forward proxy ensures that it has the keys to encrypt and decrypt the payload:

For the server, SSL forward proxy acts as a client—Because SSL forward proxy generates the shared pre-primary key, it determines the keys to encrypt and decrypt.
For the client, SSL forward proxy acts as a server—SSL forward proxy first authenticates the original server and replaces the public key in the original server certificate with a key that is known to it. It then generates a new certificate by replacing the original issuer of the certificate with its own identity and signs this new certificate with its own public key (provided as a part of the proxy profile configuration). When the client accepts such a certificate, it sends a shared pre-primary key encrypted with the public key on the certificate. Because SSL forward proxy replaced the original key with its own key, it is able to receive the shared pre-primary key. Decryption and encryption take place in each direction (client and server), and the keys are different for both encryption and decryption.

Figure 1 depicts how SSL inspection (on an existing SRX Series IPS module) is typically used to protect servers. SSL inspection requires access to private keys used by the servers so that the SRX Series device can decrypt the encrypted traffic.

Figure 1: SSL Inspection on an Existing SRX Series Firewall

Figure 2 shows how SSL forward proxy works on an encrypted payload. When application firewall (AppFW), intrusion prevention system (IPS), or application tracking (AppTrack) is configured, SSL forward proxy acts as an SSL server terminating the SSL session from the client and a new SSL session is established to the server. The device decrypts and then re-encrypts all SSL forward proxy traffic. SSL forward proxy uses the following services:

SSL-T-SSL terminator on the client side.
SSL-I-SSL initiator on the server side.
Configured AppFW, IPS, or AppTrack services use the decrypted SSL sessions.

Note:

If none of the services (AppFW, IPS, or AppTrack) are configured, then SSL forward proxy services are bypassed even if an SSL proxy profile is attached to a firewall policy. IPS does not perform SSL inspection on a session if SSL forward proxy is enabled for that session. That is, if both SSL inspection and SSL forward proxy are enabled on a session, SSL forward proxy always takes precedence.

Figure 2: SSL Proxy on an Encrypted Payload

Supported Ciphers in Proxy Mode

An SSL cipher comprises encryption ciphers, authentication method, and compression. Table 1 displays a list of supported ciphers. NULL ciphers are excluded.

The following SSL protocols are supported:

SSLv3
TLS1

Table 1: Supported Ciphers in Proxy Mode
SSL Cipher	Key Exchange Algorithm	Data Encryption	Message Integrity
RSA_WITH_RC4_128_MD5	RSA key exchange	128-bit RC4	Message Digest 5 (MD5) hash
RSA_WITH_RC4_128_SHA	RSA key exchange	128-bit RC4	Secure Hash Algorithm (SHA) hash
RSA_WITH_DES_CBC_SHA	RSA key exchange	DES CBC	SHA hash
RSA_WITH_3DES_EDE_CBC_SHA	RSA key exchange	3DES EDE/CBC	SHA hash
RSA_WITH_AES_128_CBC_SHA	RSA key exchange	128-bit AES/CBC	SHA hash
RSA_WITH_AES_256_CBC_SHA	RSA key exchange	256-bit AES/CBC	SHA hash
RSA_EXPORT_WITH_RC4_40_MD5	RSA-export	40-bit RC4	MD5 hash
RSA_EXPORT_WITH_DES40_CBC_SHA	RSA-export	40-bit DES/CBC	SHA hash
RSA_EXPORT1024_WITH_DES_CBC_SHA	RSA 1024 bit export	DES/CBC	SHA hash
RSA_EXPORT1024_WITH_RC4_56_MD5	RSA 1024 bit export	56-bit RC4	MD5 hash
RSA_EXPORT1024_WITH_RC4_56_SHA	RSA 1024 bit export	56-bit RC4	SHA hash
RSA-WITH-AES-256-GCM-SHA384	RSA key exchange	256-bit AES/GCM	SHA384 hash
RSA-WITH-AES-256-CBC-SHA256	RSA key exchange	256-bit AES/CBC	SHA256 hash
RSA-WITH-AES-128-GCM-SHA256	RSA key exchange	128-bit AES/GCM	SHA256 hash
RSA-WITH-AES-128-CBC-SHA256	RSA key exchange	128-bit AES/CBC	SHA256 hash

Server Authentication

Implicit trust between the client and the device (because the client accepts the certificate generated by the device) is an important aspect of SSL proxy. It is extremely important that server authentication is not compromised; however, in reality, self-signed certificates and certificates with anomalies are in abundance. Anomalies can include expired certificates, instances of common name not matching a domain name, and so forth.

Server authentication is governed by selecting the Ignore Server Authentication option in the SSL forward proxy profile.

If the Ignore Server Authentication option is not selected, the following scenarios occur:

If authentication succeeds, a new certificate is generated by replacing the keys and changing the issuer name to the issuer name that is configured in the root CA certificate in the proxy profile.
If authentication fails, the connection is dropped.

If the Ignore Server Authentication option is defined as an action in the SSL forward proxy profile, the following scenarios occur:

If the certificate is self-signed, a new certificate is generated by replacing the keys only. The issuer name is not changed. This ensures that the client browser displays a warning that the certificate is not valid.
If the certificate has expired or if the common name does not match the domain name, a new certificate is generated by replacing the keys and changing the issuer name to SSL-PROXY: DUMMY_CERT:GENERATED DUE TO SRVR AUTH FAILURE. This ensures that the client browser displays a warning that the certificate is not valid.

Trusted CA List

SSL forward proxy ensures secure transmission of data between a client and a server. Before establishing a secure connection, SSL forward proxy checks certificate authority (CA) certificates to verify signatures on server certificates. For this reason, a reasonable list of trusted CA certificates is required to effectively authenticate servers.

Ignore Server Authentication

You can use the Ignore Server Authentication option to ignore server authentication completely. In this case, SSL forward proxy ignores errors encountered during the server certificate verification process (such as CA signature verification failure, self-signed certificates, and certificate expiry).

We do not recommend this option for authentication, because configuring it results in websites not being authenticated at all. However, you can use this option to effectively identify the root cause for dropped SSL sessions.

Root CA

In a public key infrastructure (PKI) hierarchy, the root CA is at the top of the trust path. The root CA identifies the server certificate as a trusted certificate.

Session Resumption

An SSL session refers to the set of parameters and encryption keys created by performing a full handshake. A connection is the conversation or active data transfer that occurs within the session. The computational overhead of a complete SSL handshake and generation of primary keys is considerable. In short-lived sessions, the time taken for the SSL handshake can be more than the time for data transfer. To improve throughput and still maintain an appropriate level of security, SSL session resumption provides a session caching mechanism so that session information, such as the pre-primary secret key and agreed-upon ciphers, can be cached for both the client and server. The cached information is identified by a session ID. In subsequent connections both parties agree to use the session ID to retrieve the information rather than create a new pre-primary secret key. Session resumption shortens the handshake process and accelerates SSL transactions.

SSL Proxy Logs

When logging is enabled in an SSL proxy profile, SSL proxy can generate the messages shown in Table 2.

Table 2: SSL Proxy Logs
Log Type	Description
SSL_PROXY_SSL_SESSION_DROP	Logs generated when a session is dropped by SSL proxy.
SSL_PROXY_SSL_SESSION_ALLOW	Logs generated when a session is processed by SSL proxy even after encountering some minor errors.
SSL_PROXY_SESSION_IGNORE	Logs generated if non-SSL sessions are initially mistaken as SSL sessions.
SSL_PROXY_SESSION_ALLOWLIST	Logs generated when a session is allowlisted.
SSL_PROXY_ERROR	Logs used for reporting errors.
SSL_PROXY_WARNING	Logs used for reporting warnings.
SSL_PROXY_INFO	Logs used for reporting general information.

All logs contain similar information; the message field contains the reason for the log generation. One of three prefixes shown in Table 3 identifies the source of the message. Other fields are descriptively labeled.

Table 3: SSL Proxy Log Prefixes
Prefix	Description
system	Logs generated due to errors related to the device or an action taken as part of the SSL proxy profile. Most logs fall into this category.
openssl error	Logs generated during the handshaking process if an error is detected by the openssl library.
certificate error	Logs generated during the handshaking process if an error is detected in the certificate (x509 related errors).

Perfect Forward Secrecy

Perfect Forward Secrecy is a specific key agreement protocol that provides assurance that your session keys are not compromised even if the private key of the server is compromised. By generating a unique session key for every session a user initiates, even if a single session keys gets compromised does not affect any data other than that exchanged in a specific session protected by that particular key.

The Elliptic Curve DHE (ECDHE) cipher suits are supported to enable the perfect forward secrecy on SSL forward proxy. The SSL forward proxy still uses RSA for authentication. However, it uses EC Diffie-Hellman ephemeral key exchange to agree on a shared secret.

ECDHE cipher suites are faster than the DHE counterparts and therefore, the SSL forward proxy supports only ECDHE cipher suits. The ECDHE cipher suits are based on the elliptic curve cryptography which allows you to achieve the same level of security than RSA with smaller keys. For example, a 224 bit elliptic curve is as secure as a 2048 bit RSA key.

Table 4 shows the supported ECDHE cipher suits.

Table 4: Supported ECDHE Cipher Suits
SSL Cipher	Key Exchange Algorithm	Data Encryption	Message Integrity
ECDHE-RSA-WITH-AES-256-GCM-SHA384	ECDHE RSA	256-bit AES/GCM	SHA 384 hash
ECDHE-RSA-WITH-AES-256-CBC-SHA384	ECDHE RSA	256-bit AES/CBC	SHA 384 hash
ECDHE-RSA-WITH-AES-256-CBC-SHA	ECDHE RSA	256-bit AES/CBC	SHA hash
ECDHE-RSA-WITH-AES-3DES-EDE-CBC-SHA	ECDHE RSA	3DES AES/EDE/CBC	SHA hash
ECDHE-RSA-WITH-AES-128-GCM-SHA256	ECDHE RSA	128-bit AES/GCM	SHA 256 hash
ECDHE-RSA-WITH-AES-128-CBC-SHA256	ECDHE RSA	128-bit AES/CBC	SHA 256 hash
ECDHE-RSA-WITH-AES-128-CBC-SHA	ECDHE RSA	128-bit AES/CBC	SHA hash

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