This can happen if the Rivest, Shamir, and Adelman (RSA) keys are corrupted.
In order to resolve this issue and gain Secure Shell (SSH) access to ASA, regenerate the RSA keys. Use these commands in the global config mode:
asa(config)#crypto key zeroize rsa This command removes all RSA key pairs.asa(config)#crypto key generate rsa This command generates the new key and specifies RSA as the key type.asa(config)#ca save all This command saves the keys.
Refer to PIX/ASA 7.x: SSH on the Inside and Outside Interface Configuration Example for more information