The IPSec tunnel does not come up on the PIX Firewall 525 when configured with AES 256-bit encryption. The IPSECmanual_key_stuffing): not enough auth keymat, 20 bytes needed for addr /prot 50/spi debug message is received

[TCC_2]

Core issue

This issue is due to the presence of Cisco bug ID CSCsb48916.

When there is an attempt to configure IPSec LAN-to-LAN tunnels with manual keys and specify the Advanced Encryption Standard (AES) with a 256-bit encryption (esp-aes-256) in the transform set, the encapsulation fails.

Resolution

To resolve this issue, perform one of these steps:

• Change the IPSec keying method from IPSec to Internet Security Association and Key Management Protocol (ISAKMP).
  
  
• Change the transform set to use an encryption type other than esp-aes-256 (such as esp-aes), or use ISAKMP for tunnel negotiation.
  
  
• Upgrade to PIX Firewall version 6.3(5.103)  or the latest available version.

For more information, refer to the crypto ipsec transform-set command.