3.1.4.3.1 AES Session-Key

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If AES support is negotiated between the client and the server, the strong-key support flag is ignored and the session key is computed with the HMAC-SHA256 algorithm [RFC4634], as shown in the following pseudocode. SHA256Reset, SHA256Input, SHA256FinalBits, and SHA256Result are predicates or functions specified in [RFC4634]. MD4 is specified in [RFC1320]. 

 ComputeSessionKey(SharedSecret, ClientChallenge, 
                   ServerChallenge)
      M4SS := MD4(UNICODE(SharedSecret)) 
  
      CALL SHA256Reset(HashContext, M4SS, sizeof(M4SS));
      CALL SHA256Input(HashContext, ClientChallenge, sizeof(ClientChallenge));
      CALL SHA256FinalBits (HashContext, ServerChallenge, sizeof(ServerChallenge));
      CALL SHA256Result(HashContext, SessionKey);
      SET SessionKey to lower 16 bytes of the SessionKey;

The key produced with AES support negotiated is 128 bits (16 bytes).